Implementation and Startup of A Centrifugal Direct
Injection Rocket Engine
by
Andrew Heafitz
B.S. Mechanical Engineering
Massachusetts Institute of Technology, 1991
Submitted to the department of Mechanical Engineering in partial fulfillment of the
requirements for the degree of
Master of Science
at the
Massachusetts Institute of Technology
February, 2001
0 Andrew Heafitz, All rights reserved
The author hereby grants to MIT permission to reproduce and to distribute publicly paper
and electronic copies of this thesis document in whole or in part.
Signature of Author:
_______________
Department of Mechanical Engineering
January 19, 2001
Certified by:
Manuel Martinez-Sanchez
Professor of Aeronautics and Astronautics
Thesis supervisor
Reviewed by:
David Wallace
Professor of Mechanical Engineering
Accepted by:
Ain A. Sonin
Chairman, Department Committee on Graduate Students BARKER
MASSACHUSETTS INSTITUTE
OF TECHNOLOGY
JUL 16 2001
LIBRARIES
2
Implementation and Startup of A Centrifugal Direct Injection Rocket Engine
by
Andrew Heafitz
Submitted to the department of Mechanical Engineering on January 19, 2001 in partial
fulfillment of the requirements for the degree of Master of Science
Abstract
The Centrifugal Direct Injection Engine (CDIE) offers the possibility to reduce
the cost and complexity of liquid fueled rocket engines. A prototype CDIE engine was
designed to run on kerosene and liquid oxygen. The prototype was machined, assembled
and debugged. All of the support equipment was built in preparation for hot fire testing.
This included a 6kW electric motor to start the engine and to act as a dynamometer to
collect data while it was running. Qualification tests showed that the engine pumped the
propellants correctly.
A series of hot fire tests were conducted to develop the startup sequence. This
included how fast to spin the engine, and when to turn on the two propellants. A
successful test, which burned for 0.5 seconds, showed that the engine could be started. A
repeat test showed that the startup sequence was repeatable, but the prototype engine was
destroyed later in the test.
Further testing is required to see if the engine will run at steady state.
Thesis Supervisor: Manuel Martinez-Sanchez
Professor of Aeronautics and Astronautics
3
Table of contents
A bstract.........................................................................................................................
Introduction, background and theory of the engine................................................
A brief history of the MIT rocket team...................................................................
D escription of CD IE engine .....................................................................................
The theory behind the CDIE engine .......................................................................
Advantages of CDIE over existing turbo-pump designs.........................................
Building the engine ..............................................................
Theoretical requirem ents .......................................................................................
Design issues .....................................................
...............
Building the Startup System............................. . ..........................
Requirem ents of start-up system .............................................................................
A dvantages of m otor ..............................................................................................
A dvantages of gas startup.......................................................................................
Description of motor startup system........................................................................
Preliminary cold engine testing .....................................
Break in of rotating parts.......................................................................................
.......--.
P ow er curve ................................................................................................
......
Pre cool sequence...........................................................................................
.........................
Startup sequence.........................
Startup requirem ents .............................................................................................
...
Sequence history ..............................................................................................
Tests performed and lessons learned about startup in each test ..............................
...............Conclusions and Future objectives........................
............
Appendix: Support Equipment ..........................
...................................................................................
structure
stand
Test
..............
L OX system .............................................................................................
6
Kerosene syst m ...............................................................................................
Electronics / control system s ..................................................................................
3
6
6
8
10
10
13
13
16
36
36
36
38
38
42
42
43
54
56
56
56
60
68
70
70
70
70
71
Table of figures
7
Figure 1: Wind tunnel test article .......................................
8
Engine.............................................................................................
Figure 2: H eron's
9
Figure 3: Overall Engine Schematic ............................................................................
11
Figure 4: Expander Cycle Schematic..........................................................................
11
Figure 5: Turbo-pump for a Rocketdyne RS-27 engine ..............................................
14
............................................
....................................
..........
Figure 6: L OX C hannels
15
Figure 7: K erosene Channels.....................................................................................
.. 16
Figure 8: Engine parts ......................................................................................
.. 16
Figure 9: A ssem bled engine .................................................................................
17
Figure 10: Engine cross section................................................................................
...
18
....
Figure 11: L O X feed-lines ..............................................................................
19
Figure 12: Flow vs. Time ...........................................................................
20
Figure 13: Test 1041 LOX Flow ...................................................................
4
Figure 14: Bottom Disk, LOX heat-exchange passages (left) and chamber heat fins (right)
..............................................................................................................................
21
Figure 15: Kerosene D isk..........................................................................................
22
Figure 16: Bearing Pre-load System..........................................................................
24
Figure 17: Internal O-ring seals................................................................................
25
Figure 18: LO X N ozzle..............................................................................................
26
Figure 19: Labyrinth Seal.........................................................................................
28
Figure 20: G as Separator...........................................................................................
29
Figure 21: Pressures in the gas separator ...................................................................
29
Figure 22: Cham ber Separator...................................................................................
31
Figure 23: Separator Plum bing...................................................................................
32
Figure 24: Combustion Chamber..............................................................................
33
Figure 25: Fuel Film Cooling Schematic .......................................................................
35
Figure 26: Scoops on Top Part of Combustion Chamber ...........................................
35
Figure 27: Test 1084 high-speed coast down without electric braking ....................... 37
Figure 28: Test 1026, dV/dI behavior of the battery pack ...........................................
39
Figure 29: Solectria BRLS8 efficiency curves ..........................................................
40
Figure 30: G ears ...........................................................................................................
41
Figure 31: M otor m ount .............................................................................................
42
44
Figure 32: tests 1029 and 1030 power curve...............................................................
45
Figure 33: LOX Flow Test, LOX Flow vs. Time ........................................................
46
Figure 34: LOX Pumping Test, Amps vs. Time........................................................
46
Figure 35: LOX Pumping test, Chamber Pressure vs. Time....................
Figure 36: Test 1002, battery amps during ethanol pumping....................................... 47
48
Figure 37: 1084 Coast down power curve ................................................................
50
Figure 38: Pum p curves .............................................................................................
Figure 39: Motor Amps and speed during LOX pumping test 1091............................ 51
Figure 40: 1091 Expected power dissipation during spinning and pumping ................ 51
52
Figure 41: Power measurements vs. predicted values .................................................
Figure 42: Test 1041 power dissipation.....................................................................
53
54
Figure 43: Test 1098 power dissipated as engine self-destructs ..................................
55
Figure 44: Thermal Conduction Paths .......................................................................
58
Figure 45: 1026 Power increase as the chamber floods...............................................
Figure 46: Test 1025 Chamber pressure with GOX flowing ......................................
59
Figure 47: Test 1040 aborted because of under-speed................................................
60
Figure 48: Test 1041 Chamber Pressure .....................................................................
63
Figure 49: Hot fire test 1041- plume with shock diamonds.........................................
63
Figure 50: Test 1081 chamber pressure with LOX pumping only ..............................
65
..................................
65
51:
Time
until
pumping
as
a
function
of
LOX
pressure
Figure
66
Figure 52: Time between LOX valve turned on and full pumping flow .....................
68
.......................................................
in
test
1098
sequence
as
used
Figure 53: Startup
68
Figure 54: The results of test 1098 .............................................................................
Table 1: R otating Seals ............................................................................................
Table 2: Final Startup Sequence ................................................................................
27
67
5
Introduction, background and theory of the engine
A brief historyof the MIT rocket team
The MIT Rocket Team' was formed in December of 1998 to compete for the
Cheap Access to Space (CATS) prize. This was a $250,000 prize, put up by the Space
Frontier Foundation, for the first non-governmentally funded vehicle to deliver a 2 kg
aluminum payload to an altitude of 200km by November 8th of 2000.2 Carl Dietrich
presented an approach for building a liquid fuelled rocket, using a Centrifugal Direct
Injection Engine (CDIE) which he had thought of as a different way to compete for the
prize.
Other competitors were using large solid fuel motors or pressure-fed liquid
engines. We considered solids too large and dangerous to develop in the confined spaces
at MIT. Both solids and pressure-fed liquid engines are well-developed technologies. 3
The team agreed that the challenge of building a completely new type of engine made the
technical risk acceptable. The CDIE held the allure of being something we could develop
without expensive facilities, and it promised to be significantly cheaper to design and
build than other pump-fed, high performance liquid fuelled engines. The challenge of
building a new engine ranked equally high with the CATS prize and the team set off to
build and fly a rocket. The team was funded by the Aeronautics and Astronautics
Department, the Mechanical Engineering Department, and later on by the Edgerton
Center.
Initially I worked on designing a launch vehicle (Figure 1: Wind tunnel test
article), but after working on the engine and the rocket, it became apparent that the two
could not be developed in parallel with the given resources; the team began to focus
exclusively on engine development. Engine development started with some simple test
experiments to verify the concepts behind the engine, which will be described on page
23. These experiments proved to be over-simplistic, and did not produce many useful
results. We did, however, gain a better idea of the level of prototype completeness and
quality that would be required to obtain significant results. Because the heat transfer in
the combustion chamber is essential to the engine's operation and is difficult to simulate
without real combustion, it was decided in the middle of 1999 to develop a full test
engine. This engine was prototyped and ready to test in March of 2000. A test facility
was constructed in Thornton, NH.
http://web.mit.edu/cats/
2http://www.space-frontier.org/EVENTS/CATSPRIZE_1/
3 Sutton, George P., Rocket Propulsion Elements, John Wiley & Sons, Inc. New York, 1992, pgs. 365, 208211
6
Figure 1: Wind tunnel test article
The first few test attempts were met with mechanical troubles, and the team had
to withdraw from the CATS competition because we were too far behind schedule4 . In
September of 2000, the engine was successfully ignited for the first time. It burned for
0.5 seconds and sustained no major damage. In October, a second test ignited, but this
time for only 0.01 seconds. The last test, in November, resulted in a successful ignition,
followed immediately by an explosion and the complete destruction of the prototype.
4 The CATS prize ended on November 8 h with no one claiming the prize. The only launch attempted was
by the HARC team from California. They attempted to launch a pressure fed, liquid fuelled rocket from a
balloon at 70,000 ft. The rocket either failed to separate form the balloon correctly, or had a problem
immediately after separation.
7
Description of CDIE engine
The Centrifugal Direct Injection Engine (CDIE) burns kerosene and Liquid
Oxygen (LOX) in an expander cycle. It uses the boiling and expansion of the LOX to
power the engine. The novel part of the engine is that the turbo-pump is immersed in the
hot gas of the combustion chamber, and uses boiling liquid oxygen to power the pump
through directed jets. This engine is similar to Heron's aeolipile of 53 BC (see Figure 2:
Heron's Engine). The real advantage of the CDIE engine design is that the fuel and
oxidizer pumps are simple centrifugal pumps that are integrated together, and the pump
exhaust has replaced the turbine stage. The opportunity exists to build a high
performance engine at a fraction of the cost of existing engines.
Figure 2: Heron's Engine
The rocket was originally intended to have gravity fed propellants, so the engine
was designed to operate at very low feed-pressures. We eventually upgraded to
pressurized tanks, which were necessary to obtain the right propellant mass flows out of
the tanks.
5 James, P. and Thorpe, N.; Ancient Inventions. 1994. New York; Ballantine Books pp. 131 - 133
8
--
;;R= __ __ - __ -
As shown in Figure 3: Overall Engine Schematic, the LOX enters the engine
through a centrally located feed tube (1). It is centrifugally pumped to high pressure in
the rotating disk (6), and then passes over heat exchangers in the lower part of the disk
(7). Heat in the combustion chamber (9) boils the LOX, gasifying it before it blows out
of two tangential nozzles (8), which keep the disk rotating at high speeds.
2
3
34
5
8
6
Direction of
rotation7
Figure 3: Overall Engine Schematic
Kerosene is introduced into a tube (2) coaxially surrounding the LOX tube, and is
centrifugally pumped in the same disk (4). It is then released directly into the combustion
chamber.
This design is much simpler than other turbo-pump engines, but left many design
issues to be resolved. Some of the challenges of the CDIE engine are: sealing the LOX
and kerosene feed-tubes to the rotating section of the engine; keeping the LOX from
freezing the kerosene; getting the propellants to mix properly in the combustion chamber;
9
and providing a layer of film-coolant for the chamber walls. Another challenge is getting
the engine to operate at steady state, because getting the disk to power itself depends on
there already being heat transfer into the disk from the combustion chamber.
The theory behind the CDIE engine
Because the CDIE engine consists of a single rotating part, the various functions
of the engine are heavily integrated. Conventional engines would have separate pumps
and turbines for the fuel and oxidizer allowing them to be optimized separately. This
section will give a rough overview of the theory behind this design.
The most difficult value to calculate, and the most important, is the chamber
pressure, PC. It can be derived from mass flow through the throat of the engine. m'Throat s
a function of PC. By conservation of mass, the exhaust mass-flow out of the chamber
must equal the propellant mass-flow into the chamber, m'Throat
=
ox +
mkerosene-
m
kerosene
is a function of the rotational speed of the disk o, and PC. m'o, is a function of o, PC, and
the intermediate pressure and temperature inside the LOX heat exchanger, P7, T7 (see
Figure 3). P7 and T7 are in turn, determined by o, and q', the heat flux from the
combustion chamber into the disk. q' depends, once again, on PC and o. The speed of the
disk, (o, is dependant on the power derived from the jets (choked flow from P7 to PC)
minus the power needed to pump the propellants. This power is a function of the mass
flows, m'0 5 and m'kerosene.
As can be seen, these equations must be solved simultaneously. Carl Dietrich
completed a numerical analysis of this problem and determined that the design operating
point for the engine is at a rotational speed o of 36,000 rpm, with a chamber pressure of
500 psi. The kerosene mass flow is 0.125 liters/sec, and the LOX mass flow is 0.25
liters/sec.6
Advantages of CDIE over existing turbo-pump designs
The CDIE engine runs on an expander cycle. The liquid oxygen is used as a
working fluid. It is boiled using the heat from the combustion chamber and the
expansion of the gaseous oxygen runs the engine. The oxygen comes out of the pump,
vaporizes in a heat exchanger, and is exhausted directly into the combustion chamber.
The kerosene is pumped in the same rotating assembly.
Other expander cycle engines boil one of the propellants by running it through a
heat exchanger wrapped around the engine nozzle skirt. (American and European
engines tend to use the fuel, usually liquid hydrogen, as the working fluid, Russian
engines use the oxidizer.) The gas then goes through two turbines, which are connected
to two pumps, one for the fuel and one for the oxidizer (see Figure 4: Expander Cycle
Schematic) 7.
6 The
engine is modeled and described in Carl Dietrich's paper "An Analysis of Heat Transfer in a
Centrifugal Direct Injection Engine", This paper is available upon request, carldietrich@alum.mit.edu.
7 Sutton, pg. 213
10
Oxidizer
pump
Fuel
pump
Fuel
t urbine
Oxidizer
turbine
Figure 4: Expander Cycle Schematic
Each turbine and pump is a complex piece of machinery. Figure 5 shows a
typical geared turbo-pump assembly. This one is used on the Delta launch vehicle with
LOX and RP-1 (kerosene) as propellants.'
ina:rbin
in-xle
,xU>Pump
?~.
oft-,
Figurer
5:Tro upfo
manufactured31
fromag
losotmteilschap05aumiun
t~pe~mM
VU
aocedneR-7engn
1
tanessel
8 Sutton, pg. 329
11
and can be made on commonly available machines such as a 3 axis CNC mill and lathe.
There are no cast parts such as turbine blades, and no super-alloys are used because the
propellants actively cool the pumps. Even the pump and turbine housings have been
eliminated. All of these characteristics add up to an engine that could be less expensive
to manufacture than existing engines.'
A note on the tests referenced throughout this paper: each test has a unique
identifying number. These numbers were started at 1000 to leave room for tests that were
conducted previous to the engine prototype described in this paper. Also, most of the
graphs of test data include a tachometer trace. The tachometer trace is an easy to
recognize shape, and is there to help the reader understand how the data being presented
fits, time wise, within the test.
9 NASA / Marshall Space Flight Center, FasTrack engine reports
12
Building the engine
This section will cover the design and assembly of the prototype engine. The
engine will be broken-down into three parts, the LOX channels, the kerosene channels,
and the mechanical parts, including the bearings, seals and the combustion chamber.
Firstly, I will talk about the requirements of each section, so the that the reader
understands what each part needs to do. For the fluid passages, I will describe the parts
in the order that a drop of propellant would go through the engine. After all of the parts
have been described, I will go back and discuss the designs that were chosen, and the
issues that arose during prototyping.
Theoretical requirements
LOX channels
The LOX enters the engine through a feed tube at the top (1). (See the numbers in
Figure 6: LOX Channels). It travels through a non-contact seal between the stationary
feed tube and the rotating feed tube (2). This seal prevents the LOX from mixing with
the kerosene outside the engine. Once the LOX is in the rotating part of the engine (3), it
travels to the inlet of the LOX pump. Cavitation at this point would prevent the pump
from operating properly. From there, it is centrifugally flung to the outside of the
rotating disk in the pump channels (4), passing through an orifice to create a pressure
drop and prevent back-flow (5). Low-pressure instabilities in the heat exchange section
will have trouble passing back through this orifice in order to affect the flow in the pump.
The LOX then enters the heat exchange passages (6). As it winds back and forth across
the bottom of the disk, the LOX is heated by the heat flux from the combustion chamber
(7). Heat fins on both sides of the wall help insure that there is enough heat transfer. The
heat flux boils the LOX, and raises its temperature and volume. The high-pressure gas is
then ejected into the combustion chamber through tangential jets (8). The thrust from the
jets keeps the disk spinning along the axis of the LOX feed tube, and the oxygen injected
into the chamber combusts (9) to create the heat that boils the next volume of LOX in the
heat exchanger.
13
8
fn l Fl
F1
l~F-1
F-_1
l
I
\
Figure 6: LOX Channels
Follow through kerosene channels
The kerosene enters the engine through a feed tube coaxially outside the LOX
feed tube (10) (see Figure 7: Kerosene Channels). It travels down into the rotating disk,
through a rotating seal (11), and is centrifugally pumped to the outside of the disk (12).
Orifices (13) at the ends of the pump channels meter the kerosene into the combustion
chamber where it film-cools the chamber walls, and combusts with the oxygen from the
LOX section of the engine (9).
14
10
12
13
.>>
Figure 7: Kerosene Channels
Mechanical systems
Bearings
The bearings let the rotating part of the engine spin at high speeds. The LOX and
kerosene mass-flow and mixture ratios are determined by pressure differentials across
their pump orifices, which are related to the disk's speed. The engine is designed to
operate with the disk spinning at 36,000 rpm.
Gas seals
As the LOX and kerosene enter the engine at the top, they cannot be allowed to
mix or leak. The seal between them has to be a positive seal and it runs on a shaft with a
surface speed of 25m/s. Another seal keeps the combustion chamber from leaking
around the disk shaft. Leakage here would contaminate the bearings, which could cause
the engine to seize.
15
Combustion chamber
The combustion chamber for the flight vehicle will be designed using NASA
Fastrack engine technology. They use a silica-phenolic composite that utilizes kerosene
film cooling and an ablative throat. This engine is on the small side for using ablative
cooling at the throat, because while the ablation rate does not change with throat size
(ignoring the effects of curvature), the percentage change in throat area at a given
ablation rate does go up. For this reason, this engine cannot be scaled down to much
smaller that the one we are building.
Design issues
This section will discuss each part of the engine and the specific design issues and
choices related to each part. Figure 8 is a photo of the engine parts laid out, before their
initial assembly. Figure 9 and Figure 10 show the engine after it has been assembled.
Figure 8: Engine parts
Figure 9: Assembled engine
16
LOX inlet
LOX labyrinth seal
LOX
Kerosene
Inert Gas
Vacuum
Rotating parts
Bearing retainer ring
Belville washers
(Replaced with wavey
washer)
Bearings
Chamber separator
(old design)
Kerosene injectors
Bolt circle
0-ring seal to
copper
combustion
chamber
LOX heat exchange
channels
Holes for scoops
LOX pump
LOX jets (not shown)
Bottom disk -
Figure 10: Engine cross section
LOX channels
LOX feed-line
The feed-line connects the LOX tank to the top of the engine. It goes from the LOX run
tank (see Appendix: Support Equipment), mounted on the test cell wall, to the labyrinth
seal that separates the stationary and rotating parts of the engine. Initially, the LOX feedline was going to be open to the LOX tank, and would fill and cool as the tank was filled
(see Figure lIA). However, the gravity feed system was not sufficient to overcome the
boiling and vapor lock in the line. We switched to a pressurized LOX tank, and put a
valve between the tank and the line (Figure 1 IB). Initial tests using water instead of
LOX showed a curious spike in the flow rate before smooth flow developed (see Figure
12). I suggested that the spike was the rapid flow of water into the empty feed tube,
followed by a back-pressure as the pump primed, and then good flow as the pump began
pumping efficiently. This rapid filling of the tube was not repeatable in the next series of
tests using LN 2 as a substitute for LOX. We were not able to get good flow. I suggested
17
pre-cooling the tube by flowing LN2 through it, but not through the engine, because the
kerosene will freeze if the engine is too cold (Figure 1 IC). This system is the only one
we have gotten to work. Cooling the rotating part of the engine, which is not thermally
connected to the piping, is still difficult, and will be discussed in the sections about
testing and operating the engine.
(~N\
Pressurized
LOX tank
Gravity feed
LOX tank
Fl ow measurement point
Flow to cool
Valve
n run
wto
Engine
C
A
w0
B
Figure 11: LOX feed-lines
18
Kerosene flow ratel
1002 pumping
-
16--
-
--
14
12
E
.E 10
8
E
2
0
----94
95
96
97
98
99
100
101
102
103
104
Seconds
Figure 12: Flow vs. Time
LOX pump
Once the LOX reaches the bottom of the engine, it is centrifugally flung out along
two "T" arms to pump it up to 2100psi. This is the highest pressure in the entire engine.
The feed tube and pump section's job is to pressurize the LOX and get it to the
heat exchange section in liquid form. If it begins to boil or vaporize before then, the
pump will not develop the right pressure and it is likely that the LOX feed-line will vapor
lock, causing the mass-flow rate to drop. Chugging flow has been observed in the feedline until it is properly cooled (Figure 13: Test 1041 LOX Flow). Good flow starts around
775 seconds. The large spike at the end is a shutdown measurement transient. The risk
of low frequency instabilities involving the chamber pressure backing up the feed-lines is
a danger if the pump section does not have a uniform and large pressure rise across it.
The reasons that the LOX could boil in the feed/pump lines are from heat transfer
from the structure, heat transfer from the kerosene, and cavitation in the inlet of the
pump. Because the kerosene is fed in coaxially around the outside of the LOX feed tube,
it was important that the LOX tube be insulated as well as possible, both to avoid boiling
the LOX and freezing the kerosene. Space was very limited, because increasing the
kerosene feed-line diameter would increase the ID of the bearings, and the surface speed
of the balls that were already running near their top speed. The LOX feed-line could not
be made smaller than 0.5" because the related pressure drop would starve the engine and
could cause cavitation problems in the pump. This inner diameter requirement should be
revisited in the flight vehicle, because this calculation applied to a gravity fed LOX
system, and a pressurized start up may allow a smaller feed tube to be used. These
factors constrain the inner and outer diameters of the assembly, without leaving much
room in between. In addition to the feed-tubes, a good layer of insulation had to fit in
that space to prevent the unwanted heat transfer from the kerosene to the LOX.
19
- Tach
-LOXFlow
1041
----- 8.5
30000
25000
7.5 E
20000
0)
15000
E
6.5
10000
2
0
-J
5000
0
773
5.5
775
774
776
Seconds
Figure 13: Test 1041 LOX Flow
The highest performance type of insulation is a vacuum jacket. My original idea
consisted of a triple walled, vacuum-jacketed tube sealed with Teflon o-rings. The center
wall in the middle of the vacuum gap would reduce radiative heat transfer across the
space. An improvement on the idea, suggested by Professor Kerrebrock'0 , was to replace
the o-ring seals with welded joints, because they would be more reliable, especially at
cold temperatures. If the final weld was made in a vacuum chamber using e-beam
welding, the vacuum would be guaranteed and permanent, even after being exposed to
thermal events, such as cryogens flowing through the tube. The resulting assembly
insulated the entire feed tube and the pump in the same vacuum space. (See Figure 10:
Engine cross section)
This assembly was especially difficult to manufacture and weld. Also, the vacuum
seal was probably broken when the LOX feed tube was bent during a high-speed test
without the upper seals in place. While these seals were supposed to be non-contact
seals, in fact, they could act as backup bearing surfaces for the LOX feed tube if it got out
of balance. Given that the engine successfully pumped LOX without the vacuum, it does
not seem to be necessary, and the next redesign may replace some of the welds with orings, and use an air filled insulation gap in place of the evacuated insulating gap.
Heat exchanger
From the pump, the LOX enters the bottom disk, which has a series of heat
exchange channels (see Figure 14 left). The LOX is boiled and the gas is heated as it
passes across the bottom surface, which is directly exposed, to combustion gasses on the
" Pers Comm, Professor J. Kerrebrock, 1999
20
opposite side (Figure 14 right). The oxygen, now vaporized is directed outwards again to
a pair of right angled jets which can be seen protruding through the outer wall of the
bottom disk. The oxygen is expanded into the combustion chamber, which is at 500psi
(or less during start-up). This expansion creates two tangentially directed jets that act to
spin the disk, and provide the power to pump the propellants.
Figure 14: Bottom Disk, LOX heat-exchange passages (left) and chamber heat fins
(right)
The design criteria for the heat exchange system are described below. There were
a series of geometrical constraints on the heat exchange section that made its design
especially challenging. The following features each had to be located in the positions
listed below, and they all had to be connected together with the LOX flow.
" The inlet of the heat exchange section had to be near the edge of the disk where it is
fed by the centrifugal pump outlet.
" The outlet of the heat exchange section also had to be near the edge of the disk
because the oxygen jets that it feeds are on the outer edge.
" The best heat exchange takes place on the disk's edge because of its high velocity
within the combustion chamber.
" The entire bottom of the disk had to be cooled to prevent melting.
" A method of fastening the bottom part of the disk to the rest of the engine could not
interfere with any of the other functions, but had to be able to withstand 2100 psi of
internal pressure.
All of the criteria boil down to a geometry that is difficult to implement. The
pressurized LOX flow must start at the edge, get to the middle, covering the entire
bottom, and then get back to the edge, without crossing through itself. The final design,
which Carl Dietrich and I worked out, has two levels and a serpentine path for the LOX
to travel through. For my contribution, I figured out how to eliminate several large bolts
that were going to have to go through this disk to hold the stack of disks together. I
replaced them with a bolt circle of much smaller bolts around the edge of the disk, above
the heat exchange area. I added a single serpentine heat fin that follows the LOX flow
and improves the internal heat transfer.
21
The disk itself was made of anodized 7075 aluminum. This was chosen because
of its strength-to-weight ratio, necessary for spinning at high tip speeds. There is concern
about the bottom disk burning through in places where it is exposed to combustion
gasses, but it is cooled by liquid oxygen. The first hot fire test, which ran for 0.5
seconds, showed that some of the poorly cooled sharp corners on the outside edge started
to melt. The rest of the disk survived intact, although the chamber pressure only got up
to 300 psi out of an expected 500 psi. The increased chamber pressure at the design point
will create higher heat transfer than was experienced in this test. A CFD analysis was
decided to be too difficult to undertake, especially because we would have to verify the
results experimentally to have any confidence in them. The aluminum is anodized to
provide an extra layer of protection in case it is exposed to pockets of high-temperature
and high-pressure oxygen in the combustion chamber. We will find out if aluminum can
survive after our first extended length burn, which is planned for the spring of 2001.
Kerosene channels
The kerosene flow path is simpler than the LOX path. The kerosene drops to the
bottom of the feed tube, enters the kerosene pump, and is centrifugally pressurized and
injected into the combustion chamber through four orifices. The kerosene disk (see
Figure 15) does not have many difficult design features relating to the kerosene flow. It
does tie the rest of the engine together, but that will be discussed in the mechanical
section. This piece is also made of 7075 aluminum.
Figure 15: Kerosene Disk
22
Mechanical systems
Bearings
The engine design in some respects is quite simple. There is a rotor and a stator.
The two are connected with a pair of bearings (see Figure 10) that allow the engine to
spin at 36,000 rpm. A variety of different bearing types were tried, with minimal
success, until we settled on a set of super-precision bearings with ceramic balls.
Initially, the engine was expected to run at lower speeds, and the first rough
prototype had a pair of steel ball bearings. There were alignment problems and press
fitting problems and I never got the prototype to spin freely. We then decided to make a
static gas bearing for axial support and a jewel type bearing for the thrust load. At this
point, we were simulating the conditions in the combustion chamber with some blow
torches, so there was no chamber pressure to counteract, just the weight of the rotating
part of the engine. I designed a gas distribution system that would allow us to use
interchangeable brass sleeves on the static part of the bearing. I machined several sleeves
in the Edgerton student shop, reaching 0.0005" precision or better, which was difficult
using the equipment available. The steel shaft I had purchased turned out to be out of
round by .0005" in such a way that it was not detectable by measuring a diameter with a
micrometer, but only by measuring three points with a special gauge. This was not an
easy thing to trouble shoot. A new shaft was rounder, and we got the engine to spin up to
6000 rpm by blowing compressed gas through the jets, but there was always considerable
friction, and the LN 2, that we were trying to boil with the blow torches to make jets,
tended to vapor lock.
The LN 2 jets would not keep the disk spinning by themselves, and I had
developed a method for timing how long it took for the disk to spin down with and
without the jets. This gave us an idea of what forces were acting on the disk, although
the bearings were temperamental enough that there was some variability. There was one
test that used LN 2 and the heat source that seemed promising. The disk did not spin
down as quickly as it did without the LN 2 and blowtorches. The jet assist seemed
statistically significant, but the test rig was not performing well. Because so many
aspects of the engine were inter-linked with one another, we decided to start over with a
more completely designed engine that could be hot fired. This new engine would test all
of the systems under the most realistic situation possible, a hot combustion chamber.
The bearings in the new engine prototype are Bardon, ceramic-ball, angularcontact bearings. The angular contact allows them to take radial and axial loads.
Because the propellants have to pass through the bearings, their diameter, and resulting
radial load rating was larger than would have been otherwise necessary. This allowed us
to use the angular contact bearings to support the thrust loads as well. Because the shaft
goes through the combustion chamber wall, the bearings have to support the load of the
chamber pressure over the area of the shaft. At 500 psi, this worked out to be about
1,500 lbs. Because the bearings are rated for 1550 lbs., axially, this load is right around
their upper limits. Ceramic balls were chosen over steel balls for several reasons. Even
though the ceramic balls have a 30% lower load rating than similar steel balls, they have
a longer life, especially at high speeds and under poorly lubricated conditions. They are
lighter weight than steel, so the centrifugal forces at the high running speeds are lower.
Also, the steel balls running on steel races tend to weld under high loads, or in this case
23
high speeds. Even with less than ideal lubrication, the ceramic balls do not have the
contact problems related to metal on metal surfaces. The top rated speed for the ceramic
balls is 45,000 rpm.
The first set of bearings was damaged when the non-contact engine seals crashed.
The bearings may not have been properly preloaded at this point. The pre-load system
for the bearings is designed to keep the angular contacts in full contact, even without
chamber pressure present. The first pre-load system used a pair of Belleville washers, but
because they were so heavy, there was not much deflection. After the first set of bearings
failed, either because the improper pre-load caused the non-contact seals to crash, or the
seals crashing put large loads on the bearings, I replaced the Bellevilles with a thinner
wavy washer. This allows the bearings to move more freely in the axial direction (see
Figure 16: Bearing Pre-load System). This system has been used for all of the successful
spin, pumping and hot fire tests, and has been taken up to 40,000 rpm without problems,
unlubricated.
The reason for running the bearings unlubricated is that an oil mist system would
be difficult and heavy to implement inside the engine, and at these speeds, grease will
tend to be flung off. Also, grease tends to make bearings run hot. The manufacturer
thought that they would survive at these speeds, without lubrication, for a short time.
Since our designed burn time is only 90 seconds, we are hoping to get away without a
lubrication system. An unexpected benefit of the rotating seals, was that one of them
leaked kerosene directly into the bearing area, but this feature will be fixed on the next
design revision because leaking kerosene is an explosion hazard. It should be noted that
the bearings performed well in numerous tests that were run dry, or with only LOX or
LN, (to simulate LOX) and no kerosene. There have been no duration tests to see how
long it does take for the bearings to overheat. This type of testing has had a lower
priority than the short duration hot fire tests, and will be done by default as the burn times
are extended. We have compensated for the lack of lubrication by not running the engine
for more than about 20 seconds at a time and allowing a complete cool-down between
tests.
Wavey Washer
Kero Disk Feed tube
Figure 16: Bearing Pre-load System
24
Disk internal seals
I figured out how to design the o-ring glands that seal the LOX section from the
combustion chamber, and the kerosene sections (see Figure 17: Internal O-ring seals).
These o-rings had to be positioned in such a way as to avoid direct exposure to
combustion products. They also had to be assembleable, in terms of being able to insert
the Teflon o-rings, which are stiff and unforgiving. I chose Teflon for the o-rings because
of its compatibility with oxygen and low temperatures. It is the only polymer o-ring
material rated down to -300F. Admittedly, this is 89K, just one degree Kelvin below the
LOX temperature; however, all of the places where they are used are static seals, which
are more robust than moving seals in cold conditions. The upper seal fit in easily, but the
lower one had to be pressed into the gland after that part of the engine was assembled. It
needed to be compressed as it was inserted. This is not the standard way to pack an oring gland, but it worked.
-r
s
kero pump
1LOX pump
-I-~~I
Figure 17: Internal 0-ring seals
Disk assembly
There were two assembly steps that were very difficult. The first related to the
nozzles, which needed their own o-rings to be compressed in the X axis by the LOX
pump, which was inserted along the Z axis (see Figure 18: LOX Nozzle). The second
involved the kerosene disk where the springiness of the o-ring made it hard to seat the
aluminum disk. Further, pressing on the edges of the kerosene disk, as the bolt circle
tended to do, bowed the disk enough that the locating step would not engage at all. A
future version of the engine should have larger locating features, and the nozzles need to
have properly designed ramps that drive them into place.
25
Press in LOX pump
0
Compress 0-ring
Bottom disk
Figure 18: LOX Nozzle
Once the parts were ready to be assembled, I needed to make a number of custom
assembly tools to get all of the o-rings and tight fits to go together. The disk itself is held
together with a bolt circle. The forty-four 6-32 bolts around the edge are high strength
steel alloy to hold the disk together when it is pressurized. This was chosen over
screwing the top and bottom halves together because I judged that as being too hard to
implement. Screwing large diameter pieces together with fine threads and compressing a
static seal with a screwing motion creates a lot of friction. Attachment points for tools
capable of transmitting that much torque seemed like a difficult thing to include in the
design.
Disk balancing
The disk is designed to rotate at 36,000 rpm. I wanted to keep the disk balanced
to minimize side loads on the bearings, and to avoid unnecessary vibrations and
resonances. The tolerances on the parts were generally kept to +/- 0.001", or +1-0.0005"
on the radial alignment surfaces. By keeping the parts as symmetrical as possible during
manufacturing, the rotating parts would be pretty well balanced when they were first
assembled.
For fine-tuning, I use the bolt circle to balance the disk. Once the disk is fully
assembled, one of the bearings is slid on, and the shaft held horizontally. The disk is then
statically balanced, with the heavy side rotating to the bottom. Bolts are removed,
shortened, and reinstalled. This process can detect a 0.3 - 1.0 gram weight difference at
the edge of the disk, so the disk can be considered to be within 1 gram of balanced. The
side load produced by the worst case imbalance, equal to a 1 gram weight at the edge of
the disk, rotating at the design operating speed of 36,000 rpm, is 162 lbs., well within the
2360 lb. rating of the bearings. The engine has been successfully spun up to 40,000 rpm
and has not had any noticeable balance problems.
26
Rotating seals
There are four sets of rotating seals in the engine. Their purpose is to seal
between the stationary parts of the engine and the rotor. The engine spins at 36,000 rpm,
which at the inner diameter of the bearings is a surface speed of 56 m/s. I decided to use
non-contact seals because of the high surface speeds, extreme temperatures (either hot or
cold) and unlubricated conditions for some of the seals. Table 1: Rotating Seals, lists the
seals and their requirements.
Table 1: Rotating Seals
Seal name
Located between:
Labyrinth
LOX supply line
and LOX feed
Fluid(s)
sealed
against
LOX
Temperature
range
90K to 300K
Materials
(rotating part,
Stationary part)
Stainless steel,
Brass
tube
Gas
separator
Kerosene
Chamber
separator
LOX and kerosene
feeds
Kerosene feed and
bearings
Combustion
chamber and
LOX and
kerosene
Kerosene
Combustion
gasses
90K to 300K
273K to 300K
273K+
Stainless steel,
Aluminum
Aluminum,
graphite
Aluminum,
graphite
bearings
LOX labyrinth seal
The LOX passes through the labyrinth seal at the top of the engine. This is a
rotating seal between the LOX supply line and the rotating feed tube. I had the idea of
using a labyrinth style seal that would allow a small, but manageable amount of flow to
leak through the seal (see Figure 19: Labyrinth Seal). That flow is allowed to vent to
atmosphere, through a set of dump lines. In a flight vehicle, it may be possible to recycle
this leakage back into the propellant tanks, depending on the tank pressures that are used,
thus saving and reusing some propellant that would otherwise have to be dumped
overboard, unused. Eliminating unnecessary reductions to engine Isp due to propellant
leakage will be important in a flight engine. The labyrinth seal has the advantage of
being a non-contacting seal, so there are theoretically no frictional losses or seal wear.
In reality, this seal has been somewhat problematic because it does tend to rub
and create drag on the engine. The stationary part is made of brass, and the rotating tube
is stainless steel. The labyrinth seal piece needs to be held centered inside the engine
feed tube while it rotates at 36,000 rpm. This is difficult because the seal is mounted
directly to the LOX supply tube at the top of the engine. The weight of this plumbing
and the valves can put torque on the seal and cause it to crash. The inner part of the seal
was reinforced after this problem was identified, but it is still very delicate. Ideally, the
plumbing would be isolated from the engine by flexible tubing, but with the cooling
method shown in Figure 1 lC, the upper part of the engine must be in thermal contact
with the plumbing to help it pre-cool.
27
~~~1
Dump to
Atmosphere
Leakage
LOX Flow
Rotating Shaft
Figure 19: Labyrinth Seal
The second problem is that the rotating tube is cantilevered four inches from its
base, and tends to wobble at high speeds. The longest tolerance path in the engine is from
the LOX feed tube to the labyrinth seal. Errors in machining add up through the LOX
feed tube, the kerosene disk, the bearings, the bearing holder, the upper housing, the
labyrinth holder and the labyrinth seal itself. In fact the root cause of the engine's major
failure in test 1098 was attributed to a crack in the base of the LOX tube created months
before when it bent while running without the labyrinth seal in place. The intermittent
rubbing of this seal is evidenced by power losses, sudden temperature rises in the seal
area during high-speed operations, and smearing of the brass labyrinth material.
In the next version of the engine, I will attempt to mount the parts more securely,
and make the seal clearance as small as possible. If the seal is held securely, it will be
able to wear in and eventually stop rubbing, leaving a small clearance. The current seal
would abrade, and move, and abrade some more. By the end of testing, the seal had worn
itself out until there was 0.005 inches radial clearance. The only real problem with large
clearances is that there is excessive LOX leakage, which reduces the accuracy with which
we can measure the engine's performance.
Gas separator
The gas separator keeps the LOX and kerosene from mixing as they are fed into
the rotating part of the engine in close proximity to each other. The gas separator used a
non-contact seal similar to the labyrinth seal, but this one had helium pumped into one of
the middle cavities. The positive pressure of the helium in the middle cavity made the
LOX and kerosene unable to enter, effectively separating them (see Figure 20: Gas
Separator). I tested the gas separator by putting a piece of tracer paper in the kerosene
dump channel and introducing water into the LOX dump channel. Any water that got
through the separator would have been absorbed by the paper, and caused the ink on it to
run. There was no evidence of water having gotten through after the test. This test was
run with the He feed regulator set to 10 psi, and a pressure tap in the separator annulus
showing 2 psi. The criterion for setting the pressure in the gas separator is that the
propellants should prefer to go out the dump lines to going through the separator. Also,
there should not be so much flow as to pressurize the dump areas and cause back-flow
28
into the LOX and kerosene feed areas. The separator pressure must be higher than the
pressure drop through the dump lines, but lower than the feed pressures of the kerosene
and LOX. Ps>POd>P LOX feed, and Ps>Pkd>Per feed (see Figure 21)
LOX tent
Labyrinth seal
He feed
Small gaps
Kero vent
Rotating LOX tube
Kero feed
Kerosene seal
Rotating Kero feed tube
Figure 20: Gas Separator
r P od
Atm N.
P He
Atm
P kero
PS
-
W
-p,Nioioioi
:4zzz
A.. -
n
1
Figure 21: Pressures in the gas separator
29
Kerosene seal
The kerosene seal keeps the kerosene being fed into the disk from leaking out
through the bearings (see Figure 20). The seal consists of a small gap between a fixed
graphite ring and a lip on the top of the kerosene feed tube. Originally this seal was
aluminum on aluminum, but the outside was changed to graphite when we had trouble
with the seals crashing and galling. The main problem with this seal is that the aluminum
lip on the kerosene tube was damaged during assembly. Several nicks and dents cause
significant leak paths. Once the kerosene leaks through the seal, it goes through the
upper bearing, and hits the gear. From there it is sprayed all over the test cell in a fine
mist that tends to light off during hot fire tests. Since the test cell is not a closed space,
these fuel-air explosions have not caused any damage, although they do whiteout several
video frames at a time during the tests. An unintended advantage of having this seal leak
is that a kerosene mist on the bearings is very close to an ideal cooling and lubricating
situation. When this seal is fixed in the next engine design, the bearings will lose this
cooling feature.
Chamber separator
Another gas separator labyrinth seal was placed around the shaft between the
lower bearing and the combustion chamber to stop hot combustion gasses from melting
the bearings. The first design had a distribution groove on the outside, static part of the
seal. The design had to be changed when the aluminum surfaces started crashing and
galling. We wanted to put in a graphite insert, but machining a high-pressure distribution
groove in a small graphite insert would not work. The graphite would not be able to hold
the pressure, and the features would be too fine to allow machining the graphite without
breaking the piece. Instead I figured out a new arrangement, where the gas feeds through
a hole in the graphite, and the distribution annulus is in the aluminum of the rotating part
(see Figure 22: Chamber Separator).
This seal is pressurized with nitrogen. The pressure must be higher than that in
the combustion chamber. For the preliminary tests, we have been running it at 1 100psi
feed pressure, which produces 800 psi in the distribution annulus. There are two feed
ports, located 180' apart from each other, and a pressure tap at 900. This allowed me to
measure the pressure in the distribution annulus during operation. During pressure tests
I found that the feed regulator is choked inside, and as the tank pressure drops, the
regulator is not able to maintain the same flow. The pressure in the chamber separator
drops by 6psi/sec when fed by a single high-pressure N 2 tank. For the test firings, we are
using 3 tanks of N 2 so the pressure will only drop by 2psi/sec. With short firing times
and conservative use of the chamber separator gas, the pressure should hold up well
enough to be considered constant. The expected chamber pressure is around 500 psi, so
we may be able to lower the chamber separator pressure in the flight vehicle to save gas.
The only criterion is that it must never fall below the chamber pressure because then the
hot combustion products would vent through the lower bearing, and possibly backwards
into the gas separator feed plumbing.
30
N2
f eedEngine disk
(Rotating)
Distribution annulus
Graphite insert
Figure 22: Chamber Separator
I thought of the solution of using labyrinth seals and gas separators early on in the
engine development process, and they seemed to be a workable solution. Not realizing
how related this problem was to other engines, I was pleased to find out how similar our
solution was to that used in other engines. Pressurized gas seals like this one are used in
the space shuttle main engines", and also in jet engines12 . The biggest mistake was that
we used aluminum on both sides of these non-contact seals. We coated them with boron
nitride, which is supposed to be compatible with itself for running surfaces, and the seals
were designed to have a positive clearance at all times, so rubbing was not supposed to be
an issue.
Through a series of minor machining errors, a badly preloaded bearing, and a
series of bizarre assembly coincidences, the seals did end up rubbing and the aluminum
galled and seized. I cleaned out the seals, and tried again, but it took a number of failures
before we fixed or replaced all of the parts damaged by the original incident, including
the combustion chamber, the bearings, and the seals. The seals were redesigned, and
graphite inserts were put on the static side of two of them. (See Table 1: Rotating Seals)
To be sure that the new seals would perform well enough, a test of a rapidly spinning
aluminum wheel on a block of graphite showed that the materials were compatible at
surface speeds of 120 m/s, and with kerosene.
I reinforced the graphite seal insert on the backside with carbon fiber and glued it
in place using a flexible epoxy. I chose an epoxy that could withstand fairly high
temperatures (250C) and 300% elongation before failure. This is needed because if the
aluminum behind the graphite expands from heating, it will pull away from the seal,
compromising the seal, and causing the graphite insert to be blown downwards because
of the high pressure which would get around and above it. I also machined a groove in
the aluminum for the epoxy to fill. This is because the expected differential expansion of
the aluminum over the graphite must not be more than 300% of the thickness of the
epoxy, otherwise it will be overstrained, and come apart. This epoxy turned out not to be
compatible with the ethanol used in some of out tests to simulate kerosene, and I was
concerned about its compatibility with kerosene, so the engine is never exposed to
kerosene without the chamber separator running at least at low pressure. In order to
make this always true, I plumbed the chamber separator to its high-pressure source and
"MIT class lecture, Fall 1999, 16.512 Rocket Propulsion, Prof. Martinez-Sanchez
" Pers Comm, June 2000, Frederic Ehrich , senior lecturer GTL, MIT
31
also to the lower pressure source of the other separator through a check valve (see Figure
23). Since the low-pressure separator can be run for extended periods on a single tank, it
can be left on during start-up and shutdown. The chamber separator has such a high
flow-rate, that we only use it during the actual burn to conserve pressurized gas. When
the high pressure comes on, it fills the chamber separator, but the check valve keeps it
from over-pressurizing the gas separator. The gas separator uses Helium because it is
exposed to LOX or LN 2. Helium will not condense when exposed to either of these.
Nitrogen gas is used in the chamber separator because it is less expensive than Helium,
and is not exposed to low temperatures.
10 si
Check valve
Gas separator
900 psi
Engine
N2
N2
N2
separator
__
.Chamber
Figure 23: Separator Plumbing
In the future, a few changes to the separator designs could improve their
effectiveness. GE uses an abrasive coating on their rotating parts running against a soft
static wear ring in their jet engines". Our flight engine could use similar abradable seals
where an abrasive coating is put on the rotating side and a soft material such as graphite
or brass (if the rotor is steel) is used on the static side. The shaft expands by 0.0006"
when it spins at 36,000 rpm from centrifugal forces, and this is the diameter that the shaft
will be designed to run at. The seals will be tightest when the engine is at full speed.
Tightening up the seals this way will help use less compressed gas in the flight vehicle,
and save propellant from leaking away. This breaking in process will make the seals
tighter than is possible with separately machined parts. Temperature should not be a
problem because the seal area is cooled with kerosene and nitrogen. If there were an
effect, it would tend to open up the sealing gap.
Another design difference between our seals and non-contact seals used in other
engines is the shape of the labyrinth teeth. Typically they are sharp or triangular shaped.
The momentum of the flow going around a sharp corner creates a dynamic effect that
causes the flow to contract momentarily. As a result all of the fluid passes through an
area that is even smaller than the distance between the surfaces. We used square
labyrinth teeth because we were unaware of this phenomenon until late in the design
13
Pers Comm, John Zeiner, GE Aircraft Engines, Lynn, MA
32
stage, and we were also concerned about our ability to accurately manufacture knife
edged features, especially in aluminum.
An alternative design for the chamber separator is to simply make the chamber
separator as tight as possible and vent each section of the labyrinth to atmosphere. Fuel
film cooling in this arrangement keeps the combustion gasses from eroding the seals. If
the combustion products can be vented in a controlled manner before they reach the
lower bearing, it may not be necessary to use a compressed gas source at all. Because
this is the highest-pressure seal in the engine, it uses the most gas. The weight savings
from eliminating the gas use in the chamber separator would be considerable.
Combustion chamber
For the sake of preliminary testing, I have built a prototype chamber out of a solid
block of copper which will cool the chamber walls through heat sinking to the rest of the
mass (see Figure 24: Combustion Chamber). The test chamber weighs 27 kg and will
have enough thermal mass to conduct short test firings. Using the thermocouples in the
walls, the test chamber will show the effectiveness of the film cooling, as well as hot
spots and likely failure points. The wall thickness is over-designed from a pressure
containment point of view.
6.0
I
Pressure tap
Temp
Sensor
Ports
Ignitor hole
8.75
15.00
-!
.512
Figure 24: Combustion Chamber
33
The shape of the chamber was simplified for manufacturing reasons to conical
and straight-sided sections for the chamber walls, throat and nozzle skirt. The array of
thermocouples can be seen up the left side and two more are included in the engine that
forms the top of the chamber. The hypothesis is that the top of the chamber will be the
most fuel rich area, and therefore the coolest. The chamber walls should get hotter as
they get down towards the throat. There is a safety procedure in the hot fire test sequence
that will shut down the test if the temperature sensor near the throat gets up to 80% of the
melting temperature of copper. Since this is the strongest part of the chamber wall, at
almost 3 inches thickness, the start of a melt down here should allow enough time to shut
down the engine before the structural integrity of the chamber is lost. A short test of 5
seconds will give a good idea of the heat transfer going on in the chamber walls, and will
be the starting point for designing further tests, as well as the ablative characteristics of
the composite flight weight chamber.
I designed the nozzle using a one dimensional expansion model. A 15' straightsided expansion takes the exhaust from 35 atm in the chamber and over-expands it to
0.35 atm at the exit. This is the maximum expansion possible without having to take
flow separation along the nozzle walls into account, and is the same as that used by the
Fastrack engine. The expansion ratio came out to 1:3.37, and the throat is 13mm in
diameter.
The chamber is sealed to the top part of the engine using a Teflon o-ring and a set
of six grade 8 bolts, which are all 0.5 inch diameter. This provides enough strength to
hold the engine onto the combustion chamber with a safety factor of 10, and should also
be the failure point in the event of an over pressure.
The chamber is film cooled by the kerosene that is injected by the upper section
of the disk. The unconventional injection direction of this engine promises to offer a
challenge in balancing the film cooling versus good mixing of the propellants. The
kerosene is centrifugally pumped up to the injection pressure, and then released directly
into the combustion chamber. This will fling it straight at the walls, all the way around
the chamber (see Figure 25). First the kerosene skims along the roof of the chamber, and
then as the roof starts to curve down towards the walls, the kerosene follows and begins
to coat the walls. At this point I have devised a series of scoops protruding from the
walls, which will peal some of the kerosene flow from the walls and shoot it axially down
the length of the combustion chamber (see Figure 26: Scoops on Top Part of Combustion
Chamber). As a starting point, these deflecting scoops cover one half of the
circumferential area of the chamber. Half of the fuel should be pealed off the walls, and
half will remain coating the walls as film-cooling. This high ratio of fuel used for film
cooling was chosen because of the unconventional shape of the chamber. The chamber is
very wide and squat, compared to other chambers, because it must enclose the disk.
Hopefully, enough film-cooling kerosene will remain along the walls so that some
reaches the throat, and reduces the ablative erosion in this area.
34
Scoop
Kero deflected into mixing area
Film Cooling
Figure 25: Fuel Film Cooling Schematic
Figure 26: Scoops on Top Part of Combustion Chamber
The ratio of fuel devoted to film-cooling will be evaluated by the series of
thermocouples I have installed in the wall of the chamber. We will be able to see how far
down the chamber the film-cooling effect lasts, before the temperature starts to rise above
the melting point of the silica phenolic material that is to be used in the flight vehicle.
The film-cooling can then be adjusted by changing the configuration of the scoops.
35
Building the Startup System
The engine needs support equipment to run the hot fire tests. This includes LOX
and kerosene supply tanks, a test stand structure to hold the engine during firing, a test
cell, a control and instrumentation system, and a startup system. Because my main
design efforts were the engine and startup system, I will continue to focus on those here.
A brief overview of the other systems is included in Appendix: Support Equipment, for
reference.
Requirements of start-up system
Starting the CDIE engine is difficult because the nature of an expander cycle is
that the engine powers itself. Like the chicken and egg problem, the engine must be
running in order to pump propellants, and the propellants must be pumping in order for
the engine to run. Further, both the kerosene and LOX must be pumping with fully
developed flow before combustion starts in the chamber. The pressure increase through
the pumps is all that keeps the chamber pressure from going back up the propellant lines
and blocking them.
A start-up system is required that gets the engine as close as possible to the
expected steady-state operating conditions as possible. This includes, at a minimum,
cooling the LOX passages of the engine, and getting the disk spinning in the stable
operating regime. The lower bound is a speed of 27,000 rpm, below which heat transfer
will not be great enough to keep the disk spinning by itself, and the upper bound is
around 40,000 rpm, determined by the disk's structural speed limit and the bearings' top
rated speed. The steady state operating speed and the start-up target speed is 36,000 rpm.
I came up with two possible ways of starting the engine. The first involved
blowing pressurized gas through the disk, and using the oxygen jets to bring it up to
speed. The second uses an external electric motor to spin up the disk. The motor was
chosen for the preliminary testing. The following sections outline the arguments for
each.
Advantages of motor
The external motor was used because the focus of these preliminary tests is the
heat transfer and sustainability of the engine, not the details of the starting sequence.
Because the gas start-up would flow pressurized gas in the LOX channels, the gas would
have to be shut off before starting the LOX flow. The disk could spin down before the
heat transfer from the chamber had a chance to power the disk back up. The motor can
continue to run and keep the disk at speed through the start-up transients. If the disk spun
down, it could be due to a long start-up transient or insufficient steady-state heat flux.
Given our modeling inaccuracies, it would be impossible to tell which had happened, and
the solutions for the two situations are very different.
The motor on the other hand, can run the disk at a steady-state operating speed all
the way through the start-up transient. Additionally, the electric power used to keep the
disk at that speed gives a measurement of the power contribution from the LOX jets, and
indirectly, of the heat transfer into the disk.
In this respect, the motor can be used as a dynamometer to measure several things
inside the engine while it is running. These include: LOX and kerosene pumping, oxygen
36
flow transition from gas to liquid (liquid takes more power to pump at a given speed that
gas or mixed phase), windage and bearing losses, and contribution of the oxygen jets to
the disk's speed. Mechanical problems with the bearings and seals also show up in the
motor power data.
The motor is capable of electrically braking the disk. This is used to do fast
shutdowns, and to save run time on the engine since coast downs from high speed can
take longer than the test itself (see Figure 27: Test 1084 high-speed coast down without
electric braking). In the event that the heat transfer is greater than expected, the electric
breaking will be used to slow the disk and keep its speed from running away. A
successful test can be run in braking mode, producing more data than an emergency
shutdown would provide. This could also save the engine if the transients happen very
quickly, since an emergency shutdown takes about 0.5 seconds, as demonstrated in test
1041.
#1084 Amps
50000
--------
------------------
6000
500005000
40000
5000
4000
30000
30
300
Tach
-powerl
20000
2000
10000
1000
0
0
20
40
60
80
100
120
- -0
140
160
Seconds
Figure 27: Test 1084 high-speed coast down without electric braking
Once testing began, there were several other things for which the motor proved
useful. When new seals were being broken in, the start-up torque of the engine was
greater than that that could have been supplied by a gas start-up system. The speed of the
motor can be controlled, so running the engine at low speeds for extended periods to
break in parts, or to conduct low speed tests is easy to do. It has turned out that one of
the lesser-understood start-up transients is the length of time between starting the flow of
LOX, and getting good, single-phase liquid flow. This takes about 3 seconds, depending
on the cool down sequence, and the motor power data has been invaluable in detecting
the onset of liquid flow, and measuring this time transient. The motor keeps the disk
running during this time, because it would slow down too far in this amount of time with
a gas start-up, if the gas start-up were shut off before starting LOX flow. I hope that we
can find a way to overcome this shortcoming of the gas start-up at a later date, but the
motor was valuable in identifying and quantifying these issues.
37
Advantages of gas startup
The gas start-up idea was actually used in the team's earliest experiments. These
experiments uncovered a huge number of problems across the entire motor design, and
instigated a much more carefully thought out system approach. This first full engine
prototype is a result of that reorganization. While the gas start-up system did not survive
because it left too many variables uncovered, it does have several significant and notable
advantages.
The gas start-up system is much simpler and lighter than a motor system. It does
not put any extra stresses on the engine other than those encountered while it is running
on its own. In the case of a launch vehicle, it would be much easier to implement a gas
start-up system than to use a large external motor that would either remain on the rocket,
or be disengaged and retracted. Both of these would be difficult to implement efficiently
in a small rocket vehicle.
Once the engine has been run and characterized, many of the variables that were
in question when the two start-up systems were evaluated will be known experimentally.
The rocket engine can be optimized using the electric motor, and then a simple gas startup system can be designed around the known operating parameters. The motor could
simulate the gas start-up, using computer control, and the motor power curves would
verify that it would work before it is implemented.
Description of motor startup system
Motor/controller
The motor that I chose to use was a Solectria BRLS8 with a Solectria controller".
Solectria is a local electric car manufacturer that I worked for before coming to MIT.
The motor is on loan from Olaf Bleck, and the controller is on loan from the MIT Solar
Car Team. It is an air-cooled brushless motor, powered by a DC battery pack through the
Solectria controller that acts as a power inverter. This motor was designed to be used as a
solar car drive system, and has proven to be compact, reliable, and efficient. Its peak
power output is 6kw (8hp) and its top speed is 6600rpm. The controller typically drives
the motor based on two potentiometer inputs. One comes from the solar car's
accelerator, and the other from the brake. Byron Stanley built a torque/speed controller
that would convert the engine's desired speed, determined by our computer operating
sequence, into the torque inputs required by the Solectria controller. A speed input will
bring the engine to that speed, and hold it there, using the motor's regenerative braking if
necessary. Regenerative braking uses the motor as a generator, and puts the power back
in the battery. This motor can provide 2-3 kW of active braking power and more in autoregen mode, which occurs if the motor is run faster than its top speed.
Battery system
The battery pack chosen was a modified 96volt DC lead acid pack from a
Solectria E-10 electric pickup truck". When I worked for Solectria, I designed the
aluminum front battery box of this vehicle to hold 8 Hawker Genesis starved electrolyte
" Solectria Electric Vehicle Components Catalogue, 1994, pg. 6-7
" Solectria sales brochure, 1996, E-10 Electric Fleet Pickup
38
batteries 6 . Wired in series, these batteries put out 96 volts, 38 Ah (rated as new, this
particular pack is probably around 1/2 capacity, as the batteries are about 5 years old),
and more amps than can be used by this motor and drive system. The batteries were
cycled and tested using a couple of old Solectria headlight discharge banks, and they put
out 80-100 amps continuously without a great voltage drop. There is always some voltage
drop in a battery when a current is drawn, and this relationship can be approximated as
linear. The slope of the line is known as the dV/dI constant for a particular battery.
dV/dI refers to the change in voltage for a given change in current. The dV/dI curve for
this battery is shown in Figure 28. There is hysterisis in this curve, because the voltage
of the battery is slow to react to the changes in current, so there is a lag. It can be seen in
the graph that as regenerative braking is applied, and the current drops to -8 amps, it
takes a little while for the voltage to rise back up to 98 volts. In cases where the voltage
was not available during a test, due to voltage sensor problems, I used this curve to
estimate the voltage, based on the current, to get the power used from the battery. This
voltage - current relationship is also one of the most reliable ways to estimate the
battery's state of charge.
dl/dV 1026
t4
92
0
-10
0
10
20
30
40
50
60
amps
Figure 28: Test 1026, dV/dI behavior of the battery pack
Instrumentation/measurement (Voltage, current, speed)
The motor/controller system is instrumented to give DC battery pack voltage, DC
battery current, using a clamp style ammeter, and motor speed. The motor speed sensor
uses a magnetic pickup that detects 3 bolts on the motor's gear hub, producing 3 pulses
per motor revolution. The engine is going six times faster because of the 6:1 gear ratio.
Our tests at the beginning of the project had used an optical tachometer, and there were
problems related to frost from the LN2 covering the optical markings. The magnetic
6 Hawker
Energy Products, January 2001, http://www.hepi.congenesis.htm
39
pickup on the other hand has proven to be our most reliable sensor with the cleanest
signal.
It should be noted that the power readings, which are the voltage and amperage
measurements multiplied together, are the input into the motor controller. The motor
efficiency is not taken into account, so only 92-94% of the power measured actually gets
converted to shaft power to drive the engine. The variation is due to the torque and speed
operating point of the motor. These DC measurements are used because it is difficult to
measure the variable frequency AC power that drives the motor, or to get torque readings
off of the shaft. This motor is specifically designed for high efficiency, which is what
keeps the efficiency numbers so high. (See Figure 29: Solectria BRLS8 efficiency
curves)
SLR)U rp
6000 RPM
90
T
4000 R PM
85-
4,000
2000 R PM
90
2,000N80
00
Figure 29: Solectria BRLS8 efficiency curves 7
Gear transmission
Because the engine needs to run at 36,000 rpm, and the electric motor only turns
at 6,600 rpm, a speed increaser was required. Vendors were not happy discussing such
high speeds, so choosing a system was not easy. Chain and belt drives were ruled out
because their surface speeds were too high. The best choice seemed to be a gear drive
with a single stage 6:1 ratio. One company quoted high speed filled nylon gears with
herringbone pattern teeth, which would have been very nice, with low noise and
vibration. The cost was very high though. I settled on a set of steel spur gears. The
smaller gear had to fit over the kerosene tube, and 1.56" pitch diameter was the smallest
standard size gear that would fit. In Figure 30, the small gear can be seen on the kerosene
17 Solectria Electric Vehicle Components Catalogue, 1994, pg. 7
40
shaft, with the large gear that will go on the motor visible in the background. I did a
strength calculation that showed that there would be enough metal left around the shaft
after the teeth were cut to prevent the small gear from flying apart at these high speeds.
Figure 30: Gears
A 6:1 ratio made the larger gear on the motor 9.375" in diameter. An oil mist
system for cooling the gears was too complex and messy so I decided to run them
unlubricated. At high speed though, the force on the gear teeth was only expected to be
130N, which is a very light load. The gear face thickness was chosen to be 0.5", based
on the interface between the small gear and the kerosene tube on the engine. The tube is
thin-walled aluminum, only 0.10" thick, so securing the gear using a key or set screw was
ruled out. A micro spline could have been used, but we were running into other
machining difficulties at that time, and did not want to add the cost and complexity of a
spline. I decided to bond the gear to the smooth shaft using a Locktite cyanoacrylite
bonding compound, designed for such cylindrical bonding applications, and rated to
3,000 psi in shear. A 0.5" thick gear would provide enough surface area to allow the full
torque of the motor to be transmitted without breaking the gear loose on the shaft. Given
that our running time was on the order of minutes, the gear manufacturer thought this
design would hold up, even without lubrication.
The only problem that we have run into with the gears is that the thermal cycling
on the engine is not yet well enough controlled. There are frequently instances where we
have overcooled the engine during flow tests that did not include kerosene flow or
ignition. Either kerosene flow or ignition would keep the kerosene feed-tube warm, but
in their absence, the aluminum tube can get cold enough to shrink away from the steel
gear and break the adhesive bond between the two. The aluminum can shrink up to
0.005" if it cools all the way down to LN2 or LOX temperatures. The bond gap is only
0.0005", so the adhesive would have to stretch by 1000%. This is unrealistic for an
adhesive, and cyanoacrylites are not flexible to start with. Under these conditions, the
41
bond broke. The gear was held in place in subsequent tests by the remains of the glue
jamming between the gear and shaft. A telltale mark was used to make sure the gear
hadn't spun during each test. The next design iteration will use a very low profile key to
keep the gear from spinning and have a shoulder for the gear so that it does not slip up or
down on the shaft.
Mounting
The motor with its large gear was attached to an aluminum mounting plate that
was designed to hold the gears at the right spacing. After some rework, this was
achieved, and the gears have a small but manageable amount of backlash. The motor is
positioned so that it is above the engine (see Figure 31: Motor mount). While this made
the mounting plate more complex, it keeps an expensive, borrowed piece of equipment
out of the way of the rocket exhaust.
Electric motor
Engine axis of rotation Top of engine
Motor mount
6:1 gear increaser
Copper combustion chamber
Figure 31: Motor mount
Preliminary cold engine testing
Break in of rotating parts
When the engine was assembled for the first time, the act of compressing the oring between the engine and the combustion chamber damaged the engine. I found that
the combustion chamber was out of round by 0.0 13 inches. The chamber was probably
crushed when it was held in a vise to drill the temperature sensor holes. The 2' taper at
the top of the chamber caused metal to metal contact across the o-ring gland and crushed
the corresponding part of the engine when it was sealed in place with six 1/2" bolts, not a
very delicate operation. The distortion in the engine caused the chamber separator to
crash and gall.
After the offending and damaged parts had been cleaned up and fixed, it crashed
again. This time because the bearings had also been damaged, but not replaced. At this
point, the non-contact seals were replaced with graphite inserts to prevent aluminum on
42
aluminum galling, and new bearings were installed. The graphite seals were very tight,
and the motor was used to run the engine until they had worn away enough that they were
no longer contacting. The labyrinth seal on top of the LOX feed-line was also rubbing
because the external LOX plumbing was pulling it to the side. After the first successful
hot fire test, the Teflon holder for the labyrinth seal was damaged and the labyrinth was
replaced with a better-supported design that did not rub as much. From the motor power
curves it was clear that the engine took less and less power to spin with the motor as the
seals wore in.
Power curve
One of the functions I came up with for the electric motor was the taking of data
on the power balance of the engine as it runs. While the motor is functioning as a
dynamometer in this fashion, it is possible to tell how much power is going into pumping
kerosene and LOX, and how much power is being generated by the oxygen jets on the
spinning disk. (Actually, pumping and jets are combined into one net measurement).
The final goal being to provide the data necessary to verify the heat transfer model for the
cooled rotating disk inside the hot combustion chamber. Both Carl Dietrich and I did the
calculations to develop this model, but as I got to the point where the equations became
algebraically complex and the reasoning became circular, Carl had already developed a
numerical solving routine on the computer. Not wanting to duplicate efforts, I took on a
role of developing other parts of the engine and error checking his model instead of
finishing mine.
The following factors make up the power balance of the disk (followed by a + or if it will tend to speed the disk up, or slow it down): Motor power (+/-); windage (-);
bearing friction (-); seal rubbing friction (-); Oxygen jets (+); LOX pumping (-); and
kerosene pumping (-). Each of these factors will be discussed below. After the
explanations, I will show how the power curves are put together and used.
Motor power
The power going into the motor is measured in DC volts and DC Amps between
the battery and the motor controller (inverter). The motor electrical power is the product
of these two measurements. The shaft output is the electrical input power times the
motor's efficiency iq. r varies with speed and load, and can be figured out from the motor
efficiency curves if the operating point of the motor is known. Since this is a DC
brushless motor, it is typically very efficient. According to Solectria's published curves
for this motor (Figure 29), the efficiency should fall between 92% and 94%. Windage and
bearing losses within the motor are accounted for automatically because the motor is
always spinning, even in the calibration runs. The motor's electrical efficiency is taken
into account by a multiplier after the data have been recorded. The gear inefficiencies
would also be included in this efficiency number. A test of the motor, with the large gear,
but not driving the engine showed that motor drag losses are around 200 Watts at 6,000
rpm which is the speed of the motor when the engine is running at 36,000rpm. This is
negligible compared to the multi-kilowatt loads induced by pumping the propellants.
43
Windage, bearing friction and seal rubbing friction
These three elements are difficult to separate and will be looked at together. A
test was run with the electric motor spinning the engine disk at various speeds with no
propellant present. The results were plotted on a graph of Speed vs. Power (see Figure
32: tests 1029 and 1030 power curve). The equation for the curve fitted line is y = 3x10'0
x 2.83 . Note that this curve includes the motor's own bearing and windage losses as well.
Since this is very close to a 3 rd power curve, it seems like windage is dominating. Other
factors such as bearing and seal friction would be lower order functions of speed.
0 dry spin test 1029-1030
power points
-
5000
-
-
-
-
-
Power (dry spin test 1029-1030)
- ---
4000
3000
2000
1000
0
0
10000
20000
rpm
30000
40000
Figure 32: tests 1029 and 1030 power curve
Earlier tests took more power, and never reached a top speed of over 32,000 rpm.
This is because the labyrinth seal was rubbing and wearing badly. Once the seals wore in
more and stopped rubbing, the power consumption dropped, and became dominated by
windage. Also, tests done in late August and early September are suspect because of a
low battery in the ammeter. The power data presented in the next three sections are
assembled from data that were not affected by the rubbing seal or the low battery.
Oxygen jets
The oxygen jets at the edge of the disk should put out enough thrust during a hot fire test
to balance the pumping and other loads on the disk. They are powered by the heating of
LOX in the heat exchange section of the bottom portion of the disk. Thermal energy
from the chamber vaporizes the LOX, and heats it. When it escapes through the jets it
produces tangential thrust and keeps the disk spinning".
While assembling the engine, I noticed that water running through the jets tended
to cant out from the tangent line by 18'. Assuming that gaseous or mixed phase oxygen
'8 The relation between the pressure of the jets and the heat transfer mentioned here is modeled and
described in Carl Dietrich's paper "An Analysis of Heat Transfer in a Centrifugal Direct Injection Engine".
This model is what will be verified during the extended burn engine tests in the spring of 2001.
44
behaves the same way as liquid water, this will reduce the jet's tangential thrust
component by cos(18') or 5%.
LOX pumping
Initially, the LOX pump was tested with water, to see if pumping could be
observed. After that series of tests was successfully completed, LN 2 was used to simulate
LOX so we could practice pumping cryogenic fluid, without the safety problems
associated with LOX. The following sample data show that we can detect pumping with
a variety of measurements.
In test 1091, the disk is spun up to 38,000 rpm (see Figure 33). Once it gets there,
the LOX valve is opened and LOX is allowed to flow into the engine (time index 279).
First the LOX has to cool down the inside of the disk, and then the disk begins to pump
LOX into the engine. Around time index 282, the data show a sudden drop in the disk's
speed. This is accompanied by an increase and smoothing of the LOX flow (Figure 33),
a rise in motor amps (Figure 34), and a rise in the chamber pressure because more LOX is
being pumped into it (Figure 35).
A note about Figure 33: at time index 284.5, the LOX flow appears to increase
suddenly. This is a measurement artifact of the shutdown process. The LOX flow is
measured by looking at the differential pressure between the gas at the top of the LOX
tank, and the LOX flowing through the LOX supply line, and deriving the flow velocity.
The main LOX shutoff valve is between these two points. During the shutdown
sequence, this valve is closed. The down stream LOX pressure vents instantly through
the engine and a dump line. The pressure in the tank stays constant. The result is that the
differential pressure sensor sees a condition that looks like high flow velocity. All LOX
flow data after a normal shutdown should be ignored.
#1091 LOX Flow
40000
40
30000
-
-
30
--
E
20000
20
L-Tach
-- LOX Flow
CL
S'
10000
1
10
0
0
0
2 '0
-10000
2 -2
-----------
2-F4
2T'6
-'18
--
280
-----------
282
284
286
2! 0
2 38
--
-10
Seconds
Figure 33: LOX Flow Test, LOX Flow vs. Time
45
A note about Figure 34: The battery amps have a very sharp peak as the motor
speeds up. This is a characteristic of a brushless permanent magnet motor's torque speed
curve. On the left side of the peak, the motor is actually delivering constant torque.
When the back EMF of the motor equals that of the battery pack, as it does here at 27,000
rpm, the motor can no longer deliver full torque at higher and higher speeds, so the
battery current drops. Changing the gear ratio, between the motor and the engine, would
allow us to move the engine speed up or down at this peak power point.
#1091 Amps
40000
60
30000
40
20000
- 20
(C
E
0
10000
Tach
BatteryAmps
-
0
-20
2 '0
272
274
276
278
280
282
284
286
288
2
-10000
-40
seconds
Figure 34: LOX Pumping Test, Amps vs. Time
#1091 Chamber Pressure
--- ,----,----,----,---
40000 -
200
30000
150
20000
100
0'
10000
50
-10000 '
E
0
0
2 '0
-- Tach
ChamberPressure
2
J--
2 76
2
1-
2
78
2 10
2
2 4
2 36
2 38
2 10
-50
'-
Seconds
Figure 35: LOX Pumping test, Chamber Pressure vs. Time
46
Kerosene pumping
It was not possible to test the engine pumping kerosene in the lab, because the
expected kerosene spillage would be too messy. Water could not be used as a substitute,
because the bearings are exposed to the fuel vent pathways, and rusting the bearings was
undesirable. Ethanol was chosen as a substitute, with roughly similar density and
viscosity as kerosene. Figure 36 shows an early ethanol-pumping test. The drop in disk
speed and rise in motor amps around 96 seconds shows that pumping has begun. I found
out much later that we were probably losing a lot of power to rubbing in the seals, which
casts much of the power data from this early testing in doubt.
1002 pump test
40000
-
-
-
-
-
- -
-
-
-
-
-
80
-
60
30000
.
20000
20
E
-7:2 1:6 PM Tach
7:21:16 PM Battery Amps
0
10000
-20
0
90
---- ---- --------92
94
96
98
100
seconds
40
102
104
106
108
Figure 36: Test 1002, battery amps during ethanol pumping
The only time the engine was used to pump kerosene alone, was a series of hot
fire attempts in which we tried to start the flow of kerosene before the LOX. The
problem with these tests was that the combustion chamber was not draining fast enough,
filled to the top with kerosene, and the disk became immersed. The excess drag slowed it
down and shut down the startup sequence before engine ignition. Some of the data from
these tests are presented later.
Power curve analysis
My first attempts at power curves consisted of running the engine at several
different speeds and recording the power required to run at steady state at each speed.
These points were then graphed and connected with a smooth curve (Figure 32).
47
The next idea that I had, was to do a coast down test of the engine and get the
entire power curve from that. The engine was run up to full speed with the motor, and
then the motor was shut off and the engine coasted back down to a stop. At each point in
a coast down test, the speed is known, and the rate of deceleration can be calculated from
the data. If the moment of inertia were known, I could figure out how much power it
would take to change the speed of the disk at that rate at each speed. That power is equal
to the amount of power it takes to run the engine at each speed without pumping.
On a spreadsheet, these calculations can be done for every data point, or 50 times
per second. By running the motor at steady state at high speed, just before starting the
coast down, the first point can be determined, and the moment of inertia can be solved
for. The equation for the power dissipated by the engine is P = I o 0' (power = moment
of inertia x angular velocity x angular acceleration). Figure 27 shows the coast down test
performed in test 1084. The engine was brought up to 38,000 RPM, the amps were
allowed to level off to get a steady state reading, and then the motor was shut off for the
coast down.
The averaged steady state power at 37,950 rpm was 1686 watts, and this made the
power curve solvable. The results are plotted in Figure 37: 1084 Coast down power
curve, plotted as power vs. speed. Sometimes, in other tests, this curve seems to be a bit
off. This is because the labyrinth seal in the top of the engine rubs different amounts
based on how much tension is in the LOX supply piping. This can change with handling
as well as changes in length from cooling or heating. Often in a test, the engine is run up
to full speed and held there until the test starts. Sometimes a power point can be obtained
from this brief steady state operation. In tests where this looks like a problem, I have
linearly scaled the entire power curve to match at the point obtained during the test in
question. Hopefully this approximation will not be necessary in the next engine
prototype, when I intend to reinforce the labyrinth seal so it is not as flexible and will not
be able to rub at all.
power curve (test 1084)
3000
2500
2000
1500
1000
500
0
0
10000
20000
30000
40000
RPM
Figure 37: 1084 Coast down power curve
48
Figure 38 shows how two coast down tests compare to the power curve generated
from discrete points. The heavy lines at the bottom are these three power curves. Next,
there are eight pumping tests. Water was pumped through the LOX channels in two tests,
LN 2 was pumped through the LOX channels four times, and kerosene data was picked
out of two different hot fire attempts that did not ignite, but flowed kerosene. The data
points are each discrete steady state operating points where power and speed data were
obtained. The curves crossing through these points are the predicted power that it should
take to pump each fluid at the design mass flow, added to the power from the dry spin
tests. The pumping equation is Power = M' o2 R2 , where M' is the mass flow, 0o is the
rotational speed of the disk, and R is the radius that the fluid is pumped to. Note that
since m' is constant, the water-pumping curve is higher than the LN 2 pumping curve
because water is 20% more dense than LN. For reference, LOX is 10% more dense than
water.
It is assumed that for the LN2 predicted pump power curve, liquid is pumped all
the way out to the edge of the disk without vaporizing. This can either happen if the
pump channels are full, and the bottom disk only contains gas, or if the entire bottom disk
is full and liquid is coming out of the LOX jets. If the LN 2 is not making it all the way to
the outside of the disk, then that could explain why the two low speed points are below
the curve. This seems reasonable, because the heat capacity of the disk might be enough
to vaporize some of the LN2 at these lower pumping rates. The correlation between the
predicted and measured value shows that the electrical input to the motor can be used to
measure what is happening inside the engine. This will be very useful during hot fire
tests when it is not so clear what is going on inside the engine.
49
Pumping curves
7000
S
6000
S
I
5000
A
z
0
4000
---1061
-1084 coast down
predicted kerosene
predicted LN2
predicted H20
/V
w
(~3
3000
kero pump data
LN2 Pump data
water pump data
dry spin test 1029-10
2000
1000
0
0
10000
20000
rpm
30000
40000
Figure 38: Pump curves
Next, I will show how this kind of power analysis can be used during a nonsituation to confirm what is happening inside the engine. Test 1091 was a
state
steady
LOX pumping test. Figure 39 shows the motor power data along with the tachometer
readings from this test. The LOX valve was turned on at 279 seconds, and full flow
developed at 282 seconds. Figure 40 shows the expected power dissipation during this
test. The expected power curve shows how much power it takes to spin the disk,
calculated at each speed from the power curve in Figure 37. To this, I added the power
expected to go into LOX pumping from 282 seconds until the LOX shut off at 284.4
seconds.
50
#1091 Amps
4uuuu
60
30000
40
20000
20
UP
a-
aE
10000
0
-Tach
Amps
-Battery
0
2 0
-10000
------------
----------
272
274
276
278
280
---
282
284
286
288
-20
2! 0
40
---
seconds
Figure 39: Motor Amps and speed during LOX pumping test 1091
-Tach
-power
1091 power curves
40000
curve data
2000
-
0
-2000
30000
-4000
7' 20000
-6000
co
-
-8000
)
-10000
10000
-12000
-14000
0
270
-- 4-
-T
I
275
280
285
-16000
290
seconds
Figure 40: 1091 Expected power dissipation during spinning and pumping
51
The next graph, Figure 41 compares this to the power that was measured going in
and out of the system. The motor puts in electrical power. To this is added or subtracted
any power that is released or stored as kinetic energy in the disk (P = I o o'). The
resulting power is the amount that is dissipated by drag, pumping or other forces. During
a hot fire test, this should be able to detect power being put into the system from the
oxygen jets. If the jets produce enough power to run the engine, that will validate the
engine concept. As can be seen in Figure 41: Power measurements vs. predicted values,
there is a correlation between the expected and measured results during the LOX
pumping test. Note the jump in power when the LOX valve is turned on at 279 seconds.
This is the start of mixed phase pumping through the engine. Then the power dissipated
goes up to -14 kW, which is close to the expected value with LOX pumping.
The positive peak in the data around 275 seconds is probably due to the gear's
efficiency curve. Gears have high peak efficiency (up to 98%), but are not as good at
high speed and low torque. This peak corresponds with a medium speed and the highest
current drawn by the motor during the acceleration, which would logically be a high
efficiency region. The peaks after shutdown at 285 seconds show up in other tests, and
are probably an artifact of a data glitch produced when valves open or close. The
chamber pressure rose to 120 psi (8atm) during this test, so increased windage, at low
temperatures because the gas in the chamber was cold 02, and bearing loads could
explain why the actual values are more negative than the predicted values.
1091 power curves
40000-
2000
-
0
-2000
30000-
-4000
-6000
c
4-)
C- 20000-8000
5
-10000
10000
-
-12000
-1 4000
S-0
270
-16000
-
275
280
285
290
seconds
Figure 41: Power measurements vs. predicted values
52
When this type of analysis was applied to hot fire test 1041, (Figure 42: Test 1041
power dissipation) it can be seen that there is a correlation between the expected power
used during spin up and spin down. During the part of the test where the engine ignited,
there is a power drain of 20 kW. This is the time when there was LOX pumping and
kerosene pumping, however the oxygen jets should have offset this. Also, the chamber
pressure and temperature rose, increasing the density by up to a factor of 3, which causes
increased windage losses and bearing loads, which would be seen here. Because the
engine was shut down too early, there are not enough data here to see how everything
was going to stabilize.
1041 power usage
-
Tach
-expected
power curve
10 per. Mov. Avg. (measured power curve)
.0
40 000
-2000
35000
-4000
30000
-6000
2 500 0-
-8000
000
20000
0
-10000
15000
-14000
10000
-16000
5000
0
771
-18000
- --
773
--775
-
-
-20 00
0
777
Seconds
Figure 42: Test 1041 power dissipation
In test 1098, a power analysis shows two events that put huge loads on the engine,
as it was destroyed (see Figure 43). Note that the scale is much larger than before so that
these anomalies can be seen. The second of these, which lasted for a fraction of a second,
drew power out of the spinning disk (off the bottom of the graph) at 67 kW. Since this
happened at the time when the engine is known to have stopped, it is thought that a loose
part got jammed between the remains of the spinning disk and the combustion chamber.
The power was dissipated by shearing off some of the steel gear teeth.
53
40000
30000
20000
FRPM
10000
-
-power
curve data
-Tech
--- 3-----7
---
power used
4 per. Mov. Avg. (power used)
-10000
-20000
-30000
-40000
seconds
Figure 43: Test 1098 power dissipated as engine self-destructs
Pre cool sequence
In order for the engine to fire, the propellants must be able to flow freely through
the engine's passages. The LOX feed tubes and pump must be pre-cooled to prevent the
LOX from boiling and vapor locking. Also, the kerosene passages must be warm enough
that the kerosene does not freeze when it starts to flow. It has been difficult to properly
pre-cool this center section without overcooling the rest of the engine. If the kerosene
section gets cold, the kerosene can freeze as soon as it starts to flow. If the bearings and
gear get cold, they experience mechanical problems because of differential expansion
between themselves and the shaft or bearing holder. Fortunately, there have been no
freezing problems in the engine to date.
The first part of the LOX section is vacuum insulated, and so does not tend to
cool the rest of the engine very quickly. (It was discovered later that the vacuum may
have been broken, but the resulting air gap provided some thermal protection in the
vacuum's place.) However, as soon as the LOX gets past the pump section, the heat
exchange section is thick walled aluminum, and is directly connected, mechanically, and
thermally, to the kerosene feed section (see Figure 44). The kerosene section is, in turn,
thermally connected to the bearings and the gear, which must stay warm.
54
Bearings
Gear
Insulation
Thermal conduction
to bearings and gear
LOX Flow
Aluminum
Figure 44: Thermal Conduction Paths
This part of the engine has been designed to work under the steady state
conditions that exist while the engine is running. The LOX section is well insulated to
stay cold, and the kerosene section stays warm because of this insulation and the heat
from the kerosene flow and the combustion chamber. During the startup sequence, which
I will discuss later, it becomes necessary to cool the center section without overcooling
the rest of the engine.
I performed a test to find out how long the LOX section could be cooled without
overcooling the rest of the engine. The rotating part of the engine, the disk, was removed
from the bearings so that it could be monitored with a thermocouple. LN 2 was flowed
through the disk, while the temperature at the kerosene injector port was monitored. I
chose this point because it is the narrowest passage that the kerosene will have to flow
through, and is also closest to the LOX section as far as heat flow is concerned. The test
showed that we could flow LN2 through the engine for 160 seconds before the kerosene
injectors reached -40C, which is the freezing point of kerosene. (The heat capacities of
LN 2 and LOX are within 10% of each other.)
However, it must be noted that this test was performed with the LN 2 tank
pressurized to 35psi. Also the disk was stationary for this test. If it were rotating, and the
flow rate were increased due to pumping, this cool-down time would be proportionally
smaller. For the hot fire tests, the LOX tank was set to 100 psi, which would reduce the
cooling-time by a factor of three, just based on the feed pressure.
The object is to cool the LOX feed-lines, and the center, vacuum insulated section
of the engine, without cooling down the rest of the engine. The first part of the problem
involved getting a good flow of LOX to the engine. Good flow was achieved by precooling the supply lines as shown in Figure 11. The next problem was cooling the
insulated LOX feed tube preferentially, without overcooling the kerosene injectors. In
some of the less favorable tests, it took over 10 or 15 seconds to start pumping LOX, and
I did not want to run the engine at high speeds for long periods for fear of damaging the
55
bearings or seals. At one point, I realized that a leaky engine valve had been fixed, and
there was no longer LOX dripping into the engine during the supply line pre-cool cycle.
A low flow of LOX into the center of the engine would boil away on the inside of the
vacuum insulated tube, and since the engine was not rotating, would not be flung out into
the aluminum section which is thermally connected to the kerosene pump. This is a way
to preferentially cool the inside of the engine without cooling the outer parts. I devised a
procedure through which we would open the engine LOX valve for a second and then
close it for 5-10 seconds so the LOX had time to boil off in the insulated LOX section.
This was repeated several times immediately before starting the engine. This procedure
appeared to solve the problem the day it was implemented, but a larger problem may
have been our failure to pre-chill the LOX in the tank before starting the engine. This
will be explained further in the section about Test 1067, October 15, 2000. The pre-chill
sequence described there replaced the procedure of burping LOX into the feed tube
before the tests with pre-chilling the LOX tank.
Startup sequence
Startup requirements
In order for the engine to burn, there must be oxidizer, fuel, and an ignition source
present in the combustion chamber. The fuel and oxidizer must be injected at a pressure
higher than chamber pressure to prevent back-flow into the feed-lines.
To pressurize the oxidizer, the engine must be spinning, in order to run the LOX
pump, and the LOX flow passages must be cool enough to let LOX flow through without
boiling. If boiling occurs, the bubbles can block the flow passage and stop the flow until
they have risen out of the way. This is a slow and unsteady process, and must be
avoided.
To pressurize the kerosene the pump has to be spinning, and the kerosene pump
cannot be so cold as to freeze the kerosene in it. Pre-cooling the LOX pump without
overcooling the kerosene pump has been difficult.
An ignition source must be present inside the chamber. We started with a spark
igniter, and later replaced it with a pyrotechnic igniter.
Sequence history
This section goes through the LOX, kerosene, igniter and spin up stages of the
startup sequence in more detail and talks about the history of their development.
LOX flow
Initially, both the kerosene and the LOX were going to be gravity fed to the
engine. Since the LOX line was not perfectly insulated, bubbles forming in the line kept
good flow from ever reaching the end of the supply line. Head heights up to 5 meters
were tried but were unable to push through the chugging flow. The LOX tank was
replaced with a pressurized vessel. Since the pressure could be arbitrarily set, a long feed
line was not needed, reducing heat flux into the fluid, and the pressure could be increased
to push through any remaining bubbles. A pressure of 100psi was found to provide the
right flow rate for water (which has a similar viscosity as LOX) to the end of the feed56
line. The amount of water that flowed out of the feed-line in a measured time matched
the design rate of 0.25liters/sec. (The plumbing details had changed and smaller orifices
were used, so 100psi cannot be compared to the head height from the gravity fed system.)
In preparation for the first hot fire test it was noted that the engine was not cooling
down fast enough to allow LOX pumping to establish itself before the engine shut down.
I came up with the idea of "burping" LOX into the engine for one second and then
waiting 5 seconds before starting to rotate the engine. The reasoning is that the vacuum
jacket insulates center of the engine very well. If it could be filled with LOX, without
flowing much through the rest of the engine, the center section could be preferentially
cooled, which is exactly what was needed. The 5 second pause gives the LOX time to
boil off, and cool the engine. By doing this with the engine not turning, the tendency for
the LOX to get flung to the ends of the pumping tubes is eliminated, and LOX flow past
the pumping section is minimized.
During the first hot fire attempt, LOX pumping was established. We used my
idea to detect pumping using the motor amperage data. When the motor current
increased past 50 Amps, indicating that more power was being used at that speed than
could be justified by spinning the disk alone, pumping was assumed to have started.
However, the chamber pressure rose to several psi because of the quantity of oxygen
being injected into the chamber. When the kerosene was turned on, its head height was
insufficient to overcome the chamber pressure, and no kerosene flowed. Additionally,
GOX may have flowed up the kerosene line, which presents a fire hazard. In later tests, a
check valve was used in the kerosene line to prevent back flow under these conditions.
Kerosene flow
The next step was to get kerosene flowing into the chamber. This had to be done
before the LOX was turned on because if the kerosene pump was not primed before then,
the chamber pressure from the oxygen flow would prevent the kerosene from flowing.
The results from the next test were very puzzling (see Figure 45: 1026 Power increase as
the chamber floods). The engine spun up to speed, the kerosene started flowing, the LOX
started flowing, and then the engine bogged down and stopped. After further testing I
realized that the engine was flooding with kerosene. When the liquid level reached the
bottom of the disk, the drag forces went up, and pretty much stopped the disk, even
though the motor applied full power to try to keep it moving. The LOX takes about 3
seconds to cool down the engine enough to flow effectively, and there was just not
enough time before the engine flooded.
57
1026 Kerosene pumping
35000
-
-
-
-
-
-
--
-
-
- -
-
teryAmps
-
-
-
70
------
-
30000
60
25000
50
20000
40
-
15000
30 E
10000
20
5000
10
0
804
0
-5000 -----------
8 0
88
806
-
-
-
-
8,4
8 2
-
-
-
-
8 8
8,6
-
-
-
-
82
80
-
--
-10
Seconds
Figure 45: 1026 Power increase as the chamber floods
The real danger here is that of a hard start. With the possibility of so much
propellant in the chamber, and the throat filled with liquid kerosene, a detonation could
over-pressurize the feed-lines, or worse, the chamber walls. The only option left was to
pressurize the kerosene system, and start the LOX flow first. The LOX did not
accumulate in the chamber as the kerosene did, because it would boil immediately on
contact with the copper chamber walls, and the gaseous oxygen could escape through the
throat more easily than the liquid kerosene. The decision was made to run the LOX until
good flow was established and then start a pressurized kerosene flow.
I found that the chamber pressure is around 10psi while gaseous oxygen is
flowing (see Figure 46) from 860 seconds through 863 seconds, and jumps to a higher
value when liquid flow starts. Once LOX starts flowing in a smooth liquid flow, the
pressure jumps up to 120 psi, but this was not clear at the time. Other tests indicated that
this jump in pressure was not as great, or was caused by electronic noise in the sensors.
The jump to 120 psi was verified at a later date. I chose to initiate the kerosene and turn
on the igniter when the chamber pressure sensor detected a rise of 14 psi, indicating that
LOX flow had been established. The LOX flow reading is not used because chugging
flow can appear as full flow for short periods of time as opposed to low flow (see Figure
13: Test 1041 LOX Flow). It is difficult for the computer, or a person, to recognize full
flow without the benefit of hindsight.
58
-
Test 1025 chamber pressure
Tach
ChamberPressure
40000
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
140
120
30000
-
-
100
80
20000
60
---
40
10000
-20
0
0
855
856
857
858
859
860
861
862
863
864
-20
865
Seconds
Figure 46: Test 1025 Chamber pressure with GOX flowing
Igniter
Until this point, we had been using a continuous spark igniter. I had thought of
that because of my experience with glow plug engines. A high voltage source provided
an arc between two wires, which were inserted up the nozzle. However, it became a
concern that the spark might not carry well in an environment with too much kerosene,
and I figured that we could use a small solid propellant rocket motor screwed into the
side of the chamber.
I devised a test to time how long it would take to light a solid propellant motor
using a resistance wire igniter. If an LED was wired in parallel to the igniter, a video
camera could be used to document the time between the LED illuminating (the igniter
getting power) and the motor igniting. Two tests showed that it was 5-6 frames, (56/ 3 0 thS of a second) delay, and it was repeatable. The flame from an Estes C6-0 motor
ranges from 6" in length during the core burn at the beginning, to 4" for the end burn
section, and lasts 1.8 seconds. Either one is enough to have a flame crossing the entire
chamber from one side to the other. A test of the igniter by itself in the combustion
chamber showed no noticeable pressure reading while it was firing.
Spin up
A new idea, to start the LOX flow while the engine was coming up to speed was
tried out after favorable results using this method with LN2. The purpose was to give the
engine longer to cool without spending a lot of excess time at high speeds, which puts
wear on the engine and cools the outside parts of the engine faster. Unfortunately, the
difference in densities between LN2 and LOX was enough to keep the engine from
reaching a high enough speed and the hot fire test using this method was aborted (see
Figure 47). It seems important to let the engine get to the highest speed possible, and
59
then light it quickly. This way, kinetic energy stored in the disk can help keep the engine
at high speed during startup. The motor only has enough power to drive the disk at
24,000rpm when LOX is being pumped, and this is too low of a speed to get steady state
operation.
Test 1040 tachometer
30000
25000
20000
15000
10000
5000
9 5
917
919
921
923
925
927
929
--Seconds
-5000
Figure 47: Test 1040 aborted because of under-speed
First hot fire test, startup sequence
Tests performed and lessons learned about startup in each test
This section will discuss the data obtained during the attempted hot fire tests.
These are the tests in which the computer was programmed to flow LOX and kerosene,
and turn on the igniter, although sometimes they were aborted before ignition. These
tests occurred during the last four out of seven separate trips to the New Hampshire test
facility. The first trip, in July of 2000, highlighted the importance of when the propellant
flows are turned on. The second trip, in September, featured a successful 0.5 second
engine burn. On the third trip, in October, the burn only lasted for 0.01 seconds, and in
November, the engine started successfully, but then exploded due to unnoticed damage
sustained just before the first July test. Tests of any type done on the engine were given
an identifying number. The sequence was started at 1000 to allow room for tests prior to
this engine prototype.
Test 1025, July 13, 2000
Test plan: The engine was spun up to 36,000 rpm using the electric motor. When
the speed reached 31,000 rpm, LOX flow was initiated. When the motor current spiked
above 50 Amps, indication that the engine had cooled down and LOX was being pumped,
the kerosene and spark igniter were turned on.
60
Results and conclusions: The LOX pumping into the chamber pressurized the
chamber to over 120 psi (see Figure 46). This is higher than the kerosene feed pressure,
since the kerosene was only being gravity fed from two meters up. The oxygen filled the
kerosene pumping passages and kept the kerosene from ever starting to be pumped.
Changes: Make sure kerosene has filled the pump channels before turning on the
LOX so the chamber pressure does not back-flow up the kerosene lines.
Test 1026, July 13, 2000
Test plan: This time, the kerosene was started first, followed by the LOX and the
igniter.
Results and conclusions: The kerosene flowing first made the combustion
chamber very fuel rich, which prevented ignition. Then after about 3 seconds, the
chamber actually filled with kerosene, and the disk, immersed in liquid, slowed down to a
steady state of 4000 rpm very quickly. This scenario was confirmed with a series of
kerosene pumping tests, which convinced us that the LOX should be started first to
prevent a detonation of accumulated propellants.
Changes: Replace the gravity feed kerosene system with a pressurized system,
and start the kerosene after the LOX. Also based on advice from a contact at the Air
Force Rocket Lab, we decided to replace our spark igniter with a pyrotechnic igniter' 9. I
worked out a way to install an Estes C6-0 solid model rocket motor in the side of the
combustion chamber, which was used in all subsequent hot fire tests.
Test 1040, September 3, 2000
Test plan: A step rise in the chamber pressure was observed at the time when the
LOX flow transitioned from cooling the engine to good pumping. We would cue off of
this step to start the kerosene flow and start the igniter. The pressurizable kerosene tank
was set to 28psi, which we (incorrectly) thought would be enough to overcome the
chamber pressure with fully developed LOX flow. The chamber pressure trigger point
was set at l4psi, higher than the pressure with just gaseous oxygen flowing, but lower
than that with LOX being pumped. Also, this time we started the LOX flow at 0 rpm so
that the engine could cool down as it sped up.
Results and conclusions: The LOX provided too much pumping load, and the
motor was not able to spin the engine up to the start speed. The test was aborted.
Changes: From this point on, we tried to spin the disk as fast as possible before
ignition so that some kinetic energy would be stored in the disk. Since pumping created
such a load on the motor, our hope was that the engine would start closer to the steady
state operating speed of 36,000 rpm if it started at a high speed.
'9
Pers comm, David Barland, Air Force Rocket Laboratory, June, 2000
61
Test 1041, September 3, 2000
For the first successful hot fire startup, I have put together an analysis of the
startup sequence used along with the sequence of events that actually happened. I based
this analysis on the data, videotapes, and post fire inspection of the engine:
" The LOX feed-line leading to the engine was cooled by flowing LOX
through it for 750 seconds. This time was chosen as a conservatively long
cool-down time based on previous tests, since the temp sensor on the line
was not working.
* LOX was burped into the engine for 1 second followed by a 5 second
pause, to cool the central core of the engine. This was repeated 7 times.
* The engine was spun up to 38,000 rpm and when it passed 31,000 the
LOX valve was turned on (see Figure 48: Test 1041 Chamber Pressure,
time index 774 seconds)
* When the chamber pressure reached 14 psi, full LOX flow was assumed
(although this was just some noise triggering the sensor early, time index
774.1 seconds)
* The igniter was started, and after a brief 0.2 second delay to ensure full
ignition, the kerosene flow was turned on. (There is a 0.2 second delay
between the kerosene valve being opened and good flow. This was tested
in a similar manner to the igniter time delay.)
* Since the LOX flow had not really developed as planned it took another
0.5 seconds for LOX to start pumping, at which point combustion in the
chamber began (775 seconds).
* 0.2 seconds after ignition began, the chamber pressure rose above 200 psi
triggering the computer to switch from start to run modes. At this point, a
programming error caused the computer to immediately switch to the
emergency shutdown sequence.
" Almost coincident with the shutdown commands, A plume with shock
diamonds becomes visible and lasts for the next 0.43 seconds (see Figure
49: Hot fire test 1041- plume with shock diamonds). The engine developed
about 120 lbs. of thrust. The aluminum heat exchanger on the bottom of
the disk melted at the points where it is not getting enough cooling from
inside the disk, namely on the outer corners, and in the areas where the heat
fins become tall and narrow towards the center. The melting is
symmetrical, but not as severe in the areas where the disk has features on
the inside and thus, more thermal capacity.
" Just as the plume is fading away, one of the two LOX dump lines, which
has been blown out of its water bath because it was not tied down, swings
around far enough to kink. The pressure in the LOX dump area of the
engine doubles, blowing off the labyrinth seal holder and soon after,
ruptures two of the kerosene dump lines. During this test, the gas separator
had been set to 100psi, instead of its usual 26 psi. This last minute change
was to try to fix a problem where frost was accumulating on the kerosene
dump lines. This led us to believe that the gas separator was allowing LOX
into the kerosene system and causing the frost. (I figured out during the
test 1098 postmortem, that the frosting was due to the LOX feed-tube
62
leaking directly into the kerosene pump. This crack, created by bending
the LOX feed tube in early July, is what caused the engine to explode in
test 1098) Coincident with the unintended shutdown three vent tubes burst
because the gas separator pressure was set too high. In the end, it was
fortunate that the engine shut down when it did.
#1041
350
-
30000
300
25000
250
t 20000
2005
-
--- ach
-2ChaberPressurel
15000
1
150
10000
-
100
5000 1
50
0
1
76 6 767
768 769 770 771 772 773
Seconds
774
775 776
1
1.0
777 778
Figure 48: Test 1041 Chamber Pressure
Figure 49: Hot fire test 1041- plume with shock diamonds
63
Changes: We replaced the labyrinth seal and its holder, and reduced the gas
separator seal back to 26 psi. The next test would try to burn for the full five seconds.
Also, the engine was reliably getting up past 39,000 rpm, so we decided to start the next
test at a higher speed.
Test 1067, October 15, 2000
Test plan: Spin the engine up to full speed with the motor. When it passes
38,500, turn on the LOX flow. I then figured that we could observe pumping by
watching for the disk to slow back below 35,000 rpm. This used the tachometer as the
trigger because that was historically the cleanest signal, and nothing other than pumping
would make it slow down. At 35,000 rpm, the igniter and kerosene would be triggered,
and if the chamber pressure reached 200 psi, it was programmed to burn for five seconds.
Results and conclusions: The disk never had a clean deceleration, indicating
pumping. The LOX never started flowing and pumping correctly. Kerosene flow started
after 13 seconds, and since the igniter was running the engine lit. However, since the
LOX was not pumping well, there was only gaseous oxygen being blown into the
chamber. As the first few drops of kerosene sprayed into the chamber, the engine ignited
and a plume was visible. Then, about 0.01 seconds later, as the kerosene flow developed
into full pumping flow, the mixture ratio got so rich that it could no longer support
burning. The conclusion was that the LOX never started pumping because it was boiling
or cavitating at the pump inlet. I devised a matrix of tests, comparing pump speed and
LOX feed pressure. The goal was to find the border between the cavitating regime and
good pumping. During these tests, we figured out that we should pre-chill the LOX by
venting the tank to atmospheric pressure before pressurizing it for the test. This way, the
bulk fluid would be boiling at 90K, at 1 atm., and would then be pressurized up to 100
psi. At this pressure, 90K is below the boiling point, so there is some margin before the
fluid boils or cavitates in the engine. The cavitation tests showed that if we pre-cooled in
this manner, we would get reliable pumping.
Changes: After eliminating the cavitation problem, the new sequence, was going
to be based on detecting pumping again. I realized that once pumping begins, the
chamber pressure jumped up over 100 psi (see Figure 50: Test 1081 chamber pressure
with LOX pumping only). This meant that the kerosene system would have to be
pressurized to over 100 psi. This would create large leakage problems. A solution was
needed that would allow the kerosene pressure to remain lower. We could not turn on the
kerosene more than 0.5 seconds before the LOX flow fully developed, because that
would risk flooding the chamber, as happened in test 1026. We needed a way to predict
when the LOX flow would develop, and turn on the kerosene and igniter just before. I
realized that this was the sequence that happened accidentally in 1041, our most
successful test to date. I graphed the time from turning on the LOX valve until fully
developed flow, measured by when the combustion chamber pressure rose above 30psi,
vs. LOX tank pressure (see Figure 51). The times had some correlation with pressure,
but the three data points at 95 psi showed that it was not the only correlation. Since the
tests at 95psi got consecutively lower times, it was hypothesized that the engine was
cooling down, and the pump time was related to the disk's initial temperature as well as
the LOX tank pressure. During the next trip to New Hampshire, it was decided that we
64
would duplicate this data using LOX, and see if there was a correlation between pumping
time and warm up time between tests.
#1081 Chamber Pressure
45000
120
-1
50 00
3
30000 -- -----
-
25000
--
20000
00
-
-
-- --
_-
60
-Chber_PreSUF
--
2000
40
----
15000 -
i-
000----
-.-
.
--
0
__
--
- - -
--2
--1--
_
-000
--- 20
Figure 50: Test 1081 chamber pressure with LOX pumping only
Time until
pumping
5
4
4.5
4
3.5
4
3
3
02.5
2
1.5
1
0.5
0
60
80
100
120
LOX pressure
140
160
180
(psi)
Figure 51: Time until pumping as a function of LOX pressure
65
Test 1098 Test session, November 4-5, 2000
A series of tests were conducted to find out how long it would take to cool the
engine and for LOX pumping to develop into smooth flow. The plan was to conduct
pumping tests until we could successfully predict the pumping delay time in the next test.
After the first LOX pumping test, we measured how long it took the disk to warm up, by
holding a thermocouple up the throat of the engine and placing it on the bottom of the
disk. It took 60 minutes for the disk and the labyrinth seal to return to ambient
temperature.
Figure 52 shows the relationship between the warm up times between tests and
how long it took to develop good LOX flow. The two points on the far right were the
first tests of each day. 500 minutes was arbitrarily chosen to represent a long time since
the previous test. The other high test in the middle, at four seconds, should be ignored
because there were three aborted tests immediately before it, which would have tended to
warm up the engine.
Time till pumping
4.5
4
3.5
2.5
E2
2
S1.5
0.5
0
T
0
100
200
300
400
500
Time since last test (minutes)
Figure 52: Time between LOX valve turned on and full pumping flow
Using the data from Figure 52, and the time interval between tests, we set the
igniter and kerosene to turn on at 2.4 seconds after the LOX was turned on. There is a
0.2 second delay between the commands and the actual igniter ignition or the kerosene
flow. If the chamber pressure spiked from the LOX pumping before 2.4 seconds the
computer would shut down. If the LOX started after the 0.2 second delay, the engine
would light. As luck would have it, the LOX pressure built up exactly half way through
the 0.2 second time delay, and the kerosene was turned on, but didn't flow. After
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replacing the igniter and moving the ignition time back to 2.0 seconds, test 1098
successfully lit (see Table 2 and Figure 53: Startup sequence as used in test 1098).
Table 2: Final Startup Sequence
Sequence Actual time
index from
Time
(seconds) test 1098
(Figure
Criteria for next
sequence step
Commanded Event
What is happening in the engine
Cool the LOX feed
plumbing
Command full speed
with the motor
labyrinth seal cools, the rest of the
engine stays warm
Engine is spinning up to speed
Turn on LOX flow
LOX is cooling the LOX pump
53)
T -600
T -5
983.5
00.0
989.35
02.0
991.35
02.0
02.2
02.2
991.4
991.6
991.6
02.5
991.9
02.5
991.9
02.6
02.7
992.0
992.2
02.7
992.5
when labyrinth seal
temperature has
leveled out
When the engine has
reached 35,000 rpm
Wait 2.0 seconds
(based on time since
last test, Figure 52)
immediately
Turn on Igniter
Turn on kerosene
The igniter lights
Kerosene starts flowing
The LOX pump finishes cooling, and
LOX flow is established in LOX pump
Chamber pressure rises from LOX
pumping
Engine ignites
Chamber pressure continues to rise
from combustion
Chamber pressure
rises above 200 psi
transition t o
extended burn phase
#1098 Chamber Pressure
40000
200
-Chamber
--
Start I db
Extended burn
_Pressure
35000
Tach
150i--
30000
Engi e reaches
35,0 irpm
25000
100
1
20000
15000
E
-
CD
50
kero ;ene on
0
10000
LOX
on
0
9
w
5000
9 35
9i 6
91 '7
9 38
9 39
9i
0
9 H1
9 )2
Igniter
on
-50 '---J---
9 13
0
-5000
Seconds
67
Figure 53: Startup sequence as used in test 1098
The rest of test 1098 did not go as well as the startup. At ignition, the high-speed
video camera showed a burst of sparks coming out of the nozzle. These were probably
the first bits of melted engine material being shot out of the engine. After a few seconds
of quiet, a series of explosions whited-out the video screens, and left the test stand in
flames. The post test analysis showed that a crack had developed at the base of the LOX
feed tube. This probably happened four months previously, when the LOX tube got bent
during a test, and we straightened it back out. Stainless steel is not a good material to
bend and straighten, as it fatigues and cracks easily. This crack allowed LOX to get into
the kerosene pump, explaining why the kerosene dump vent lines were getting cold. This
propellant mixture lit, burning a bigger hole in the LOX tube, and eroding the kerosene
pump channels. The burning mixture no longer provided film cooling for the combustion
chamber, which burnt away until it burned through a temperature sensor port. That
temperature sensor goes off-line near the end of the test. A significant amount of the
melted engine was blown out of this hole in the combustion chamber and sprayed against
the back wall of the test stand. The bottom disk was not getting the right amount of LOX
cooling, and it melted off, the pieces jamming into the rest of the disk, and stopping it
quickly enough to shear off some of the gear teeth. The rest of the engine was deposited
on the blast deflector (see Figure 54).
Figure 54: The results of test 1098
Conclusions and Future objectives
The Centrifugal Direct Injection Engine (CDIE) offers to be a low cost, high
performance engine. It runs on an expander cycle, although this prototype implements
that without a conventional turbine section.
A prototype engine was built using low cost materials such as aluminum and
stainless steel. The various parts of the engine, such as the rotating seals were tested and
debugged before the first hot fire test. An external electric motor was used to spin the
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engine up to its operating speed and the LOX, kerosene, and igniter were turned on in the
order determined by this research to start the engine. The external electric motor also acts
as a dynamometer for the rocket engine and produces data that can be used to measure
what is happening inside the engine.
The engine ran for 0.5 seconds during the first successful hot fire test, and was
destroyed in a later test. These tests proved that the startup sequence could start the
engine repeatably. The engine was not run for an extended duration, however, so the
steady-state operating point was not reached. A test at the steady state operating point is
needed to verify the heat transfer models used in the design.
In future research, the engine will be rebuilt and run for an extended period, and
the dynamometer will be used to measure the power balance in the engine. This data will
allow the heat flux from the combustion chamber into the LOX flow to be calculated and
compared to the expected values. Once the design of the engine has been verified, a
flight weight engine will be built, along with a launch vehicle. The engine and vehicle
combination will be flight tested to verify the performance of the engine under flight
conditions.
Ideally, in a flight vehicle, it would be possible to start the engine using the gas
startup described on page 38. Blowing high-pressure gas through the LOX channels
could spin the engine up to its operating speed. The challenge then would be to get the
engine started before it lost too much speed. The current startup sequence requires a 2-3
second pause between the spin up and ignition, as the internals of the engine cool down.
A way would have to be found to either keep it spinning during this cool-down, or to
shorten the cool-down. The motor could be used to start the flight vehicle, but a method
to disconnect the motor while it is running at high speeds would be required. This is
certainly possible, but the gas startup would be much simpler if it could be made to work.
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Appendix: Support Equipment
Test stand structure
The LOX tests and hot fire tests were all conducted in a special test facility built
for these tests in New Hampshire. It is located in the field behind my parents' house in
Thornton, NH, 60 miles north of Concord. It consists of a 4 inch thick concrete pad, with
a three sided cinderblock structure built on it. The structure has a removable roof, and is
designed to direct the blast from any explosions upwards or towards the farthest end of
the field.
The engine is secured in a test stand that holds it vertically. A blast deflector
shield angles the rocket's exhaust out the open side of the enclosure. Small kerosene and
LOX run tanks are filled before each test, and are attached to the outsides of the walls, on
opposite sides of the structure. This keeps the tanks out of a blast, and keeps the
propellants separate in the event of a leak or spill. The electronics and compressed gas
cylinders are attached to the back wall, on the outside of the structure.
LOX system
The LOX run tank is a 30-liter stainless steel tank that has been hydrostaticly
tested to over 300 psi. It has solenoid valves that control fill, pressurization, and vent
ports at the top, and a feed port at the bottom that goes to the engine, or through a coil
heat exchanger in a large bucket of water and then a vent. This can be used to dump the
LOX out of the tank without sending it through the engine. A differential pressure sensor
measures the static pressure in the engine feed-line flow compared to the static pressure
in the tank. This is used to get the dynamic pressure of the flow and a LOX mass-flow
measurement. There are also load cells under the tank's feet. The tank weight is used to
tell how full the tank is, and as a backup flow measurement. The tank can be pressurized
with helium that will not condense when exposed to LOX or LN 2. It also has a 240 psi
pressure relief valve incase the tank self pressurizes as the LOX or LN 2 boil off.
The run tank is filled via a 150 foot Teflon tube from a LOX dewar, which is
delivered by truck and left on a small concrete pad near the road.
Kerosene system
The kerosene run tank is a 4-gallon steel tank. It is filled by hand, using a funnel.
There are pressurization and vent ports on top, and a feed port at the bottom, with a
manual dump valve at the bottom of the line. Load cells are used to measure the tank's
weight and this is used to calculate the flow rate out of the tank.
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Electronics/ controlsystems
The control and instrumentation system monitors the engine during tests, and
using a computer, runs the control sequences. This system monitors motor speed, battery
voltage and current, LOX and kerosene tank weight, LOX flow and pressure, LOX feedline temperature, combustion chamber pressure, and temperature at 12 different locations,
and engine thrust.
The computer runs a control sequence, written in Lab View. This program uses
input from the sensors to control the motor speed, and the positions of the kerosene and
LOX valves. It also controls the igniter and the gas separator valves.
The kerosene, LOX and electronics systems are each a modular unit. They can be
carried up to the test site on a specially modified trailer. They mount to the test stand
walls with bolts and quick release fittings and connectors. The same units are used at
MIT for the laboratory testing of the engine.
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