Propulsion Basics
Fundamental principle behind rocket
propulsion is Newton’s action-reaction law
For every action there is an equal and opposite
reaction
Escaping exhaust gas of the rocket motor
drives the rocket in the opposite direction
with an equal force – forward thrust
This is equivalent to the forward momentum of
the rocket being the same as the momentum
of the exhaust, but in the opposite direction
Propulsion Basics
Momentum = mass x velocity = m v
Momentum forward = momentum rearward =
massrocket x velocityrocket =
-massexhaust gas x velocityexhaust gas
MrVr = -meve or Vr = -ve(me/Mr)
Since the mass of the rocket is much greater
than the mass of the exhaust gas, the
velocity of the exhaust gas must be much
greater than the forward velocity of the
rocket
Propulsion Basics
Thrust and momentum are not the same
Thrust = force = weight (in dimensions)
Momentum times mass flow rate
(1/seconds) has the same dimensions
as thrust (mass length/time2)
For measuring rocket thrust, we
therefore need exhaust momentum
time exhaust mass flow rate
Propulsion Basics
Mass flow rate is the amount of fuel that
is consumed (or combusted) and
expelled as exhaust
Larger rockets consume more fuel than
smaller rocket in the same time interval
 Larger rockets produce more thrust than
smaller rockets


All very obvious
The size of a rocket roughly determines
its thrust, or lift capacity
Propulsion Basics
Propulsion performance is measured primarily as
thrust and exhaust velocity

Thrust is determined by mass flow rate and exhaust
velocity

Exhaust velocity is a measure of thrust efficiency

Higher exhaust velocity = higher thrust efficiency
Propulsion Types
Rocket propulsion types

Chemical

Liquid propellant







Unheated
Heated
Electric


Solid propellant – combined fuel & oxidizer
Compressed gas


Single (mono) – combined fuel & oxidizer
Dual (bi) - separate fuel & oxidizer
Ion
Electrothermal
Nuclear
Solar pressure
Propulsion Types
Rocket propulsion
Simplest propulsion type is compressed
gas






Simple
Inexpensive
Inefficient – low exhaust velocity
Low energy content
Used on small satellites
Balloon is the simplest example
Propulsion Types
Most common rocket propulsion
type is the solid rocket






Simple
Inexpensive
Modest exhaust velocity
Can be scaled from small model
rocket and fireworks to large Solid
Rocket Boosters used on the
space shuttle
Single use
Used primarily for first stage
boosters and separation motors
Propulsion Types
Large rocket engines used for
most launchers are liquid
bipropellant engines





Complex
Relatively expensive
Higher exhaust velocity
Difficult to scale from small to
large
Can be restartable and/or
reusable
Propulsion Types
Liquid monopropellant engines





Simple
Relatively inexpensive
Modest exhaust velocity
Can be scaled up to moderate
thrust
Often restartable and used in a
variety of roles (attitude control,
orbit booster, deorbit motor,
etc.)
Propulsion Types
Electric propulsion engines






Relatively complex
Expensive
Very low thrust
Very high exhaust velocity
Useable only in space (vacuum)
A developing technology,
although used on interplanetary
boosters and for satellite
stationkeeping
Propulsion Types
Electric propulsion – ion
engine






Electric and or magnetic
fields used to accelerate
charged atoms (ions)
Heavy nuclei better than
light nuclei (Xe commonly
used)
Extremely low thrust
10-6 N – 0.01 N
Very high exhaust velocity
10 – 100 times chemical
rocket exhaust velocity
Propulsion Types
Electric propulsion –
electrothermal (heated
gas)






Electric current used to
heat cold gas
Heating reactive or inert gas
increases exhaust velocity
and thrust
Low to modest thrust
Moderate exhaust velocity
Resistojet – electric heater
Arc jet – electric arc heating
Propulsion Types
Electric propulsion
Other electric propulsion types include:



Magnetoplasmadynamic engine
Variable Specific Impulse Magnetoplasma
Rocket (VASIMR) engine
Nuclear ion engine

Heated gas by hot nuclear reactor core
Propulsion Types
Nuclear propulsion
Nuclear reactors used to heat a cold gas
to very high temperatures




Hydrogen gas is the most efficient
propellant
High exhaust gas velocities
Heated gas by hot nuclear reactor core
Nuclear ion engine is a variation with
greater effeciency
Propulsion Types
Nuclear propulsion – Nerva program (1957-1972)
Propulsion Types
Solar pressure propulsion
Solar photon pressure can be used for
propulsion (solar sailing) with certain
limitations






Low thrust
Low payload mass
Very large reflective “sail” needed
Operable only in vacuum of space
Limited to inner solar system
Prototype launches failed, but several are
being readied for flight
Propulsion Performance
Lift/payload performance
A rocket's lift or payload performance is
a function of three measures:



Thrust
Thrust efficiency (Isp)
Thrust duration
These three components also determine
the total propulsive energy of the
rocket and its propellants
Propulsion Performance
Thrust
Thrust is a measure of the forward force
produced by the rocket




Thrust has the same dimensions of both force
and weight
Thrust units are typically lbf (meaning force in
lbs), or in Newtons (or kgf meaning kg force)
Thrust is proportional to exhaust momentum
times exhaust mass flow rate
Thrust can be increased or decreased in some
rocket motor designs by increasing or
decreasing the propellant flow rate
Propulsion Performance
Thrust
Three factors dominate thrust performance of the
chemical rocket motor
1. Fuel flow rate - Higher fuel flow rates increase
forward thrust
2. Pressure difference between internal nozzle pressure
and external (ambient) pressure

Maximum pressure difference is in space (vacuum
pressure)
3. Exhaust velocity - Higher velocity produces greater
forward thrust
Propulsion Performance
Thrust efficiency - Specific Impulse (Isp)
Specific impulse is a measure of the thrust
produced for a given fuel weight flow
An equation expressing Isp in relation to the
thrust produced and the fuel consumed
(fuel flow rate) would be:
Isp = Thrust produced / fuel weight flow rate
(dimensions and units are seconds)
Propulsion Performance
Thrust efficiency - Specific Impulse (Isp)
Isp value is a measure of how efficient the
propellant (or engine) is converted into
thrust
Isp could also be described as the burn time of a
fuel for a specified mass at a specified thrust

A fuel with an Isp that is two times another would
burn twice as long with the same thrust
Propulsion Performance
Approximate Isp ranges
Very high
1,000-10,000 sec Ion and plasma engines
High
350-500 sec
Liquid bipropellant (liquid fuel + liquid
oxidizer)
Moderate
200-350 sec
Solid fuel or liquid monopropellant (liquid
fuel combined with oxidizer)
Low
0 -200 sec
Cold (compressed) gas
Propulsion Performance
Engine
Space Shuttle Main Engine
(SSME)
Space Shuttle Solid Rocket
Boosters (SRB)
Saturn V F-1 first stage
engine
Isp
453 s (vac) 363 s
(sea level)
269 s (vac) 237 s
(sea level)
260 s (sea level)
Thrust
233,295 kgf (513,250 lbf, 2.3 MN)
(vac)
1,500,000 kgf (3,300,000 lbf, 14.8
MN) (sea level)
681,180 kgf (1,500,000 lbf, 6.7
MN) (sea level)
Propulsion Performance
Specific impulse that is a measure of thrust efficiency
is also proportional to exhaust velocity
An approximation of the relationship between thrust
efficiency and exhaust velocity is:
Vexhaust = g Isp where g is the gravitational acceleration at the
Earth’s surface = 9.8 m/s2
For example, a rocket engine with an Isp of 300 s
would have an exhaust velocity of 300 s x 9.8 m/s2
= 2,940 m/s (6,580 mph)
Propulsion Performance
For chemical rockets, exhaust velocity is influenced primarily by
the following factors:

Exhaust gas molecular weight - Lower is better (hydrogen is one of
the optimum fuels)

Combustion temperature - Higher is better


Limited by the combustions chamber strength
Combustion chamber pressure - Higher is better

Limited by the combustions chamber strength

Specific heat ratio (chemical energy available to convert fuel into
exhaust gas based on the reaction chemistry of the fuel and oxidizer) Some fuels are better than others

Exhaust nozzle geometry – maximizes exhaust velocity using both the
kinetic and potential energies of the exhaust gas flow
Propulsion Performance
Propulsion Performance
Exhaust nozzle
To optimize the exit velocity in a chemical rocket:


Subsonic gas will increase speed if flowing through a converging
exit nozzle (decrease flowing through a diverging nozzle
Supersonic gas will increase its flow speed through a diverging
nozzle (decrease flowing through a converging nozzle)
Maximum exhaust velocity comes form a subsonic flow through a
converging interior nozzle with supersonic flow through the
exterior diverging nozzle
Also important in optimizing exhaust velocity include the nozzle's
convergent and divergent angles, and the throat-to-exit area
ratio
Propulsion Performance
Exhaust nozzle
Expansion of the exhaust from the
combustion chamber and the nozzle
throat should ideally conform to an even
flow into the outside gas (or vacuum)
Four conditions showing correct expansion,
underexpansion, and overexpansion
from the nozzle with respect to the
ambient air/vacuum are shown on the
right

Under expanded (top)

Ideal

Over expanded

Far over expanded (bottom)
Propellants
Propellant selection is important, not only for
combustion energy and exhaust gas velocity, but
also for density, storage, handling, and cost
The simplest rocket fuels are solids which consist of a
combined fuel and oxidizer compound that is
stabilized into a fast-burning propellant

The solid fuel is generally cast in the combustion chamber
as a unit
Liquid propellants called monopropellants can also
have a combined fuel and oxidizer compound

The single (mono) liquid propellant is simpler and much
less costly to store and handle than cryogenic propellants
Liquid Propellants
Propellant choice is not only based on performance and
density, but also on a variety of characteristics that
include important safety and handling criteria, some of
which are:











Specific impulse (Isp)
Cost
Toxicity & health hazards
Explosion and fire hazard
Corrosion characteristics
Handling safety
Propulsion system computability
Freezing/boiling point temperatures
Stability
Heat transfer properties
Ignition, flame and combustion properties
Propellants – Common types of liquid propellants
Oxidizer
Fuel
Isp (theoretical)
Liquid oxygen (LOX)
Liquid hydrogen (LH2)
477 s
LOX
Kerosene (RP-1)
370 s
LOX
Monomethyl hydrazine
365 s
LOX
Methane (CH4)
368 s
Liquid ozone (O3)
Hydrogen
580 s
Nitrogen tetroxide (N2O4)
Hydrazine (N2H4)
334 s
Hydrogen peroxide (H2O2)
Monopropellant
154 s (90% H2O2)
H2O2
Fluorine
Fluorine
Hydrazine
Lithium
542 s
Hydrogen
580 s
Liquid Propellant Engine Basics
Liquid bipropellant engine diagram showing the
major elements, and several of the thrust
parameters that include exhaust and ambient
pressures (Pe and Po), exhaust velocity (Ve), and
mass flow rate (dm/dt) (Courtesy NASA-Exploration)
Solid Propellants
Solid rocket propellants contain a variey of
chemicals in addition to the basic fuel and
oxidizer, part for stabilization and part for
performance
Solid fuels used for larger rockets are composite
mixtures containing separate granulated or
powdered fuel and oxidizer, with a chemical
binder, a stabilizer, and often an accelerant or
catalyst added for improved performance and
stability
The final mixture of solid fuel used today is a dense,
rubber-like material that is cast into the
combustion chamber
Solid Propellants
The cast fuel has a central cavity
to allow burning throughout
the length of the rocket
motor, in shapes that can be
anything from a simple
cylinder to a star
Solid rocket fuel is typically
identified by the type of
chemical binder used - either
HTPB or PBAN
Hydroxyl-terminated
polybutadiene, or HTPB, is a
rubber-like binder that is
stronger, more flexible, and
faster-curing than PBAN, but
suffers from a slightly lower
Isp, and uses fast-curing, toxic
isocynates
Solid Propellants
Polybutadiene acrylic acid
acrylonitrile (PBAN), has a
slightly higher Isp, is less
costly, and less toxic,
which makes it popular for
amateur rocket-makers
PBAN Is also used in the large
boosters, including the
Titan III, the Space Shuttle
SRBs, and NASA's new
Constellation Ares I and
Ares V launchers
HTPB is or has been used in
the Delta II, Delta III, Delta
IV, Titan IVB and Ariane
launchers
Hybrid Rockets
Hybrid chemical rocket motors, motors with solid fuel and liquid oxidizer, are
used for intermediate sized boosters
The most familiar hybrid engine is the one used to power Burt Rutan's
SpaceShipOne (test article shown below)
The cast solid fuel core is contained in a combustion chamber
Nitrous oxide stored as a liquid is injected over the solid fuel core

The oxidizer flow is used to start, regulate, and stop the combustion process
Rocket Stability
Two types of stability of a rocket are needed for its
successful flight

Static – stability during initial launch

Dynamic – stability during powered and unpowered
flight
Static stability

The force of thrust, or even simple gravity on a rocket
or on an upright pencil produces stable lift, unstable
lift, or a neutral stable lift

An analogy is an upright pencil with the force of
gravity pulling downward which is equivalent to a
propulsion thrust pushing upwards

The rocket must be stabilized in its initial launch or
the force of thrust will immediately rotate the rocket
(pencil)

Upward force below the center of mass is
unstable

Upward force above the center of gravity is
stable
Rocket Stability
Static stability
Passive

Traditionally provided on very small
rockets with a guide rail attached to
the launch pad and a guide
attachment on the rocket
Active

A guidance control system creates
thrust vectoring of the rocket
exhaust for launch
 Used on larger rockets and
missiles (Atlas missile shown on
the right)
 Can be provided by control of the
main engine thrust, or by smaller
augmentation guidance engines
Rocket Stability
Dyanmic stability
Passive
Aerodynamic fins create
restoring force to
align the rocket in
the direction of
motion during flight

Requires
aerodynamic force
aft of the center of
mass
Rocket Stability
Dynamic stability
Active

A guidance control system creates
thrust vectoring of the rocket
exhaust for launch
 Used on larger rockets and
missiles
 Can be provided by control of
the main engine thrust, or by
smaller augmentation guidance
engines
Soviet RD-107 shown on the right with four
primary thrust engines and four small
outboard vernier guidance thrusters
Early missiles
V-2
The German V-2 developed by
the Nazis in WW-II

Alcohol fuel (ethanol 75%,
water 25% for cooling and
stability)

Liquid oxygen oxidizer

Employed double-wall
combustion chamber
 Inner cavity allowed fuel
to circulate to cool
combustion chamber

55,000 lbf thrust (24,958 N)
Early missiles
Redstone
Developed by Wernher von Braun and the Army
Ballistic Missile Agency (ABMA)

Intermediate range ballistic missile (IRBM)

Used similar design features of the V-2 with
improved performance

Engine design by North American (Rocketdyne)




NAA 75-110
Based on Navajo cruise missile engine
Alcohol fuel
Liquid oxygen oxidizer

Payload 6,300 lb

78,000-83,000 lbf thrust
Early missiles
Jupiter missile

Intermediate range ballistic missile
(IRBM)

Combined Army-Navy project

Limited use as IRBM missile


Converted to use by NASA for the early
interplanetary missions



Navy rejected design for submarine
missiles
Juno II
Juno I was Redstone spacecraft launcher
150,000 lbf thrust
Early missiles
Thor missile (IRBM)

Initial intermediate-range missile for U.S.
and European deployment during the Cold
War

Single Rocketdyne LR-79 (SD-3) engine
used later on Atlas
 150,000 lbf thrust

Single-stage missile was augmented with
multiple stage for increased payload and
range, and launching first reconnissance
satellites

Four-stage version later renamed Delta
(4th letter of Greek alphabet)

Delta booster now a family of launchers
Early missiles
Delta rocket family
Early missiles
Atlas ICBM

First American
intercontinental ballistic
missile (ICBM)

USAF project

Still used as commercial
launcher
 Medium- and heavy-lift
versions
 Atlas V heavy-lift booster
uses Russian RD-180
engine

Liquid oxygen (LOX) and
kerosene first stage
Early missiles
Atlas ICBM

Used three primary engines on
original ICBM design
 Sustainer (central)
 Steering and augmented
thrust engines outboard

Later used for Mercury orbital
missions

Used for Gemini’s Agena target
vehicle launch

Developed into family of
launchers
Early missiles
Atlas rocket family (+ Titans)
Early missiles
Navy Viking missile

Developed by Naval Research Labs as a
missile prototype

Used Reaction Motor’s Company XLR10RM engine

First to use integrated tanks and
structure (monocoque)

First to use thrust-vectored engines

Used as first stage for Vanguard rocket


First satellite launch attempt
Alcohol and liquid oxygen propellants
Early missiles
Titan missile

Developed originally for the Air
Force as a backup ICBM to
supplement the Atlas

Original Titan I design used
RP-1 (kerosene) and LOX

Later Titan II used nitrogen
tetroxide (NTO) and
unsymmetrical dimethyl
hydrazine (UDMH)

Titan III and Titan IV used solid
rocket boosters to augment
thrust on first stage
 Prototype for SRBs used on
Space Shuttle
Early missiles
Titan missile

Titan III and IV
used for both
military satellite
launches and
civil
interplanetary
launches

Heaviest-lift
launcher before
Delta IV Heavy
Current Rockets
Delta launcher family
Current Rockets
Atlas launcher
family
Rocket Stability
Delta IV Heavy used for spacecraft launches
Atlas V Heavy to be used for Orion capsule
tests
Newest launcher is NASA’s heavy-lift
launcher designated Space Launch
System (SLS)
Rocket Stability
Space Launch System (SLS)

3-stage booster

Payload capacity
 LEO
70,000 kg - 129,000 kg
(150,000 lb – 280,000 lb)
1st stage – five segment SRB
boosters
2nd stage (core) – LOX-LH2 fueled
RS-25E engines (5)
3rd stage – 1 RL 10B-2 engine or 3
J-2X engines
Rocket Stability
Falcon 9 rocket

2-stage booster

Payload capacity
 LEO
10,450 kg (23,000 lb)

1st stage – nine Merlin engines

2nd stage – 1 Merlin engines

Propellants LOX & RP-1 (refined
kerosene)
The End