Implementation and Startup of A Centrifugal DirectInjection Rocket Engine

byAndrew Heafitz

B.S. Mechanical EngineeringMassachusetts Institute of Technology, 1991

Submitted to the department of Mechanical Engineering in partial fulfillment of therequirements for the degree of

Master of Scienceat 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 paperand electronic copies of this thesis document in whole or in part.

Signature of Author: _______________

Department of Mechanical EngineeringJanuary 19, 2001

Certified by:Manuel Martinez-Sanchez

Professor of Aeronautics and AstronauticsThesis supervisor

Reviewed by:David Wallace

Professor of Mechanical Engineering

Accepted by:Ain A. Sonin

Chairman, Department Committee on Graduate Students BARKER

MASSACHUSETTS INSTITUTEOF TECHNOLOGY

JUL 16 2001

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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 partialfulfillment of the requirements for the degree of Master of Science

AbstractThe Centrifugal Direct Injection Engine (CDIE) offers the possibility to reduce

the cost and complexity of liquid fueled rocket engines. A prototype CDIE engine wasdesigned to run on kerosene and liquid oxygen. The prototype was machined, assembledand 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 tocollect data while it was running. Qualification tests showed that the engine pumped thepropellants correctly.

A series of hot fire tests were conducted to develop the startup sequence. Thisincluded how fast to spin the engine, and when to turn on the two propellants. Asuccessful test, which burned for 0.5 seconds, showed that the engine could be started. Arepeat test showed that the startup sequence was repeatable, but the prototype engine wasdestroyed later in the test.

Further testing is required to see if the engine will run at steady state.

Thesis Supervisor: Manuel Martinez-SanchezProfessor of Aeronautics and Astronautics

3

Table of contents

A bstract......................................................................................................................... 3Introduction, background and theory of the engine................................................ 6

A brief history of the MIT rocket team................................................................... 6D escription of CD IE engine ..................................................................................... 8The theory behind the CDIE engine ....................................................................... 10Advantages of CDIE over existing turbo-pump designs......................................... 10

Building the engine .............................................................. 13Theoretical requirem ents ....................................................................................... 13Design issues ..................................................... 16

Building the Startup System............................. . .......................... ............... 36Requirem ents of start-up system ............................................................................. 36A dvantages of m otor .............................................................................................. 36A dvantages of gas startup....................................................................................... 38Description of motor startup system........................................................................ 38

Preliminary cold engine testing ..................................... 42Break in of rotating parts....................................................................................... 42P ow er curve ................................................................................................ .......--. 4 3Pre cool sequence........................................................................................... ...... 54

Startup sequence......................... ......................... 56Startup requirem ents ............................................................................................. 56Sequence history .............................................................................................. ... 56Tests performed and lessons learned about startup in each test .............................. 60

Conclusions and Future objectives........................ ...............- 68Appendix: Support Equipment .......................... ............ 70

Test stand structure ................................................................................... 70L O X system ............................................................................................. .............. 70K erosene syst6 m ............................................................................................... 70Electronics / control system s .................................................................................. 71

Table of figuresFigure 1: Wind tunnel test article ....................................... 7Figure 2: H eron's Engine............................................................................................. 8Figure 3: Overall Engine Schematic ............................................................................ 9Figure 4: Expander Cycle Schematic.......................................................................... 11Figure 5: Turbo-pump for a Rocketdyne RS-27 engine .............................................. 11Figure 6: L O X C hannels .......... .................................... ............................................ 14Figure 7: K erosene Channels..................................................................................... 15Figure 8: Engine parts ...................................................................................... .. 16Figure 9: A ssem bled engine ................................................................................. .. 16Figure 10: Engine cross section................................................................................ 17Figure 11: L O X feed-lines .............................................................................. .... ... 18Figure 12: Flow vs. Time ........................................................................... 19Figure 13: Test 1041 LOX Flow ................................................................... 20

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Figure 14: Bottom Disk, LOX heat-exchange passages (left) and chamber heat fins (right).............................................................................................................................. 2 1

Figure 15: K erosene D isk.......................................................................................... 22Figure 16: Bearing Pre-load System.......................................................................... 24Figure 17: Internal O -ring seals................................................................................ 25Figure 18: LO X N ozzle.............................................................................................. 26Figure 19: Labyrinth Seal......................................................................................... 28Figure 20: G as Separator........................................................................................... 29Figure 21: Pressures in the gas separator ................................................................... 29Figure 22: Cham ber Separator................................................................................... 31Figure 23: Separator Plum bing................................................................................... 32Figure 24: Combustion Chamber.............................................................................. 33Figure 25: Fuel Film Cooling Schematic ....................................................................... 35Figure 26: Scoops on Top Part of Combustion Chamber ........................................... 35Figure 27: Test 1084 high-speed coast down without electric braking ....................... 37Figure 28: Test 1026, dV/dI behavior of the battery pack ........................................... 39Figure 29: Solectria BRLS8 efficiency curves .......................................................... 40F igure 30: G ears ........................................................................................................... 4 1Figure 31: M otor m ount ............................................................................................. 42Figure 32: tests 1029 and 1030 power curve............................................................... 44Figure 33: LOX Flow Test, LOX Flow vs. Time ........................................................ 45Figure 34: LOX Pumping Test, Amps vs. Time........................................................ 46Figure 35: LOX Pumping test, Chamber Pressure vs. Time.................... 46Figure 36: Test 1002, battery amps during ethanol pumping....................................... 47Figure 37: 1084 Coast down power curve ................................................................ 48Figure 38: Pum p curves ............................................................................................. 50Figure 39: Motor Amps and speed during LOX pumping test 1091............................ 51Figure 40: 1091 Expected power dissipation during spinning and pumping ................ 51Figure 41: Power measurements vs. predicted values ................................................. 52Figure 42: Test 1041 power dissipation..................................................................... 53Figure 43: Test 1098 power dissipated as engine self-destructs .................................. 54Figure 44: Thermal Conduction Paths ....................................................................... 55Figure 45: 1026 Power increase as the chamber floods............................................... 58Figure 46: Test 1025 Chamber pressure with GOX flowing ...................................... 59Figure 47: Test 1040 aborted because of under-speed................................................ 60Figure 48: Test 1041 Chamber Pressure ..................................................................... 63Figure 49: Hot fire test 1041- plume with shock diamonds......................................... 63Figure 50: Test 1081 chamber pressure with LOX pumping only .............................. 65Figure 51: Time until pumping as a function of LOX pressure .................................. 65Figure 52: Time between LOX valve turned on and full pumping flow ..................... 66Figure 53: Startup sequence as used in test 1098 ....................................................... 68Figure 54: The results of test 1098 ............................................................................. 68

Table 1: R otating Seals ............................................................................................ 27Table 2: Final Startup Sequence ................................................................................ 67

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Introduction, background and theory of the engine

A brief history of the MIT rocket teamThe 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 SpaceFrontier Foundation, for the first non-governmentally funded vehicle to deliver a 2 kgaluminum payload to an altitude of 200km by November 8th of 2000.2 Carl Dietrichpresented an approach for building a liquid fuelled rocket, using a Centrifugal DirectInjection Engine (CDIE) which he had thought of as a different way to compete for theprize.

Other competitors were using large solid fuel motors or pressure-fed liquidengines. We considered solids too large and dangerous to develop in the confined spacesat 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 thetechnical risk acceptable. The CDIE held the allure of being something we could developwithout expensive facilities, and it promised to be significantly cheaper to design andbuild than other pump-fed, high performance liquid fuelled engines. The challenge ofbuilding a new engine ranked equally high with the CATS prize and the team set off tobuild and fly a rocket. The team was funded by the Aeronautics and AstronauticsDepartment, the Mechanical Engineering Department, and later on by the EdgertonCenter.

Initially I worked on designing a launch vehicle (Figure 1: Wind tunnel testarticle), but after working on the engine and the rocket, it became apparent that the twocould not be developed in parallel with the given resources; the team began to focusexclusively on engine development. Engine development started with some simple testexperiments to verify the concepts behind the engine, which will be described on page23. These experiments proved to be over-simplistic, and did not produce many usefulresults. We did, however, gain a better idea of the level of prototype completeness andquality that would be required to obtain significant results. Because the heat transfer inthe combustion chamber is essential to the engine's operation and is difficult to simulatewithout real combustion, it was decided in the middle of 1999 to develop a full testengine. This engine was prototyped and ready to test in March of 2000. A test facilitywas constructed in Thornton, NH.

http://web.mit.edu/cats/2 http://www.space-frontier.org/EVENTS/CATSPRIZE_1/

3 Sutton, George P., Rocket Propulsion Elements, John Wiley & Sons, Inc. New York, 1992, pgs. 365, 208-211

6

Figure 1: Wind tunnel test article

The first few test attempts were met with mechanical troubles, and the team hadto withdraw from the CATS competition because we were too far behind schedule4 . InSeptember of 2000, the engine was successfully ignited for the first time. It burned for0.5 seconds and sustained no major damage. In October, a second test ignited, but thistime 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 wasby the HARC team from California. They attempted to launch a pressure fed, liquid fuelled rocket from aballoon at 70,000 ft. The rocket either failed to separate form the balloon correctly, or had a problemimmediately after separation.

7

Description of CDIE engineThe Centrifugal Direct Injection Engine (CDIE) burns kerosene and Liquid

Oxygen (LOX) in an expander cycle. It uses the boiling and expansion of the LOX topower the engine. The novel part of the engine is that the turbo-pump is immersed in thehot gas of the combustion chamber, and uses boiling liquid oxygen to power the pumpthrough 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 andoxidizer pumps are simple centrifugal pumps that are integrated together, and the pumpexhaust has replaced the turbine stage. The opportunity exists to build a highperformance 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 enginewas designed to operate at very low feed-pressures. We eventually upgraded topressurized tanks, which were necessary to obtain the right propellant mass flows out ofthe tanks.

5 James, P. and Thorpe, N.; Ancient Inventions. 1994. New York; Ballantine Books pp. 131 - 133

8

As shown in Figure 3: Overall Engine Schematic, the LOX enters the enginethrough a centrally located feed tube (1). It is centrifugally pumped to high pressure inthe 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 outof two tangential nozzles (8), which keep the disk rotating at high speeds.

2

3

3 4 5

8

6

Direction ofrotation7

Figure 3: Overall Engine Schematic

Kerosene is introduced into a tube (2) coaxially surrounding the LOX tube, and iscentrifugally pumped in the same disk (4). It is then released directly into the combustionchamber.

This design is much simpler than other turbo-pump engines, but left many designissues to be resolved. Some of the challenges of the CDIE engine are: sealing the LOXand kerosene feed-tubes to the rotating section of the engine; keeping the LOX fromfreezing the kerosene; getting the propellants to mix properly in the combustion chamber;

9

-- ;;R= __ __ - __ -

and providing a layer of film-coolant for the chamber walls. Another challenge is gettingthe engine to operate at steady state, because getting the disk to power itself depends onthere already being heat transfer into the disk from the combustion chamber.

The theory behind the CDIE engineBecause the CDIE engine consists of a single rotating part, the various functions

of the engine are heavily integrated. Conventional engines would have separate pumpsand turbines for the fuel and oxidizer allowing them to be optimized separately. Thissection will give a rough overview of the theory behind this design.

The most difficult value to calculate, and the most important, is the chamberpressure, 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 chambermust 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, andthe intermediate pressure and temperature inside the LOX heat exchanger, P7, T7 (seeFigure 3). P7 and T7 are in turn, determined by o, and q', the heat flux from thecombustion chamber into the disk. q' depends, once again, on PC and o. The speed of thedisk, (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 massflows, m'0 5 and m'kerosene.

As can be seen, these equations must be solved simultaneously. Carl Dietrichcompleted a numerical analysis of this problem and determined that the design operatingpoint for the engine is at a rotational speed o of 36,000 rpm, with a chamber pressure of500 psi. The kerosene mass flow is 0.125 liters/sec, and the LOX mass flow is 0.25liters/sec.6

Advantages of CDIE over existing turbo-pump designsThe 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 theexpansion 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 aheat exchanger wrapped around the engine nozzle skirt. (American and Europeanengines tend to use the fuel, usually liquid hydrogen, as the working fluid, Russianengines use the oxidizer.) The gas then goes through two turbines, which are connectedto two pumps, one for the fuel and one for the oxidizer (see Figure 4: Expander CycleSchematic) 7.

6 The engine is modeled and described in Carl Dietrich's paper "An Analysis of Heat Transfer in aCentrifugal Direct Injection Engine", This paper is available upon request, carldietrich@alum.mit.edu.7 Sutton, pg. 213

10

Fuelpump

Fuelt urbine

Oxidizerpump

Oxidizerturbine

Figure 4: Expander Cycle Schematic

Each turbine and pump is a complex piece of machinery.typical geared turbo-pump assembly. This one is used on the DeltaLOX and RP-1 (kerosene) as propellants.'

Figure 5 shows alaunch vehicle with

ina:rbin in-xle

,xU>Pump?~. t~pe~mM

oft-, VU

Figurer 5:Tro upfo aocedneR-7engn

manufactured31 fromag losotmteilschap05aumiun 1 tanessel

11

8 Sutton, pg. 329

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 thepropellants actively cool the pumps. Even the pump and turbine housings have beeneliminated. All of these characteristics add up to an engine that could be less expensiveto manufacture than existing engines.'

A note on the tests referenced throughout this paper: each test has a uniqueidentifying number. These numbers were started at 1000 to leave room for tests that wereconducted previous to the engine prototype described in this paper. Also, most of thegraphs of test data include a tachometer trace. The tachometer trace is an easy torecognize shape, and is there to help the reader understand how the data being presentedfits, time wise, within the test.

9 NASA / Marshall Space Flight Center, FasTrack engine reports

12

Building the engineThis 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 readerunderstands what each part needs to do. For the fluid passages, I will describe the partsin the order that a drop of propellant would go through the engine. After all of the partshave been described, I will go back and discuss the designs that were chosen, and theissues that arose during prototyping.

Theoretical requirements

LOX channelsThe 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 stationaryfeed tube and the rotating feed tube (2). This seal prevents the LOX from mixing withthe kerosene outside the engine. Once the LOX is in the rotating part of the engine (3), ittravels to the inlet of the LOX pump. Cavitation at this point would prevent the pumpfrom operating properly. From there, it is centrifugally flung to the outside of therotating disk in the pump channels (4), passing through an orifice to create a pressuredrop and prevent back-flow (5). Low-pressure instabilities in the heat exchange sectionwill 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 acrossthe 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. Theheat flux boils the LOX, and raises its temperature and volume. The high-pressure gas isthen ejected into the combustion chamber through tangential jets (8). The thrust from thejets keeps the disk spinning along the axis of the LOX feed tube, and the oxygen injectedinto the chamber combusts (9) to create the heat that boils the next volume of LOX in theheat exchanger.

13

8

fn l Fl F1 l~ F-1 F-_1 l

I \

Figure 6: LOX Channels

Follow through kerosene channelsThe 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 combustionchamber where it film-cools the chamber walls, and combusts with the oxygen from theLOX section of the engine (9).

14

10

12

13

Figure 7: Kerosene Channels

Mechanical systems

BearingsThe 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 acrosstheir pump orifices, which are related to the disk's speed. The engine is designed tooperate with the disk spinning at 36,000 rpm.

Gas sealsAs 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 asurface speed of 25m/s. Another seal keeps the combustion chamber from leakingaround the disk shaft. Leakage here would contaminate the bearings, which could causethe engine to seize.

15

.>>

Combustion chamberThe combustion chamber for the flight vehicle will be designed using NASA

Fastrack engine technology. They use a silica-phenolic composite that utilizes kerosenefilm cooling and an ablative throat. This engine is on the small side for using ablativecooling 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 givenablation rate does go up. For this reason, this engine cannot be scaled down to muchsmaller that the one we are building.

Design issuesThis 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 theirinitial assembly. Figure 9 and Figure 10 show the engine after it has been assembled.

Figure 8: Engine parts

Figure 9: Assembled engine

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LOX inlet

LOX labyrinth seal

LOX

Kerosene

Inert Gas

Vacuum

Rotating parts

Bearing retainer ring

Bearings

Chamber separator(old design)

Bolt circle

Belville washers(Replaced with waveywasher)

Kerosene injectors

0-ring seal tocoppercombustionchamber

LOX heat exchangechannels

LOX jets (not shown)

Holes for scoopsLOX pump

Bottom disk -

Figure 10: Engine cross section

LOX channels

LOX feed-lineThe 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 feed-

line 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 LN2 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 thekerosene will freeze if the engine is too cold (Figure 1 IC). This system is the only onewe have gotten to work. Cooling the rotating part of the engine, which is not thermallyconnected to the piping, is still difficult, and will be discussed in the sections abouttesting and operating the engine.

(~N\

Gravity feedLOX tank

Fl

Valve

Engine

A

PressurizedLOX tank

ow measurement point

B

Figure 11: LOX feed-lines

18

Flow to cool

n wto run

C w0

Kerosene flow ratel

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 pumpOnce 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, thepump will not develop the right pressure and it is likely that the LOX feed-line will vaporlock, causing the mass-flow rate to drop. Chugging flow has been observed in the feed-line until it is properly cooled (Figure 13: Test 1041 LOX Flow). Good flow starts around775 seconds. The large spike at the end is a shutdown measurement transient. The riskof low frequency instabilities involving the chamber pressure backing up the feed-lines isa 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 transferfrom the structure, heat transfer from the kerosene, and cavitation in the inlet of thepump. 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 boilingthe LOX and freezing the kerosene. Space was very limited, because increasing thekerosene feed-line diameter would increase the ID of the bearings, and the surface speedof the balls that were already running near their top speed. The LOX feed-line could notbe made smaller than 0.5" because the related pressure drop would starve the engine andcould cause cavitation problems in the pump. This inner diameter requirement should berevisited in the flight vehicle, because this calculation applied to a gravity fed LOXsystem, and a pressurized start up may allow a smaller feed tube to be used. Thesefactors constrain the inner and outer diameters of the assembly, without leaving muchroom in between. In addition to the feed-tubes, a good layer of insulation had to fit inthat space to prevent the unwanted heat transfer from the kerosene to the LOX.

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1002 pumping

1041 - Tach-LOXFlow

--- -- 8.5

30000

25000

7.5 E

20000

0)

15000 E

6.5 210000 0

-J

5000

0 5.5773 774 775 776

Seconds

Figure 13: Test 1041 LOX Flow

The highest performance type of insulation is a vacuum jacket. My original ideaconsisted 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 replacethe 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 vacuumseal was probably broken when the LOX feed tube was bent during a high-speed testwithout the upper seals in place. While these seals were supposed to be non-contactseals, in fact, they could act as backup bearing surfaces for the LOX feed tube if it got outof balance. Given that the engine successfully pumped LOX without the vacuum, it doesnot seem to be necessary, and the next redesign may replace some of the welds with o-rings, and use an air filled insulation gap in place of the evacuated insulating gap.

Heat exchangerFrom 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 itpasses 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 toa pair of right angled jets which can be seen protruding through the outer wall of thebottom 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 tospin 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 werea series of geometrical constraints on the heat exchange section that made its designespecially challenging. The following features each had to be located in the positionslisted 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 ofinternal pressure.

All of the criteria boil down to a geometry that is difficult to implement. Thepressurized LOX flow must start at the edge, get to the middle, covering the entirebottom, 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 LOXto travel through. For my contribution, I figured out how to eliminate several large boltsthat were going to have to go through this disk to hold the stack of disks together. Ireplaced them with a bolt circle of much smaller bolts around the edge of the disk, abovethe heat exchange area. I added a single serpentine heat fin that follows the LOX flowand improves the internal heat transfer.

21

The disk itself was made of anodized 7075 aluminum. This was chosen becauseof its strength-to-weight ratio, necessary for spinning at high tip speeds. There is concernabout the bottom disk burning through in places where it is exposed to combustiongasses, but it is cooled by liquid oxygen. The first hot fire test, which ran for 0.5seconds, showed that some of the poorly cooled sharp corners on the outside edge startedto melt. The rest of the disk survived intact, although the chamber pressure only got upto 300 psi out of an expected 500 psi. The increased chamber pressure at the design pointwill create higher heat transfer than was experienced in this test. A CFD analysis wasdecided to be too difficult to undertake, especially because we would have to verify theresults experimentally to have any confidence in them. The aluminum is anodized toprovide an extra layer of protection in case it is exposed to pockets of high-temperatureand high-pressure oxygen in the combustion chamber. We will find out if aluminum cansurvive after our first extended length burn, which is planned for the spring of 2001.

Kerosene channelsThe 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 andinjected into the combustion chamber through four orifices. The kerosene disk (seeFigure 15) does not have many difficult design features relating to the kerosene flow. Itdoes tie the rest of the engine together, but that will be discussed in the mechanicalsection. This piece is also made of 7075 aluminum.

Figure 15: Kerosene Disk

22

Mechanical systems

BearingsThe 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 tospin at 36,000 rpm. A variety of different bearing types were tried, with minimalsuccess, 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 roughprototype had a pair of steel ball bearings. There were alignment problems and pressfitting problems and I never got the prototype to spin freely. We then decided to make astatic gas bearing for axial support and a jewel type bearing for the thrust load. At thispoint, we were simulating the conditions in the combustion chamber with some blowtorches, so there was no chamber pressure to counteract, just the weight of the rotatingpart of the engine. I designed a gas distribution system that would allow us to useinterchangeable brass sleeves on the static part of the bearing. I machined several sleevesin the Edgerton student shop, reaching 0.0005" precision or better, which was difficultusing the equipment available. The steel shaft I had purchased turned out to be out ofround by .0005" in such a way that it was not detectable by measuring a diameter with amicrometer, but only by measuring three points with a special gauge. This was not aneasy thing to trouble shoot. A new shaft was rounder, and we got the engine to spin up to6000 rpm by blowing compressed gas through the jets, but there was always considerablefriction, 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 haddeveloped a method for timing how long it took for the disk to spin down with andwithout the jets. This gave us an idea of what forces were acting on the disk, althoughthe bearings were temperamental enough that there was some variability. There was onetest that used LN 2 and the heat source that seemed promising. The disk did not spindown as quickly as it did without the LN 2 and blowtorches. The jet assist seemedstatistically significant, but the test rig was not performing well. Because so manyaspects of the engine were inter-linked with one another, we decided to start over with amore completely designed engine that could be hot fired. This new engine would test allof the systems under the most realistic situation possible, a hot combustion chamber.

The bearings in the new engine prototype are Bardon, ceramic-ball, angular-contact bearings. The angular contact allows them to take radial and axial loads.Because the propellants have to pass through the bearings, their diameter, and resultingradial load rating was larger than would have been otherwise necessary. This allowed usto use the angular contact bearings to support the thrust loads as well. Because the shaftgoes through the combustion chamber wall, the bearings have to support the load of thechamber pressure over the area of the shaft. At 500 psi, this worked out to be about1,500 lbs. Because the bearings are rated for 1550 lbs., axially, this load is right aroundtheir upper limits. Ceramic balls were chosen over steel balls for several reasons. Eventhough the ceramic balls have a 30% lower load rating than similar steel balls, they havea longer life, especially at high speeds and under poorly lubricated conditions. They arelighter 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 thecontact problems related to metal on metal surfaces. The top rated speed for the ceramicballs 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 systemfor the bearings is designed to keep the angular contacts in full contact, even withoutchamber pressure present. The first pre-load system used a pair of Belleville washers, butbecause they were so heavy, there was not much deflection. After the first set of bearingsfailed, either because the improper pre-load caused the non-contact seals to crash, or theseals crashing put large loads on the bearings, I replaced the Bellevilles with a thinnerwavy washer. This allows the bearings to move more freely in the axial direction (seeFigure 16: Bearing Pre-load System). This system has been used for all of the successfulspin, 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 wouldbe difficult and heavy to implement inside the engine, and at these speeds, grease willtend to be flung off. Also, grease tends to make bearings run hot. The manufacturerthought 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 alubrication system. An unexpected benefit of the rotating seals, was that one of themleaked kerosene directly into the bearing area, but this feature will be fixed on the nextdesign revision because leaking kerosene is an explosion hazard. It should be noted thatthe bearings performed well in numerous tests that were run dry, or with only LOX orLN, (to simulate LOX) and no kerosene. There have been no duration tests to see howlong it does take for the bearings to overheat. This type of testing has had a lowerpriority than the short duration hot fire tests, and will be done by default as the burn timesare extended. We have compensated for the lack of lubrication by not running the enginefor more than about 20 seconds at a time and allowing a complete cool-down betweentests.

Wavey Washer

Kero Disk Feed tube

Figure 16: Bearing Pre-load System

24

Disk internal sealsI 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 tocombustion products. They also had to be assembleable, in terms of being able to insertthe Teflon o-rings, which are stiff and unforgiving. I chose Teflon for the o-rings becauseof its compatibility with oxygen and low temperatures. It is the only polymer o-ringmaterial rated down to -300F. Admittedly, this is 89K, just one degree Kelvin below theLOX temperature; however, all of the places where they are used are static seals, whichare more robust than moving seals in cold conditions. The upper seal fit in easily, but thelower one had to be pressed into the gland after that part of the engine was assembled. Itneeded to be compressed as it was inserted. This is not the standard way to pack an o-ring gland, but it worked.

-r s

kero pump

1LOX pump

-I-~~I

Figure 17: Internal 0-ring seals

Disk assemblyThere 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 LOXpump, which was inserted along the Z axis (see Figure 18: LOX Nozzle). The secondinvolved the kerosene disk where the springiness of the o-ring made it hard to seat thealuminum disk. Further, pressing on the edges of the kerosene disk, as the bolt circletended to do, bowed the disk enough that the locating step would not engage at all. Afuture version of the engine should have larger locating features, and the nozzles need tohave 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 heldtogether with a bolt circle. The forty-four 6-32 bolts around the edge are high strengthsteel 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 astatic 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 balancingThe 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 duringmanufacturing, 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 thenstatically 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 atthe edge of the disk, so the disk can be considered to be within 1 gram of balanced. Theside load produced by the worst case imbalance, equal to a 1 gram weight at the edge ofthe disk, rotating at the design operating speed of 36,000 rpm, is 162 lbs., well within the2360 lb. rating of the bearings. The engine has been successfully spun up to 40,000 rpmand has not had any noticeable balance problems.

26

Rotating sealsThere 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 usenon-contact seals because of the high surface speeds, extreme temperatures (either hot orcold) and unlubricated conditions for some of the seals. Table 1: Rotating Seals, lists theseals and their requirements.

Table 1: Rotating Seals

Seal name Located between: Fluid(s) Temperature Materialssealed range (rotating part,against Stationary part)

Labyrinth LOX supply line LOX 90K to 300K Stainless steel,and LOX feed Brass

tube

Gas LOX and kerosene LOX and 90K to 300K Stainless steel,separator feeds kerosene AluminumKerosene Kerosene feed and Kerosene 273K to 300K Aluminum,

bearings graphiteChamber Combustion Combustion 273K+ Aluminum,separator chamber and gasses graphite

bearings

LOX labyrinth seal

The LOX passes through the labyrinth seal at the top of the engine. This is arotating seal between the LOX supply line and the rotating feed tube. I had the idea ofusing a labyrinth style seal that would allow a small, but manageable amount of flow toleak through the seal (see Figure 19: Labyrinth Seal). That flow is allowed to vent toatmosphere, through a set of dump lines. In a flight vehicle, it may be possible to recyclethis 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 dumpedoverboard, unused. Eliminating unnecessary reductions to engine Isp due to propellantleakage will be important in a flight engine. The labyrinth seal has the advantage ofbeing 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 ruband create drag on the engine. The stationary part is made of brass, and the rotating tubeis stainless steel. The labyrinth seal piece needs to be held centered inside the enginefeed tube while it rotates at 36,000 rpm. This is difficult because the seal is mounteddirectly to the LOX supply tube at the top of the engine. The weight of this plumbingand the valves can put torque on the seal and cause it to crash. The inner part of the sealwas reinforced after this problem was identified, but it is still very delicate. Ideally, theplumbing would be isolated from the engine by flexible tubing, but with the coolingmethod shown in Figure 1 lC, the upper part of the engine must be in thermal contactwith the plumbing to help it pre-cool.

27

~~~1

Dump toAtmosphere

Leakage

LOX Flow

Rotating Shaft

Figure 19: Labyrinth Seal

The second problem is that the rotating tube is cantilevered four inches from itsbase, and tends to wobble at high speeds. The longest tolerance path in the engine is fromthe LOX feed tube to the labyrinth seal. Errors in machining add up through the LOXfeed tube, the kerosene disk, the bearings, the bearing holder, the upper housing, thelabyrinth holder and the labyrinth seal itself. In fact the root cause of the engine's majorfailure in test 1098 was attributed to a crack in the base of the LOX tube created monthsbefore 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 sealarea 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 beable to wear in and eventually stop rubbing, leaving a small clearance. The current sealwould abrade, and move, and abrade some more. By the end of testing, the seal had wornitself out until there was 0.005 inches radial clearance. The only real problem with largeclearances is that there is excessive LOX leakage, which reduces the accuracy with whichwe can measure the engine's performance.

Gas separator

The gas separator keeps the LOX and kerosene from mixing as they are fed intothe rotating part of the engine in close proximity to each other. The gas separator used anon-contact seal similar to the labyrinth seal, but this one had helium pumped into one ofthe middle cavities. The positive pressure of the helium in the middle cavity made theLOX and kerosene unable to enter, effectively separating them (see Figure 20: GasSeparator). I tested the gas separator by putting a piece of tracer paper in the kerosenedump channel and introducing water into the LOX dump channel. Any water that gotthrough the separator would have been absorbed by the paper, and caused the ink on it torun. There was no evidence of water having gotten through after the test. This test wasrun with the He feed regulator set to 10 psi, and a pressure tap in the separator annulusshowing 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 thepressure drop through the dump lines, but lower than the feed pressures of the keroseneand LOX. Ps>POd>P LOX feed, and Ps>Pkd>Per feed (see Figure 21)

tent

He feed

Kero vent

ero feed

Labyrinth seal

Small gaps

Rotating LOX tube

Figure 20: Gas Separator

Atm -

P HeAtm

P kero

N.

r

1

Figure 21: Pressures in the gas separator

LOX

K

29

Kerosene sealRotating Kero feed tube

-

W-p-

,Nioioioi: 4zzz

A.. -n

P od

PS

Kerosene seal

The kerosene seal keeps the kerosene being fed into the disk from leaking outthrough the bearings (see Figure 20). The seal consists of a small gap between a fixedgraphite ring and a lip on the top of the kerosene feed tube. Originally this seal wasaluminum 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 aluminumlip on the kerosene tube was damaged during assembly. Several nicks and dents causesignificant leak paths. Once the kerosene leaks through the seal, it goes through theupper bearing, and hits the gear. From there it is sprayed all over the test cell in a finemist 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 severalvideo frames at a time during the tests. An unintended advantage of having this seal leakis that a kerosene mist on the bearings is very close to an ideal cooling and lubricatingsituation. When this seal is fixed in the next engine design, the bearings will lose thiscooling 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 meltingthe bearings. The first design had a distribution groove on the outside, static part of theseal. 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 holdthe 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 feedports, located 180' apart from each other, and a pressure tap at 900. This allowed me tomeasure the pressure in the distribution annulus during operation. During pressure testsI found that the feed regulator is choked inside, and as the tank pressure drops, theregulator is not able to maintain the same flow. The pressure in the chamber separatordrops by 6psi/sec when fed by a single high-pressure N2 tank. For the test firings, we areusing 3 tanks of N2 so the pressure will only drop by 2psi/sec. With short firing timesand conservative use of the chamber separator gas, the pressure should hold up wellenough to be considered constant. The expected chamber pressure is around 500 psi, sowe 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 thehot combustion products would vent through the lower bearing, and possibly backwardsinto the gas separator feed plumbing.

30

N2f eed-

Engine 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 theengine development process, and they seemed to be a workable solution. Not realizinghow related this problem was to other engines, I was pleased to find out how similar oursolution was to that used in other engines. Pressurized gas seals like this one are used inthe space shuttle main engines", and also in jet engines12 . The biggest mistake was thatwe used aluminum on both sides of these non-contact seals. We coated them with boronnitride, which is supposed to be compatible with itself for running surfaces, and the sealswere designed to have a positive clearance at all times, so rubbing was not supposed to bean issue.

Through a series of minor machining errors, a badly preloaded bearing, and aseries of bizarre assembly coincidences, the seals did end up rubbing and the aluminumgalled 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, includingthe combustion chamber, the bearings, and the seals. The seals were redesigned, andgraphite 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 spinningaluminum wheel on a block of graphite showed that the materials were compatible atsurface speeds of 120 m/s, and with kerosene.

I reinforced the graphite seal insert on the backside with carbon fiber and glued itin place using a flexible epoxy. I chose an epoxy that could withstand fairly hightemperatures (250C) and 300% elongation before failure. This is needed because if thealuminum 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 becauseof the high pressure which would get around and above it. I also machined a groove inthe aluminum for the epoxy to fill. This is because the expected differential expansion ofthe aluminum over the graphite must not be more than 300% of the thickness of theepoxy, otherwise it will be overstrained, and come apart. This epoxy turned out not to becompatible 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 tokerosene 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 Figure23). Since the low-pressure separator can be run for extended periods on a single tank, itcan be left on during start-up and shutdown. The chamber separator has such a highflow-rate, that we only use it during the actual burn to conserve pressurized gas. Whenthe high pressure comes on, it fills the chamber separator, but the check valve keeps itfrom over-pressurizing the gas separator. The gas separator uses Helium because it isexposed to LOX or LN2. 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

__ .Chamber separator

Figure 23: Separator Plumbing

In the future, a few changes to the separator designs could improve theireffectiveness. GE uses an abrasive coating on their rotating parts running against a softstatic wear ring in their jet engines". Our flight engine could use similar abradable sealswhere an abrasive coating is put on the rotating side and a soft material such as graphiteor 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 shaftwill 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 sealstighter than is possible with separately machined parts. Temperature should not be aproblem because the seal area is cooled with kerosene and nitrogen. If there were aneffect, it would tend to open up the sealing gap.

Another design difference between our seals and non-contact seals used in otherengines 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 thatcauses the flow to contract momentarily. As a result all of the fluid passes through anarea that is even smaller than the distance between the surfaces. We used squarelabyrinth 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 knifeedged features, especially in aluminum.

An alternative design for the chamber separator is to simply make the chamberseparator as tight as possible and vent each section of the labyrinth to atmosphere. Fuelfilm cooling in this arrangement keeps the combustion gasses from eroding the seals. Ifthe combustion products can be vented in a controlled manner before they reach thelower bearing, it may not be necessary to use a compressed gas source at all. Becausethis is the highest-pressure seal in the engine, it uses the most gas. The weight savingsfrom eliminating the gas use in the chamber separator would be considerable.

Combustion chamberFor 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 themass (see Figure 24: Combustion Chamber). The test chamber weighs 27 kg and willhave enough thermal mass to conduct short test firings. Using the thermocouples in thewalls, the test chamber will show the effectiveness of the film cooling, as well as hotspots and likely failure points. The wall thickness is over-designed from a pressurecontainment point of view.

6.0

TempSensorPorts

15.00

Pressure tap

Ignitor hole

8.75

Figure 24: Combustion Chamber

33

I

-! .512

The shape of the chamber was simplified for manufacturing reasons to conicaland straight-sided sections for the chamber walls, throat and nozzle skirt. The array ofthermocouples can be seen up the left side and two more are included in the engine thatforms the top of the chamber. The hypothesis is that the top of the chamber will be themost fuel rich area, and therefore the coolest. The chamber walls should get hotter asthey get down towards the throat. There is a safety procedure in the hot fire test sequencethat will shut down the test if the temperature sensor near the throat gets up to 80% of themelting temperature of copper. Since this is the strongest part of the chamber wall, atalmost 3 inches thickness, the start of a melt down here should allow enough time to shutdown the engine before the structural integrity of the chamber is lost. A short test of 5seconds will give a good idea of the heat transfer going on in the chamber walls, and willbe the starting point for designing further tests, as well as the ablative characteristics ofthe composite flight weight chamber.

I designed the nozzle using a one dimensional expansion model. A 15' straight-sided expansion takes the exhaust from 35 atm in the chamber and over-expands it to0.35 atm at the exit. This is the maximum expansion possible without having to takeflow separation along the nozzle walls into account, and is the same as that used by theFastrack engine. The expansion ratio came out to 1:3.37, and the throat is 13mm indiameter.

The chamber is sealed to the top part of the engine using a Teflon o-ring and a setof six grade 8 bolts, which are all 0.5 inch diameter. This provides enough strength tohold the engine onto the combustion chamber with a safety factor of 10, and should alsobe the failure point in the event of an over pressure.

The chamber is film cooled by the kerosene that is injected by the upper sectionof the disk. The unconventional injection direction of this engine promises to offer achallenge in balancing the film cooling versus good mixing of the propellants. Thekerosene is centrifugally pumped up to the injection pressure, and then released directlyinto the combustion chamber. This will fling it straight at the walls, all the way aroundthe chamber (see Figure 25). First the kerosene skims along the roof of the chamber, andthen as the roof starts to curve down towards the walls, the kerosene follows and beginsto coat the walls. At this point I have devised a series of scoops protruding from thewalls, which will peal some of the kerosene flow from the walls and shoot it axially downthe length of the combustion chamber (see Figure 26: Scoops on Top Part of CombustionChamber). As a starting point, these deflecting scoops cover one half of thecircumferential area of the chamber. Half of the fuel should be pealed off the walls, andhalf will remain coating the walls as film-cooling. This high ratio of fuel used for filmcooling was chosen because of the unconventional shape of the chamber. The chamber isvery 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 somereaches 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 ofthermocouples I have installed in the wall of the chamber. We will be able to see how fardown the chamber the film-cooling effect lasts, before the temperature starts to rise abovethe 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 SystemThe 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 testcell, a control and instrumentation system, and a startup system. Because my maindesign 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, forreference.

Requirements of start-up systemStarting 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 berunning in order to pump propellants, and the propellants must be pumping in order forthe engine to run. Further, both the kerosene and LOX must be pumping with fullydeveloped flow before combustion starts in the chamber. The pressure increase throughthe pumps is all that keeps the chamber pressure from going back up the propellant linesand blocking them.

A start-up system is required that gets the engine as close as possible to theexpected 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 stableoperating regime. The lower bound is a speed of 27,000 rpm, below which heat transferwill not be great enough to keep the disk spinning by itself, and the upper bound isaround 40,000 rpm, determined by the disk's structural speed limit and the bearings' toprated 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 involvedblowing pressurized gas through the disk, and using the oxygen jets to bring it up tospeed. The second uses an external electric motor to spin up the disk. The motor waschosen for the preliminary testing. The following sections outline the arguments foreach.

Advantages of motorThe 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 wouldhave to be shut off before starting the LOX flow. The disk could spin down before theheat transfer from the chamber had a chance to power the disk back up. The motor cancontinue to run and keep the disk at speed through the start-up transients. If the disk spundown, 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, andthe solutions for the two situations are very different.

The motor on the other hand, can run the disk at a steady-state operating speed allthe way through the start-up transient. Additionally, the electric power used to keep thedisk at that speed gives a measurement of the power contribution from the LOX jets, andindirectly, of the heat transfer into the disk.

In this respect, the motor can be used as a dynamometer to measure several thingsinside 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 thatgas or mixed phase), windage and bearing losses, and contribution of the oxygen jets tothe disk's speed. Mechanical problems with the bearings and seals also show up in themotor power data.

The motor is capable of electrically braking the disk. This is used to do fastshutdowns, and to save run time on the engine since coast downs from high speed cantake longer than the test itself (see Figure 27: Test 1084 high-speed coast down withoutelectric braking). In the event that the heat transfer is greater than expected, the electricbreaking will be used to slow the disk and keep its speed from running away. Asuccessful test can be run in braking mode, producing more data than an emergencyshutdown would provide. This could also save the engine if the transients happen veryquickly, since an emergency shutdown takes about 0.5 seconds, as demonstrated in test1041.

#1084 Amps

50000 -------- ------------------ 6000

500005000

40000 5000

4000

30000

30 Tach300 -powerl

20000

2000

100001000

0 - -0

0 20 40 60 80 100 120 140 160

Seconds

Figure 27: Test 1084 high-speed coast down without electric braking

Once testing began, there were several other things for which the motor proveduseful. When new seals were being broken in, the start-up torque of the engine wasgreater than that that could have been supplied by a gas start-up system. The speed of themotor can be controlled, so running the engine at low speeds for extended periods tobreak in parts, or to conduct low speed tests is easy to do. It has turned out that one ofthe lesser-understood start-up transients is the length of time between starting the flow ofLOX, and getting good, single-phase liquid flow. This takes about 3 seconds, dependingon the cool down sequence, and the motor power data has been invaluable in detectingthe onset of liquid flow, and measuring this time transient. The motor keeps the diskrunning during this time, because it would slow down too far in this amount of time witha gas start-up, if the gas start-up were shut off before starting LOX flow. I hope that wecan find a way to overcome this shortcoming of the gas start-up at a later date, but themotor was valuable in identifying and quantifying these issues.

37

Advantages of gas startupThe 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, andinstigated a much more carefully thought out system approach. This first full engineprototype is a result of that reorganization. While the gas start-up system did not survivebecause it left too many variables uncovered, it does have several significant and notableadvantages.

The gas start-up system is much simpler and lighter than a motor system. It doesnot put any extra stresses on the engine other than those encountered while it is runningon its own. In the case of a launch vehicle, it would be much easier to implement a gasstart-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 efficientlyin a small rocket vehicle.

Once the engine has been run and characterized, many of the variables that werein 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 start-up system can be designed around the known operating parameters. The motor couldsimulate the gas start-up, using computer control, and the motor power curves wouldverify that it would work before it is implemented.

Description of motor startup system

Motor/controllerThe 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 SolarCar Team. It is an air-cooled brushless motor, powered by a DC battery pack through theSolectria controller that acts as a power inverter. This motor was designed to be used as asolar car drive system, and has proven to be compact, reliable, and efficient. Its peakpower output is 6kw (8hp) and its top speed is 6600rpm. The controller typically drivesthe motor based on two potentiometer inputs. One comes from the solar car'saccelerator, and the other from the brake. Byron Stanley built a torque/speed controllerthat would convert the engine's desired speed, determined by our computer operatingsequence, into the torque inputs required by the Solectria controller. A speed input willbring the engine to that speed, and hold it there, using the motor's regenerative braking ifnecessary. Regenerative braking uses the motor as a generator, and puts the power backin the battery. This motor can provide 2-3 kW of active braking power and more in auto-regen mode, which occurs if the motor is run faster than its top speed.

Battery systemThe 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 thealuminum 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

batteries6 . Wired in series, these batteries put out 96 volts, 38 Ah (rated as new, thisparticular 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 werecycled and tested using a couple of old Solectria headlight discharge banks, and they putout 80-100 amps continuously without a great voltage drop. There is always some voltagedrop in a battery when a current is drawn, and this relationship can be approximated aslinear. 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 forthis battery is shown in Figure 28. There is hysterisis in this curve, because the voltageof the battery is slow to react to the changes in current, so there is a lag. It can be seen inthe graph that as regenerative braking is applied, and the current drops to -8 amps, ittakes a little while for the voltage to rise back up to 98 volts. In cases where the voltagewas not available during a test, due to voltage sensor problems, I used this curve toestimate the voltage, based on the current, to get the power used from the battery. Thisvoltage - current relationship is also one of the most reliable ways to estimate thebattery's state of charge.

dl/dV 1026

t4

92

0

-10 0 10 20 30 40 50 60amps

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 sensoruses a magnetic pickup that detects 3 bolts on the motor's gear hub, producing 3 pulsesper 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 wereproblems 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 cleanestsignal.

It should be noted that the power readings, which are the voltage and amperagemeasurements multiplied together, are the input into the motor controller. The motorefficiency is not taken into account, so only 92-94% of the power measured actually getsconverted to shaft power to drive the engine. The variation is due to the torque and speedoperating point of the motor. These DC measurements are used because it is difficult tomeasure the variable frequency AC power that drives the motor, or to get torque readingsoff of the shaft. This motor is specifically designed for high efficiency, which is whatkeeps the efficiency numbers so high. (See Figure 29: Solectria BRLS8 efficiencycurves)

6000 RPM

T 4000 R PM4,000 85-

2000 R PM

2,000 N80

00

90

Figure 29: Solectria BRLS8 efficiency curves 7

Gear transmissionBecause 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 suchhigh speeds, so choosing a system was not easy. Chain and belt drives were ruled outbecause their surface speeds were too high. The best choice seemed to be a gear drivewith a single stage 6:1 ratio. One company quoted high speed filled nylon gears withherringbone pattern teeth, which would have been very nice, with low noise andvibration. The cost was very high though. I settled on a set of steel spur gears. Thesmaller gear had to fit over the kerosene tube, and 1.56" pitch diameter was the smalleststandard 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

SLR)U rp

90

shaft, with the large gear that will go on the motor visible in the background. I did astrength calculation that showed that there would be enough metal left around the shaftafter 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 mistsystem for cooling the gears was too complex and messy so I decided to run themunlubricated. At high speed though, the force on the gear teeth was only expected to be130N, which is a very light load. The gear face thickness was chosen to be 0.5", basedon the interface between the small gear and the kerosene tube on the engine. The tube isthin-walled aluminum, only 0.10" thick, so securing the gear using a key or set screw wasruled out. A micro spline could have been used, but we were running into othermachining difficulties at that time, and did not want to add the cost and complexity of aspline. I decided to bond the gear to the smooth shaft using a Locktite cyanoacrylitebonding compound, designed for such cylindrical bonding applications, and rated to3,000 psi in shear. A 0.5" thick gear would provide enough surface area to allow the fulltorque of the motor to be transmitted without breaking the gear loose on the shaft. Giventhat our running time was on the order of minutes, the gear manufacturer thought thisdesign would hold up, even without lubrication.

The only problem that we have run into with the gears is that the thermal cyclingon the engine is not yet well enough controlled. There are frequently instances where wehave overcooled the engine during flow tests that did not include kerosene flow orignition. Either kerosene flow or ignition would keep the kerosene feed-tube warm, butin their absence, the aluminum tube can get cold enough to shrink away from the steelgear and break the adhesive bond between the two. The aluminum can shrink up to0.005" if it cools all the way down to LN2 or LOX temperatures. The bond gap is only0.0005", so the adhesive would have to stretch by 1000%. This is unrealistic for anadhesive, 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 gluejamming between the gear and shaft. A telltale mark was used to make sure the gearhadn't spun during each test. The next design iteration will use a very low profile key tokeep the gear from spinning and have a shoulder for the gear so that it does not slip up ordown on the shaft.

MountingThe 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 wasachieved, and the gears have a small but manageable amount of backlash. The motor ispositioned so that it is above the engine (see Figure 31: Motor mount). While this madethe mounting plate more complex, it keeps an expensive, borrowed piece of equipmentout of the way of the rocket exhaust.

Engine axis of rotation - Electric motor

Top of engineMotor mount

6:1 gear increaser

Copper combustion chamber

Figure 31: Motor mount

Preliminary cold engine testing

Break in of rotating partsWhen the engine was assembled for the first time, the act of compressing the o-

ring between the engine and the combustion chamber damaged the engine. I found thatthe combustion chamber was out of round by 0.0 13 inches. The chamber was probablycrushed when it was held in a vise to drill the temperature sensor holes. The 2' taper atthe top of the chamber caused metal to metal contact across the o-ring gland and crushedthe corresponding part of the engine when it was sealed in place with six 1/2" bolts, not avery delicate operation. The distortion in the engine caused the chamber separator tocrash and gall.

After the offending and damaged parts had been cleaned up and fixed, it crashedagain. This time because the bearings had also been damaged, but not replaced. At thispoint, 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 wereno longer contacting. The labyrinth seal on top of the LOX feed-line was also rubbingbecause the external LOX plumbing was pulling it to the side. After the first successfulhot fire test, the Teflon holder for the labyrinth seal was damaged and the labyrinth wasreplaced with a better-supported design that did not rub as much. From the motor powercurves it was clear that the engine took less and less power to spin with the motor as theseals wore in.

Power curveOne 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 adynamometer in this fashion, it is possible to tell how much power is going into pumpingkerosene and LOX, and how much power is being generated by the oxygen jets on thespinning 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 thecooled rotating disk inside the hot combustion chamber. Both Carl Dietrich and I did thecalculations to develop this model, but as I got to the point where the equations becamealgebraically complex and the reasoning became circular, Carl had already developed anumerical solving routine on the computer. Not wanting to duplicate efforts, I took on arole of developing other parts of the engine and error checking his model instead offinishing 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 (-); andkerosene pumping (-). Each of these factors will be discussed below. After theexplanations, I will show how the power curves are put together and used.

Motor powerThe 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 productof these two measurements. The shaft output is the electrical input power times themotor's efficiency iq. r varies with speed and load, and can be figured out from the motorefficiency curves if the operating point of the motor is known. Since this is a DCbrushless motor, it is typically very efficient. According to Solectria's published curvesfor this motor (Figure 29), the efficiency should fall between 92% and 94%. Windage andbearing losses within the motor are accounted for automatically because the motor isalways spinning, even in the calibration runs. The motor's electrical efficiency is takeninto account by a multiplier after the data have been recorded. The gear inefficiencieswould 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 6,000rpm which is the speed of the motor when the engine is running at 36,000rpm. This isnegligible compared to the multi-kilowatt loads induced by pumping the propellants.

43

Windage, bearing friction and seal rubbing frictionThese 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 nopropellant present. The results were plotted on a graph of Speed vs. Power (see Figure32: tests 1029 and 1030 power curve). The equation for the curve fitted line is y = 3x10'0

x2.83 . Note that this curve includes the motor's own bearing and windage losses as well.Since this is very close to a 3rd power curve, it seems like windage is dominating. Otherfactors such as bearing and seal friction would be lower order functions of speed.

0 dry spin test 1029-1030power points - Power (dry spin test 1029-1030)

5000 - - - - - - ---

4000

3000

2000

1000

00 10000 20000 30000 40000

rpm

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 inmore and stopped rubbing, the power consumption dropped, and became dominated bywindage. Also, tests done in late August and early September are suspect because of alow battery in the ammeter. The power data presented in the next three sections areassembled from data that were not affected by the rubbing seal or the low battery.

Oxygen jetsThe oxygen jets at the edge of the disk should put out enough thrust during a hot fire testto balance the pumping and other loads on the disk. They are powered by the heating ofLOX in the heat exchange section of the bottom portion of the disk. Thermal energyfrom the chamber vaporizes the LOX, and heats it. When it escapes through the jets itproduces tangential thrust and keeps the disk spinning".

While assembling the engine, I noticed that water running through the jets tendedto 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 anddescribed 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 thrustcomponent by cos(18') or 5%.

LOX pumpingInitially, the LOX pump was tested with water, to see if pumping could be

observed. After that series of tests was successfully completed, LN2 was used to simulateLOX so we could practice pumping cryogenic fluid, without the safety problemsassociated with LOX. The following sample data show that we can detect pumping witha 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 pumpLOX into the engine. Around time index 282, the data show a sudden drop in the disk'sspeed. 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 isbeing pumped into it (Figure 35).

A note about Figure 33: at time index 284.5, the LOX flow appears to increasesuddenly. This is a measurement artifact of the shutdown process. The LOX flow ismeasured by looking at the differential pressure between the gas at the top of the LOXtank, 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 shutdownsequence, this valve is closed. The down stream LOX pressure vents instantly throughthe engine and a dump line. The pressure in the tank stays constant. The result is that thedifferential pressure sensor sees a condition that looks like high flow velocity. All LOXflow data after a normal shutdown should be ignored.

#1091 LOX Flow

40000 40

30000 - - -- 30

E

20000 20

CL L-Tach 1S' -- LOX Flow

10000 10

0

0 02 '0 2 -2 2-F4 2T'6 -'18 280 282 284 286 2 38 2! 0

-10000 ----------- -- ----------- -- -10Seconds

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 motorspeeds up. This is a characteristic of a brushless permanent magnet motor's torque speedcurve. 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 27,000rpm, the motor can no longer deliver full torque at higher and higher speeds, so thebattery current drops. Changing the gear ratio, between the motor and the engine, wouldallow us to move the engine speed up or down at this peak power point.

#1091 Amps

(CE

- Tach- BatteryAmps

Figure 34: LOX Pumping Test, Amps vs. Time

#1091 Chamber Pressure

40000 -

30000

20000

0'

10000

02 '0 2 2

-10000 ' J-- 1- '-

36 276 22

200

150

100

50 E

010

-- Tach- ChamberPressure

-50Seconds

Figure 35: LOX Pumping test, Chamber Pressure vs. Time

46

40000

30000

20000

10000

02

-10000

60

40

- 20

0

'0 272 274 276 278 280 282 284 286 288 2

seconds

-20

-40

--- ,----,----,----,---

4 210 2 38 278 2 2

Kerosene pumpingIt 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 wasundesirable. Ethanol was chosen as a substitute, with roughly similar density andviscosity as kerosene. Figure 36 shows an early ethanol-pumping test. The drop in diskspeed and rise in motor amps around 96 seconds shows that pumping has begun. I foundout much later that we were probably losing a lot of power to rubbing in the seals, whichcasts much of the power data from this early testing in doubt.

1002 pump test

40000 - - - - - - - - - - - - - 80

60

30000

. -7:2 1:6 PM Tach20000

20 E 7:21:16 PM Battery Amps

0

10000

-20

0 -- -- ---- ----- -- --- 4 090 92 94 96 98 100 102 104 106 108

seconds

Figure 36: Test 1002, battery amps during ethanol pumping

The only time the engine was used to pump kerosene alone, was a series of hotfire attempts in which we tried to start the flow of kerosene before the LOX. Theproblem 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 itdown and shut down the startup sequence before engine ignition. Some of the data fromthese tests are presented later.

Power curve analysisMy 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 theentire power curve from that. The engine was run up to full speed with the motor, andthen the motor was shut off and the engine coasted back down to a stop. At each point ina coast down test, the speed is known, and the rate of deceleration can be calculated fromthe data. If the moment of inertia were known, I could figure out how much power itwould take to change the speed of the disk at that rate at each speed. That power is equalto 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 timesper second. By running the motor at steady state at high speed, just before starting thecoast down, the first point can be determined, and the moment of inertia can be solvedfor. The equation for the power dissipated by the engine is P = I o 0' (power = momentof inertia x angular velocity x angular acceleration). Figure 27 shows the coast down testperformed in test 1084. The engine was brought up to 38,000 RPM, the amps wereallowed to level off to get a steady state reading, and then the motor was shut off for thecoast down.

The averaged steady state power at 37,950 rpm was 1686 watts, and this made thepower curve solvable. The results are plotted in Figure 37: 1084 Coast down powercurve, plotted as power vs. speed. Sometimes, in other tests, this curve seems to be a bitoff. This is because the labyrinth seal in the top of the engine rubs different amountsbased on how much tension is in the LOX supply piping. This can change with handlingas well as changes in length from cooling or heating. Often in a test, the engine is run upto full speed and held there until the test starts. Sometimes a power point can be obtainedfrom this brief steady state operation. In tests where this looks like a problem, I havelinearly scaled the entire power curve to match at the point obtained during the test inquestion. Hopefully this approximation will not be necessary in the next engineprototype, when I intend to reinforce the labyrinth seal so it is not as flexible and will notbe able to rub at all.

power curve (test 1084)

3000

2500

2000

1500

1000

500

00 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 generatedfrom 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 pickedout of two different hot fire attempts that did not ignite, but flowed kerosene. The datapoints are each discrete steady state operating points where power and speed data wereobtained. The curves crossing through these points are the predicted power that it shouldtake to pump each fluid at the design mass flow, added to the power from the dry spintests. The pumping equation is Power = M' o2 R2 , where M' is the mass flow, 0o is therotational speed of the disk, and R is the radius that the fluid is pumped to. Note thatsince m' is constant, the water-pumping curve is higher than the LN 2 pumping curvebecause water is 20% more dense than LN. For reference, LOX is 10% more dense thanwater.

It is assumed that for the LN2 predicted pump power curve, liquid is pumped allthe way out to the edge of the disk without vaporizing. This can either happen if thepump channels are full, and the bottom disk only contains gas, or if the entire bottom diskis full and liquid is coming out of the LOX jets. If the LN2 is not making it all the way tothe outside of the disk, then that could explain why the two low speed points are belowthe curve. This seems reasonable, because the heat capacity of the disk might be enoughto vaporize some of the LN2 at these lower pumping rates. The correlation between thepredicted and measured value shows that the electrical input to the motor can be used tomeasure what is happening inside the engine. This will be very useful during hot firetests when it is not so clear what is going on inside the engine.

49

Pumping curves

7000

6000

5000

4000

3000

2000

1000

020000

rpm30000 40000

Figure 38: Pump curves

Next, I will show how this kind of power analysis can be used during a non-steady state situation to confirm what is happening inside the engine. Test 1091 was aLOX pumping test. Figure 39 shows the motor power data along with the tachometerreadings from this test. The LOX valve was turned on at 279 seconds, and full flowdeveloped at 282 seconds. Figure 40 shows the expected power dissipation during thistest. 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 powerexpected to go into LOX pumping from 282 seconds until the LOX shut off at 284.4seconds.

50

w(~3

10000

SS

I

/V

0

A kero pump dataz LN2 Pump data0 water pump data

--- dry spin test 1029-10-1061-- 1084 coast down

predicted kerosenepredicted LN2predicted H20

-

#1091 Amps

4uuuu

30000

20000

10000

02

---------- ------------ ---

0 272 274 276 278 280 282 284 286 288 2!

60

40

20UPa-E

0-Tach-Battery Amps

-200

-10000 --- 40seconds

Figure 39: Motor Amps and speed during LOX pumping test 1091

1091 power curves -Tach

-power curve

2000

0

-2000

-4000

-6000 co

-8000 )

-10000

-12000

-14000

-16000290

seconds

Figure 40: 1091 Expected power dissipation during spinning and pumping

51

a-

40000 -

30000

data

7' 20000 -

10000

0270 275 280 285

I -T --4-

The next graph, Figure 41 compares this to the power that was measured going inand out of the system. The motor puts in electrical power. To this is added or subtractedany power that is released or stored as kinetic energy in the disk (P = I o o'). Theresulting power is the amount that is dissipated by drag, pumping or other forces. Duringa hot fire test, this should be able to detect power being put into the system from theoxygen jets. If the jets produce enough power to run the engine, that will validate theengine 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 LOXpumping 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 dissipatedgoes 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'sefficiency curve. Gears have high peak efficiency (up to 98%), but are not as good athigh speed and low torque. This peak corresponds with a medium speed and the highestcurrent drawn by the motor during the acceleration, which would logically be a highefficiency region. The peaks after shutdown at 285 seconds show up in other tests, andare probably an artifact of a data glitch produced when valves open or close. Thechamber pressure rose to 120 psi (8atm) during this test, so increased windage, at lowtemperatures because the gas in the chamber was cold 02, and bearing loads couldexplain why the actual values are more negative than the predicted values.

1091 power curves

280 285

2000

- 0

-2000

-4000

-6000 c4-)

-8000 5

-10000

-12000

-1 4000

- -16000

290

seconds

Figure 41: Power measurements vs. predicted values

52

40000-

30000-

C- 20000-

10000 -

S- 0270 275

When this type of analysis was applied to hot fire test 1041, (Figure 42: Test 1041power dissipation) it can be seen that there is a correlation between the expected powerused 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 andkerosene pumping, however the oxygen jets should have offset this. Also, the chamberpressure and temperature rose, increasing the density by up to a factor of 3, which causesincreased windage losses and bearing loads, which would be seen here. Because theengine was shut down too early, there are not enough data here to see how everythingwas going to stabilize.

1041 power usage - Tach

-expected power curve

10 per. Mov. Avg. (measured power curve)

40 000 .0

-200035000

-400030000

-6000

2 500 0- 000-8000

0 20000 -10000

15000

-1400010000

-16000

5000 -18000

0 -- - -- - -- - - -20 00 0771 773 775 777

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 thatthese 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 thishappened at the time when the engine is known to have stopped, it is thought that a loosepart 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 sequenceIn 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 theLOX from boiling and vapor locking. Also, the kerosene passages must be warm enoughthat the kerosene does not freeze when it starts to flow. It has been difficult to properlypre-cool this center section without overcooling the rest of the engine. If the kerosenesection gets cold, the kerosene can freeze as soon as it starts to flow. If the bearings andgear get cold, they experience mechanical problems because of differential expansionbetween themselves and the shaft or bearing holder. Fortunately, there have been nofreezing problems in the engine to date.

The first part of the LOX section is vacuum insulated, and so does not tend tocool the rest of the engine very quickly. (It was discovered later that the vacuum mayhave been broken, but the resulting air gap provided some thermal protection in thevacuum's place.) However, as soon as the LOX gets past the pump section, the heatexchange section is thick walled aluminum, and is directly connected, mechanically, andthermally, 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 conductionto bearings and gear

LOX Flow

Aluminum

Figure 44: Thermal Conduction Paths

This part of the engine has been designed to work under the steady stateconditions that exist while the engine is running. The LOX section is well insulated tostay cold, and the kerosene section stays warm because of this insulation and the heatfrom the kerosene flow and the combustion chamber. During the startup sequence, whichI will discuss later, it becomes necessary to cool the center section without overcoolingthe rest of the engine.

I performed a test to find out how long the LOX section could be cooled withoutovercooling the rest of the engine. The rotating part of the engine, the disk, was removedfrom the bearings so that it could be monitored with a thermocouple. LN2 was flowedthrough the disk, while the temperature at the kerosene injector port was monitored. Ichose this point because it is the narrowest passage that the kerosene will have to flowthrough, and is also closest to the LOX section as far as heat flow is concerned. The testshowed that we could flow LN2 through the engine for 160 seconds before the keroseneinjectors reached -40C, which is the freezing point of kerosene. (The heat capacities ofLN 2 and LOX are within 10% of each other.)

However, it must be noted that this test was performed with the LN 2 tankpressurized to 35psi. Also the disk was stationary for this test. If it were rotating, and theflow rate were increased due to pumping, this cool-down time would be proportionallysmaller. For the hot fire tests, the LOX tank was set to 100 psi, which would reduce thecooling-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 sectionof the engine, without cooling down the rest of the engine. The first part of the probleminvolved getting a good flow of LOX to the engine. Good flow was achieved by pre-cooling the supply lines as shown in Figure 11. The next problem was cooling theinsulated LOX feed tube preferentially, without overcooling the kerosene injectors. Insome of the less favorable tests, it took over 10 or 15 seconds to start pumping LOX, andI 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, andthere 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 thevacuum insulated tube, and since the engine was not rotating, would not be flung out intothe aluminum section which is thermally connected to the kerosene pump. This is a wayto preferentially cool the inside of the engine without cooling the outer parts. I devised aprocedure through which we would open the engine LOX valve for a second and thenclose 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 procedureappeared to solve the problem the day it was implemented, but a larger problem mayhave been our failure to pre-chill the LOX in the tank before starting the engine. Thiswill be explained further in the section about Test 1067, October 15, 2000. The pre-chillsequence described there replaced the procedure of burping LOX into the feed tubebefore the tests with pre-chilling the LOX tank.

Startup sequence

Startup requirementsIn 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 pressurehigher 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 LOXpump, and the LOX flow passages must be cool enough to let LOX flow through withoutboiling. If boiling occurs, the bubbles can block the flow passage and stop the flow untilthey have risen out of the way. This is a slow and unsteady process, and must beavoided.

To pressurize the kerosene the pump has to be spinning, and the kerosene pumpcannot be so cold as to freeze the kerosene in it. Pre-cooling the LOX pump withoutovercooling the kerosene pump has been difficult.

An ignition source must be present inside the chamber. We started with a sparkigniter, and later replaced it with a pyrotechnic igniter.

Sequence historyThis 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 flowInitially, 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 keptgood flow from ever reaching the end of the supply line. Head heights up to 5 meterswere tried but were unable to push through the chugging flow. The LOX tank wasreplaced with a pressurized vessel. Since the pressure could be arbitrarily set, a long feedline was not needed, reducing heat flux into the fluid, and the pressure could be increasedto push through any remaining bubbles. A pressure of 100psi was found to provide theright flow rate for water (which has a similar viscosity as LOX) to the end of the feed-

56

line. The amount of water that flowed out of the feed-line in a measured time matchedthe design rate of 0.25liters/sec. (The plumbing details had changed and smaller orificeswere 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 coolingdown 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 thenwaiting 5 seconds before starting to rotate the engine. The reasoning is that the vacuumjacket insulates center of the engine very well. If it could be filled with LOX, withoutflowing much through the rest of the engine, the center section could be preferentiallycooled, which is exactly what was needed. The 5 second pause gives the LOX time toboil off, and cool the engine. By doing this with the engine not turning, the tendency forthe LOX to get flung to the ends of the pumping tubes is eliminated, and LOX flow pastthe pumping section is minimized.

During the first hot fire attempt, LOX pumping was established. We used myidea to detect pumping using the motor amperage data. When the motor currentincreased past 50 Amps, indicating that more power was being used at that speed thancould 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 oxygenbeing injected into the chamber. When the kerosene was turned on, its head height wasinsufficient 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, acheck valve was used in the kerosene line to prevent back flow under these conditions.

Kerosene flowThe 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 asthe chamber floods). The engine spun up to speed, the kerosene started flowing, the LOXstarted flowing, and then the engine bogged down and stopped. After further testing Irealized that the engine was flooding with kerosene. When the liquid level reached thebottom of the disk, the drag forces went up, and pretty much stopped the disk, eventhough the motor applied full power to try to keep it moving. The LOX takes about 3seconds to cool down the engine enough to flow effectively, and there was just notenough time before the engine flooded.

57

1026 Kerosene pumping teryAmps

35000 - - - - - - - -- - - - - - - - ------ - 70

30000 60

25000 50

20000 - 40

15000 30 E

10000 20

5000 10

0 0804 806 88 8 0 8 2 8,4 8,6 8 8 80 82

-5000 ----------- - - - - - - - - - - - - - -- -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 muchpropellant in the chamber, and the throat filled with liquid kerosene, a detonation couldover-pressurize the feed-lines, or worse, the chamber walls. The only option left was topressurize the kerosene system, and start the LOX flow first. The LOX did notaccumulate in the chamber as the kerosene did, because it would boil immediately oncontact with the copper chamber walls, and the gaseous oxygen could escape through thethroat more easily than the liquid kerosene. The decision was made to run the LOX untilgood flow was established and then start a pressurized kerosene flow.

I found that the chamber pressure is around 10psi while gaseous oxygen isflowing (see Figure 46) from 860 seconds through 863 seconds, and jumps to a highervalue when liquid flow starts. Once LOX starts flowing in a smooth liquid flow, thepressure jumps up to 120 psi, but this was not clear at the time. Other tests indicated thatthis 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 turnon the igniter when the chamber pressure sensor detected a rise of 14 psi, indicating thatLOX flow had been established. The LOX flow reading is not used because chuggingflow can appear as full flow for short periods of time as opposed to low flow (see Figure13: Test 1041 LOX Flow). It is difficult for the computer, or a person, to recognize fullflow without the benefit of hindsight.

58

Test 1025 chamber pressure - Tach

- ChamberPressure40000 - - - - - - - - - - - - - - - - - - - - - - 140

120

30000 - - 100

80

20000 --- 60

40

10000 -20

0

0 -20855 856 857 858 859 860 861 862 863 864 865

Seconds

Figure 46: Test 1025 Chamber pressure with GOX flowing

IgniterUntil 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 providedan arc between two wires, which were inserted up the nozzle. However, it became aconcern 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 theside of the chamber.

I devised a test to time how long it would take to light a solid propellant motorusing a resistance wire igniter. If an LED was wired in parallel to the igniter, a videocamera could be used to document the time between the LED illuminating (the ignitergetting power) and the motor igniting. Two tests showed that it was 5-6 frames, (5-6/ 3 0 thS of a second) delay, and it was repeatable. The flame from an Estes C6-0 motorranges from 6" in length during the core burn at the beginning, to 4" for the end burnsection, and lasts 1.8 seconds. Either one is enough to have a flame crossing the entirechamber from one side to the other. A test of the igniter by itself in the combustionchamber showed no noticeable pressure reading while it was firing.

Spin upA 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 theengine longer to cool without spending a lot of excess time at high speeds, which putswear on the engine and cools the outside parts of the engine faster. Unfortunately, thedifference in densities between LN2 and LOX was enough to keep the engine fromreaching a high enough speed and the hot fire test using this method was aborted (seeFigure 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 engineat high speed during startup. The motor only has enough power to drive the disk at24,000rpm when LOX is being pumped, and this is too low of a speed to get steady stateoperation.

Test 1040 tachometer

30000

25000

20000

15000

10000

5000

9 5 917 919 921 923 925 927 929

-5000 ---

Seconds

Figure 47: Test 1040 aborted because of under-speed

First hot fire test, startup sequence

Tests performed and lessons learned about startup in each testThis 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. Thesetests occurred during the last four out of seven separate trips to the New Hampshire testfacility. The first trip, in July of 2000, highlighted the importance of when the propellantflows are turned on. The second trip, in September, featured a successful 0.5 secondengine burn. On the third trip, in October, the burn only lasted for 0.01 seconds, and inNovember, the engine started successfully, but then exploded due to unnoticed damagesustained just before the first July test. Tests of any type done on the engine were givenan identifying number. The sequence was started at 1000 to allow room for tests prior tothis engine prototype.

Test 1025, July 13, 2000Test 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 spikedabove 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 thechamber 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 thekerosene pumping passages and kept the kerosene from ever starting to be pumped.

Changes: Make sure kerosene has filled the pump channels before turning on theLOX so the chamber pressure does not back-flow up the kerosene lines.

Test 1026, July 13, 2000Test plan: This time, the kerosene was started first, followed by the LOX and the

igniter.

Results and conclusions: The kerosene flowing first made the combustionchamber very fuel rich, which prevented ignition. Then after about 3 seconds, thechamber actually filled with kerosene, and the disk, immersed in liquid, slowed down to asteady state of 4000 rpm very quickly. This scenario was confirmed with a series ofkerosene pumping tests, which convinced us that the LOX should be started first toprevent 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 AirForce Rocket Lab, we decided to replace our spark igniter with a pyrotechnic igniter' 9. Iworked out a way to install an Estes C6-0 solid model rocket motor in the side of thecombustion chamber, which was used in all subsequent hot fire tests.

Test 1040, September 3, 2000Test 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 ofthis step to start the kerosene flow and start the igniter. The pressurizable kerosene tankwas set to 28psi, which we (incorrectly) thought would be enough to overcome thechamber pressure with fully developed LOX flow. The chamber pressure trigger pointwas set at l4psi, higher than the pressure with just gaseous oxygen flowing, but lowerthan that with LOX being pumped. Also, this time we started the LOX flow at 0 rpm sothat the engine could cool down as it sped up.

Results and conclusions: The LOX provided too much pumping load, and themotor 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 beforeignition so that some kinetic energy would be stored in the disk. Since pumping createdsuch a load on the motor, our hope was that the engine would start closer to the steadystate operating speed of 36,000 rpm if it started at a high speed.

'9 Pers comm, David Barland, Air Force Rocket Laboratory, June, 2000

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Test 1041, September 3, 2000For 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 basedthis analysis on the data, videotapes, and post fire inspection of the engine:

" The LOX feed-line leading to the engine was cooled by flowing LOXthrough it for 750 seconds. This time was chosen as a conservatively longcool-down time based on previous tests, since the temp sensor on the linewas not working.

* LOX was burped into the engine for 1 second followed by a 5 secondpause, 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 theLOX 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 index774.1 seconds)

* The igniter was started, and after a brief 0.2 second delay to ensure fullignition, the kerosene flow was turned on. (There is a 0.2 second delaybetween the kerosene valve being opened and good flow. This was testedin a similar manner to the igniter time delay.)

* Since the LOX flow had not really developed as planned it took another0.5 seconds for LOX to start pumping, at which point combustion in thechamber began (775 seconds).

* 0.2 seconds after ignition began, the chamber pressure rose above 200 psitriggering the computer to switch from start to run modes. At this point, aprogramming error caused the computer to immediately switch to theemergency shutdown sequence.

" Almost coincident with the shutdown commands, A plume with shockdiamonds becomes visible and lasts for the next 0.43 seconds (see Figure49: Hot fire test 1041- plume with shock diamonds). The engine developedabout 120 lbs. of thrust. The aluminum heat exchanger on the bottom ofthe disk melted at the points where it is not getting enough cooling frominside the disk, namely on the outer corners, and in the areas where the heatfins become tall and narrow towards the center. The melting issymmetrical, but not as severe in the areas where the disk has features onthe inside and thus, more thermal capacity.

" Just as the plume is fading away, one of the two LOX dump lines, whichhas been blown out of its water bath because it was not tied down, swingsaround far enough to kink. The pressure in the LOX dump area of theengine doubles, blowing off the labyrinth seal holder and soon after,ruptures two of the kerosene dump lines. During this test, the gas separatorhad been set to 100psi, instead of its usual 26 psi. This last minute changewas to try to fix a problem where frost was accumulating on the kerosenedump lines. This led us to believe that the gas separator was allowing LOXinto the kerosene system and causing the frost. (I figured out during thetest 1098 postmortem, that the frosting was due to the LOX feed-tube

62

leaking directly into the kerosene pump. This crack, created by bendingthe LOX feed tube in early July, is what caused the engine to explode intest 1098) Coincident with the unintended shutdown three vent tubes burstbecause the gas separator pressure was set too high. In the end, it wasfortunate that the engine shut down when it did.

#1041

771 772 773 774 775Seconds

- 350

300

250

- 2005

150

- 100

--- ach 1-2ChaberPressurel

50

1 1 1. 0776 777 778

Figure 48: Test 1041 Chamber Pressure

Figure 49: Hot fire test 1041- plume with shock diamonds

63

30000

25000

t 20000

15000

10000

5000 1

076 6 767 768 769 770

Changes: We replaced the labyrinth seal and its holder, and reduced the gasseparator 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 nexttest at a higher speed.

Test 1067, October 15, 2000Test 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 bywatching for the disk to slow back below 35,000 rpm. This used the tachometer as thetrigger because that was historically the cleanest signal, and nothing other than pumpingwould 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, indicatingpumping. The LOX never started flowing and pumping correctly. Kerosene flow startedafter 13 seconds, and since the igniter was running the engine lit. However, since theLOX was not pumping well, there was only gaseous oxygen being blown into thechamber. As the first few drops of kerosene sprayed into the chamber, the engine ignitedand a plume was visible. Then, about 0.01 seconds later, as the kerosene flow developedinto full pumping flow, the mixture ratio got so rich that it could no longer supportburning. The conclusion was that the LOX never started pumping because it was boilingor cavitating at the pump inlet. I devised a matrix of tests, comparing pump speed andLOX feed pressure. The goal was to find the border between the cavitating regime andgood pumping. During these tests, we figured out that we should pre-chill the LOX byventing the tank to atmospheric pressure before pressurizing it for the test. This way, thebulk fluid would be boiling at 90K, at 1 atm., and would then be pressurized up to 100psi. At this pressure, 90K is below the boiling point, so there is some margin before thefluid boils or cavitates in the engine. The cavitation tests showed that if we pre-cooled inthis manner, we would get reliable pumping.

Changes: After eliminating the cavitation problem, the new sequence, was goingto be based on detecting pumping again. I realized that once pumping begins, thechamber pressure jumped up over 100 psi (see Figure 50: Test 1081 chamber pressurewith LOX pumping only). This meant that the kerosene system would have to bepressurized to over 100 psi. This would create large leakage problems. A solution wasneeded that would allow the kerosene pressure to remain lower. We could not turn on thekerosene more than 0.5 seconds before the LOX flow fully developed, because thatwould risk flooding the chamber, as happened in test 1026. We needed a way to predictwhen the LOX flow would develop, and turn on the kerosene and igniter just before. Irealized that this was the sequence that happened accidentally in 1041, our mostsuccessful test to date. I graphed the time from turning on the LOX valve until fullydeveloped 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 thetests at 95psi got consecutively lower times, it was hypothesized that the engine wascooling down, and the pump time was related to the disk's initial temperature as well asthe 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 pumpingtime and warm up time between tests.

#1081 Chamber Pressure

45000 120

3 5 0 0 0 -1 0 - - 0

30000 -- ----- - -- _- --

25000 -- 60

20000 -- -Chber_PreSUF

15000 - ---- 40

000 i- -- -- -.- . -- __ -- - - - --22000

0 --1--

_

-000 --- 20

Figure 50: Test 1081 chamber pressure with LOX pumping only

Time until pumping

5

4.5 4

4

3.5

3 43

02.5

2

1.5

1

0.5

060 80 100 120 140 160 180

LOX pressure (psi)

Figure 51: Time until pumping as a function of LOX pressure

65

Test 1098 Test session, November 4-5, 2000A 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 conductpumping 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, byholding a thermocouple up the throat of the engine and placing it on the bottom of thedisk. It took 60 minutes for the disk and the labyrinth seal to return to ambienttemperature.

Figure 52 shows the relationship between the warm up times between tests andhow long it took to develop good LOX flow. The two points on the far right were thefirst tests of each day. 500 minutes was arbitrarily chosen to represent a long time sincethe previous test. The other high test in the middle, at four seconds, should be ignoredbecause there were three aborted tests immediately before it, which would have tended towarm up the engine.

Time till pumping

4.5

4

3.5

2.5

E22

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 theigniter and kerosene to turn on at 2.4 seconds after the LOX was turned on. There is a0.2 second delay between the commands and the actual igniter ignition or the keroseneflow. If the chamber pressure spiked from the LOX pumping before 2.4 seconds thecomputer would shut down. If the LOX started after the 0.2 second delay, the enginewould light. As luck would have it, the LOX pressure built up exactly half way throughthe 0.2 second time delay, and the kerosene was turned on, but didn't flow. After

66

replacing the igniter and moving the ignition time back to 2.0 seconds, test 1098successfully lit (see Table 2 and Figure 53: Startup sequence as used in test 1098).

Table 2: Final Startup Sequence

Sequence Actual time Criteria for next Commanded Event What is happening in the engineTime index from sequence step(seconds) test 1098

(Figure 53)T -600 Cool the LOX feed labyrinth seal cools, the rest of the

plumbing engine stays warmT -5 983.5 when labyrinth seal Command full speed Engine is spinning up to speed

temperature has with the motorleveled out

00.0 989.35 When the engine has Turn on LOX flow LOX is cooling the LOX pumpreached 35,000 rpm

02.0 991.35 Wait 2.0 seconds Turn on Igniter(based on time since

last test, Figure 52)02.0 991.4 immediately Turn on kerosene02.2 991.6 The igniter lights02.2 991.6 Kerosene starts flowing

02.5 991.9 The LOX pump finishes cooling, andLOX flow is established in LOX pump

02.5 991.9 Chamber pressure rises from LOXpumping

02.6 992.0 Engine ignites02.7 992.2 Chamber pressure continues to rise

from combustion02.7 992.5 Chamber pressure transition t o

rises above 200 psi extended burn phase

#1098 Chamber Pressure

200

-Chamber

-- Tach

150i--

100 1

E

0

50

09

-50 '---J---

_Pressure

9 35 9i 6 91 '7 9 38 9

Start I dbExtended burn

;ene on

H1 9

Igniteron

Engi e reaches35,0 irpm

LOXon

39 9i

kero

0 9 )2 9

40000

35000

30000

25000

20000 -

15000 CD

w10000

5000

130

-5000Seconds

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-speedvideo camera showed a burst of sparks coming out of the nozzle. These were probablythe first bits of melted engine material being shot out of the engine. After a few secondsof quiet, a series of explosions whited-out the video screens, and left the test stand inflames. The post test analysis showed that a crack had developed at the base of the LOXfeed tube. This probably happened four months previously, when the LOX tube got bentduring a test, and we straightened it back out. Stainless steel is not a good material tobend and straighten, as it fatigues and cracks easily. This crack allowed LOX to get intothe kerosene pump, explaining why the kerosene dump vent lines were getting cold. Thispropellant mixture lit, burning a bigger hole in the LOX tube, and eroding the kerosenepump channels. The burning mixture no longer provided film cooling for the combustionchamber, which burnt away until it burned through a temperature sensor port. Thattemperature sensor goes off-line near the end of the test. A significant amount of themelted engine was blown out of this hole in the combustion chamber and sprayed againstthe back wall of the test stand. The bottom disk was not getting the right amount of LOXcooling, and it melted off, the pieces jamming into the rest of the disk, and stopping itquickly enough to shear off some of the gear teeth. The rest of the engine was depositedon the blast deflector (see Figure 54).

Figure 54: The results of test 1098

Conclusions and Future objectivesThe Centrifugal Direct Injection Engine (CDIE) offers to be a low cost, high

performance engine. It runs on an expander cycle, although this prototype implementsthat without a conventional turbine section.

A prototype engine was built using low cost materials such as aluminum andstainless steel. The various parts of the engine, such as the rotating seals were tested anddebugged before the first hot fire test. An external electric motor was used to spin the

68

engine up to its operating speed and the LOX, kerosene, and igniter were turned on in theorder determined by this research to start the engine. The external electric motor also actsas a dynamometer for the rocket engine and produces data that can be used to measurewhat is happening inside the engine.

The engine ran for 0.5 seconds during the first successful hot fire test, and wasdestroyed in a later test. These tests proved that the startup sequence could start theengine repeatably. The engine was not run for an extended duration, however, so thesteady-state operating point was not reached. A test at the steady state operating point isneeded to verify the heat transfer models used in the design.

In future research, the engine will be rebuilt and run for an extended period, andthe dynamometer will be used to measure the power balance in the engine. This data willallow the heat flux from the combustion chamber into the LOX flow to be calculated andcompared to the expected values. Once the design of the engine has been verified, aflight weight engine will be built, along with a launch vehicle. The engine and vehiclecombination will be flight tested to verify the performance of the engine under flightconditions.

Ideally, in a flight vehicle, it would be possible to start the engine using the gasstartup described on page 38. Blowing high-pressure gas through the LOX channelscould spin the engine up to its operating speed. The challenge then would be to get theengine started before it lost too much speed. The current startup sequence requires a 2-3second 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 toshorten the cool-down. The motor could be used to start the flight vehicle, but a methodto disconnect the motor while it is running at high speeds would be required. This iscertainly possible, but the gas startup would be much simpler if it could be made to work.

69

Appendix: Support Equipment

Test stand structureThe 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 inThornton, NH, 60 miles north of Concord. It consists of a 4 inch thick concrete pad, witha three sided cinderblock structure built on it. The structure has a removable roof, and isdesigned to direct the blast from any explosions upwards or towards the farthest end ofthe field.

The engine is secured in a test stand that holds it vertically. A blast deflectorshield angles the rocket's exhaust out the open side of the enclosure. Small kerosene andLOX run tanks are filled before each test, and are attached to the outsides of the walls, onopposite sides of the structure. This keeps the tanks out of a blast, and keeps thepropellants separate in the event of a leak or spill. The electronics and compressed gascylinders are attached to the back wall, on the outside of the structure.

LOX systemThe 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 ventports at the top, and a feed port at the bottom that goes to the engine, or through a coilheat exchanger in a large bucket of water and then a vent. This can be used to dump theLOX out of the tank without sending it through the engine. A differential pressure sensormeasures the static pressure in the engine feed-line flow compared to the static pressurein the tank. This is used to get the dynamic pressure of the flow and a LOX mass-flowmeasurement. There are also load cells under the tank's feet. The tank weight is used totell how full the tank is, and as a backup flow measurement. The tank can be pressurizedwith helium that will not condense when exposed to LOX or LN2. It also has a 240 psipressure 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 isdelivered by truck and left on a small concrete pad near the road.

Kerosene systemThe 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 amanual dump valve at the bottom of the line. Load cells are used to measure the tank'sweight and this is used to calculate the flow rate out of the tank.

70

Electronics / control systemsThe control and instrumentation system monitors the engine during tests, and

using a computer, runs the control sequences. This system monitors motor speed, batteryvoltage and current, LOX and kerosene tank weight, LOX flow and pressure, LOX feed-line 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 usesinput from the sensors to control the motor speed, and the positions of the kerosene andLOX valves. It also controls the igniter and the gas separator valves.

The kerosene, LOX and electronics systems are each a modular unit. They can becarried up to the test site on a specially modified trailer. They mount to the test standwalls with bolts and quick release fittings and connectors. The same units are used atMIT for the laboratory testing of the engine.

71