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,^ ARCHIVES-REKRENCE
«'^^«'YES-R£gRENCECEf
CENTER
by William R. Corliss and Francis C. Schwe.nk
'i,
An Understanding the Atom Series Booklet
^^H^JX? !!
The Understanding the Atom Series
Nuclear energy is playing a vital role in thfe life of every
man, woman, and child in the United States today In the
years ahead it will affect increasingly all the peoples of the
earth. It is essential that all Americans gam an understanding
of this vital force if they are to discharge thoughtfully their
responsibilities as citizens and if they are to realize fully the
myriad benefits that nuclear energy offers them.
This booklet is one of the "Understanding the Atom"
Series Comments a r e invited on this booklet and others
in the s e r i e s , please send them to the Division of Technical
Information, U. S Atomic Energy Commission, Washington,
D. C 20545.
Published as part of the AEC's educational assistance
program, the s e r i e s includes these titles:
The United States Atomic Energy Commission provides
this booklet to help you achieve such understanding.
'^
Edward J Brunenkant, Director
Division of Technical Information
UNITED STATES ATOMIC ENERGY COMMISSION
Dr Glenn T Seaborg, Chairman
James T Ramey
Wilfrid E. Johnson
Dr Clarence E. Larson
THE COVER
The cover shows a reusable nuclear
stage, powered by the NERVA engine,
leaving an earth orbit on a round-trip
mission to the moon. The NERVA engine " f i r e s " at least four times on
this mission To leave earth orbit, to
go into orbit around the moon, to leave
the moon, and to return to orbit around
earth. This stage was delivered to orbit,
where it was assembled, and refueled
by multiple flights of the two stage
space shuttle. To provide sufficient payload to support a lunar station, the
nuclear stage c a r r i e s 150 tons of liquid
hydrogen in eight tanks After the stage
r e t u r n s to earth orbit, it will be r e fueled, repaired, if needed, and checkedout for additional missions to the moon,
to synchronous orbit, or to interplanetary space.
Accelerators
Anavals tn Atomic Research
Atomic Fuel
Atomic POHcy Safety
Atoms at the Science laii
Atoms m Agiicultnie
Atoms Wature and Van
Book^ on Atomic Energy foy Adults
and Children
Calipers m Atomic Energy
Computers
Controlled Sudt ar I usion
Cryogenics the incommon Cold
Diiect Conicyiion o/ Pnetgy
Fallout From N'uclear Tests
Food Preservation by li radiation
Genetic Effects of Radiation
Index to the UAS Series
Lasers
Microstructure of Matter
"Neutron Activation Analysis
Nondestructiie Te'iting
Nuclear Clocks
Nuclear Energy for Desalting
Nuclear Power and Merchant Shipping
Nuclear Power Plants
Nuclecn Propulsion for Space
Niicleat Rcactots
Nuclear Terms A Biicf Glossary
Our Momic World
Ploushars.
Plutonium
Poller from Radioisotopes
Pouei Reactoys in Small Packages
Radioactni l^astes
Radioisotopes and Life P)ocesses
Radioisotopes in Indwitry
Radioisotopes m Mtdicme
Rat e Earths
Research Reactors
SVAP Nuclcay Space Reactors
Sources of Nuclear Fuel
Space Radiation
Spectroscopy
Synthetic Transuranium Clements
The Atom and the Ocean
The Chemistry of the Noble Gases
The Elusive Neutrino
The First Reactoi
The Natui al Radiation Environment
Whole Body Counters
Your Body and Radiation
A Single copy of any one booklet, or of no more than three
different booklets, may be obtained free by writing to
USAEC, P 0 BOX 62, OAK RIDGE, TENNESSEE
37830
Complete sets of the s e r i e s a r e available to school and
public librarians, and to teachers who can make them
available for reference or for use by groups. Requests
should be made on school or library letterheads and indicate the proposed u s e .
Students and teachers who need other material on specific aspects of nuclear science, or references to other
reading material, may also write to the Oak Ridge address
Requests should state the topic of interest exactly, and the
use intended.
In all requests, include " Z i p Code" in return addiess
Printed in the United States of America
USAEC Division of Technical Information Extension Oak fUdge, Tennessee
r
by William R. Corliss and Francis C. Schwenk
CONTENTS
ENERGY FOR SPACE TRAVEL
HOW A NUCLEAR ROCKET WORKS
FROM THE FIRST SPECULATIONS TO NERVA
BUILDING AN ENGINE FOR SPACE
The Biggest, Hottest Reactors Ever Built
The Artificial Maelstrom
Where the Work is Done
Holding the Pieces Together
Bootstrapping and the Full Flow Turbine Drive
NINETY MILES OUT IN THE DESERT
ENGINES FORTHE MOON AND BEYOND
GLOSSARY OF ACRONYMS AND CODE NAMES
READING LIST
MOTION PICTURES
United States Atomic Energy Commission
Division of Technical Information
Library of Congress Catalog Card Number 79 171030
1968 1971(rev i
Interest mounts high as technicians and engineers monitor a space propulsion test m the Reactor
Control building at the Nuclear Rocket Development
Station, Nevada Instruments
record performance data
By William R. Corliss
and F r a n c i s C. Schwenk
ENERGY FOR SPACE TRAVEL
The secret of space travel is energy—immense
amounts
of energy. The first stage of the Satiirn-V moon rocket generates as much energy each second as a million
automobile
engines. The energy leverage of nuclear fission
eventually
will give lis spaceships
that can sweep across the solar
system and carry men to the farthest
planets.
But the uranium nucleus is not broken to man's bidding
easily. Our quest is for a means of uniting the almost
limitless energy of the atomic nucleus and the rocket's
unique
ability to thrust through the vacuum of space. This difficult
wedding of nuclear fission
(age, 30 years) to the rocket
(age, 1000 years) is the subject of this booklet.
In the words of Dr. Glenn T. Seaborg, Chairman of the
U. S. Atomic Energy Commission:
"What we a r e attempting to make is a flyable compact r e a c t o r , not much bigger
than an office desk, that will produce the power of Hoover
Dam from a cold start in a matter of minutes."
1
HOW A NUCLEAR ROCKET WORKS
The rocket concept was grasped by the first caveman
when he pushed off from a lakeshore on a raft; every action
has an equal and opposite reaction according to Newton's
Third Law of Motion.* The caveman's action was pushing
the shore away with his foot; the reaction was the surge of
the raft onto the lake.
Actually, it is not n e c e s s a r y to have something solid to
push against. The caveman could have propelled himself out
on the lake by hurling rocks shoreward; as each rock left
his hand, he and the raft would have moved a bit farther.
It is the same way in a i r l e s s space; propulsion in a given
direction means throwing something away in the opposite
direction. Instead of rocks, the ordinary chemical rocket
expels a roaring jet of hot combustion g a s e s . But the effect
is the s a m e . To make a nuclear rocket, the uranium nucleus must be fissioned! in such a way that something
is expelled from the spaceship. We'll explain how later in
this booklet.
Chemical rocket engines, jet engines, automobile eng i n e s — in fact, most of mankind's engines — extract heat
from a fuel and turn it into motion through the expansion
of hot gases. The nuclear rocket sprouts from the same
family t r e e ; it also c r e a t e s hot h i g h - p r e s s u r e gas and turns
it into thrust. The nuclear rocket is a direct descendent of
the aeolipile, a steam-spouting "reaction engine" reputedly
built a century or two before Christ by the famous Alexandrian engineer, Hero.
It is relatively easy to see how a hot gas expands against
a piston in an internal combustion engine to do useful work.
The piston p r e s e n t s the gas with something solid to push
against. And the rocket engine operates in much the same
way, except that the piston is replaced by the rocket
nozzle. The hot gases created by chemical combustion or
nuclear heaters issue from the throat of the rocket nozzle
mim
FRANCIS C. SCHWENK has 20 y e a r s experience in aeronautical and
space propulsion r e s e a r c h and development—the last 11 y e a r s in
the Nuclear Rocket P r o g r a m . In his present position of Assistant
Manager for Vehicles and Propulsion Technology in the A E C NASA Space Nuclear Systems Office, he is responsible for p r o g r a m s on nuclear rocket stages, advanced propulsion r e s e a r c h ,
and applications studies. Mr. Schwenk received a B.S. degree from
the Pennsylvania State University and an M.S. degree from the
University of Illinois, both in the field of mechanical engineering.
While at the NACA Lewis Flight Propulsion Laboratory, he authored
many technical r e s e a r c h reports on c o m p r e s s o r s for turbojet
engines and received additional education in nuclear physics and
r e a c t o r engineering. When the National Aeronautics and Space
Administration was formed, he served in the NASA headquarters
office, planning for development of launch vehicles for space m i s sions. He was coauthor with M. W. Rosen of the 1959 article,
"A Rocket for Manned Lunar Exploration" (IRE Transactions,
December 1969), which first described the basic characteristics
of a lunar mission — later to become Apollo, and coauthor with
E. W. Hall of "Current Trends in Large Booster Development",
(Aerospace Engineering, May 1960). Since joining the Nuclear
Rocket Program, he has written numerous articles and speeches,
the most recent being " P r o s p e c t s for High-Thrust Nuclear Propulsion S y s t e m s ' ' with C. E. Franklin (Chemical Propulsion Information Agency).
*Sir Isaac Newton, the great British intellect who lived from
1642 to 1727, formalized the basic laws of motion as we know
them today.
t F o r an explanation of nuclear fission, see Our Atomic World,
another booklet in this s e r i e s .
2
59
a n d e x p a n d a g a i n s t i t s f l a r e d s i d e s , p u s h i n g the n o z z l e
(and t h e w h o l e r o c k e t ) u p w a r d . T h e p r e s s u r e a g a i n s t t h e
n o z z l e w a l l s i s t h e r e a c t i o n to t h e e x p u l s i o n o r p u s h i n g
a w a y of t h e hot e x h a u s t g a s e s .
WILLIAM R. CORLISS is an atomic energy consultant and writer with
12 y e a r s of industrial experience including service as Director of
Advanced P r o g r a m s for the Martin Company's Nuclear Division.
P r i o r industrial connections were with the Flight Propulsion
Laboratory of the General Electric Company and with Pratt &
Whitney Aircraft Company. Mr. Corliss has B.S. and M.S. degrees
in Physics from Rensselaer Polytechnic Institute and the University of Colorado, respectively. He has taught at those two institutions and at the University of Wisconsin. He is the author of
Propulsion Systems for Space Flight
(McGraw-Hill 1960), Space
Probes and Planetary Exploration
(Van Nostrand 1965), Sc/e^ziZ/Vc
Satellites (Government Printing Office, 1967), Mysteries
of the
Universe (Crowell, 1967), Mysteries Beneath the Sea (Crowell,
1970), and coauthor of Radioisotopic Power Generation (PrenticeHall, 1964), Human Factors Applications
in
TeleoperatorDesign
and Operation (Wiley, 1971), and Man and Atom (Dutton, 1971), as
well as numerous articles and papers for technical journals and
conferences. In this s e r i e s he has written Neutron
Activation
Analysis, Power Reactors in Small Packages, Direct
Conversion
of Energy, Computers, Space Radiation,
and was coauthor ol
Power from
Radioisotopes.
The first reaction engine, bi Hero's aeolipile,
squirting from the two pipes caused rotation.
steam
Looking at it a n o t h e r way, e a c h m o l e c u l e in t h e e x h a u s t
i s l i k e a s m a l l b u l l e t s h o t f r o m a b i g gun (the r o c k e t e n g i n e ) . I n d e e d , if we think of t h e r o c k e t e n g i n e a s a c o n t i n u o u s l y f i r i n g s h o t g u n with m o l e c u l a r a m m u n i t i o n we a r e
p r e t t y close to the truth.
If t h e m a s s e s of t h e s e m o l e c u l a r " b u l l e t s " and t h e i r
" m u z z l e v e l o c i t i e s " a r e known, t h e r o c k e t t h r u s t c a n b e
computed from the s i m p l e equation:
F = rhv
w h e r e F = t h r u s t ( m e a s u r e d in n e w t o n s * ) ,
m = t h e m a s s flow r a t e of t h e b u l l e t s o r p r o p e l l a n t ,
(in k i l o g r a m s p e r s e c o n d ) , and
V = the a v e r a g e m u z z l e velocity or exhaust velocity
a l o n g t h e r o c k e t a x i s (in m e t e r s p e r s e c o n d ) .
*Named for Sir Isaac and equal to the force necessary to a c c e l e r a t e 1 kilogram of mass 1 meter per second per second; a
force of one newton = .225 lbs. Newton also introduced the idea of
placing a dot over a symbol to indicate rate of change. For example, if V = velocity, v = acceleration.
58
3
Dividing F by m reveals that the thrust produced per
unit m a s s flow of propellant is equal to v. In other words,
the higher the bullet or propellant velocity, the more thrust
t
F
F
1
t
Frontispiece courtesy Nuclear Rocket Development Statioii'WRDST
Page 9
F= net pressure x area
Los Alamos Scientific Laboratory (LASL)
Page 15
LASL
Page 17
LASL
Page 21
NRDS
Page 42
NRDS
Pages 44 •46
LASL
Page 46
NRDS
Page 47
NRDS
Page 48
NRDS
Page 53
LASL
F= mv
Hot gases expand against a piston, left, pushing it upward.
Center,
hot gases expand against a rocket nozzle, pushing it upward. The
reaction thrust, F, can be computed either from the total net pressure against the nozzle and combustion chamber area, or from mv
(right). The two F's are identical.
we get from each kilogram of gas that r o a r s out the nozzle
each second. We want to have a high exhaust velocity for
good rocket performance because we can thereby a c complish a space mission with l e s s propellant. We will
show later that the nuclear rocket produces about twice the
exhaust velocity of the best chemical rocket. High exhaust
velocity is the greatest advantage of the nuclear rocket.
The question then is: How does a nuclear rocket gene r a t e high exhaust velocities? The key to the nuclear
rocket's success lies in a simple equation from t h e r m o dynamics
4
57
Preparing to Go Past the Moon: Testing Nuclear Rocket System
for Deep Space Probes, Business Week, 136 (February 12,1966).
The Nuclear Rocket, Space/Aeronautics,
43: 32 (April 1965).
The Next 20 Years of Interplanetary Exploration, Wernher von
Braun, Astronautics and Aeronautics, 3: 24 (November 1965).
Astronautics and Aeronautics, i (June 1965). Several articles of
interest.
Beyond Apollo with Nuclear Propulsion, Paul G. Johnson, Astronautics and Aeronautics, 2: 22 (December 1964).
Motion Pictures
Available for loan without charge from the AEC Headquarters
Film Library, Division of Public Information, U. S. Atomic Energy Commission, Washington, D. C. 20545 and from other AEC
film l i b r a r i e s .
Power for Propulsion,
15 minutes, color, 1965. Produced by
Aerojet-General Corporation. Major steps in the history of
power sources for propulsion a r e traced. Animation sequences
a r e used to illustrate the principles of rocketry, Newton's Law
of Motion, and operation of nuclear rocket engines. Development
of NERVA is covered, including its first test firing at the testing
station in Jackass Flats, Nevada. Developments for deep-space
missions to the moon and beyond a r e shown.
Project Rover, 2iy2 minutes, color, 1963. Produced by the AEC's
Los Alamos Scientific Laboratory. This is a 1962 p r o g r e s s
report on Project Rover, a program to develop a nuclear rocket
for spacecraft propulsion. Design, fabrication, and testing of a
Kiwi a r e detailed.
Atomic Energy for Space, 17 minutes, color, 1967. Produced by
the Handel Film Corporation with the cooperation of the AEC
and NASA. This film explains the two basic ways in which nuclear energy for space is being developed: A nuclear rocket
for space propulsion, and isotopic generators and reactor power
plants to produce electricity for spacecraft operations. Project
Rover is covered through animation and film shots of Kiwi and
NERVA t e s t s . The efficiencies of nuclear and chemical rockets
a r e compared. The last part of the film discusses the SNAP
(Systems for Nuclear Auxiliary Power) program.
Nuclear Propulsion in Space, 19 minutes, color, 1969. Produced by
the National Aeronautics and Space Administration and the AEC.
This film presents the story of the development of a nuclear
rocket engine for space exploration. Conventional chemical
rockets are compared with nuclear rockets through the use of
graphs, c h a r t s , and animation that show that the nuclear rocket
can be twice as efficient as its chemical counterpart. The film
explains the principles and operating characteristics of a
nuclear rocket and how its power and thrust will be controlled.
Tests are shown of the KIWI r e a c t o r in Nevada and the NERVA
(Nuclear Engine for Rocket Vehicle Application), which will
complete the technology for a nuclear rocket engine application
in space missions of the late 1970s and 1980s.
56
V oc ^fT7M
which s a y s t h a t t h e e x h a u s t v e l o c i t y , v, of any r o c k e t i s
p r o p o r t i o n a l to V T / M ;
w h e r e T = t h e t e m p e r a t u r e of t h e hot g a s e s j u s t b e f o r e t h e y
e n t e r t h e n o z z l e t h r o a t , and
M = t h e a v e r a g e m o l e c u l a r w e i g h t of t h e e x h a u s t
gases.
It i s o b v i o u s t h a t for a high v v a l u e we w i s h to m a x i m i z e
the quantity
-JTJM.
Knowing t h a t T and M c o n t r o l v, l e t u s f i r s t t r y to m a n i p u l a t e T . S i n c e T i s u n d e r t h e s q u a r e r o o t s i g n , it will h a v e
t o b e q u a d r u p l e d to double v, t h e e x h a u s t v e l o c i t y . C h e m i c a l
Nuclear rocket
Chemical rocket
Nozzle
Wm
F = 1
F = 1
V = 1
V = 2
m= 1
P= 1
Nuclear rockets have twice the exhaust velocity of
chemical rockets.
Thus, for the same level of
thrust, propellant mass flow is halved and power
is doubled.
5
r o c k e t s a l r e a d y o p e r a t e at t e m p e r a t u r e s c l o s e to 3000°K;*
if the nuclear fuel i s to s t a y in s o l i d f o r m , it i s apparent
it cannot go to 12,000 °K, which i s hotter than the s u n ' s
s u r f a c e . In fact, today's nuclear r o c k e t r e a c t o r fuels b a r e l y
s u r v i v e at 3000 °K, the s a m e g e n e r a l t e m p e r a t u r e l e v e l
achieved in c h e m i c a l r o c k e t s .
T o avoid this i m p a s s e , s o m e t h i n g m u s t be done to r e duce M, the m o l e c u l a r weight, rather than to i n c r e a s e T.
In a c h e m i c a l (combustion) r o c k e t , M d o e s not drop much
below 18, b e c a u s e the m o s t convenient o x i d i z e r s have quite
heavy a t o m s . For e x a m p l e , the advanced Centaur engine
burns hydrogen with oxygen to f o r m water (H2O) for which
M = 18. C h e m i c a l r o c k e t exhaust v e l o c i t i e s a r e thus l i m i t e d
m a i n l y by the high m o l e c u l a r w e i g h t s of the c o m b u s t i o n
p r o d u c t s . A s long a s c h e m i c a l f u e l s m u s t be burned with
oxygen or fluorine, c h e m i c a l rocket exhaust v e l o c i t i e s c a n not be g r e a t l y i m p r o v e d .
In a nuclear rocket, h o w e v e r , c o m b u s t i o n i s not required:
The nucleus f i s s i o n s without c h e m i c a l s t i m u l a t i o n , and the
propellant i s not an engine fuel but a s e p a r a t e s u b s t a n c e
that IS heated by the f i s s i o n i n g nuclei in a n u c l e a r r e a c t o r .
A nuclear rocket d e s i g n e r can heat up any propellant he
w i s h e s a s long a s it d o e s not c h e m i c a l l y attack his r e a c t o r
fuel e l e m e n t s . T h e r e i n l i e s the s e c r e t of the n u c l e a r
r o c k e t ' s high exhaust v e l o c i t y — it can make u s e of a
propellant with a low m o l e c u l a r weight.
In designing a n u c l e a r r o c k e t then, the f i r s t inclination
•would be to c h o o s e hydrogen g a s a s the propellant b e c a u s e
hydrogen i s the l i g h t e s t of all m o l e c u l e s , with M = 2. Other
p o s s i b i l i t i e s — such a s water ( H 2 0 - M = 18), methane (CH4:
M = 16), and a m m o n i a (NH3: M = 1 7 ) — o f f e r e d m o d e s t i m provements over early kerosene-burning chemical rockets,
but show no advantage o v e r the n e w e r c h e m i c a l e n g i n e s
that burn hydrogen with oxygen. In the e a r l y days of n u c l e a r
r o c k e t r y , though, a m m o n i a and methane w e r e c o m p e t i t o r s
of hydrogen b e c a u s e hydrogen w a s f e a r e d a s a f i c k l e ,
e x p l o s i o n - p r o n e m a t e r i a l that had to be s t o r e d a s a liquid
at a t e m p e r a t u r e of only 20°K ( ~ 2 5 3 ° C ) . In addition, it
*Or 2727° centigrade; degrees Kelvin a r e measured above absolute zero and a r e equal in size to centigrade degrees. 0°C =
273°K.
6
READING LIST
Technical Books
Nuclear Rocket Propulsion, Robert W. Bussard and Richard D.
DeLauer, McGraw-Hill Book Company, New York, 1958, 370 pp.,
$11.50.
Fundamentals of Nuclear Flight, Robert W. Bussard and Richard D.
DeLauer, McGraw-Hill Book Company, New York, 1965, 453 pp.,
$15.50.
Propulsion Systems jor Space Flight, WiUiamR. Corliss, McGrawHill Book Company, New York, 1960, 300 pp., $12.50, translated
and published in French as Le Vol dans L' E'ipace, Systimes de
Pt opulsion, Dunod, 1963.
Realities of Space Traiel, British Interplanetary Society, L. J.
C a r t e r (Ed.), McGraw-Hill Book Company, New York, 1957,
431 pp., $10.75. The Atomic Rocket, L. R. Shepherd and A. V.
Cleaver, p. 136. Originally published m the Journal of the British
Interplanetary Society in a s e r i e s of 1948 and 1949 articles.
Jet, Rocket, Nuclear. Ion and Electric Propulsion Theory and Design, W. H. T Loh, Ed., Spnnger-Verlag New York Inc , New
York, 1968, 779 pp., $24.80.
Principles of Spaceflight Propulsion, E. M. Goodger, Pergamon
P r e s s , Inc., Elmsford, New York, 1970, 186 pp., $8.50.
Technical Articles
The Nuclear Shuttle for Interorbital and Transplanetary Applications, K. P. Johnson and R. J. HoU, American Institute of
Aeronautics and \stronautics paper 70-679, 1970.
Manned Planetary Missions with Reusable Nuclear Shuttles, R. G.
Riedesel and R. S. Cowls, American Institute of Aeronautics
and Astronautics paper 70-040, 1970.
Nuclear Rocket Engine P r o g r a m Status —1970, D. S Gabriel and
I. L. Helms, American Institute of Aeronautics and Astronautics
paper 70-711, 1970.
Nuclear Propulsion for Exploration of the Solar System, American
Astronautical Society paper 69-330, 1969.
Popular Articles
Nuclear R o c k e t r y - T h e F i r s t Bright Hopes, Robert W Bussard,
Astronautics,
7 32 (December 1962).
The Development of Nuclear Rocket Propulsion in the United States,
William C House, Journal of the British Inter plane lay y Society,
19 306 ( M a r c h - A p r i l 1964).
Isotopic Propulsion, John S Mdrtmez, Space'Astronautics,
42. 77
(November 1964).
Astronautics,
7 (December 1962). Special issue on nuclear rockets
Nucleonics, 19 (April 1961). Special issue on nuclear rockets
55
GLOSSARY OF ACRONYMS AND CODE NAMES
AEC
ANP
Atlas
Atomic Energy Commission.
Aircraft Nuclear Propulsion program.
An Air Force ICBM using kerosene and liquid oxygen
propellants.
C e n t a u r A chemical rocket system intended for unmanned space
missions, burning liquid hydrogen —liquid oxygen.
Condor Early code name for the nuclear rocket, also the name
of an early committee studying nuclear rockets.
Dumbo An early experimental rocket r e a c t o r .
ElV Engine Installation Vehicle at NRDS.
E-MAD Engine-Maintenance, A s s e m b l y , and Disassembly
building, at NRDS.
EST Engine System Test.
ETS-1 Engine Test Stand 1 at NRDS
ICBM Intercontinental Ballistic Missile.
Kiwi A s e r i e s of rocket test r e a c t o r s built by Los Alamos
Scientific Laboratory
LASL Los Alamos Scientific Laboratory in New Mexico.
MCC Manned Control Car, prime mover for the EIV, at
NRDS
NASA National Aeronautics and Space Administration.
NERVA Nuclear Engine for Rocket Vehicle Application, an
AEC-NASA program.
NRDS Nuclear Rocket Development Station in Nevada.
NRTS National Reactor Testing Station m Idaho.
NRX NERVA Reactor Experiment.
NTS Nevada Test Site, including NRDS.
Orion Nuclear bomb propulsion study, now discontinued.
P e e w e e A r e a c t o r for testing nuclear rocket fuel elements.
Phoebus An advanced s e r i e s of nuclear rocket reactor experiments under development at LASL.
Pluto Program for developing nuclear r a m - j e t propulsion;
now discontinued.
Poodle A very small nuclear rocket concept employing decaying radioisotopes as the heat source
RIFT Reactor In-FIight Test program, now discontinued
R-MAD Reactor-Maintenance, Assembly, a n d Disassembly
building at NRDS
Rover General name for the L. S nuclear rocket program.
Saturn-5 The key launch vehicle for NASA's manned moon
exploration system, powered by clustered liquid
oxygen-liquid hydrogen engines
SNPO Space Nuclear Propulsion Office, a joint NASA-AEC
organization.
TNT Transient Nuclear Test
XE Ground experimental engine.
w a s known t h a t a m m o n i a a n d m e t h a n e d i s s o c i a t e * to a l a r g e
e x t e n t a t t e m p e r a t u r e s a r o u n d 3000 °K, a s t h e i r m o l e c u l e s
c o l l i d e v i o l e n t l y w i t h e a c h o t h e r . C o m p l e t e d i s s o c i a t i o n of
m e t h a n e into i t s five c o n s t i t u e n t a t o m s would b r i n g M down
f r o m 16 t o 3 . 2 , a n d t h i s would m a k e m e t h a n e m u c h m o r e
10,000
5000
•K
4000 —
3000
2000
1000
2000
3000
4000
Temperature T
Liquid hydrogen-l iquid oxygen engine operating at 500 pounds per square inch pressure
Plots oj exhaust velocity
Methane and
ammonia nuclear rockets do not appear
to be much better than a hydrogen-oxygen
chemical engine
A hydrogen
nuclear
rocket IS superior to all
a t t r a c t i v e a s a p r o p e l l a n t . N e v e r t h e l e s s , h y d r o g e n finally
did win out over a l l competition, mainly b e c a u s e t h e
dangerous handling p r o b l e m s w e r e licked and b e c a u s e the
n u c l e a r r o c k e t needed t h e even higher exhaust velocity
p r o m i s e d by p u r e h y d r o g e n if it w a s t o a c h i e v e i t s full
potential.
M o l e c u l a r h y d r o g e n ! a-lso d i s s o c i a t e s with h e a t ; b u t ,
b e c a u s e t h e H - H c h e m i c a l bond i s s o s t r o n g , d i s s o c i a t i o n
*Or decompose into the constituent atoms making up their
molecules.
t Hydrogen gas, H2, is a molecule containing 2 hydrogen atoms.
54
7
is negligible at today's nuclear rocket t e m p e r a t u r e s . If
atomic hydrogen could be used as a propellant, M would
equal 1, and the exhaust velocity factor would multiply that
of molecular hydrogen by V2. This is a development that
will have to wait for future exploitation, however. The
first operational nuclear rockets will spew hot molecular
hydrogen out of their nozzles. With M = 2 instead of 18, as
in hydrogen-oxygen chemical engines, the nuclear rocket
exhaust velocity still will be more than double that of the
best chemical rocket for the same t e m p e r a t u r e .
Doubling the nuclear r o c k e t ' s exhaust velocity has one
further consequence: It r e q u i r e s the nuclear heat source to
generate more power. A simple equation for the power, P,
in the exhaust gases comes from the kinetic energy (KE)
equation:
Despite the superiority indicated in these particular
missions, the r e a l significance of the nuclear rocket lies
in the fact that it r e p r e s e n t s a true advance in our overall
propulsion capability; perhaps as big a jump as the first
automobiles were over the h o r s e . We can only s u r m i s e
where such an advance in propulsion capability eventually
will take us.
but, since power is the rate of energy production, m is r e placed by m and KE by P. Thus,
mv'^
Using this equation, and F = mv, we can see that if thrust is
held fixed and exhaust velocity is doubled, propellant flow
will be halved (as desired), but the power required to do
this will be doubled. The price of increasing the exhaust
velocity is increased power. From this relationship a r i s e s
another important advantage of the nuclear rocket: The
great r e s e r v o i r of energy contained in its nuclear fuel can
be turned into high exhaust velocity.
If we keep in mind t h r e e basic facts about nuclear
rockets:
1. They convert fission-generated heat into the kinetic
energy of rocket propellant,
2. Chemical combustion is not needed, and they can use
low molecular weight propellants to attain high exhaust
velocities,
3. Their reactor fuel has a great deal of energy packed
in it,
Giant dewars containing liquid hydrogen and liquid oxygen and a
maze of pipes, valves, and other equipment are needed for the reactor tests. This is Test Cell C at NRDS.
we can understand better the thoughts of the pioneers in
8
53
nuclear rocketry as they tried to channel the power of the
atom into h i g h - t e m p e r a t u r e , lightweight reaction engines
that would c a r r y men toward the planets.
X
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U S Senator Clinton P Anderson of Neu Mexico, Chait man oj the
Senate Aei onautical and Space Sciences Committee, and a member
of the Joint (Congressional) Committee on Atomic Energy, examines a Kiwi reactor during a visit to Los Alam.os Scientific
Laboratory
52
9
Chronology of Rocket and Reactor Technology
Rocket
development
First Chinese rockets -
Konstantin Ziolkovsky
rocket work
Key Robert Goddard paper
A Method of Reaching
Extreme Altitudes (1919) —
1200 A.D.
I
18 7 5
Reactor
development
S
19 2 0
19 2 0
19 3 0
- Ernest Rutherford makes
first controlled nuclear
transmutation
19 3 0
- James Chadwick discovers
the neutron
- O Hahn-F Strassmann
"fission experiments"
19 4 0
V - 2 rocket developed by
Wernher von Braun and
associates
19 4 0
F. de Hoffman-S Ulam
Speculations (44)
- L Meitner fission paper
(1-39)
- E Fermi " p i l e " critical
(12-42)
Alamagordo A - b o m b test (7-45) • NAA, Rand reports (7-46)
- L Shepard-A. Cleaver papers (48-49)
19 5 0
- LASL Nuclear Propulsion Div formed (6-55)
NASA formed ( 1 0 - 5 8 1 SNPQ formed (8-60) —
1960
- K I W I - A test (7-59)
Kiwi-B4Atest (11-62)-Suscessful test of redesigned Kiwi (5-64)
N R X - E S T multiple restarts
NERVA XE (experimental engine)
tested at full power (6-69)
19 7 0
10
-Technology development
program complete (1969)
many different s o l a r - s y s t e m missions by simply adding or
subtracting propellant tanks to and from the spacecraft.
A particularly interesting mission in the next decade or
two will involve the automated collection and return of
geological and possibly biological samples from Mars by
an unmanned spacecraft. A nuclear-propelled spacecraft
with five standard propellant tanks could deliver 17 to
32 tons to an orbit about Mars beginning from a low
earth orbit. With this kind of payload, 160 to 240 pounds
(73 to 110 kg) of samples could be selected by a roving
automaton and then returned to earth. Sample-return m i s sions to Mercury, Venus, and the asteroids are also possible with nuclear spacecraft. Such automated missions
would be far cheaper than long, complex manned missions.
The superlative propulsion capabilities of the nuclear
rocket can be applied to reducing travel time to the planets
as well as increasing the payloads. Shorter, high-energy
t r a j e c t o r i e s could cut a year or two off trip times to the
outer planets. Time savings such as these may be critical
on long missions where the reliability of complex spacecraft is being pushed to its upper limits.
A final bonus derived from nuclear rocket propulsion
a r i s e s from the tremendous quantity of potential energy
locked in the uranium-235 in the reactor core. Only a
small fraction of this energy is used in engine operations.
By inserting separate coolant loops in the reactor, some
of this unused energy can be tapped to generate electricity.
Power levels of 15 to 25 kilowatts could easily be generated for long periods of time. Large quantities of electrical power are just as essential to the exploration of
the solar system as the high-performance nuclear rocket
engine.
In all these c l a s s e s of missions, it is the high exhaust
velocity of the nuclear rocket that confers superiority. Of
course, if the nuclear engines on a lunar ferry were employed on many round t r i p s , the second major advantage
of the nuclear rocket, high-energy density in the r e a c t o r
core (see p. 8), may also come to the fore. Lastly, the
same type of modular nuclear stage could, of course,
undertake all types of missions, promoting system standardization.
51
Jupiter, and the other denizens of the solar system, and
the l a r g e - s c a l e ferrying of supplies to the moon a r e right
down the nuclear rocket's alley. Generally speaking, the
more "ambitious" the mission, the better the nuclear
rocket looks.
Nuclear rockets a r e an integral and indispensable part
of the space transportation
system that has been proposed
for A m e r i c a ' s space ventures following the completion of
the Apollo p r o g r a m . The "first stage" of this system is
the chemical rocket Launch vehicles, such as those m the
Saturn class or the proposed new space shuttle, will use
chemical energy to boost payloads into low earth orbits,
nuclear stages, already m space, will pick up these payloads and c a r r y t h e m — w i t h low propellant consumption
(equivalent to low c o s t ) — t o their final destinations The
NERVA engine is the only practical advanced propulsion
system that can meet the requirements of the space t r a n s portation system in the 1980s and beyond.
A Space Transportation System. Lunar operations beyond
Apollo may involve the establishment of permanent or
semipermanent b a s e s on the moon. Obviously, a great deal
of hardware would have to be injected into earth orbit
and then ferried to the moon by the space transportation
system. A NERVA-powered vehicle with a usable propellant
capacity of 150 tons (distributed in eight propellant modules,
as illustrated in the figure) could support a modest p r o g r a m
of manned lunar exploration with six round t r i p s per year.
The nuclear shuttle could c a r r y 60 tons of payload on the
outward leg and return with 13 tons. The total shuttle weight
p r i o r to its departure for the moon would be about 250 tons.
An equivalent, two-stage shuttle with chemical rockets
would weigh about 350 tons in earth orbit. In support of
various other manned and unmanned space missions, the
NERVA-propelled vehicle could transport from 5 to 60 tons
from a low earth orbit to a synchronous orbit almost six
earth radii above the e a r t h ' s surface.
Automated spacecraft will be m a n ' s p r e c u r s o r s to Mars,
Jupiter, and t a r g e t s beyond. Here, too, the nuclear rocket's
high specific impulse gives it an advantage over the chemical rocket. The basic NERVA engine can be applied to
50
FROM THE FIRST SPECULATIONS TO NERVA
"The Italian navigator has landed in the new world." This
was the text of the cryptic message sent by Arthur Compton
to other American scientists working on nuclear r e s e a r c h
when Enrico Fermi* attained the first self-sustaining nuc l e a r chain reaction The p r e c i s e moment had come at
3:20 p.m. on December 2, 1942, in a squash court at the
University of Chicago, as neutron-absorbing cadmium rods
were pulled slowly from F e r m i ' s "pile" of graphite and
uranium. In the context of this booklet, Compton's telephone
call was even more prophetic than he intended, because
F e r m i had not only discovered the new world of the atom
but had also given man an energy source with which he
will be able to explore worlds far beyond the earth
F e r m i ' s s u c c e s s set scientists and engineers to wondering about what constructive things could be done with the
r e s u l t s of nuclear r e s e a r c h . In 1944, Stanislaus Ulam and
F r e d e r i c k de Hoffman, at a New Mexico center that later
became the Los Alamos Scientific Laboratory (LASL), mused
that the power of the atomic explosive might somehow be
controlled to launch space vehicles. Ulam and de Hoffman
were following the thoughts of the great m a s t e r of science
fiction, Jules Verne, who, in an amazing 1865 novel,
De la Terre a la Lune (From the Earth to the Moon), wrote
that the Baltimore Gun Club fired a manned projectile to
the moon from a huge cannon emplaced near Cape Canaveral,
now Cape Kennedy, Florida. The use of nuclear explosions
for propulsion never proved feasible, but Ulam and de
Hoffman started people thinking about a l e s s violent source
of nuclear energy — the nuclear r e a c t o r — i n which heat is
r e l e a s e d slowly and controllably.
But ideas r a r e l y come to fruition unless a practical
need beckons. There was no such thing as a "space p r o g r a m " in 1945. But near the close of World War II, German
V-2 rockets had proved they could c a r r y 1600 pounds of
amatol explosive 200 miles from Nazi-held t e r r i t o r y in
* Fermi, an Italian-born Nobel Prize winner, came to the United
States in 1938 and headed the research team that built the first
nuclear reactor. See The first Reactor, a companion booklet in
this series.
II
Europe to London What if the V-2s had c a r r i e d atomic
bombs •? The thought was unsettling Soon designs for
Intercontinental Ballistic Missiles (ICBMs) began to take
shape on drawing boards around this country. Some of
these big rocket designs were nuclear at both e n d s — t h e y
had nuclear warheads and nuclear engines. Secret r e p o r t s
issued in July 1946 by North American Aviation, Inc , and
Douglas Aircraft Company (Project Rand) a r e landmarks
in the history of nuclear rockets The r e p o r t s underlined
the great p r o m i s e of the '"heat t r a n s f e r " nuclear rocket,
noting its high exhaust velocity, its attainment of very high
t e m p e r a t u r e s , and its high rate of heat transfer The r e ports also noted that there were problems to be solved.
The military rocket work naturally was classified as
s e c r e t by the Air Force However, at the Applied Physics
Laboratory of the Johns Hopkins University in Maryland
(near Washington, D. C ) a group of engineers, who were
unaware of the ICBM work and did not have access to the
s e c r e t r e p o r t s , innocently proceeded to duplicate all the
important findings of North American and Douglas. Their
unclassified report was published in January 1947 It was
obvious that any engineer with a slide rule and a few pieces
of paper could discover the essentials of the nuclear rocket
without much help.
In 1948 and 1949, two British space buffs, A. V. Cleaver
and L. R. Shepherd, again duplicated most of the s e c r e t
nuclear rocket fundamentals in a classic s e r i e s of papers
published m the Journal of the British Interplanetary
Society. Not long before the English r e p o r t appeared, the
American-educated Chinese scientist, H. S. Tsien, had
reported his studies on the application of nuclear energy
to rockets and other " t h e r m a l j e t s " at a Massachusetts
Institute of Technology s e m i n a r . The basic principles of the
nuclear rocket could be concealed no longer As a s i d e light, it IS interesting that Shepherd went on to become a
key man in Britain's atomic energy p r o g r a m , and Tsien
later returned to China where he was a principal figure in
the development of the Peking government's atomic bomb
Although the bonds of secrecy had been broken, interest
in nuclear rockets declined precipitously in the late 1940s.
The chief " u n d e r t a k e r " at the funeral was a careful,
competent technical r e p o r t by scientists at North American
12
ENGINES FOR THE MOON AND BEYOND
The big "selling" point of the nuclear rocket is its higher
exhaust velocity, which is approximately double that of the
best chemical rockets. But this advantage is partially offset
by the fact that the nuclear engine is considerably heavier
and more costly than a chemical engine of the same thrust
rating. Admittedly, nuclear rocket engines are in an e a r l i e r
stage of development than chemical engines, too. Comparing
engine weights and costs at a fixed thrust level does not
tell the whole story. The nuclear r o c k e t ' s doubled exhaust
velocity means that it uses only half as much propellant
for each second of operation. Thus, the nuclear rocket can
do much more than a chemical rocket with equal thrust.
How much more depends a great deal upon the type of m i s sion being contemplated.
NERVA's thrust of 37.5 tons is considerably s m a l l e r
than that of the biggest chemical rockets, which can gene r a t e as much as 750 tons each. The role of the nuclear
rocket is not in journeys beginning at the launch pad, where
chemical engines have been so highly successful, but
r a t h e r in outer space where the nuclear rocket's high exhaust velocity IS a great asset. We should think of nuclear
rockets then, as prime movers for missions beginning
from an earth orbit and moving outward toward higher earth
orbits, the moon, and the planets. Very high thrust levels
a r e not n e c e s s a r y for nuclear rockets because they a r e
not required to lift payloads off the launch pad.
The g r e a t propellant economy of the nuclear rocket
tends to make it superior for those missions in which much
of the spacecraft m a s s is allotted to propellant. This is
subject to the condition that the payload m a s s be a sizeable
fraction of the engine m a s s , otherwise the advantage conf e r r e d by the higher exhaust velocity would be negated to
a large extent by the nuclear engine's 12-ton m a s s . Neither
would the nuclear rocket be the best engine where only
small velocity changes a r e desired, such as in the maneuvering of an orbital space station. But l a r g e - s c a l e orbital
maneuvers, fast t r i p s to the outer planets, the delivery of
payloads to the surfaces of, or orbits about, Venus, Mars,
49
Aviation concluding that the nuclear rocket did not seem a p plicable to ICBMs. F u r t h e r m o r e , the North American r e port mentioned the (then) fantastic temperature of 5700 °F
(3400°!^ as n e c e s s a r y for the nuclear r e a c t o r . Few engin e e r s could swallow the idea of building a reactor that
would have to operate at t e m p e r a t u r e s twice the melting
point of steel. So, the nuclear rocket languished, nourished
only in a desultory and indirect way by the Atomic Energy
Commission—Air Force nuclear aircraft program. Robe r t W. Bussard, one of the early champions of the nuclear
rocket, put the situation well:
"Nuclear rocketry had been effectively written off as a
dead end by most of the missile and rocket people (who
didn't really understand nuclear energy and liked chemical
energy better anjrway) and most of the reactor people
(who thought the whole idea of nuclear flight of any sort
was generally loony).'"
It was Bussard, in fact, who did much to r e s u r r e c t
nuclear rocketry. Working in the nuclear aircraft development p r o g r a m at Oak Ridge National Laboratory (ORNL) in
Tennessee in the early 1950s, he was able to show that the
e a r l i e r nuclear rocket studies had been too negative and
too conservative. He was convinced that nuclear rockets
could successjully
compete with chemical rockets on long
flights with heavy payloads. B u s s a r d ' s studies and personal
salesmanship were decisive. The Air Force in early 1955
decided to reexamine nuclear rockets as I C B M t h r u s t e r s .
The Air Force scientists and engineers recognized,
along with everyone else, that the most critical problem
of the nuclear rocket was that of developing a hight e m p e r a t u r e - r e s i s t a n t m a t e r i a l . What material would hang
together at 3000 °K? To help answer this question, Los
Alamos Scientific Laboratory formed the Nuclear Propulsion Division under Raemer E. Schreiber. Concurrently,
a similar group was created at the Lawrence Radiation
Laboratory in California.* In mid-1956, budget cutbacks
forced consolidation at Los Alamos of all the nuclear
rocket work, while Livermore took on the task of building
a nuclear ramjet engine. In an unintentional canine p a r a l *Both the Los Alamos and Livermore laboratories are operated
for the AEC by the University of California.
13
lelism, the rocket p r o g r a m was dubbed Project Rover and
the ramjet was labelled Project Pluto.
Curious code names have always been part of nuclear
and space l o r e . As part of Rover, there was Dumbo, a
" p a c h y d e r m a l " reactor concept, and a huge nuclear-powered
" b i r d " named Condor. Then came the Kiwis. A kiwi, as
every New Zealander knows, is a t a i l l e s s , hairy-feathered
bird that the native Maori people named for its shrill call.
The kiwi cannot fly, and its nuclear namesake could not
either. The first Kiwis (Kiwi-A's) comprised a s e r i e s of
heavy, "battleship" test r e a c t o r s that were fired nozzle up,
a s if to emphasize their inability to leave the earth.*
The Kiwis were an essential prelude to practical nuclear
rockets. Materials t e s t s in Los Alamos laboratories had
shown that graphite (like the m a t e r i a l in pencil leads) was
a likely s t r u c t u r a l material to withstand 3000 °K. Indeed,
graphite's strength increases with t e m p e r a t u r e up to about
3300°K; at 3900 °K, it s u b l i m e s ! instead of melting. What
better way was there to test this strange m a t e r i a l than to
disperse uranium carbide fuel in it and build a reactor ?
Kiwi-A was the result.
Kiwi-A was fired for 5 minutes by Los Alamos at Nevada
Test Site in July 1959, using p r e s s u r i z e d hydrogen gas as
the "propellant". It generated 70 megawatts of heat power
and reached t e m p e r a t u r e s as high as 1777°K. Kiwi-A was
the first attempt to t r a n s l a t e a decade of calculations and
paper studies into h a r d w a r e .
Two more Kiwi-A r e a c t o r s were tested in 1959 and 1960
in order to (1) check reactor design, and (2) test the
practicality of coating the graphite fuel elements with
niobium carbide for protection against corrosion by the hot
hydrogen propellant. In essence, the Kiwis were "pilot
plants" that showed engineers how to build better engines.
The climate of official support for the nuclear rocket
project changed markedly in the late 1950s. F i r s t , the
Atlas (chemical) ICBM had by that time proved itself
capable of carrying powerful nuclear warheads to just
*They were not intended to fly, so a downward exhaust would
only have caused problems; the interest at that time was only in
the reactor operation. However, full-scale nuclear rocket engines
have since been tested nozzle-down.
t Turning directly to vapor without going through a liquid phase.
14
Once an engine or reactor has been placed in one of the
cavernous " bays " in a MAD building, engineers can strip it
down piece by piece with remote manipulators that simulate
human a r m s and hands. The NERVA engine, for example,
has s e v e r a l thousands of p a r t s , including bolts, fuel e l e ments, pipe connectors, etc., that all must be checked after
a test.
Although this booklet concentrates attention on the nuclear rocket engine, the scope of NRDS facilities confirms
the general rule of thumb that most of the money used in
the space p r o g r a m stays right on t e r r a firma in the form
of launch pads, tracking facilities, r e s e a r c h - a n d - d e v e l o p ment equipment and s a l a r i e s , and, of course, NRDS. Succ e s s in space depends first of all upon success on the
ground.
Interior of the E-MAD Building, showing the huge overhead and
wall-mounted manipulators used jor disassembling
radioactive nuclear rocket engines. When m actual use, no workmen would be
present m this portion of the building, because oj the high radiation
levels.
47
^JH (immf¥
Engine Test Stand 1 (ETS-1), 140feet (43 cm) high, where NERVA
engines are tested at NRDS. White spheres at right are dewars jor
liquid hydrogen and liquid oxygen. The hot exhaust rushe<; down
through a duct to the deflector, at left, below where men are standing. Tracks permit railroad to bring m engine and shields.
46
Cutaway drawing of a Kiiii-A reactor,
basic design oj the graphite core
showing the
about any spot on earth. Therefore, the Air Force no
longer needed nuclear-powered rockets to deliver weapons.
Sav3 for another event, the nuclear rocket engine might
have died a second time then and t h e r e . But on October 3,
1957, the Russians signalled the beginning of the international space r a c e by orbiting Sputnik I, the first man-made
earth satellite. A month later they placed a live dog in
orbit.
15
The National Aeronautics and Space Administration (NASA)
was created in this country on October 1, 1958, by President
Eisenhower's Executive Order 10783, to cope with this
Russian technological t h r e a t . In the p r o c e s s , the A E C - A i r
F o r c e partnership in nuclear rocketry was transformed
into an AEC-NASA activity. In August 1960, the joint
AEC-NASA Space Nuclear Propulsion Office (SNPO) was
formed* to push toward an operational nuclear engine that
would aid the U. S. in the race to the moon and planets.
Unperturbed by the administrative manipulations, nuclear
rocket engineers were greatly encouraged by the Kiwi-A
s u c c e s s e s . Plans were laid for a s e r i e s of more powerful
test r e a c t o r s , the Kiwi-B's, which would run on frigid liquid
hydrogenf rather than gaseous hydrogen. The optimistic
outlook also set SNPO workers to looking for industrial
contractors to build a flyable nuclear engine. The NERVA
(Nuclear Engine for Rocket Vehicle Application) P r o g r a m
began in the fall of 1960, when SNPO requested bids from
interested companies. After a close competition, AerojetGeneral Corporation was selected in July 1961 to build the
engine (structure, pumps, etc.), and the Westinghouse
Electric Corporation's Astronuclear Laboratory was chosen
to construct the nuclear r e a c t o r , based on the Kiwi work of
Los Alamos. In May 1962, the NASA Marshall Space Flight
Center at Huntsville, Alabama, took another step by signing
the RIFT (Reactor In-Flight Test) contract with the Lockheed Missiles and Space Company. Lockheed was to build
a flight test vehicle for the Aerojet NERVA engine. This
aggressive planning was commendable, but it proved to be
p r e m a t u r e because no operational missions were assigned
to the RIFT system.
Nuclear Space Propulsion Tests
Moving a Kiwi reactor from
its assembly bay to a test
cell. This "Jackass & Western RR" locoTUotive can be
remotely controlled.
Workers swarm over a Kiwi-B
reactor prior to its test in the
series that established the
value of liquid hydrogen propellant.
The new Kiwi-B r e a c t o r s were designed to run at about
2300°K at 1100 megawatts, ten times the Kiwi-A power,
sufficient to generate about 27.5 tons of thrust in space.
*In 1970, the joint office was renamed the Space Nuclear Systems Office, when it was assigned the additional responsibility
of providing nuclear-electric power for space missions.
t Hydrogen is liquid at temperatures less than 20 °K. For a
description of the extensive low-temperature technology that supports nuclear propulsion for space, see Cryogenics, the Uncommon Cold, another booklet in this series.
16
A Kiwi-A ready for a test
with gaseous hydrogen propellant.
Test Cell C m 1967 Movable shields on test stand,
right center, surround reactors during test Building, lower right, can be
moved on rails over reactors for weather protection.
Exhaust blast J>om a successful teacto) test shoots high m the air
oiey the Lest cell propellant tanks (Telescopic lens photo )
'
,..
JI
T»~-' fei-—^1.; iifti
» r
- ^ ^ '•i'^J^
The Phoebus-IA reactor after arrival on its special railroad car at
the test cell
The E-MAD Building, where
engines are put together,
or disassembled
after
tests
The test s e r i e s started successfully enough when Kiwi-BlA
was fired with gaseous hydrogen in December 1961. T e s t s
with liquid hydrogen commenced in September 1962 in
Kiwi-BlB. This firing proved that a nuclear rocket could
operate successfully with liquid hydrogen,* but the test
had to be cut short when some of the internal graphite
p a r t s cracked and failed. Was this failure due just to
faulty m a t e r i a l s or components, or was the whole reactor
design s u s p e c t ? The Kiwi-B4A—the core design favored
for NERVA — was tested on November 30, 1962 As the
r e a c t o r power was raised, o b s e r v e r s watchin*'- from a
distance saw flashes of light in the hydrogen exhaust plume
rising from the inverted nozzle. More flashes appeared and
the test was purposely terminated. Unquestionably, s o m e thing fundamental was wrong with the Kiwi-B design.
*An important finding indeed, since rocket propellant tanks for
compressed gaseous hydrogen at room temperature would have to
be 2000 times as heavy as tanks for the same weight of liquid
hydrogen.
17
Kiwi-B4A was towed off to the R-MAD Building (see
p. 41) where engineers picked it apart with remote manipul a t o r s . The reactor core showed widespread damage. Many
opinions were ventured about the cause, but the most
reasonable pointed to "dynamic" failure. That i s , the
h i g h - p r e s s u r e hydrogen gas had caused the graphite fuel
elements to v i b r a t e — t h a t i s , spaces in the r e a c t o r a l t e r nately became pressurized and depressurized. This caused
the elements to vibrate and impact each other.
A good engineer views testing philosophically. After all,
it was far better to liave a failure out on the Nevada d e s e r t
than on a spaceship halfway to M a r s . The path ahead was
c l e a r . All of 1963 was devoted to confirming the diagnosis
of vibration-induced failure. Reactor components were
systematically tested in s t r e a m s of hot high velocity
hydrogen. "Cold-flow" t e s t s with complete r e a c t o r s , but
generating no heat, were c a r r i e d out. Gradually, a new
vibration-resistant reactor was designed. In this sense,
the Kiwi-B4A test was successful.
Hot testing of the new Kiwi nuclear r e a c t o r s r e s u m e d in
May 1964, only 18 months after Kiwi-B4A. Since then, a
long s e r i e s of highly successful t e s t s has proved the r e designed nuclear core to be sound. Included in the s e r i e s
were the following:
May 1964
August-September 1964
September-October 1964
January 1965
A p r i l - M a y 1965
June 1965
FebruaryMarch 1966
June 1966
February 1967
December 1967
J u n e - J u l y 1968
November—
December 1968
December 1 9 6 8 August 1969
One power test, Kiwi-B4D
Two power tests, Kiwi-B4E
Two power tests, NRX-A2*
Kiwl-TNT, a nuclear safety test
Three power tests, NRX-A3
One power test, Phoebus-IA
Ten s t a r t s . E n g i n e System
Test, NRX-EST
Two power t e s t s . NRX-A5
Two power t e s t s . Phoebus-lB
One power test, NRX-A6
Three power tests ot Phoebus-2A.
Peak power reached: 4200
megawatts.
Two power tests of P e w e e - 1 , a
fuel-element, test-bed r e a c t o r .
Total of 28 s t a r t s of the NERVA
ground experimental engine (XE).
*NRX s t a n d s f o r NERVA R e a c t o r E x p e r i m e n t . See G l o s s a r y ,
p a g e 54 f o r e x p l a n a t i o n s of o t h e r c o d e n a m e s .
18
Arrangement
Nevada.
of facilities
at
the
Nuclear
Rocket
Development
Station,
All the Kiwis, Phoebuses, Pewees, and NRXs were tested
on special railroad c a r s that c a r r i e d them between the test
cells and R-MAD and also served as mobile testbeds.
Thrust and exhaust velocity were engine p a r a m e t e r s of
little interest in those reactor t e s t s ; actually, the nozzles
were not designed to produce thrust efficiently. The r e s u l t s
sought in testing a complete engine, though, dictate that
the engine be placed in a special structure on the ETS-1
that can measure the reaction forces that a r e generated.
A unique railroad car called the Engine Installation Vehicle
(ElV) has been constructed to c a r r y the complete engine
up to the ETS-1 and lift it into place on the test stand with
hydraulic "hands". After a test, the p r o c e s s is r e v e r s e d
and the hot engine is returned to E-MAD.
The MAD buildings a r e p r i m a r i l y immense "hot c e l l s "
where engineers can perform work on radioactive r e a c t o r s
and engines, but remain protected by thick concrete walls.
43
"W^?T^^-"r«,»«
required to hold the big engine (one is about 7 m e t e r s
long) in a vertical position and keep it from soaring off
into space as its thrust builds up. A spherical t h e r m o s like dewar (pronounced d o o ' - e r ) 14 m e t e r s in diameter,
holding up to 260,000 gallons (1,000,000 liters) of liquid
hydrogen, stands nearby as a substitute for the rocket
propellant tanks that in flight would tower high over the
relatively diminutive engine on the launch pad.
Beneath the engine nozzle is a large, water-cooled exhaust duct that turns the hydrogen stream by 90° and d i r e c t s it down a concrete-lined artificial canyon. (The
water is pumped up from deep wells.) During a test, this
canyon becomes an inferno as the hot hydrogen spontaneously ignites upon contacting the air and burns to form
water. Unlike the spectacular conflagration created during
the firing of a kerosene-burning rocket, a nuclear rocket
engine flame is invisible save for any incandescent i m purities and the heat aberrations it produces in the air.
Engine Test Stand 1,
from a different
vantage that shows
the exhaust deflector "canyon",
foreground, incised
m the desert floor
lo permit escape of
burning hydrogen.
Note upright semicylindrical shields
on either side (near
left, far right),
uhich are moved
around the engine
iihile test IS in
progress
42
vfV»»»
The rocket r e a c t o r test program achieved a major milestone
when the NRX-A6 r e a c t o r was operated at full-power for
60 minutes, a running-time sufficient for many space
missions.
Of great significance were the Engine Systems Tests
(ESTs). For the N R X - E S T s e r i e s , all major engine components were assembled—turbopump, hydrogen-cooled nozzle, control equipment, e t c . — i n t o a "breadboard"* engine. This close approximation to a real engine operating
in space was started, shut down, and r e s t a r t e d in different
ways. In short, the complete engine was put through its
paces just as it would be on a real space mission. The real
import of the N R X - E S T s e r i e s is that it showed that
a complete nuclear rocket system can start on its own
power and operate stably over a wide range of conditions.
*A breadboard engine is one that includes all principal components of a flight-test system, but is arranged to be convenient
for the test, and not as It would be in flight.
The Pewee-2 reactor on railroad car at NRDS in Nevada.
Pewee-2 is a reactor in which fuel elements can be tested
at conditions very close to (or even worse than) those encountered in an actual nuclear rocket engine.
19
Further details of this EST test s e r i e s are listed in the
table on p. 22.
The test in June 1965 introduced the Phoebus nuclear
rocket r e a c t o r s e r i e s . The Phoebus r e a c t o r s a r e best
thought of as "tools" to advance nuclear rocket technology.
For example, they incorporate significant advances over
the Kiwis in t e r m s of higher t e m p e r a t u r e s , higher power
density in the core, and longer life — all factors that will
lead to better nuclear rockets.
A most important s e r i e s of engine t e s t s commenced in
December 1968, when the NERVA ground experimental
engine, called the XE for short, was first put through its
paces. The XE was similar to the NRX/EST but its components were arranged in a configuration closer to that
which would be used in an actual flight system. For example, the liquid-hydrogen run tank was closely coupled
and the test compartment was partially evacuated to simulate space conditions more closely. In fact, the XE t e s t s
were as much a test of the ETS-1 (Engine Test Stand 1)
as of the engine itself. The XE s e r i e s was concluded in
late August 1969, logging a total of 28 power t e s t s at v a r i ous power levels. The total engine operating time was
just under four hours. The XE was started successfully
from a wide variety of starting conditions, demonstrating
that a nuclear rocket can meet the requirements of a space
transportation system.
In addition to developing better graphite r e a c t o r s such
as Phoebus, NASA-AEC experts have studied new types of
nuclear rockets in which the fuel elements a r e made from
tungsten, another material with good strength at very high
t e m p e r a t u r e s . Even more adventurous a r e the studies of the
so-called " g a s - c o r e " r e a c t o r s in which the nuclear fuel
itself is in gaseous form, retained within the rocket by
centrifugal forces exerted as the fuel and propellant gases
are spun about the rocket a x i s .
Although such advanced concepts undoubtedly will play
a role in nuclear rockets of the distant future, we must
focus our attention now on the graphite-core nuclear rocket.
By the end of 1969, it was obvious that the graphite nuclear
rocket had weathered all administrative and technical
20
NINETY MILES OUT IN THE DESERT
A four-lane dual highway runs northwest from Las Vegas,
Nevada, toward Death Valley. After 90 miles of sand, rocks,
Joshua t r e e s , and creosote bush, the road abruptly n a r rows to two lanes. A few miles (kilometers) to the north
a r e the AEC's Camp Mercury site for underground nuclear weapons testing* and the AEC-NASA Nuclear Rocket
Development Station (NRDS). NRDS is surrounded by b a r r e n
mountains and located in a flat basin, which is called
J a c k a s s Flats after some of the indigenous local inhabitants. Both r e a c t o r s and complete engines can be tested
safely at NRDS.
NRDS has three major test a r e a s : Test Cells A and C
plus Engine Test Stand No. 1 (ETS-1). The Kiwi, NERVA,
and advanced Phoebus r e a c t o r s have been put on trial at
the two Test Cells. ETS-1 is r e s e r v e d for engine e x p e r i ments and NERVA engine firings. The three test a r e a s are
connected by road and railroad to the R-MAD and E-MAD
Buildings (Reactor and Engine Maintenance, Assembly, and
Disassembly). Typically, a r e a c t o r or engine is put together
in one of the MAD buildings and then c a r r i e d via the
" J a c k a s s and Western Railroad "(jokingly called the world's
shortest and slowest) to one of the test s i t e s . After the
test, while the engine or reactor is still highly r a d i o active, a heavily shielded railroad engine tows it back to
the proper MAD building for dissection. The map gives
little feeling for the great distances between the test i n s t a l lations, MAD buildings, and other support facilities. An
o b s e r v e r at any point in NRDS can see only minute outlines
of the next building shimmering in the desert heat.
The ETS-1 is the most impressive s t r u c t u r e at the
station. It r i s e s approximately 121 feet (37 m e t e r s ) from
the desert floor and looks like a s t r u c t u r e that would be at
home at Cape Kennedy. Although only engines—not complete rockets — are tested h e r e , a heavy s u p e r s t r u c t u r e is
*See Plowshare, another booklet in this s e r i e s , for an account of
this testing. The name " M e r c u r y ' " has no relation to the NASA
" P r o j e c t M e r c u r y " for manned space flight; it perpetuates an old
mercury-mining town name.
41
drives the turbopump a little faster. The turbopump pumps
more liquid hydrogen into the engine. More warmed hydrogen drives the turbopump faster, and so on, until full
power is reached. The t e r m " bootstrapping" is applied to
this type of cold start, and the engine really does pull
itself up to full power, in effect "by its own bootstraps".
The turbopump cycle, the structure, and the engine cont r o l s transcend any one of the other major components.
They unify the engine subsystems and show that one part
cannot be designed without careful regard for its effect on
the r e s t .
The Xe installed in ETS-1 in
Nevada. The XE has been the
engine closest to actual flight
configuration.
s t o r m s and reached the point where a flyable NERVA engine could be developed.
BUILDING AN ENGINE FOR SPACE
A nuclear rocket engine is considerably more than a
heater of hydrogen. It is true that the engine is built around
the r e a c t o r c o r e — t h e wellspring of e n e r g y — b u t s o m e thing else has to transport hydrogen propellant from tank
to r e a c t o r ; something else has to convert heat to thrust.
The engine, in fact, has five major segments:
1. The nuclear reactor heat source.
2. The pump that pulls liquid hydrogen from its tanks
and forces it through the reactor.
3. The nozzle, the prime mover that t r a n s f o r m s heat
to thrust.
4. The structure that physically holds all the pieces
together.
40
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ing or contracting, changes the reactor power in a way that
keeps t e m p e r a t u r e constant. In reactor engineering, hydrogen is said to have a negative temperature coefficient of reactimlv.
The full-flow turbine-drive cycle was selected over
several other potential cycles because it yields the highest
performance at a given r e a c t o r operating t e m p e r a t u r e .
The other major contender was the hot bleed cv( Ie, m
which a small portion of the hot hydrogen rushing out of
the reactor is fed back to the turbopump. The diverted
hydrogen, after it has passed on some of its energy to the
turbine, is jettisoned and does not produce useful thrust.
The hot bleed cycle is therefore wasteful of hydrogen and,
for the same r e a c t o r temperature, has a specific impulse*
about 25 seconds lower than a topping cycle using the same
r e a c t o r . Because the hot bleed cycle's turbine uses only
a small fraction (about 3%) of the hydrogen stream, it is
small and lightweight compared to the topping cycle t u r bine. It IS, however, a high t e m p e r a t u r e turbine and subject
to all the ills that b e f a l l hot, high-speed rotating m a chinery— in other words, its reliability would be lower
than that of the bigger but cooler topping cycle turbine.
Engine designers also had to find a scheme that would
s t a r t the engine, given a cold reactor and an inactive turbopump The first thought is for some energy source to turn
over the turbopump. In an automobile, a storage battery
and s t a r t e r motor suffice, but a similar system that was
big enough to turn over the turbopump would weigh too much.
The answer to the startup problem lies in the energy
contained in the p r e s s u r i z e d hydrogen in the propellant
tank. Even the very cold liquid hydrogen, when admitted to
the turbopump under p r e s s u r e , can turn it over enough to
push a little of the hydrogen into the r e a c t o r . Here the
hydrogen picks up some heat and is turned to gas, which
* Propulsion engineers often also speak of an artificial quantity
called specific impulse,which is equal to the exhaust velocity
divided by the acceleration due to gravity, gQ, (9.8 meters per
second^, or 32.2 feet per second^) This is also equal to F/ggrfi.
39
away from the core.) The answer to this also i s simplicity
itself: As the reactor power r i s e s , so does the t e m p e r a ture of the hydrogen. The hydrogen expands with t e m p e r a t u r e , and there is l e s s of it in the core at any one moment,
even though it is being pumped through faster. L e s s hydrogen means that reactor criticality is affected l e s s , and the
power level begins to level off.
In fact, the power will become constant when the average
hydrogen t e m p e r a t u r e is exactly what it was before the pilot
commanded the engine to provide more thrust. Since t e m p e r a t u r e stabilizes at the old value, exhaust velocity, v, is
unchanged. The things that have changed are rh, the hydrogen flow r a t e , and F, the thrust. Because F = riiv, the
engine t h r u s t is directly proportional to the flow rate (v is
fixed because the t e m p e r a t u r e is fixed) and, m in turn, to
how far the pilot closes the valve m the bypass line. In
other words, the hydrogen gas in the core, by either expand-
Turbopump
Turbopump
exhaust
"Warm"
hydrogen
bled from
this plenum
5. The controls that force all engine components to
m a r c h in step at the command of the spacecraft pilot.
Besides the system unity imposed by the s t r u c t u r e and
controls, the engine p a r t s have to fit together thermodynamically. To breathe "life" into a nuclear rocket there
must be a " s t a r t e r " , like that in an automobile, intrinsic
in the system. The engine must "catch", become selfsustaining, and generate useful power. For this to happen,
two energy sources must be found: One to s t a r t the engine
and another to power the pump that keeps propellant flowing through the engine. A nuclear reactor heat source all
by itself would be no more useful than an unharnessed
gasoline flame would be in driving an automobile.
The Biggest, Hottest Reactors Ever Built
The heart of the nuclear rocket engine i s , of course,
the r e a c t o r . In the NERVA engine the reactor must gene r a t e upwards of 1500 megawatts of t h e r m a l power — more
than the output of 15,000 Volkswagens or 50,000 homeheating furnaces—and still not weigh the i^ocket down.
A t e m p e r a t u r e of 2500°K can be reached, according to
r e a c t o r and full-scale engine t e s t s in Nevada. Power
levels of 4200 megawatts have already been attained.
NERVA is as powerful and three t i m e s hotter than the
r e a c t o r s in any earthbound commercial nuclear-electric
generating plant in the U. S. The NERVA engine, however,
does not have to operate for y e a r s like its commercial
counterparts; it can run at 2500°K only because a few
hours of operation are sufficient for most space applications.*
A pile of uranium and graphite does not make a rocket
r e a c t o r , although this was the way Enrico F e r m i achieved
the first self-sustaining chain reaction in 1942. Four r e quirements control reactor design: (1) The need to attain
Cold bleed cycle
A third type of engine cycle is the "cold bleed cycle". This mode
of operation makes inefficient use of hydrogen propellant because
some of it IS thrown away.
38
*Another AEC program develops space reactors that operate at
low power levels tor long periods of time. See Nuclear Reactors
for Space Power, a companion booklet in this series.
23
Nozzle coolant pipe
(carries entire
hydrogen flow)-^
Warm hydrogen drives
turbopump (bypass line
not shown)
Reflector
Hydrogen flow diagram tn the full-flou^ cycle. The numbers
to the description of the operation of this cycle on page 36.
•
Neutron absorption
Q Uranium - 235 Fission
f ® Neutron generation #1
Fissions < ,-. .
[CD Neutron generation #2
Reactor criticality occurs when exactly one neutron from each fission reaction goes on to cause another fission reaction.
Sketch
shows five neutrons from Generation No. 1 producing two new fissions and five neutrons in Generation No. 2. Three neutrons in
Generation No. 1 are lost through escape and absorption.
a critical m a s s ; * (2) The need to remove all generated heat;
(3) The need to raise and lower power at will, or control
the reactor; and (4) The need to maintain s t r u c t u r a l integrity
at high t e m p e r a t u r e and under the forces exerted by the
high velocity hydrogen gas.
Attainment of a critical m a s s is a m a t t e r of "neutron
economics", because it is the cloud of invisible neutrons
*The smallest mass of fissionable material that will sustain
a chain reaction. See Nuclear Terms, A Glossary, a companion
booklet in this s e r i e s , for detinitions of unlamiliar t e r m s .
24
refer
glance one would think that the pilot should also signal
the reactor control drums to r a i s e the r e a c t o r power. It
t u r n s out, however, that this is unnecessary. Nuclear
rocket control is simplified by the fact that the hydrogen
itself affects reactor criticality because of its ability to
collide with neutrons and slow them down to speeds where
they can more readily cause uranium fissions.* In other
words, shooting more hydrogen into the nuclear rocket
core is equivalent to a slight outward rotation of the control drums. The pilot, then, by closing the valve in the
turbine bypass line will effectively i n c r e a s e both hydrogen
flow and r e a c t o r power with the same action. His lever
will be as direct and simple as the throttle on an automobile
engine.
But what keeps the reactor power from increasing
indefinitely once the additional hydrogen is pumped in?
(Remember that r e a c t o r s a r e rate-controlled
and that their
power level keeps rising once neutron poisons are pulled
*The hydrogen thereby serves as a reactor moderator.
37
A final structural note: Heavy though the nuclear engine
may be, it is still but a small appendage on a much l a r g e r
s t r u c t u r e consisting mainly of huge propellant tanks.
Bootstrapping and the Full-Flow Turbine Drive
The NERVA engine is really two engines in s e r i e s . The
main engine is, of course, the rocket nozzle that converts
most of the energy in the hot hydrogen coming from the
r e a c t o r proper into exhaust kinetic energy. The other
engine is the turbopump, which e x t r a c t s a portion of the
energy in the ivarm hydrogen returning from cooling the
nozzle and reflector and converts it into the rotary motion
of the main hydrogen pump. Following the diagram on
page 37, the sequence of events is: (1) the hydrogen steam
picks up energy from the nozzle and reflector; (2) the
turbine attached to the pump converts p a r t of this energy
into pump energy; (3) the hydrogen is pumped into the
reactor; (4) the reactor adds energy to the hydrogen stream;
and (5) the nozzle converts a fraction (90%) of the hydrogen
energy into exhaust kinetic energy. The overall p r o c e s s is
called the "full-flow " o r "topping cycle "because there a r e
two separate additions of energy to the hydrogen — one on
top of the other. Some logical questions at this point a r e :
How is engine thrust controlled and how can we get the
engine started ?
Suppose that some spaceship pilot of the future wants
more thrust from his nuclear rocket engine, all he will
have to do is pull a lever that partially closes a valve in
a pipe that c a r r i e s some of the warm hydrogen around the
turbine driving the pump. When hydrogen flow is r e s t r i c t e d
in this " bypass line", more of the warm hydrogen p a s s e s
through the turbine. The turbine now has more power to
convert into pump power and consequently speeds up. Thus,
more liquid hydrogen flows into the reactor for thrust
production. But in a rocket engine, unless the reactor
power is increased simultaneously, the only result will be
more, but cooler, hydrogen leaving the reactor. At first
36
coursing through the reactor that stimulates nuclear fission
and thence power production. Each fissioned atom of
uranium-235 produces 2V2 new neutrons on the average. If
the rate at which fissions occur (and consequently the power
level) is to remain constant, exactly one of these 2V2 neut r o n s has to go on and cause another fission. Reactor
"criticality" occurs at just this point. This balance sheet
leaves IV2 neutrons per fission that can escape the reactor
altogether or be absorbed in nonfission nuclear reactions.*
To prevent too many neutrons from escaping, a material
is placed around the reactor to reflect some errant neutrons back into the core. Excessive neutron absorption can
be avoided by using core and reflector m a t e r i a l s that have
*See Nuclear Reactors, a companion booklet in this s e r i e s , for
an account of reactor construction.
\
Fission fragments
fly apart
Fission
time= 1 microsecond
Thermal
"spike"
Nuclear fuel
A few microseconds later
Heat removed by
propellant
The kinetic energies of the fission fragments appear as a pulse
of heat that flows outward to the
walls of the fuel, where it heats
the rocket
propellant.
25
little appetite for neutrons; fortunately, graphite has just
such a "low neutron absorption c r o s s section". Naturally,
enough uranium-235 atoms must be dispersed throughout
the graphite core so that the questing neutrons can find and
fission them. But if there is too much uranium mixed with
the graphite, fuel element structural strength is reduced.
Core design requires a balancing of all these considerations,
plus one more.
Almost all the energy released by uranium is first
incorporated in the kinetic energy of two large fission
fragments that fly off in opposite directions as a nucleus
splits (Newton's Third Law again), peppering surrounding
atoms. In a few millionths of a second, the kinetic energy
of the heavy fragments is t r a n s f e r r e d to the nearby atoms,
setting them to vibrating. A wave of heat s p r e a d s out from
the fission site. If this heat is not removed, core t e m p e r a t u r e s will quickly r i s e beyond the sublimation point of the
graphite fuel.
The reactor core, therefore, has to be designed in such
a way that all this fission-generated heat is t r a n s f e r r e d
to the hydrogen gas that is to be driven through the reactor
by the pump. This means that the reactor must be perforated
with holes that c a r r y the hydrogen — serving here as a
reactor coolant—through and past the hot fuel to the nozzle. If the coolant p a s s a g e s are too large, the size of the
whole engine becomes so great it is unacceptable for space
use. Holes that a r e too small cause high friction losses that
impose powerful forces on the reactor s t r u c t u r e . The
designer must avoid these extremes and still provide
enough coolant passage a r e a to remove all the heat. (Note
that the rate of heat transfer to the hydrogen is p r o p o r tional to the a r e a inside the holes and to the t e m p e r a t u r e
difference between the hole wall and the center of the
hydrogen stream.)
The basic fuel s t r u c t u r e within the reactor core is
called a fuel "element". The NERVA fuel element is a long
hexagonal piece of graphite pierced by 19 holes that c a r r y
the hydrogen gas lengthwise through the reactor. The fuel
elements a r e stacked together in a close a r r a y so that
the hydrogen gas driven into the reactor by the pump encounters thousands of long, but very narrow, holes about
26
centerline. The nozzle is bolted to the reactor p r e s s u r e
shell, which in turn is bolted to a truncated conical shell
that supports the turbopump. Thus, a continuous shell of
strength encapsulates core, reflector, control drums, t u r b o pump, and miscellaneous a c c e s s o r i e s .
A critical structural problem a r i s e s at the junction b e tween the engine and rocket body, however. Across this
junction must be transmitted the entire engine thrust
that is conveyed upward from the nozzle through the engine
exoskeleton. Ordinarily, such a joint would present no engineering difficulties. But in this case, the joint must be
flexible'. All big rockets have their engines mounted on
gimbals that permit the " d r i v e r " to s t e e r them. During a
launch, for example, a gust of wind might deflect an
ascending rocket off course: A gimballed engine can bring
it back on the desired flight path. Gimballing chemical
engines is relatively easy because they a r e lightweight.
But, in a nuclear rocket, the heavy reactor replaces the
empty combustion chamber of the chemical rocket. Despite
the added weight, suitable flexible joints now have been
designed.
NOZZLE SKIRT EXTENSION
EXTERNAL
DISC SHIELD
REFLECTOR
Drawing of the complete
NERVA
engine.
35
)!»**•*•
technique by pumping the supercold liquid hydrogen through
the nozzle tubes, then up through the reactor reflector, and
finally down through the core proper. Nozzle cooling thus
forces a down-up-down hydrogen flow pattern. Without this
cooling the nozzle would not survive more than a few
seconds.
Holding the Pieces Together
Holding the various pieces of the engine together is the
obvious task of the engine s t r u c t u r e . All major components
are arranged along a centerline that extends from the
nozzle, through reactor and turbopump, into the rocket body
proper. The engine s t r u c t u r e is essentially an exoskeleton
(like a b e e t l e ' s hard shell) built symmetrically around the
Turbopump
Shield
Pressure shell
Motors
Control drums
"T
one-tenth of an inch (0.25 cm) in diameter. It is in these
holes that the gas picks up the heat generated in the body
of the fuel element by the fissioning uranium.
Nuclear rocket fuel elements operate under extremely
severe conditions: (1) t e m p e r a t u r e s that reach over 3000°K
internally; (2) power densities of kilowatts per cubic centim e t e r ; (3) intense levels of nuclear radiation; and (4) many
thermal " c y c l e s " (transients) that can crack the fuel elements just as boiling water cracks a heavy glass tumbler.
On top of these problems, the hydrogen gas driven into the
holes of the fuel element will react chemically and c o r rode the graphite structure unless protective coatings are
added. Because the success of the nuclear rocket depends
upon fuel elements that retain their integrities despite
these infernal conditions, much of the AEC effort has gone
into extending the life of graphite fuel elements at high
temperatures.
Excessive hydrogen corrosion can be prevented by
coating the fuel element holes with a metal carbide, such
as niobium carbide. C e r a m i c i s t s solve the other problems
by trying different combinations of graphite powder, u r a nium carbide, and binder m a t e r i a l s until they have a fuel
element that can hold together under some of the worst
conditions man can c r e a t e . During the nuclear rocket
program, there has been a p r o g r e s s i o n of better and better
fuel m a t e r i a l s . The goal of the NERVA program has been
a fuel element that will heat hydrogen to 2360°K and s u r vive for 10 hours.
T"
Core and reflector
= reactor
Nozzle
34
Exploded viewoj a nuclear rocket
engine. Control drums Jit in the
reflector,
the reactor inside the
pressure shell, and the turbopump
and nozzle go at top and bottom,
respectively.
Sketch of a NERVA fuel element.
The hexagon
0.75 inch (1.8 cm) across the flats (the sides of the
Each hole is 0.1 inch (0.25 cm) in diameter.
measures
element).
27
10
—
7
i 6
^^^^
o
a>'
o
^B
.5
(O
•D
c
4
—
UJ
3
2
—
1
Temperature, K
Exhaust velocity,
V, m/sec
Lifetimes
peratures.
1
^-
9
8
goal
y
oj NERVA
Progress
• during
1
1
B 1I
1970
w>
H
2200
8000
juel
A nuclear rocket
nozzle
under construction,
showing coolant tubes welded
to the basic
bell-shaped
structure.
Krogre
> durinc
^^P
J 1970
1 4, H
2360
8250
elements
2500
8450
at different
hydrogen
tem-
Actually, there are two factors that control the size of
the reactor core. Sometimes, the core has to be made
bigger so that enough hydrogen can be forced through it to
remove all the heat generated; it is then termed "heattransfer limited". On the other hand, if the size of the core
has to be increased just so enough fuel can be added to make
it critical, it is called "criticality-limited".
To raise or lower the reactor power, the neutron economy
must be upset, or altered. Control drums help perform this
task in the NERVA reactor. The control drums are cylinders
covered on one side with a neutron "poison" such as boron.
When all the drums' absorbing faces are turned inward,
neutrons that would otherwise be reflected back into the
core to cause new fissions are absorbed by the poison
instead. To start the reactor, motors slowly rotate the
control drums, moving the poisons away from the core
regions, thus giving the neutron economy a boost. If the
drums are turned far enough so that each neutron in
28
alloy. Rocket engineers call this a regeneratively cooled
nozzle. Most liquid chemical rockets use this sort of nozzle, too.
In the chemical rocket, the nozzle coolant is usually the
liquid fuel or oxidizer, which is diverted through the
nozzles' cooling tubes on its way from the large fuel tanks
to the combustion chamber. The nuclear rocket copies this
Structural details oj a
regeneratively
cooled
nozzle.
Cross-section
oj nozzle "skin",
showing the phalanx of hydrogen-carrying
cooling
tubes behind the highlemperature alloy surjaces IS shown al right
33
Reactor hot end
Convergent
section
Region of
transition to
supersonic flow
Generation No. 1 causes 1.1 fissions in Generation No. 2,
1.21 in Generation No. 3, and so on in a continuing p r o g r e s sion, the r e a c t o r power will r i s e very rapidly. Unless the
control d r u m s a r e returned to the exact point where
criticality occurs, reactor power will r i s e toward infinity.
X
\
y^
'^'^~Nr
\
/ I
\
/
-f^*"^^ • \
fJS^^'''^
Reflector /
\
\
rrJ
/
rf ® Neutron generation # 1
Fissions <
I (D Neutron generation #2
\
\
Corp
I—I Absorption
\
O Fission
\
(Y)
3}
Directed hydrogen
exhaust stream
The rocket nozzle converts the heat m the exhaust
gas to directed kinetic energy.
is a compromise between these factors to attain high
exhaust velocity.
As the hydrogen leaves the end of the nozzle, it still
contains some of the heat energy that the reactor added to
it. This i s "waste heat" and cannot be converted into the
kinetic energy of the exhaust gas. Like all other engines
that convert heat into kinetic energy, the nuclear rocket
engine must throw away some of the heat it g e n e r a t e s .
Powerful forces act on the nozzle because it has to c a r r y
the entire thrust load (37.5 tons in NERVA) up to the
rocket body proper. What material can withstand this force
in the presence of the super-hot hydrogen rushing past it
at supersonic s p e e d s ? Graphite i s too weak and steel would
melt. Tungsten i s probably strong enough, but no one yet
knows how to fabricate big s t r u c t u r e s out of tungsten. The
solution i s a high temperature alloy such as stainless steel,
covered by a solid phalanx of cooling tubes that keeps the
nozzle t e m p e r a t u r e s well below the melting point of the
32
Neutron absorption occurs when
the neutron "poison"
sides oj
the control drums are turned toward the reactor core. In an actual control system
all drums
turn together.
Because neutron generations a r e only milliseconds apart,
neutron "population explosions" (and reactor power changes,
too) can come about very quickly.
An interesting feature of practical reactor control, however, i s that almost identical control drum settings can
produce stable power levels of 1 watt o r , say, 10,000
megawatts. Reactors a r e "rate-controlled", that i s , doubling the amount of displacement of the drum does not
double the power level, but rather doubles the rate at which
the power level changes.
The reactor core i s supported by a support plate, tie
rods, and side r e s t r a i n t components. This side or lateral
support system h a s been developed to eliminate vibration
and to accommodate the change in core dimensions arising
from the t h e r m a l expansion of the c o r e .
29
The Artificial Maelstrom
Overall hydrogen
flow pattern
The NERVA pump must r a i s e the p r e s s u r e of liquid
hydrogen by about 97 atmospheres (1300 lbs per square
inch) while delivering nearly 3 tons of it a minute to the
reactor. Not only is this a huge volume of fluid to handle
(liquid hydrogen has only about one-tenth the density of
water), but the p r o c e s s consumes a huge quantity of power.
Two types of pumps can be considered: Centrifugal and
axial-flow. In a centrifugal pump, liquid hydrogen at the
intake (near the pump shaft) is caught by spinning blades
(impellers) and flung outward toward the pump rim by
centrifugal force, where it leaves through a diffuser at a
higher p r e s s u r e . An axial-flow pump is something like a
s e r i e s of fans mounted on the same shaft, driving the
liquid hydrogen along parallel to the pump shaft.
The centrifugal pump employed during the Engine System T e s t s consumed about 5 million watts of shaft power.
Where does all this power come from? In space, the only
answer can be: From the r e a c t o r itself. Some of the energy
imparted to the hydrogen by the r e a c t o r must drive the
turbine that turns the pump. As we shall see a little later,
this energy comes from the engine nozzle and the outer
sections of the r e a c t o r in a mode of engine operation called
the "full-flow " or "topping cycle ".
3%
Turbine power
control valve
- 3 % of
reactor
efflux—
— Nozzle coolant pipe
carries entire
hydrogen flow)
(100%)
Where the Work is Done
The hydrogen leaving the hot end of the nuclear rocket
reactor is laden with t h e r m a l energy that first must be
converted into gas kinetic energy and then into rocket
kinetic energy. This is the job of the nozzle. A c o n s t r i c tion called the nozzle " t h r o a t " s t a r t s the p r o c e s s nicely.
F i r s t the throat speeds up the hydrogen velocity until it is
travelling at the speed of sound (Mach 1). Beyond the
throat, the nozzle opens up into a carefully contoured
"divergent" section. Here, the hydrogen expands and cools
rapidly as heat energy is converted to gas velocity (kinetic
energy). The hydrogen, now travelling at supersonic velocit i e s , pushes against the divergent sides of the nozzle,
pushing the rocket in the opposite direction through a i r l e s s
space. The expansion of hot gas in the nozzle is analogous
to gas expansion against a piston or a turbine blade.
30
A possible method of diverting
to drive the propellant pump is
This particular arrangement is
the so-called "full-floiv cycle"
will be described.
f
a small fraclion oj reactor
power
shown in this engine
arrangement.
called the"hot bleed cycle".
Later,
which is used in the NERVA engine
To achieve a high exhaust velocity, the exit a r e a of the
divergent section must be as large as possible in comparison to the throat a r e a . For booster rocket engines,
the ratio of exit a r e a to throat a r e a is usually about 15 to 1.
For space, the ratio can be as high as 100 to 1, limited
only by the length and weight of the nozzle. Nozzle design
31
The Artificial Maelstrom
The NERVA pump must r a i s e the p r e s s u r e of liquid
hydrogen by about 97 atmospheres (1300 lbs per square
inch) while delivering nearly 3 tons of it a minute to the
reactor. Not only is this a huge volume of fluid to handle
(liquid hydrogen has only about one-tenth the density of
water), but the p r o c e s s consumes a huge quantity of power.
Two types of pumps can be considered: Centrifugal and
axial-flow. In a centrifugal pump, liquid hydrogen at the
intake (near the pump shaft) is caught by spinning blades
(impellers) and flung outward toward the pump rim by
centrifugal force, where it leaves through a diffuser at a
higher p r e s s u r e . An axial-flow pump is something like a
s e r i e s of fans mounted on the same shaft, driving the
liquid hydrogen along parallel to the pump shaft.
The centrifugal pump employed during the Engine System T e s t s consumed about 5 million watts of shaft power.
Where does all this power come from? In space, the only
answer can be: From the reactor itself. Some of the energy
imparted to the hydrogen by the r e a c t o r must drive the
turbine that t u r n s the pump. As we shall see a little later,
this energy comes from the engine nozzle and the outer
sections of the r e a c t o r in a mode of engine operation called
the "full-flow " or "topping cycle ".
^
Liquid
hydrogen tank
,'
loos'
Overall fiydrogen
flow pattern
3%
Turbine powe
control valve
97%
- 3 % of
reactor
efflux—
Nozzle coolant pipe
carries entire
fiydrogen flow)
(100%)
Where the Work is Done
The hydrogen leaving the hot end of the nuclear rocket
reactor is laden with t h e r m a l energy that first must be
converted into gas kinetic energy and then into rocket
kinetic energy. This is the job of the nozzle. A c o n s t r i c tion called the nozzle " t h r o a t " s t a r t s the p r o c e s s nicely.
F i r s t the throat speeds up the hydrogen velocity until it i s
travelling at the speed of sound (Mach 1). Beyond the
throat, the nozzle opens up into a carefully contoured
"divergent" section. Here, the hydrogen expands and cools
rapidly as heat energy is converted to gas velocity (kinetic
energy). The hydrogen, now travelling at supersonic velocities, pushes against the divergent sides of the nozzle,
pushing the rocket in the opposite direction through a i r l e s s
space. The expansion of hot gas in the nozzle is analogous
to gas expansion against a piston or a turbine blade.
30
A possible method oj diverting
lo drive the propellant pump is
This particular arrangement is
the so-called "full-flow cycle"
will be described.
a small /racl/on of reactor
power
shown in this engine
arrangement.
called the"hot lileed cycle".
Later,
rvhich is used in the NERVA engine
To achieve a high exhaust velocity, the exit a r e a of the
divergent section must be as large as possible in comparison to the throat a r e a . For booster rocket engines,
the ratio of exit area to throat a r e a is usually about 15 to 1.
For space, the ratio can be as high as 100 to 1, limited
only by the length and weight of the nozzle. Nozzle design
31
Reactor tiot end
Convergent
section
Region of
transition to
supersonic flow
Generation No. 1 causes 1.1 fissions in Generation No. 2,
1.21 in Generation No. 3, and so on in a continuing p r o g r e s sion, the reactor power will r i s e very rapidly. Unless the
control d r u m s are returned to the exact point where
criticality occurs, reactor power will r i s e toward infinity,
y^
X
^"^^iT —X
/ (
/
\
\
rjrJ
\
\
/
\
/
y ^
R^S^^''''^
Reflector /
~^
Core
is a compromise between these factors to attain high
exhaust velocity.
As the hydrogen leaves the end of the nozzle, it still
contains some of the heat energy that the reactor added to
it. This IS "waste heat" and cannot be converted into the
kinetic energy of the exhaust gas. Like all other engines
that convert heat into kinetic energy, the nuclear rocket
engine must throw away some of the heat it generates.
Powerful forces act on the nozzle because it has to c a r r y
the entire thrust load (37.5 tons in NERVA) up to the
rocket body proper. What material can withstand this force
in the presence of the super-hot hydrogen rushing past it
at supersonic s p e e d s ? Graphite is too weak and steel would
melt. Tungsten is probably strong enough, but no one yet
knows how to fabricate big s t r u c t u r e s out of tungsten. The
solution is a high temperature alloy such as stainless steel,
covered by a solid phalanx of cooling tubes that keeps the
nozzle t e m p e r a t u r e s well below the melting point of the
32
\
n Absorption
\
® Fission
\
\
rjj")
^^'"'-sl'
The rocket nozzle converts the heat m the exhaust
gas to directed kinetic energy
_
f ® Neutron generation #1
Fissions \
I ® Neutron generation #2
\
U!^
'*
Neutron absorption occurs iihen
the neutron
"poison"
sides of
the control drums are turned toward the reactor core. In an actual control system
all drums
turn together.
Because neutron generations a r e only milliseconds apart,
neutron "population explosions" (and reactor power changes,
too) can come about very quickly.
An interesting feature of practical reactor control, however, IS that almost identical control drum settings can
produce stable power levels of 1 watt or, say, 10,000
megawatts. Reactors are " rate-controlled ", that is, doubling the amount of displacement of the drum does not
double the power level, but rather doubles the rate at which
the power level changes.
The reactor core is supported by a support plate, tie
rods, and side r e s t r a i n t components. This side or lateral
support system has been developed to eliminate vibration
and to accommodate the change m core dimensions arising
from the t h e r m a l expansion of the c o r e .
29
10
Luny-Biiuurcim-e yudi
^
9
8
7
i
6
o
Progress
* during
1970
i 5
CD
•o
=
4
A nuclear rocket nozzle
under construction, shoii ing coolant tubes welded
to lite basic
bell-shaped
structure.
UJ
3
Progress
during
1970
2
1
Temperature, °K
Exhaust velocity,
V, m/sec
Lifetimes
peratures.
of NERVA
2200
8000
fuel
2360
8250
elements
2500
8450
at different
hydrogen
tem-
Actually, there are two factors that control the size of
the reactor c o r e . Sometimes, the core has to be made
bigger so that enough hydrogen can be forced through it to
remove all the heat generated; it is then t e r m e d " h e a t transfer limited". On the other hand, if the size of the core
has to be increased just so enough fuel can be added to make
it critical, it is called "criticality-limited".
To r a i s e or lower the r e a c t o r power, the neutron economy
must be upset, or altered. Control d r u m s help perform this
task in the NERVA r e a c t o r . The control drums a r e cylinders
covered on one side with a neutron " p o i s o n " such as boron.
When all the d r u m s ' absorbing faces are turned inward,
neutrons that would otherwise be reflected back into the
core to cause new fissions are absorbed by the poison
instead. To s t a r t the r e a c t o r , motors slowly rotate the
control d r u m s , moving the poisons away from the core
regions, thus giving the neutron economy a boost. If the
drums are turned far enough so that each neutron in
28
alloy. Rocket engineers call this a regeneratively cooled
nozzle. Most liquid chemical rockets use this sort of nozzle, too.
In the chemical rocket, the nozzle coolant i s usually the
liquid fuel or oxidizer, which is diverted through the
nozzles' cooling tubes on its way from the large fuel tanks
to the combustion chamber. The nuclear rocket copies this
Structural details of a
regeneratively
cooled
nozzle.
Cross-section
of nozzle "skin",
showing the phalanx of hydrogen-carrying
cooling
tubes behind the highlemperature alloy surfaces is shown at right.
33
)!»**•*•
technique by pumping the supercold liquid hydrogen through
the nozzle tubes, then up through the reactor reflector, and
finally down through the core proper. Nozzle cooling thus
forces a down-up-down hydrogen flow pattern. Without this
cooling the nozzle would not survive more than a few
seconds.
Holding the Pieces Together
Holding the various pieces of the engine together is the
obvious task of the engine s t r u c t u r e . All major components
are arranged along a centerline that extends from the
nozzle, through reactor and turbopump, into the rocket body
proper. The engine s t r u c t u r e is essentially an exoskeleton
(like a b e e t l e ' s hard shell) built symmetrically around the
Turbopump
Shield
Pressure shell
Motors
Control drums
"T
one-tenth of an inch (0.25 cm) in diameter. It is in these
holes that the gas picks up the heat generated in the body
of the fuel element by the fissioning uranium.
Nuclear rocket fuel elements operate under extremely
severe conditions: (1) t e m p e r a t u r e s that reach over 3000°K
internally; (2) power densities of kilowatts per cubic centim e t e r ; (3) intense levels of nuclear radiation; and (4) many
thermal " c y c l e s " (transients) that can crack the fuel elements just as boiling water cracks a heavy glass tumbler.
On top of these problems, the hydrogen gas driven into the
holes of the fuel element will react chemically and c o r rode the graphite structure unless protective coatings are
added. Because the success of the nuclear rocket depends
upon fuel elements that retain their integrities despite
these infernal conditions, much of the AEC effort has gone
into extending the life of graphite fuel elements at high
temperatures.
Excessive hydrogen corrosion can be prevented by
coating the fuel element holes with a metal carbide, such
as niobium carbide. C e r a m i c i s t s solve the other problems
by trying different combinations of graphite powder, u r a nium carbide, and binder m a t e r i a l s until they have a fuel
element that can hold together under some of the worst
conditions man can c r e a t e . During the nuclear rocket
program, there has been a p r o g r e s s i o n of better and better
fuel m a t e r i a l s . The goal of the NERVA program has been
a fuel element that will heat hydrogen to 2360°K and s u r vive for 10 hours.
T"
Core and reflector
= reactor
Nozzle
34
Exploded viewoj a nuclear rocket
engine. Control drums Jit in the
reflector,
the reactor inside the
pressure shell, and the turbopump
and nozzle go at top and bottom,
respectively.
Sketch of a NERVA fuel element.
The hexagon
0.75 inch (1.8 cm) across the flats (the sides of the
Each hole is 0.1 inch (0.25 cm) in diameter.
measures
element).
27
little appetite for neutrons; fortunately, graphite has just
such a "low neutron absorption c r o s s section". Naturally,
enough uranium-235 atoms must be dispersed throughout
the graphite core so that the questing neutrons can find and
fission them. But if there is too much uranium mixed with
the graphite, fuel element structural strength is reduced.
Core design requires a balancing of all these considerations,
plus one more.
Almost all the energy released by uranium is first
incorporated in the kinetic energy of two large fission
fragments that fly off in opposite directions as a nucleus
splits (Newton's Third Law again), peppering surrounding
atoms. In a few millionths of a second, the kinetic energy
of the heavy fragments is t r a n s f e r r e d to the nearby atoms,
setting them to vibrating. A wave of heat s p r e a d s out from
the fission site. If this heat is not removed, core t e m p e r a t u r e s will quickly r i s e beyond the sublimation point of the
graphite fuel.
The reactor core, therefore, has to be designed in such
a way that all this fission-generated heat is t r a n s f e r r e d
to the hydrogen gas that is to be driven through the reactor
by the pump. This means that the reactor must be perforated
with holes that c a r r y the hydrogen — serving here as a
reactor coolant—through and past the hot fuel to the nozzle. If the coolant p a s s a g e s are too large, the size of the
whole engine becomes so great it is unacceptable for space
use. Holes that a r e too small cause high friction losses that
impose powerful forces on the reactor s t r u c t u r e . The
designer must avoid these extremes and still provide
enough coolant passage a r e a to remove all the heat. (Note
that the rate of heat transfer to the hydrogen is p r o p o r tional to the a r e a inside the holes and to the t e m p e r a t u r e
difference between the hole wall and the center of the
hydrogen stream.)
The basic fuel s t r u c t u r e within the reactor core is
called a fuel "element". The NERVA fuel element is a long
hexagonal piece of graphite pierced by 19 holes that c a r r y
the hydrogen gas lengthwise through the reactor. The fuel
elements a r e stacked together in a close a r r a y so that
the hydrogen gas driven into the reactor by the pump encounters thousands of long, but very narrow, holes about
26
centerline. The nozzle is bolted to the reactor p r e s s u r e
shell, which in turn is bolted to a truncated conical shell
that supports the turbopump. Thus, a continuous shell of
strength encapsulates core, reflector, control drums, t u r b o pump, and miscellaneous a c c e s s o r i e s .
A critical structural problem a r i s e s at the junction b e tween the engine and rocket body, however. Across this
junction must be transmitted the entire engine thrust
that is conveyed upward from the nozzle through the engine
exoskeleton. Ordinarily, such a joint would present no engineering difficulties. But in this case, the joint must be
flexible'. All big rockets have their engines mounted on
gimbals that permit the " d r i v e r " to s t e e r them. During a
launch, for example, a gust of wind might deflect an
ascending rocket off course: A gimballed engine can bring
it back on the desired flight path. Gimballing chemical
engines is relatively easy because they a r e lightweight.
But, in a nuclear rocket, the heavy reactor replaces the
empty combustion chamber of the chemical rocket. Despite
the added weight, suitable flexible joints now have been
designed.
NOZZLE SKIRT EXTENSION
EXTERNAL
DISC SHIELD
REFLECTOR
Drawing of the complete
NERVA
engine.
35
A final structural note: Heavy though the nuclear engine
may be, it is still but a small appendage on a much l a r g e r
s t r u c t u r e consisting mainly of huge propellant tanks.
Bootstrapping and the Full-Flow Turbine Drive
The NERVA engine is really two engines in s e r i e s . The
main engine is, of course, the rocket nozzle that converts
most of the energy in the hot hydrogen coming from the
r e a c t o r proper into exhaust kinetic energy. The other
engine is the turbopump, which e x t r a c t s a portion of the
energy in the ivarm hydrogen returning from cooling the
nozzle and reflector and converts it into the rotary motion
of the main hydrogen pump. Following the diagram on
page 37, the sequence of events is: (1) the hydrogen steam
picks up energy from the nozzle and reflector; (2) the
turbine attached to the pump converts p a r t of this energy
into pump energy; (3) the hydrogen is pumped into the
reactor; (4) the reactor adds energy to the hydrogen stream;
and (5) the nozzle converts a fraction (90%) of the hydrogen
energy into exhaust kinetic energy. The overall p r o c e s s is
called the "full-flow " o r "topping cycle "because there a r e
two separate additions of energy to the hydrogen — one on
top of the other. Some logical questions at this point a r e :
How is engine thrust controlled and how can we get the
engine started ?
Suppose that some spaceship pilot of the future wants
more thrust from his nuclear rocket engine, all he will
have to do is pull a lever that partially closes a valve in
a pipe that c a r r i e s some of the warm hydrogen around the
turbine driving the pump. When hydrogen flow is r e s t r i c t e d
in this " bypass line", more of the warm hydrogen p a s s e s
through the turbine. The turbine now has more power to
convert into pump power and consequently speeds up. Thus,
more liquid hydrogen flows into the reactor for thrust
production. But in a rocket engine, unless the reactor
power is increased simultaneously, the only result will be
more, but cooler, hydrogen leaving the reactor. At first
36
coursing through the reactor that stimulates nuclear fission
and thence power production. Each fissioned atom of
uranium-235 produces 2V2 new neutrons on the average. If
the rate at which fissions occur (and consequently the power
level) is to remain constant, exactly one of these 2V2 neut r o n s has to go on and cause another fission. Reactor
"criticality" occurs at just this point. This balance sheet
leaves IV2 neutrons per fission that can escape the reactor
altogether or be absorbed in nonfission nuclear reactions.*
To prevent too many neutrons from escaping, a material
is placed around the reactor to reflect some errant neutrons back into the core. Excessive neutron absorption can
be avoided by using core and reflector m a t e r i a l s that have
*See Nuclear Reactors, a companion booklet in this s e r i e s , for
an account of reactor construction.
\
Fission fragments
fly apart
Fission
time= 1 microsecond
Thermal
"spike"
Nuclear fuel
A few microseconds later
Heat removed by
propellant
The kinetic energies of the fission fragments appear as a pulse
of heat that flows outward to the
walls of the fuel, where it heats
the rocket
propellant.
25
Nozzle coolant pipe
(carries entire
hydrogen flow)-^
Warm hydrogen drives
turbopump (bypass line
not shown)
Reflector
Hydrogen flow diagram tn the full-flou^ cycle. The numbers
to the description of the operation of this cycle on page 36.
•
Neutron absorption
Q Uranium - 235 Fission
f ® Neutron generation #1
Fissions < ,-. .
[CD Neutron generation #2
Reactor criticality occurs when exactly one neutron from each fission reaction goes on to cause another fission reaction.
Sketch
shows five neutrons from Generation No. 1 producing two new fissions and five neutrons in Generation No. 2. Three neutrons in
Generation No. 1 are lost through escape and absorption.
a critical m a s s ; * (2) The need to remove all generated heat;
(3) The need to raise and lower power at will, or control
the reactor; and (4) The need to maintain s t r u c t u r a l integrity
at high t e m p e r a t u r e and under the forces exerted by the
high velocity hydrogen gas.
Attainment of a critical m a s s is a m a t t e r of "neutron
economics", because it is the cloud of invisible neutrons
*The smallest mass of fissionable material that will sustain
a chain reaction. See Nuclear Terms, A Glossary, a companion
booklet in this s e r i e s , for detinitions of unlamiliar t e r m s .
24
refer
glance one would think that the pilot should also signal
the reactor control drums to r a i s e the r e a c t o r power. It
t u r n s out, however, that this is unnecessary. Nuclear
rocket control is simplified by the fact that the hydrogen
itself affects reactor criticality because of its ability to
collide with neutrons and slow them down to speeds where
they can more readily cause uranium fissions.* In other
words, shooting more hydrogen into the nuclear rocket
core is equivalent to a slight outward rotation of the control drums. The pilot, then, by closing the valve in the
turbine bypass line will effectively i n c r e a s e both hydrogen
flow and r e a c t o r power with the same action. His lever
will be as direct and simple as the throttle on an automobile
engine.
But what keeps the reactor power from increasing
indefinitely once the additional hydrogen is pumped in?
(Remember that r e a c t o r s a r e rate-controlled
and that their
power level keeps rising once neutron poisons are pulled
*The hydrogen thereby serves as a reactor moderator.
37
away from the core.) The answer to this also i s simplicity
itself: As the reactor power r i s e s , so does the t e m p e r a ture of the hydrogen. The hydrogen expands with t e m p e r a t u r e , and there is l e s s of it in the core at any one moment,
even though it is being pumped through faster. L e s s hydrogen means that reactor criticality is affected l e s s , and the
power level begins to level off.
In fact, the power will become constant when the average
hydrogen t e m p e r a t u r e is exactly what it was before the pilot
commanded the engine to provide more thrust. Since t e m p e r a t u r e stabilizes at the old value, exhaust velocity, v, is
unchanged. The things that have changed are rh, the hydrogen flow r a t e , and F, the thrust. Because F = riiv, the
engine t h r u s t is directly proportional to the flow rate (v is
fixed because the t e m p e r a t u r e is fixed) and, m in turn, to
how far the pilot closes the valve m the bypass line. In
other words, the hydrogen gas in the core, by either expand-
Turbopump
Turbopump
exhaust
"Warm"
hydrogen
bled from
this plenum
5. The controls that force all engine components to
m a r c h in step at the command of the spacecraft pilot.
Besides the system unity imposed by the s t r u c t u r e and
controls, the engine p a r t s have to fit together thermodynamically. To breathe "life" into a nuclear rocket there
must be a " s t a r t e r " , like that in an automobile, intrinsic
in the system. The engine must "catch", become selfsustaining, and generate useful power. For this to happen,
two energy sources must be found: One to s t a r t the engine
and another to power the pump that keeps propellant flowing through the engine. A nuclear reactor heat source all
by itself would be no more useful than an unharnessed
gasoline flame would be in driving an automobile.
The Biggest, Hottest Reactors Ever Built
The heart of the nuclear rocket engine i s , of course,
the r e a c t o r . In the NERVA engine the reactor must gene r a t e upwards of 1500 megawatts of t h e r m a l power — more
than the output of 15,000 Volkswagens or 50,000 homeheating furnaces—and still not weigh the i^ocket down.
A t e m p e r a t u r e of 2500°K can be reached, according to
r e a c t o r and full-scale engine t e s t s in Nevada. Power
levels of 4200 megawatts have already been attained.
NERVA is as powerful and three t i m e s hotter than the
r e a c t o r s in any earthbound commercial nuclear-electric
generating plant in the U. S. The NERVA engine, however,
does not have to operate for y e a r s like its commercial
counterparts; it can run at 2500°K only because a few
hours of operation are sufficient for most space applications.*
A pile of uranium and graphite does not make a rocket
r e a c t o r , although this was the way Enrico F e r m i achieved
the first self-sustaining chain reaction in 1942. Four r e quirements control reactor design: (1) The need to attain
Cold bleed cycle
A third type of engine cycle is the "cold bleed cycle". This mode
of operation makes inefficient use of hydrogen propellant because
some of it IS thrown away.
38
*Another AEC program develops space reactors that operate at
low power levels tor long periods of time. See Nuclear Reactors
for Space Power, a companion booklet in this series.
23
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Engine designers also had to find a scheme that would
s t a r t the engine, given a cold reactor and an inactive turbopump The first thought is for some energy source to turn
over the turbopump. In an automobile, a storage battery
and s t a r t e r motor suffice, but a similar system that was
big enough to turn over the turbopump would weigh too much.
The answer to the startup problem lies in the energy
contained in the p r e s s u r i z e d hydrogen in the propellant
tank. Even the very cold liquid hydrogen, when admitted to
the turbopump under p r e s s u r e , can turn it over enough to
push a little of the hydrogen into the r e a c t o r . Here the
hydrogen picks up some heat and is turned to gas, which
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ing or contracting, changes the reactor power in a way that
keeps t e m p e r a t u r e constant. In reactor engineering, hydrogen is said to have a negative temperature coefficient of reactimlv.
The full-flow turbine-drive cycle was selected over
several other potential cycles because it yields the highest
performance at a given r e a c t o r operating t e m p e r a t u r e .
The other major contender was the hot bleed cv( Ie, m
which a small portion of the hot hydrogen rushing out of
the reactor is fed back to the turbopump. The diverted
hydrogen, after it has passed on some of its energy to the
turbine, is jettisoned and does not produce useful thrust.
The hot bleed cycle is therefore wasteful of hydrogen and,
for the same r e a c t o r temperature, has a specific impulse*
about 25 seconds lower than a topping cycle using the same
r e a c t o r . Because the hot bleed cycle's turbine uses only
a small fraction (about 3%) of the hydrogen stream, it is
small and lightweight compared to the topping cycle t u r bine. It IS, however, a high t e m p e r a t u r e turbine and subject
to all the ills that b e f a l l hot, high-speed rotating m a chinery— in other words, its reliability would be lower
than that of the bigger but cooler topping cycle turbine.
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* Propulsion engineers often also speak of an artificial quantity
called specific impulse,which is equal to the exhaust velocity
divided by the acceleration due to gravity, gQ, (9.8 meters per
second^, or 32.2 feet per second^) This is also equal to F/ggrfi.
39
drives the turbopump a little faster. The turbopump pumps
more liquid hydrogen into the engine. More warmed hydrogen drives the turbopump faster, and so on, until full
power is reached. The t e r m " bootstrapping" is applied to
this type of cold start, and the engine really does pull
itself up to full power, in effect "by its own bootstraps".
The turbopump cycle, the s t r u c t u r e , and the engine cont r o l s transcend any one of the other major components.
They unify the engine subsystems and show that one part
cannot be designed without careful regard for its effect on
the r e s t .
The Xe installed in ETS-1 m
Nevada. The XE has been the
engine closest to actual flight
configuration.
s t o r m s and reached the point where a flyable NERVA engine could be developed.
BUILDING AN ENGINE FOR SPACE
A nuclear rocket engine is considerably more than a
heater of hydrogen. It is true that the engine is built around
the r e a c t o r c o r e — t h e wellspring of e n e r g y — b u t s o m e thing else has to transport hydrogen propellant from tank
to r e a c t o r ; something else has to convert heat to thrust.
The engine, in fact, has five major segments:
1. The nuclear r e a c t o r heat source.
2. The pump that pulls liquid hydrogen from its tanks
and forces it through the reactor.
3. The nozzle, the p r i m e mover that transforms heat
to thrust.
4. The structure that physically holds all the pieces
together.
40
21
Further details of this EST test s e r i e s are listed in the
table on p. 22.
The test in June 1965 introduced the Phoebus nuclear
rocket reactor s e r i e s . The Phoebus r e a c t o r s a r e best
thought of as "tools" to advance nuclear rocket technology.
For example, they incorporate significant advances over
the Kiwis in t e r m s of higher t e m p e r a t u r e s , higher power
density in the core, and longer life — all factors that will
lead to better nuclear rockets.
A most important s e r i e s of engine t e s t s commenced in
December 1968, when the NERVA ground experimental
engine, called the XE for short, was first put through its
paces. The XE was similar to the NRX/EST but its components were arranged in a configuration closer to that
which would be used in an actual flight system. For example, the liquid-hydrogen run tank was closely coupled
and the test compartment was partially evacuated to simulate space conditions more closely. In fact, the XE t e s t s
were as much a test of the ETS-1 (Engine Test Stand 1)
as of the engine itself. The XE s e r i e s was concluded in
late August 1969, logging a total of 28 power t e s t s at v a r i ous power levels. The total engine operating time was
just under four hours. The XE was started successfully
from a wide variety of starting conditions, demonstrating
that a nuclear rocket can meet the requirements of a space
transportation system.
In addition to developing better graphite r e a c t o r s such
as Phoebus, NASA-AEC experts have studied new types of
nuclear rockets in which the fuel elements a r e made from
tungsten, another material with good strength at very high
t e m p e r a t u r e s . Even more adventurous a r e the studies of the
so-called " g a s - c o r e " r e a c t o r s in which the nuclear fuel
itself IS in gaseous form, retained within the rocket by
centrifugal forces exerted as the fuel and propellant gases
are spun about the rocket axis.
Although such advanced concepts undoubtedly will play
a role in nuclear rockets of the distant future, we must
focus our attention now on the graphite-core nuclear rocket.
By the end of 1969, it was obvious that the graphite nuclear
rocket had weathered all administrative and technical
20
NINETY MILES OUT IN THE DESERT
A four-lane dual highway runs northwest from Las Vegas,
Nevada, toward Death Valley. After 90 miles of sand, rocks,
Joshua t r e e s , and creosote bush, the road abruptly n a r rows to two lanes A few miles (kilometers) to the north
a r e the AEC's Camp Mercury site for underground nuclear weapons testing* and the AEC-NASA Nuclear Rocket
Development Station (NRDS). NRDS is surrounded by b a r r e n
mountains and located m a flat basin, which is called
J a c k a s s Flats after some of the indigenous local inhabitants. Both r e a c t o r s and complete engines can be tested
safely at NRDS
NRDS has three major test areas- Test Cells A and C
plus Engine Test Stand No. 1 (ETS-1). The Kiwi, NERVA,
and advanced Phoebus r e a c t o r s have been put on trial at
the two Test Cells. ETS-1 is r e s e r v e d for engine experiments and NERVA engine firings. The three test a r e a s are
connected by road and railroad to the R-MAD and E-MAD
Buildings (Reactor and Engine Maintenance, Assembly, and
Disassembly). Typically, a r e a c t o r or engine is put together
in one of the MAD buildings and then carried via the
" J a c k a s s and Western Railroad "(jokingly called the world's
shortest and slowest) to one of the test s i t e s . After the
test, while the engine or reactor is still highly r a d i o active, a heavily shielded railroad engine tows it back to
the proper MAD building for dissection. The map gives
little feeling for the great distances between the test installations, MAD buildings, and other support facilities. An
observer at any point in NRDS can see only minute outlines
of the next building shimmering in the desert heat.
The ETS-1 is the most impressive structure at the
station. It r i s e s approximately 121 feet (37 meters) from
the desert floor and looks like a s t r u c t u r e that would be at
home at Cape Kennedy. Although only engines—not complete rockets — are tested h e r e , a heavy s u p e r s t r u c t u r e is
*See Plonshaie another booklet in this series, for an account of
this testing. The name "Mercury has no relation to the NASA
"Project Mercury" for manned space flight, it peipetuates an old
mercury-mining town name
41
"W^?T^^-"r«,»«
required to hold the big engine (one is about 7 m e t e r s
long) in a vertical position and keep it from soaring off
into space as its thrust builds up. A spherical t h e r m o s like dewar (pronounced d o o ' - e r ) 14 m e t e r s in diameter,
holding up to 260,000 gallons (1,000,000 liters) of liquid
hydrogen, stands nearby as a substitute for the rocket
propellant tanks that in flight would tower high over the
relatively diminutive engine on the launch pad.
Beneath the engine nozzle is a large, water-cooled exhaust duct that turns the hydrogen stream by 90° and d i r e c t s it down a concrete-lined artificial canyon. (The
water is pumped up from deep wells.) During a test, this
canyon becomes an inferno as the hot hydrogen spontaneously ignites upon contacting the air and burns to form
water. Unlike the spectacular conflagration created during
the firing of a kerosene-burning rocket, a nuclear rocket
engine flame is invisible save for any incandescent i m purities and the heat aberrations it produces in the air.
Engine Test Stand 1,
from a different
vantage that shows
the exhaust deflector "canyon",
foreground, incised
m the desert floor
lo permit escape of
burning hydrogen.
Note upright semicylindrical shields
on either side (near
left, far right),
uhich are moved
around the engine
iihile test IS in
progress
42
vfV»»»
The rocket r e a c t o r test program achieved a major milestone
when the NRX-A6 r e a c t o r was operated at full-power for
60 minutes, a running-time sufficient for many space
missions.
Of great significance were the Engine Systems Tests
(ESTs). For the N R X - E S T s e r i e s , all major engine components were assembled—turbopump, hydrogen-cooled nozzle, control equipment, e t c . — i n t o a "breadboard"* engine. This close approximation to a real engine operating
in space was started, shut down, and r e s t a r t e d in different
ways. In short, the complete engine was put through its
paces just as it would be on a real space mission. The real
import of the N R X - E S T s e r i e s is that it showed that
a complete nuclear rocket system can start on its own
power and operate stably over a wide range of conditions.
*A breadboard engine is one that includes all principal components of a flight-test system, but is arranged to be convenient
for the test, and not as It would be in flight.
The Pewee-2 reactor on railroad car at NRDS in Nevada.
Pewee-2 is a reactor in which fuel elements can be tested
at conditions very close to (or even worse than) those encountered in an actual nuclear rocket engine.
19
Kiwi-B4A was towed off to the R-MAD Building (see
p. 41) where engineers picked it apart with remote manipul a t o r s . The reactor core showed widespread damage. Many
opinions were ventured about the cause, but the most
reasonable pointed to "dynamic" failure. That i s , the
h i g h - p r e s s u r e hydrogen gas had caused the graphite fuel
elements to v i b r a t e — t h a t i s , spaces in the r e a c t o r a l t e r nately became pressurized and depressurized. This caused
the elements to vibrate and impact each other.
A good engineer views testing philosophically. After all,
it was far better to liave a failure out on the Nevada d e s e r t
than on a spaceship halfway to M a r s . The path ahead was
c l e a r . All of 1963 was devoted to confirming the diagnosis
of vibration-induced failure. Reactor components were
systematically tested in s t r e a m s of hot high velocity
hydrogen. "Cold-flow" t e s t s with complete r e a c t o r s , but
generating no heat, were c a r r i e d out. Gradually, a new
vibration-resistant reactor was designed. In this sense,
the Kiwi-B4A test was successful.
Hot testing of the new Kiwi nuclear r e a c t o r s r e s u m e d in
May 1964, only 18 months after Kiwi-B4A. Since then, a
long s e r i e s of highly successful t e s t s has proved the r e designed nuclear core to be sound. Included in the s e r i e s
were the following:
May 1964
August-September 1964
September-October 1964
January 1965
A p r i l - M a y 1965
June 1965
FebruaryMarch 1966
June 1966
February 1967
December 1967
J u n e - J u l y 1968
November—
December 1968
December 1 9 6 8 August 1969
One power test, Kiwi-B4D
Two power tests, Kiwi-B4E
Two power tests, NRX-A2*
Kiwl-TNT, a nuclear safety test
Three power tests, NRX-A3
One power test, Phoebus-IA
Ten s t a r t s . E n g i n e System
Test, NRX-EST
Two power t e s t s . NRX-A5
Two power t e s t s . Phoebus-lB
One power test, NRX-A6
Three power tests ot Phoebus-2A.
Peak power reached: 4200
megawatts.
Two power tests of P e w e e - 1 , a
fuel-element, test-bed r e a c t o r .
Total of 28 s t a r t s of the NERVA
ground experimental engine (XE).
*NRX s t a n d s f o r NERVA R e a c t o r E x p e r i m e n t . See G l o s s a r y ,
p a g e 54 f o r e x p l a n a t i o n s of o t h e r c o d e n a m e s .
18
Arrangement
Nevada.
of facilities
at
the
Nuclear
Rocket
Development
Station,
All the Kiwis, Phoebuses, Pewees, and NRXs were tested
on special railroad c a r s that c a r r i e d them between the test
cells and R-MAD and also served as mobile testbeds.
Thrust and exhaust velocity were engine p a r a m e t e r s of
little interest in those reactor t e s t s ; actually, the nozzles
were not designed to produce thrust efficiently. The r e s u l t s
sought in testing a complete engine, though, dictate that
the engine be placed in a special structure on the ETS-1
that can measure the reaction forces that a r e generated.
A unique railroad car called the Engine Installation Vehicle
(ElV) has been constructed to c a r r y the complete engine
up to the ETS-1 and lift it into place on the test stand with
hydraulic "hands". After a test, the p r o c e s s is r e v e r s e d
and the hot engine is returned to E-MAD.
The MAD buildings a r e p r i m a r i l y immense "hot c e l l s "
where engineers can perform work on radioactive r e a c t o r s
and engines, but remain protected by thick concrete walls.
43
Test Cell C m 1967 Movable shields on test stand,
right center, surround reactors during test Building, loner right, can be
moved on rails over reactors for weather protection
Exhaust blast from a successful reactor test shoots high m the air
over the test cell propellant tanks (Telescopic lens photo)
^_^^__„t "'*^" - » • '"^{T
I J, rl'l
^ l\
---fi^'SiTjfc
The Phoebus-IA reactor after arrival on its special railroad car at
the test cell
The E-MAD Building, where
engines are put together,
or disassembled
after
tests
The test s e r i e s started successfully enough when Kiwi-BlA
was fired with gaseous hydrogen in December 1961. Tests
with liquid hydrogen commenced in September 1962 in
Kiwi-BlB. This firing proved that a nuclear rocket could
operate successfully with liquid hydrogen,* but the test
had to be cut short when some of the internal graphite
p a r t s cracked and failed. Was this failure due just to
faulty m a t e r i a l s or components, or was the whole reactor
design s u s p e c t ? The Kiwi-B4A—the core design favored
for NERVA — was tested on November 30, 1962 As the
r e a c t o r power was raised, o b s e r v e r s watchmr from a
distance saw flashes of light in the hydrogen exhaust plume
rising from the inverted nozzle. More flashes appeared and
the test was purposely terminated. Unquestionably, s o m e thing fundamental was wrong with the Kiwi-B design.
*An important finding indeed, since rocket propellant tanks for
compressed gaseous hydrogen at room temperature would have to
be 2000 times as heavy as tanks for the same weight of liquid
hydrogen.
17
The National Aeronautics and Space Administration (NASA)
was created in this country on October 1, 1958, by President
Eisenhower's Executive Order 10783, to cope with this
Russian technological threat. In the p r o c e s s , the AEC—Air
Force partnership in nuclear rocketry was transformed
into an AEC-NASA activity. In August 1960, the joint
AEC-NASA Space Nuclear Propulsion Office (SNPO) was
formed* to push toward an operational nuclear engine that
would aid the U. S. in the race to the moon and planets.
Unperturbed by the administrative manipulations, nuclear
rocket engineers were greatly encouraged by the Kiwi-A
s u c c e s s e s . Plans were laid for a s e r i e s of more powerful
test r e a c t o r s , the Kiwi-B's, which would run on frigid liquid
hydrogent rather than gaseous hydrogen. The optimistic
outlook also set SNPO workers to looking for industrial
contractors to build a flyable nuclear engine. The NERVA
(Nuclear Engine for Rocket Vehicle Application) P r o g r a m
began in the fall of 1960, when SNPO requested bids from
interested companies. After a close competition, AerojetGeneral Corporation was selected in July 1961 to build the
engine (structure, pumps, etc.), and the Westinghouse
Electric Corporation's Astronuclear Laboratory was chosen
to construct the nuclear r e a c t o r , based on the Kiwi work of
Los Alamos. In May 1962, the NASA Marshall Space Flight
Center at Huntsville, Alabama, took another step by signing
the RIFT (Reactor In-Flight Test) contract with the Lockheed Missiles and Space Company. Lockheed was to build
a flight test vehicle for the Aerojet NERVA engine. This
aggressive planning was commendable, but it proved to be
p r e m a t u r e because no operational missions were assigned
to the RIFT system.
Nuclear Space Propulsion Tests
Moving a Kiwi reactor from
its assembly bay to a test
cell. This "Jackass & Westem RR" loco7notive can be
remotely controlled.
Workers swarm, over a Kiwi-B
reactor prior to its test in the
series that established the
value of liquid hydrogen propellant.
The new Kiwi-B r e a c t o r s were designed to run at about
2300°K at 1100 megawatts, ten t i m e s the Kiwi-A power,
sufficient to generate about 27.5 tons of thrust in space.
*In 1970, the joint office was renamed the Space Nuclear Systems Office, when it was assigned the additional responsibility
of providing nuclear-electric power for space missions.
tHydrogen is liquid at temperatures less than 20°K. For a
description of the extensive low-temperature technology that supports nuclear propulsion for space, see Cryogenics, the Uncommon Cold, another booklet in this series.
16
A Kiivi-A ready for a test
with gaseous hydrogen propellant.
Cutaway drawing of a Kiwi-A reactor,
basic design of the graphite core
Engine Test Stand 1 (ETS-1), 110feet (43 cm) high, where NERVA
engines are tested at NRDS. White spheres at right are
dewarsfor
liquid hydrogen and liquid oxygen. The hot exhaust rushes down
through a duct to the deflector, at left, below where m.en are standing. Tracks permit railroad to bring in engine and shields.
46
showing the
about any spot on earth. Therefore, the Air Force no
longer needed nuclear-powered rockets to deliver weapons.
Save for another event, the nuclear rocket engine might
have died a second time then and t h e r e . But on October 3,
1957, the Russians signalled the beginning of the international space r a c e by orbiting Sputnik 1, the first man-made
earth satellite. A month later they placed a live dog in
orbit.
15
lelism, the rocket p r o g r a m was dubbed Project Rover and
the ramjet was labelled Project Pluto.
Curious code names have always been part of nuclear
and space l o r e . As part of Rover, there was Dumbo, a
" p a c h y d e r m a l " reactor concept, and a huge nuclear-powered
" b i r d " named Condor. Then came the Kiwis. A kiwi, as
every New Zealander knows, is a t a i l l e s s , hairy-feathered
bird that the native Maori people named for its shrill call.
The kiwi cannot fly, and its nuclear namesake could not
either. The first Kiwis (Kiwi-A's) comprised a s e r i e s of
heavy, "battleship" test r e a c t o r s that were fired nozzle up,
a s if to emphasize their inability to leave the earth.*
The Kiwis were an essential prelude to practical nuclear
r o c k e t s . Materials t e s t s in Los Alamos laboratories had
shown that graphite (like the m a t e r i a l in pencil leads) was
a likely s t r u c t u r a l material to withstand 3000 °K. Indeed,
graphite's strength increases with t e m p e r a t u r e up to about
3300°K; at 3900 °K, it sutalimest instead of melting. What
better way was there to test this strange material than to
disperse uranium carbide fuel in it and build a reactor ?
Kiwi-A was the result.
Kiwi-A was fired for 5 minutes by Los Alamos at Nevada
Test Site in July 1959, using p r e s s u r i z e d hydrogen gas as
the "propellant". It generated 70 megawatts of heat power
and reached t e m p e r a t u r e s as high as 1777°K. Kiwi-A was
the first attempt to t r a n s l a t e a decade of calculations and
paper studies into hardware.
Two more Kiwi-A r e a c t o r s were tested in 1959 and 1960
in order to (1) check reactor design, and (2) test the
practicality of coating the graphite fuel elements with
niobium carbide for protection against corrosion by the hot
hydrogen propellant. In essence, the Kiwis were "pilot
plants" that showed engineers how to build better engines.
The climate of official support for the nuclear rocket
project changed markedly in the late 1950s. F i r s t , the
Atlas (chemical) ICBM had by that time proved itself
capable of carrying powerful nuclear warheads to just
*They were not intended to fly, so a downward exhaust would
only have caused problems; the Interest at that time was only in
the reactor operation. However, full-scale nuclear rocket engines
have since been tested nozzle-down.
tTurning directly to vapor without going through a liquid phase.
14
Once an engine or reactor has been placed in one of the
cavernous " b a y s " i n a MAD building, engineers can strip it
down piece by piece with remote manipulators that simulate
human a r m s and hands. The NERVA engine, for example,
has s e v e r a l thousands of p a r t s , including bolts, fuel e l e ments, pipe connectors, etc., that all must be checked after
a test.
Although this booklet concentrates attention on the nuclear rocket engine, the scope of NRDS facilities confirms
the general rule of thumb that most of the money used in
the space p r o g r a m stays right on t e r r a firma in the form
of launch pads, tracking facilities, r e s e a r c h - a n d - d e v e l o p ment equipment and s a l a r i e s , and, of course, NRDS. Succ e s s in space depends first of all upon success on the
ground.
Interior of the E-MAD Building, showing the huge overhead and
wall-mounted manipulators used for disassembling
radioactive nuclear rocket engines. When in actual use, no workmen would be
present in this portion of the building, because of the high radiation
levels.
47
Aviation concluding that the nuclear rocket did not seem a p plicable to ICBMs. F u r t h e r m o r e , the North American r e port mentioned the (then) fantastic t e m p e r a t u r e of 5700 °F
(3400°K) as n e c e s s a r y for the nuclear reactor. Few engin e e r s could swallow the idea of building a reactor that
would have to operate at t e m p e r a t u r e s twice the melting
point of steel. So, the nuclear rocket languished, nourished
only in a desultory and indirect way by the Atomic Energy
Commission—Air Force nuclear aircraft p r o g r a m . Robe r t W. Bussard, one of the early champions of the nuclear
rocket, put the situation well:
"Nuclear rocketry had been effectively written off as a
dead end by most of the missile and rocket people (who
didn't really understand nuclear energy and liked chemical
energy better anyway) and most of the reactor people
(who thought the whole idea of nuclear flight of any sort
was generally loony).''
It was Bussard, in fact, who did much to r e s u r r e c t
nuclear rocketry. Working in the nuclear aircraft development program at Oak Ridge National Laboratory (ORNL) in
Tennessee in the early 1950s, he was able to show that the
e a r l i e r nuclear rocket studies had been too negative and
too conservative. He was convinced that nuclear rockets
could successfully
compete with chemical rockets on long
flights with heavy payloads. B u s s a r d ' s studies and personal
salesmanship were decisive. The Air F o r c e in early 1955
decided to reexamine nuclear rockets as ICBMthrusters.
The Air Force scientists and engineers recognized,
along with everyone else, that the most critical problem
of the nuclear rocket was that of developing a hight e m p e r a t u r e - r e s i s t a n t m a t e r i a l . What material would hang
together at 3000 °K? To help answer this question, Los
Alamos Scientific Laboratory formed the Nuclear Propulsion Division under Raemer E. Schreiber. Concurrently,
a similar group was created at the Lawrence Radiation
Laboratory in California.* In mid-1956, budget cutbacks
forced consolidation at Los Alamos of all the nuclear
rocket work, while Livermore took on the task of building
a nuclear ramjet engine. In an unintentional canine p a r a l -
*Both the Los Alamos and Livermore laboratories are operated
for the AEC by the University of California.
13
Europe to London What if the V-2s had c a r r i e d atomic
bombs"? The thought was unsettling Soon designs for
Intercontinental Ballistic Missiles (ICBMs) began to take
shape on drawing boards around this country. Some of
these big rocket designs were nuclear at both ends —they
had nuclear warheads and nuclear engines Secret r e p o r t s
issued in July 1946 by North American Aviation, Inc., and
Douglas Aircraft Company (Project Rand) a r e landmarks
in the history of nuclear rockets The r e p o r t s underlined
the great p r o m i s e of the 'heat transfer " nuclear rocket,
noting its high exhaust velocity, its attainment of very high
t e m p e r a t u r e s , and its high rate of heat transfer The r e p o r t s also noted that there were problems to be solved.
The military rocket work naturally was classified as
s e c r e t by the Air Force However, at the Applied Physics
Laboratory of the Johns Hopkins University m Maryland
(near Washington, D. C.) a group of engineers, who were
unaware of the ICBM work and did not have access to the
s e c r e t r e p o r t s , innocently proceeded to duplicate all the
important findings of North American and Douglas. Their
unclassified report was published in January 1947 It was
obvious that any engineer with a slide rule and a few pieces
of paper could discover the essentials of the nuclear rocket
without much help.
In 1948 and 1949, two British space buffs, A. V. Cleaver
and L. R. Shepherd, again duplicated most of the s e c r e t
nuclear rocket fundamentals in a classic s e r i e s of papers
published in the Journal of the British Inleiplanetarv
Societv. Not long before the English report appeared, the
American-educated Chinese scientist, H. S. Tsien, had
reported his studies on the application of nuclear energy
to rockets and other " t h e r m a l j e t s " at a Massachusetts
Institute of Technology s e m i n a r . The basic principles of the
nuclear rocket could be concealed no longer As a s i d e light, it IS interesting that Shepherd went on to become a
key man in Britain's atomic energy p r o g r a m , and Tsien
later returned to China where he was a principal figure in
the development of the Peking government's atomic bomb.
Although the bonds of secrecy had been broken, interest
in nuclear rockets declined precipitously in the late 1940s.
The chief " u n d e r t a k e r " at the funeral was a careful,
competent technical report by scientists at North American
12
ENGINES FOR THE MOON AND BEYOND
The big "selling" point of the nuclear rocket is its higher
exhaust velocity, which is approximately double that of the
best chemical rockets. But this advantage is partially offset
by the fact that the nuclear engine is considerably heavier
and more costly than a chemical engine of the same thrust
rating. Admittedly, nuclear rocket engines are in an e a r l i e r
stage of development than chemical engines, too. Comparing
engine weights and costs at a fixed thrust level does not
tell the whole story. The nuclear rocket's doubled exhaust
velocity means that it uses only half as much propellant
for each second of operation. Thus, the nuclear rocket can
do much more than a chemical rocket with equal thrust.
Hou' much more depends a great deal upon the type of m i s sion being contemplated.
NERVA's thrust of 37.5 tons is considerably s m a l l e r
than that of the biggest chemical rockets, which can gene r a t e as much as 750 tons each. The role of the nuclear
rocket is not in journeys beginning at the launch pad, where
chemical engines have been so highly successful, but
r a t h e r in outer space where the nuclear rocket's high exhaust velocity IS a great asset. We should think of nuclear
rockets then, as prime movers for missions beginning
from an earth orbit s^nA moving outward toward higher earth
orbits, the moon, and the planets. Very high thrust levels
a r e not n e c e s s a r y for nuclear rockets because they a r e
not required to lift payloads off the launch pad.
The great propellant economy of the nuclear rocket
tends to make it superior for those missions m which much
of the spacecraft m a s s is allotted to propellant. This is
subject to the condition that the payload m a s s be a sizeable
fraction of the engine m a s s , otherwise the advantage conferred by the higher exhaust velocity would be negated to
a large extent by the nuclear engine's 12-ton m a s s . Neither
would the nuclear rocket be the best engine where only
small velocity changes are desired, such as in the maneuvering of an orbital space station. But l a r g e - s c a l e orbital
maneuvers, fast t r i p s to the outer planets, the delivery of
payloads to the surfaces of, or orbits about, Venus, Mars,
49
Jupiter, and the other denizens of the solar system, and
the l a r g e - s c a l e ferrying of supplies to the moon a r e right
down the nuclear rocket's alley. Generally speaking, the
more "ambitious" the mission, the better the nuclear
rocket looks.
Nuclear rockets a r e an integral and indispensable part
of the space transportation
s\stem that has been proposed
for A m e r i c a ' s space ventures following the completion of
the Apollo p r o g r a m . The "first stage" of this system i s
the chemical rocket. Launch vehicles, such as those in the
Saturn class or the proposed new space shuttle, will use
chemical energy to boost payloads into low earth orbits;
nuclear stages, already in space, will pick up these payloads and c a r r y t h e m — w i t h low propellant consumption
(equivalent to low c o s t ) — t o their final destinations The
NERVA engine is the only practical advanced propulsion
system that can meet the requirements of the space t r a n s portation system in the 1980s and beyond.
A Space Transportation System. Lunar operations beyond
Apollo may involve the establishment of permanent or
semipermanent b a s e s on the moon. Obviously, a great deal
of hardware would have to be injected into earth orbit
and then ferried to the moon by the space transportation
system. A NERVA-powered vehicle with a usable propellant
capacity of 150 tons (distributed m eight propellant modules,
as illustrated in the figure) could support a modest program
of manned lunar exploration with six round t r i p s per year.
The nuclear shuttle could c a r r y 60 tons of payload on the
outward leg and return with 13 tons. The total shuttle weight
p r i o r to its departure for the moon would be about 250 tons.
An equivalent, two-stage shuttle with chemical rockets
would weigh about 350 tons in earth orbit. In support of
various other manned and unmanned space missions, the
NERVA-propelled vehicle could transport from 5 to 60 tons
from a low earth orbit to a synchronous orbit almost six
earth radii above the e a r t h ' s surface.
Automated spacecraft will be m a n ' s p r e c u r s o r s to Mars,
Jupiter, and t a r g e t s beyond. Here, too, the nuclear rocket's
high specific impulse gives it an advantage over the chemical rocket. The basic NERVA engine can be applied to
50
FROM THE FIRST SPECULATIONS TO NERVA
"The Italian navigator has landed in the new world." This
was the text of the cryptic message sent by Arthur Compton
to other American scientists working on nuclear r e s e a r c h
when Enrico Fermi* attained the first self-sustaining nuclear chain reaction The p r e c i s e moment had come at
3:20 p.m. on December 2, 1942, in a squash court at the
University of Chicago, as neutron-absorbing cadmium rods
w e r e pulled slowly from F e r m i ' s "pile" of graphite and
uranium. In the context of this booklet, Compton's telephone
call was even more prophetic than he intended, because
F e r m i had not only discovered the new world of the atom
but had also given man an energy source with which he
will be able to explore worlds far beyond the earth.
F e r m i ' s s u c c e s s set scientists and engineers to wondering about what constructive things could be done with the
r e s u l t s of nuclear r e s e a r c h . In 1944, Stanislaus Ulam and
F r e d e r i c k de Hoffman, at a New Mexico center that later
became the Los Alamos Scientific Laboratory (LASL), mused
that the power of the atomic explosive might somehow be
controlled to launch space vehicles. Ulam and de Hoffman
were following the thoughts of the great m a s t e r of science
fiction, Jules Verne, who, in an amazing 1865 novel,
De la Terre a la Lime (From the Earth to the Moon), wrote
that the Baltimore Gun Club fired a manned projectile to
the moon from a huge cannon emplaced near Cape Canaveral,
now Cape Kennedy, Florida. The use of nuclear explosions
for propulsion never proved feasible, but Ulam and de
Hoffman started people thinking about a l e s s violent source
of nuclear e n e r g y — t h e nuclear reactor — in which heat is
released slowly and controllably.
But ideas r a r e l y come to fruition unless a practical
need beckons. There was no such thing as a "space p r o g r a m " in 1945. But near the close of World War II, German
V-2 rockets had proved they could c a r r y 1600 pounds of
amatol explosive 200 miles from Nazi-held t e r r i t o r y in
* Fermi, an Italian-born Nobel Prize winner, came to the United
States in 19C8 and headed the research team that built the first
nuclear reactor. See The First Reactor, a companion booklet in
this series.
U
Chronology of Rocket and Reactor Technology
Rocket
development
First Chinese rockets -
Konstantin Ziolkovsky
rocket work
Key Robert Goddard paper
A Method of Reaching
Extreme Altitudes (19191 —
1200 A.D.
I
18 7 5
Reactor
development
S
19 2 0
19 2 0
19 3 0
• Ernest Rutherford makes
first controlled nuclear
transmutation
19 3 0
• James Chadwick discovers
the neutron
- 0 Hahn-F Strassmann
"fission experiments"
V - 2 rocket developed by
Wernher von Braun and
associates
19 4 0
19 4 0
F de Hoffman-S Ulam
Speculations (44)
- L Meitner fission paper
(1-39)
• E Fermi " p i l e " critical
(12-42)
Alamagordo A-bomb test ( 7 - 4 5 ) • NAA Rand reports (7-46)
- L Shepard-A Cleaver papers (48-49)
19 5 0
• LASL Nuclear Propulsion Div formed (6-55)
NASA formed ( 1 0 - 5 8 ) SNPO formed (8-60) —
1960
- K i w i - A test (7-59)
Kiwi-B4Atest (11-62)-Suscessful test of redesigned K I W I (5-64)
N R X - E S T multiple restartsNERVA XE (experimental engine)
tested at full pow/er (6-69)
19 7 0
10
-Technology development
program complete (1969)
many different s o l a r - s y s t e m missions by simply adding or
subtracting propellant tanks to and from the spacecraft.
A particularly interesting mission in the next decade or
two will involve the automated collection and return of
geological and possibly biological samples from Mars by
an unmanned spacecraft. A nuclear-propelled spacecraft
with five standard propellant tanks could deliver 17 to
32 tons to an orbit about Mars beginning from a low
earth orbit. With this kind of payload, 160 to 240 pounds
(73 to 110 kg) of samples could be selected by a roving
automaton and then returned to earth. Sample-return m i s sions to Mercury, Venus, and the asteroids are also p o s sible with nuclear spacecraft. Such automated missions
would be far cheaper than long, complex manned missions.
The superlative propulsion capabilities of the nuclear
rocket can be applied to reducing travel time to the planets
as well as increasing the payloads. Shorter, high-energy
t r a j e c t o r i e s could cut a year or two off trip t i m e s to the
outer planets. Time savings such as these may be critical
on long missions where the reliability of complex spacecraft i s being pushed to its upper limits.
A final bonus derived from nuclear rocket propulsion
a r i s e s from the tremendous quantity of potential energy
locked m the uranium-235 in the reactor core. Only a
small fraction of this energy is used in engine operations.
By inserting separate coolant loops in the reactor, some
of this unused energy can be tapped to generate electricity.
Power levels of 15 to 25 kilowatts could easily be generated for long periods of time. Large quantities of electrical power are just as essential to the exploration of
the solar system as the high-performance nuclear rocket
engine.
In all these c l a s s e s of missions, it is the high exhaust
velocity of the nuclear rocket that confers superiority. Of
course, if the nuclear engines on a lunar ferry were e m ployed on many round trips, the second major advantage
of the nuclear rocket, high-energy density in the r e a c t o r
core (see p. 8), may also come to the fore. Lastly, the
same type of modular nuclear stage could, of course,
undertake all types of missions, promoting system standardization.
51
52
o
"3
K
to
SS
o
?^
to
to
ft;
s
o
B
O
•a
nuclear rocketry as they tried to channel the power of the
atom into h i g h - t e m p e r a t u r e , lightweight reaction engines
that would c a r r y men toward the planets.
U S Senator Clinton P Anderson of Nen Mexico, Chait man of the
Senate Aeronautical and Space Sciences Committee, and a member
of the Joint (Congressional) Committee on Atomic Energy,
examines a KIWI reactor during a visit to Los Alamos Scientific
Laboratory
9
is negligible at today's nuclear rocket t e m p e r a t u r e s . If
atomic hydrogen could be used as a propellant, M would
equal 1, and the exhaust velocity factor would multiply that
of molecular hydrogen by 42. This is a development that
will have to wait for future exploitation, however. The
first operational nuclear rockets will spew hot molecular
hydrogen out of their nozzles. With M = 2 instead of 18, as
in hydrogen —oxygen chemical engines, the nuclear rocket
exhaust velocity still will be more than double that of the
best chemical rocket for the same t e m p e r a t u r e .
Doubling the nuclear rocket's exhaust velocity has one
further consequence: It r e q u i r e s the nuclear heat source to
generate more power. A simple equation for the power, P,
in the exhaust gases comes from the kinetic energy (KE)
equation:
Despite the superiority indicated in these particular
missions, the real significance of the nuclear rocket lies
in the fact that it r e p r e s e n t s a true advance in our overall
propulsion capability; perhaps as big a jump as the first
automobiles were over the h o r s e . We can only s u r m i s e
where such an advance in propulsion capability eventually
will take us.
but, since power is the rate of energy production, m is r e placed by m and KE by P. Thus,
mv
?
Using this equation, and F = mv, we can see that if thrust is
held fixed and exhaust velocity is doubled, propellant flow
will be halved (as desired), but the power required to do
this will be doubled. The price of increasing the exhaust
velocity IS increased power. From this relationship a r i s e s
another important advantage of the nuclear rocket: The
great r e s e r v o i r of energy contained in its nuclear fuel can
be turned into high exhaust velocity.
If we keep in mind t h r e e basic facts about nuclear
rockets:
1. They convert fission-generated heat into the kinetic
energy of rocket propellant,
2. Chemical combustion is not needed, and they can use
low molecular weight propellants to attain high exhaust
velocities,
3. Their reactor fuel has a great deal of energy packed
in it,
Giant dewars containing liquid hydrogen and liquid oxygen and a
maze of pipes, valves, and other equipment are needed for the reactor tests. This is Test Cell C at NRDS.
we can understand better the thoughts of the pioneers in
53
8
GLOSSARY OF ACRONYMS AND CODE NAMES
AEC
ANP
Atlas
Centaur
Condor
Dumbo
ElV
E-MAD
EST
ETS-1
ICBM
Kiwi
LASL
MCC
NASA
NERVA
NRDS
NRTS
NRX
NTS
Orion
Peewee
Phoebus
Pluto
Poodle
RIFT
R-MAD
Rover
Saturn-5
SNPO
TNT
XE
Atomic Energy Commission.
Aircraft Nuclear Propulsion p r o g r a m .
An Air Force ICBM using kerosene and liquid oxygen
propellants.
A chemical rocket system intended for unmanned space
missions, burning liquid hydrogen—liquid oxygen.
Early code name for the nuclear rocket, also the name
of an early committee studying nuclear rockets.
An early experimental rocket r e a c t o r .
Engine Installation Vehicle at NRDS.
Engine-Maintenance, A s s e m b l y , and Disassembly
building, at NRDS.
Engine System Test.
Engine Test Stand 1 at NRDS.
Intercontinental Ballistic Missile.
A s e r i e s of rocket test r e a c t o r s built by Los Alamos
Scientific Laboratory.
Los Alamos Scientific Laboratory in New Mexico.
Manned Control Car, prime mover for the EIV, at
NRDS.
National Aeronautics and Space Administration.
Nuclear Engine for Rocket Vehicle Application, an
AEC-NASA program.
Nuclear Rocket Development Station in Nevada.
National Reactor Testing Station m Idaho.
NERVA Reactor Experiment.
Nevada Test Site, including NRDS.
Nuclear bomb propulsion study, now discontinued.
A r e a c t o r for testing nuclear rocket fuel elements.
An advanced s e r i e s of nuclear rocket reactor experiments under development at LASL.
Program for developing nuclear r a m - j e t propulsion;
now discontinued.
A very small nuclear rocket concept employing decaying radioisotopes as the heat source.
Reactor In-f light Test program, now discontinued.
Reactor-Maintenance, Assembly, a n d Disassembly
building at NRDS
General name for the U. S. nuclear rocket program.
The key launch vehicle for NASA's manned moon
exploration system, powered by clustered liquid
oxygen-liquid hydrogen engines
Space Nuclear Propulsion Office, a joint NASA-AEC
organization.
Transient Nuclear Test.
Ground experimental engine.
was known that ammonia and methane dissociate* to a large
extent at t e m p e r a t u r e s around 3000 °K, as their molecules
collide violently with each other. Complete dissociation of
methane into its five constituent atoms would bring M down
from 16 to 3.2, and this would make methane much m o r e
10 000 —
5000
-R 4000
3000
2000
1000
2000
Temperature T
3000
4000
Liquid hydrogen-liquid oxygen engine operating at 500 pounds per square inch pressure
Plots of exhaust velocity. Methane and
ammonia nuclear rockets do not appear
to be much better than a hydrogen-oxygen
chemical engine
A hydrogen
nuclear
rocket IS superior to all
a t t r a c t i v e a s a p r o p e l l a n t . N e v e r t h e l e s s , h y d r o g e n finally
did win out o v e r a l l c o m p e t i t i o n , m a i n l y b e c a u s e t h e
d a n g e r o u s h a n d l i n g p r o b l e m s w e r e l i c k e d and b e c a u s e t h e
n u c l e a r r o c k e t needed the even higher exhaust velocity
p r o m i s e d by p u r e h y d r o g e n if it w a s t o a c h i e v e i t s full
potential.
M o l e c u l a r h y d r o g e n t ^-^so d i s s o c i a t e s with h e a t ; b u t ,
b e c a u s e t h e H—H c h e m i c a l b o n d i s s o s t r o n g , d i s s o c i a t i o n
*Or decompose into the constituent atoms making up their
molecules.
t Hydrogen gas, H2, is a molecule containing 2 hydrogen atoms.
54
7
r o c k e t s a l r e a d y o p e r a t e at t e m p e r a t u r e s c l o s e to 3000°K;*
if the n u c l e a r fuel i s to s t a y in s o l i d f o r m , it i s apparent
it cannot go to 12,000°K, which i s hotter than the s u n ' s
s u r f a c e . In fact, today's n u c l e a r r o c k e t r e a c t o r fuels b a r e l y
s u r v i v e at 3000°K, the s a m e g e n e r a l t e m p e r a t u r e l e v e l
achieved in c h e m i c a l r o c k e t s .
T o avoid this i m p a s s e , s o m e t h i n g m u s t be done to r e duce M, the m o l e c u l a r weight, rather than to i n c r e a s e T.
In a c h e m i c a l (combustion) rocket, M d o e s not drop much
below 18, b e c a u s e the m o s t convenient o x i d i z e r s have quite
heavy a t o m s . For e x a m p l e , the advanced Centaur engine
burns hydrogen with oxygen to f o r m water (H2O) for which
M = 18. C h e m i c a l r o c k e t exhaust v e l o c i t i e s a r e thus limited
m a i n l y by the high m o l e c u l a r w e i g h t s of the c o m b u s t i o n
p r o d u c t s . A s long a s c h e m i c a l fuels m u s t be burned with
oxygen or fluorine, c h e m i c a l rocket exhaust v e l o c i t i e s c a n not be g r e a t l y i m p r o v e d .
In a n u c l e a r rocket, h o w e v e r , c o m b u s t i o n i s not required:
The nucleus f i s s i o n s without c h e m i c a l s t i m u l a t i o n , and the
propellant i s not an engine fuel but a s e p a r a t e s u b s t a n c e
that i s heated by the f i s s i o n i n g nuclei in a n u c l e a r r e a c t o r .
A n u c l e a r rocket d e s i g n e r can heat up any propellant he
w i s h e s a s long a s it d o e s not c h e m i c a l l y attack h i s r e a c t o r
fuel e l e m e n t s . T h e r e i n l i e s the s e c r e t of the n u c l e a r
r o c k e t ' s high exhaust v e l o c i t y — it can make u s e of a
propellant with a low m o l e c u l a r weight.
In designing a n u c l e a r r o c k e t then, the f i r s t inclination
would be to c h o o s e hydrogen g a s a s the propellant b e c a u s e
hydrogen i s the l i g h t e s t of all m o l e c u l e s , with M = 2. Other
p o s s i b i l i t i e s — such a s water ( H 2 0 : M = 18), methane (CH4:
M = 16), and a m m o n i a (NH3: M = 1 7 ) — o f f e r e d m o d e s t i m provements over early kerosene-burning chemical rockets,
but show no advantage o v e r the newer c h e m i c a l e n g i n e s
that burn hydrogen with oxygen. In the e a r l y days of n u c l e a r
r o c k e t r y , though, a m m o n i a and methane w e r e c o m p e t i t o r s
of hydrogen b e c a u s e hydrogen w a s f e a r e d a s a f i c k l e ,
e x p l o s i o n - p r o n e m a t e r i a l that had to be s t o r e d a s a liquid
at a t e m p e r a t u r e of only 20°K ( - 2 5 3 ° C ) . In addition, it
*Or 2727° centigrade, degrees Kelvin are measured above absolute zero and a r e equal in size to centigrade degrees. 0°C 273°K.
6
READING LIST
Technical Books
Nuclear Rocket Propulsion, Robert W. Bussard and Richard D.
DeLauer, McGraw-Hill Book Company, New York, 1958, 370 pp.,
$11.50.
Fundamentals of Nuclear Flight, Robert W. Bussard and Richard D.
DeLauer, McGraw-Hill Book Company, New York, 1965, 453 pp.,
$15.50.
Propulsion Systems for Space Flight, William R. Corliss, McGrawHill Book Company, New York, 1960, 300 pp., $12.50, translated
and published in French as Le Vol dans L'E'ipace,
Sy<;fimes de
Pi opulston, Dunod, 1963.
Realities of Space Trai el, British Interplanetary Society, L. J.
C a r t e r (Ed.), McGraw-Hill Book Company, New York, 1957,
431 pp., $10.75. The Atomic Rocket, L. R. Shepherd and A. V.
Cleaver, p. 136. Originally published m the Journal of the British
Interplanetary Society in a s e r i e s of 1948 and 1949 articles.
Jet, Rocket, Nuclear, Ion and Electric Propulsion Theory and Design,^fj. H. T. Loh Ed., Springer-Verlag New York Inc , New
York, 1968, 779 pp., $24.80.
Principles of Spaceflight Propulsion, E M. Goodger, Pergamon
P r e s s , Inc., Elmsford, New York, 1970, 186 pp., $8.50.
Technical Articles
The Nuclear Shuttle for Interorbital and Transplanetary Applications, K. P. Johnson and R. J. Holl, American Institute of
Aeronautics and \stronautics paper 70-679, 1970.
Manned Planetary Missions with Reusable Nuclear Shuttles, R G.
Riedesel and R. S. Cowls, American Institute of Aeronautics
and Astronautics paper 70-040, 1970.
Nuclear Rocket Engine P r o g r a m Status —1970, D. S Gabriel and
I. L. Helms, American Institute of Aeronautics and Astronautics
paper 70-711, 1970.
Nuclear Propulsion for Exploration of the Solar System, American
Astronautical Society paper 69-330, 1969
Popular Articles
Nuclear R o c k e t r y - T h e I irst Bright Hopes, Robert \V Bussard,
Astronautics,
7 32 (December 1962).
The Development of Nuclear Rocket Propulsion m the United States,
William C House, Journal of the British Interplanelar\
Society,
19 306 ( M a r c h - A p r i l 1964).
Isotopic Propulsion, John S MAVtmez, Space'Asty onautics, 42- 77
(November 1964).
Astyonaiilics,
7(December 1962). Special issue on nuclear rockets.
Nucleonics, 19 (April 1961). Special issue on nuclear lockets
55
Preparing to Go Past the Moon: Testing Nuclear Rocket System
for Deep Space Probes, Business Week, 136 (February 12,1966).
The Nuclear Rocket, Space/Aeronautics,
43: 32 (April 1965).
The Next 20 Years of Interplanetary Exploration, Wernher von
Braun, AsLronautics and Aeronautics, 3: 24 (November 1965).
Astronautics and Aeronautics, 2, (June 1965). Several articles of
interest.
Beyond Apollo with Nuclear Propulsion, Paul G. Johnson, Astronautics and Aeronautics, 2: 22 (December 1964).
Motion Pictures
Available for loan without charge from the AEC Headquarters
Film Library, Division of Public Information, U. S. Atomic Energy Commission, Washington, D. C. 20545 and from other AEC
film l i b r a r i e s .
Power for Propulsion,
15 minutes, color, 1965. Produced by
Aerojet-General Corporation. Major steps in the history of
power sources for propulsion a r e traced. Animation sequences
a r e used to illustrate the principles of rocketry, Newton's Law
of Motion, and operation of nuclear rocket engines. Development
of NERVA is covered, including its first test firing at the testing
station in Jackass Flats, Nevada. Developments for deep-space
missions to the moon and beyond a r e shown.
Project Rover, 21V2 minutes, color, 1963. Produced by the AEC's
Los Alamos Scientific Laboratory. This is a 1962 progress
report on Project Rover, a program to develop a nuclear rocket
for spacecraft propulsion. Design, fabrication, and testing of a
Kiwi a r e detailed.
Atomic Energy for Space, 17 minutes, color, 1967. Produced by
the Handel Film Corporation with the cooperation of the AEC
and NASA. This film explains the two basic ways in which nuclear energy for space is being developed: A nuclear rocket
for space propulsion, and isotopic generators and reactor power
plants to produce electricity for spacecraft operations. Project
Rover is covered through animation and film shots of Kiwi and
NERVA tests. The efficiencies of nuclear and chemical rockets
a r e compared. The last part of the film discusses the SNAP
(Systems for Nuclear Auxiliary Power) program.
Nuclear Propulsion in Space, 19 minutes, color, 1969. Produced by
the National Aeronautics and Space Administration and the AEC.
This film presents the story of the development of a nuclear
rocket engine for space exploration. Conventional chemical
rockets are compared with nuclear rockets through the use of
graphs, c h a r t s , and animation that show that the nuclear rocket
can be twice as efficient as its chemical counterpart. The film
explains the principles and operating characteristics of a
nuclear rocket and how its power and thrust will be controlled.
Tests are shown of the KIWI reactor in Nevada and the NERVA
(Nuclear Engine for Rocket Vehicle Application), which will
complete the technology for a nuclear rocket engine application
in space missions of the late 1970s and 1980s.
56
V oc V T T M
•which says that the exhaust velocity, v, of any rocket is
proportional to V T / M ;
•where T = the temperature of the hot gases just before they
enter the nozzle throat, and
M = the average molecular •weight of the exhaust
gases.
It is obvious that for a high v value •we •wish to maximize
the quantity / T T M .
Kno-wing that T and M control v, let us first try to manipulate T. Since T is under the square root sign, it •will have
to be quadrupled to double v, the exhaust velocity. Chemical
Chemical rocket
Nozzle
mm:
F= 1
F= 1
V = 1
V =2
m= 1
m='/2
P= 1
Nuclear rochets have twice the exhaust velocity of
chemical rockets.
Thus, for the same level of
thrust, propellant mass flow is halved and power
is doubled.
5
Dividing F by in reveals that the thrust produced per
unit m a s s flo^w of propellant is equal to v. In other •words,
the higher the bullet or propellant velocity, the more thrust
F
F
PHOTO CREDITS
Frontispiece courtesy Nuclear Rocket Development Station (NRDS)
Piston
Page 9
Los Alamos Scientific Laboratory (LASL)
Page 15
LASL
r-
/
rn
F= net pressure x area
Page 17
LASL
Page 21
NRDS
Page 42
NRDS
Pages 44-45
LASL
Page 46
NRDS
Page 47
NRDS
Page 48
NRDS
Page 53
LASL
F= rriv
Hot gases expand against a piston, left, pushing it upward.
Center,
hot gases expand against a rocket nozzle, pushing it upward. The
reaction thrust, F, can be computed either from the total net pressure against the nozzle and combustion chamber area, or from inv
(right). The two F's are identical.
•we get from each kilogram of gas that r o a r s out the nozzle
each second. We •want to have a high exhaust velocity for
good rocket performance because •we can thereby a c complish a space mission •with l e s s propellant. We will
show later that the nuclear rocket produces about twice the
exhaust velocity of the best chemical rocket. High exhaust
velocity is the greatest advantage of the nuclear rocket.
The question then is: How does a nuclear rocket gene r a t e high exhaust velocities? The key to the nuclear
r o c k e t ' s s u c c e s s lies in a simple equation from t h e r m o dynamics
4
57
and expand against its flared s i d e s , pushing the nozzle
(and t h e whole r o c k e t ) u p w a r d . T h e p r e s s u r e a g a i n s t t h e
n o z z l e w a l l s i s t h e r e a c t i o n to t h e e x p u l s i o n o r p u s h i n g
a w a y of t h e hot e x h a u s t g a s e s .
WILLIAM R. CORLISS is an atomic energy consultant and writer with
12 y e a r s of industrial experience including service as Director of
Advanced P r o g r a m s for the Martin Company's Nuclear Division.
P r i o r industrial connections were with the Flight Propulsion
Laboratory of the General Electric Company and with Pratt &
Whitney Aircraft Company. Mr. Corliss has B.S. and M.S. degrees
in Physics from Rensselaer Polytechnic Institute and the University of Colorado, respectively. He has taught at those two institutions and at the University of Wisconsin. He is the author of
Propulsion Systems for Space Flight
(McGraw-Hill 1960), Space
Probes and Planetary Exploration (Van Nostrand 1965), Sczew/Z/ic
Satellites (Government Printing Office, 1967), Mysteries
of the
Universe (Crowell, 1967), Mysteries
Beneath the Sea (Crowell,
1970), and coauthor of Radioisotopic Power Generation (PrenticeHall, 1964), Human Factors Applications
in
TeleoperatorDesign
and Operation (Wiley, 1971), and Man and Atom (Dutton, 1971), as
well as numerous articles and papers for technical journals and
conferences. In this s e r i e s he has written Neutron
Activation
Analysis, Power Reactors in Small Packages, Direct
Conversion
of Energy, Computers, Space Radiation,
and was coauthor of
Power from
Radioisotopes.
The Jirst reaction engine. In Hero's aeolipile,
squirting Jrom the two pipes caused rotation.
steam
Looking at it a n o t h e r way, e a c h m o l e c u l e in t h e e x h a u s t
i s l i k e a s m a l l b u l l e t s h o t f r o m a b i g gun (the r o c k e t e n g i n e ) . I n d e e d , if we think of t h e r o c k e t e n g i n e a s a c o n t i n u o u s l y f i r i n g s h o t g u n with m o l e c u l a r a m m u n i t i o n we a r e
p r e t t y c l o s e to t h e t r u t h .
If t h e m a s s e s of t h e s e m o l e c u l a r " b u l l e t s " and t h e i r
" m u z z l e v e l o c i t i e s " a r e known, t h e r o c k e t t h r u s t c a n b e
computed from the s i m p l e equation:
F = rhv
w h e r e F = t h r u s t ( m e a s u r e d in n e w t o n s * ) ,
m = t h e m a s s flow r a t e of t h e b u l l e t s o r p r o p e l l a n t ,
(in k i l o g r a m s p e r s e c o n d ) , and
v = the a v e r a g e muzzle velocity or exhaust velocity
a l o n g t h e r o c k e t a x i s (in m e t e r s p e r s e c o n d ) .
*Named for Sir Isaac and equal to the force necessary to a c c e l e r a t e 1 kilogram of mass 1 meter per second per second; a
force of one newton = .225 lbs. Newton also Introduced the idea of
placing a dot over a symbol to indicate rate of change. For example, if V = velocity, v = acceleration.
58
3
HOW A NUCLEAR ROCKET WORKS
The rocket concept was grasped by the first caveman
when he pushed off from a lakeshore on a raft; every action
has an equal and opposite reaction according to Ne^wton's
Third Law of Motion.* The caveman's action was pushing
the shore away with his foot; the reaction was the surge of
the raft onto the lake.
Actually, it is not n e c e s s a r y to have something solid to
push against. The caveman could have propelled himself out
on the lake by hurling rocks shoreward; as each rock left
his hand, he and the raft would have moved a bit farther.
It is the same way in a i r l e s s space; propulsion in a given
direction means throwing something away in the opposite
direction. Instead of rocks, the ordinary chemical rocket
expels a roaring jet of hot combustion g a s e s . But the effect
is the s a m e . To make a nuclear rocket, the uranium nucleus must be fissioned! in such a way that something
is expelled from the spaceship. We'll explain how later in
this booklet.
Chemical rocket engines, jet engines, automobile eng i n e s — i n fact, most of mankind's engines — extract heat
from a fuel and turn it into motion through the expansion
of hot gases. The nuclear rocket sprouts from the same
family t r e e ; it also c r e a t e s hot h i g h - p r e s s u r e gas and turns
it into thrust. The nuclear rocket is a direct descendent of
the aeolipile, a steam-spouting "reaction engine" reputedly
built a century or two before Christ by the famous Alexandrian engineer, Hero.
It is relatively easy to see how a hot gas expands against
a piston in an internal combustion engine to do useful work.
The piston p r e s e n t s the gas with something solid to push
against. And the rocket engine operates in much the s a m e
way, except that the piston is replaced by the rocket
nozzle. The hot gases created by chemical combustion or
nuclear heaters issue from the throat of the rocket nozzle
FRANCIS C.SCHWENK has 20 years experience in aeronautical and
space propulsion research and development — the last 11 years in
the Nuclear Rocket Program. In his present position of Assistant
Manager for Vehicles and Propulsion Technology in the AEC NASA Space Nuclear Systems Office, he is responsible for programs on nuclear rocket stages, advanced propulsion research,
and applications studies. Mr. Schwenk received a B.S. degree from
the Pennsylvania State University and an M.S. degree from the
University of Illinois, both in the field of mechanical engineering.
While at the NACA Lewis Flight Propulsion Laboratory, he authored
many technical research reports on compressors for turbojet
engines and received additional education in nuclear physics and
reactor engineering. When the National Aeronautics and Space
Administration was formed, he served in the NASA headquarters
office, planning for development of launch vehicles for space missions. He was coauthor with M. W. Rosen of the 1959 article,
"A Rocket for Manned Lunar Exploration" (IRE Transactions,
December 1969), which first described the basic characteristics
of a lunar mission — later to become Apollo, and coauthor with
E. W. Hall of "Current Trends in Large Booster Development",
(Aerospace Engineerifig, May 1960). Since joining the Nuclear
Rocket Program, he has written numerous articles and speeches,
the most recent being "Prospects for High-Thrust Nuclear Propulsion Systems'' with C. E. Franklin (Chemical Propulsion Information Agency).
*Sir Isaac Newton, the great British intellect who lived from
1642 to 1727, formalized the basic laws of motion as we know
them today.
tFor an explanation of nuclear fission, see Our Atomic World,
another booklet in this series.
2
59
By William R. Corliss
and F r a n c i s C. Schwenk
ENERGY FOR SPACE TRAVEL
The secret of space travel is energy—immense
amounts
of energy. The first stage of the Saturn-V moon rocket generates as much energy each second as a million
automobile
engines. The energy leverage of nuclear fission
eventually
will give us spaceships
that can sweep across the solar
system and carry men to tlie farthest
planets.
But the uranium nucleus is not broken to man's bidding
easily. Our quest is for a means of uniting the almost
limitless energy of the atomic nucleus and the rocket's
unique
ability to thrust through the vacuum of space. This difficult
wedding of nuclear fission
(age, 30 years) to the rocket
(age, 1000 years) is the subject of this booklet.
In the words of Dr. Glenn T. Seaborg, Chairman of the
U. S. Atomic Energy Commission:
"What we a r e attempting to make is a flyable compact r e a c t o r , not much bigger
than an office desk, that will produce the power of Hoover
Dam from a cold start in a matter of minutes."
1
Interest mounts high as technicians and engineers monitor a space propulsion test m the Reactor
Control building at the Nuclear Rocket Development Station, Nevada Instyuments record perJorinance data
by William R. Corliss and Francis C. Schwenk
CONTENTS
ENERGY FOR SPACE TRAVEL
HOW A NUCLEAR ROCKET WORKS
FROM THE FIRST SPECULATIONS TO NERVA
BUILDING AN ENGINE FOR SPACE
The Biggest, Hottest Reactors Ever Built
The Artificial Maelstrom
Where the Work is Done
Holding the Pieces Together
Bootstrapping and the Full-Flow Turbine Drive
NINETY MILES OUT IN THE DESERT
ENGINESFORTHEMOON AND BEYOND
GLOSSARY OF ACRONYMS AND CODE NAMES
READING LIST
MOTION PICTURES
United States Atomic Energy Commission
Division of Technical Information
Library of Congress Catalog Card Number: 79-171030
1968;1971(rev.)
1
2
11
21
23
30
30
34
36
41
49
54
55
56
The Understanding the Atom Series
Nuclear energy is playing a vital role in the life of every
man, woman, and child in the United States today. In the
years ahead it will affect increasingly all the peoples of the
earth. It is essential that all Americans gain an understanding
of this vital force if they are to discharge thoughtfully their
responsibilities as citizens and if they are to realize fully the
myriad benefits that nuclear energy offers them.
The United States Atomic Energy Commission provides
this booklet to help you achieve such understanding.
Edward J. Brunenkant, Director
Division of Technical Information
UNITED STATES ATOMIC ENERGY COMMISSION
Dr. Glenn T. Seaborg, Chairman
James T. Ramey
Wilfrid E. Johnson
Dr. Clarence E. Larson
THE COVER
The cover shows a reusable nuclear
stage, powered by the NERVA engine,
leaving an earth orbit on a round-trip
mission to the moon. The NERVA engine " f i r e s " at least four times on
this mission: To leave earth orbit, to
go into orbit around the moon, to leave
the moon, and to r e t u r n to orbit around
earth. This stage was delivered to orbit,
where it was assembled, and refueled
by multiple flights of the two stage
space shuttle. To provide sufficient payload to support a lunar station, the
nuclear stage c a r r i e s 150 tons of liquid
hydrogen in eight tanks. After the stage
r e t u r n s to earth orbit, it will be r e fueled, repaired, if needed, and checkedout for additional missions to the moon,
to synchronous orbit, or to interplanetary space.
This booklet is one of the "Understanding the Atom"
Series. Comments a r e invited on this booklet and others
in the s e r i e s ; please send them to the Division of Technical
Information, U. S. Atomic Energy Commission, Washington,
D. C. 20545.
Published as part of the AEC's educational assistance
program, the s e r i e s includes these titles:
*" "Accelerators
Animals tn Atomic Research
Atomic Fuel
Atomic Pouter Safety
Atoms at the Science Fair
Atoms m Agriculture
Atoms, Nature, and Man
Books on Atomic Energy for Adults
and Children
Careers m Atomic Energy
Computers
Controlled Nuclear Fusion
Cryogenics, the Uncommon Cold
Diiect Conversion of Eneigy
Fallout From Nuclear Tests
Food Preservation by Irradiation
Genetic Effects of Radiation
Index to the UAS Series
Lasers
Microsti-ucture of Matter
Neutron Actiuation Analysis
Nondestructive Testing
Nuclear Clocks
Nuclear Energy for Desalting
Nuclear Power and Merchant Shipping
Nuclear Power Plants
Nuclear Propulsion for Space
Nuclear Reactors
Nuclear Terms, A Biief Glossary
Our Atomic World
Ploushaye
Plutonium
Power from Radioisotopes
Pouer Reactors in Small Packages
Radtoactii e Wastes
Radioisotopes and Life Processes
Radioisotopes m Industry
Radioisotopes in Medicine
Rare Earths
Research Reactors
SNAP, Nuclear Space Reactois
Sources of Nuclear Fuel
Space Radiation
Spectroscopy
Synthetic Transuranium Elements
The Atom and the Ocean
The Chemistry of the Noble Gases
The Elusive Neutrino
The First Reactor
The Natural Radiation Environment
Whole Body Counters
Your Body and Radiation
A single copy of any one booklet, or of no more than three
different booklets, may be obtained free by writing toUSAEC, P. 0. BOX 62, OAK RIDGE, TENNESSEE
37830
Complete sets of the s e r i e s a r e available to school and
public librarians, and to teachers who can make them
available for reference or for use by groups. Requests
should be made on school or library letterheads and indicate the proposed u s e .
Students and teachers who need other material on specific aspects of nuclear science, or references to other
reading material, may also write to the Oak Ridge address.
Requests should state the topic of interest exactly, and the
use intended.
In all requests, include " Z i p Code" in return address.
Printed m the United States of America
USAEC Division of Technical Information Extension, Oak Rjdge, Tennessee
. |'•wJ^J'^i'«^H w j !9!;i*!^r»jyp.jP«^'-'.fliV .^r^^^-'i- ^,n M T ' - - ^ T ™ - r •. .^
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•
"'J"»T;^F^
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,^ ARCHIVES-REKRENCE
«'^^«'YES-R£gRENCECEf
CENTER
by William R. Corliss and Francis C. Schwe.nk
'i,
An Understanding the Atom Series Booklet