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AlAA 92-3301Results of Labscale Hybrid RocketMotor Investigation6. Greiner and R. A. Frederick, Jr.Propulsion Research CenterUniversity of Alabam a in Huntsvil leHuntsville, AL 35899

A1AAJSA %/ASM%/ASEE28th Joint PropulsionConference and ExhibitFor permlsslon to copy or republish, contact the A m erk an Instflute of Aeronautics and Astronautlcs370 L'En fam Promenad e, S.W.. Washlngton, D.C. 20024

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Results of Labscale Hybrid Rocket Motor Investigati on

vB. Greiner* and R. A. Frederick, Jr.**

Propulsion Research Center, University of Alabama in HuntsvilleHuntsville, AL 35899

ABSTRACTThis work was performed to

establish a labscale hybridrocket motor test and evaluationcapability at NASA Marshall SpaceFlight Center. The scopeincluded activation of a LabscaleHybrid Motor, determination ofbaseline burning rates for PVWAfuel, and replication of pressureoscillations for HTPB fuel. The0.820-in.-diameter port, 10-in.-long fuel grains were burned fortwo seconds with gaseous oxygen.PMMA fuels were tested at oxygenfluxes from 0.047 lbm/sec,in2 to0.378 lbm/sec,inz, and the HTPBfuel was evaluated at 0.378lbm/sec.inZ. The experimentswere instrumented with pressuretransducers to determine oxygen

'W flow rates and chamberconditions. Cartridge loadedgrains were weighed before andafter the experiments todetermine fuel flow rates. Theresults showed that the labscalehybrid motor replicatedpreviously reported PMMA fuelregression rates. The resultsalso replicated low-frequency(

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in which the fuel regressionrates and motor chamber pressuresfor various fuel compositions

W could be evaluated. The firsttask in this effort was to test astandard fuel, Poly (methyl-methacrylate) (PMMA), to confirmthat the LHM produced resultsconsistent with previousinvestigations. The second taskwas to determine if chamberpressure oscillations previouslyobserved in this type of motor[11 could be replicated.Specifically, two different aftclosure geometries were testedfor a hydroxyl-terminatedpolybutadiene (HTPB) fuel.Success on these two tasks wouldwould verify the system allowingprogression to detailed fuel burnrate studies and pressureoscillation investigations.

Several fuel regressionrate expressions have beenproposed for hybrid rocket motorsin the past. Most are attemptsa t following the behaviorobserved in solid rocket motorsgiven byv

i. = aPc"

In hybrid type motors, it hasbeen observed [21 that the semi-emperical regression rateequation

i = aGox"

provides an accurate descriptionof hybrid regression behaviorunder some conditions. It hasbeen noted, however, that somehybrid configurations, mostnotably those with metalizedfuels, have a pressure dependance[ 2 1 . To account for thisdependance, an alternateregression equation has beenproposed

t = aGoxnPcm

Even more complicated forms havebeen put forward, however forthis investigation the simpleaGox" form was used.

In the past, oscillationshave been observed in chamberpressure of hybrid motors[l, 3 , 4 1 . Oscillations have beenobserved for both gaseous andliquid oxidizer systems. Someexplanations have been offeredfor the liquid oxidizerinstabilities. However, anunderstanding of this phenomenonfor gaseous oxidizers has not yetbeen verified. Thisinvestigation attempted toduplicate a particular conditionthat has produced pressureoscillations as well as one thatattenuated them for gaseousoxygen.

The LHM was selected as thetest motor for its simple,flexible design. The motor u s e scartridge loaded grains allowingmultiple tests to be performed ina single day. The motor designalso allows adjustment of grainlength, throat diameter, and aftclosure geometry. A wide rangeof oxygen flow rates are possibleand chamber pressures up to 1100psia are allowable. All of thesefeatures provide an efficient wayof evaluating fuels.

The results of this workshow that the LHM reproducedregression rates that areconsistent with past results forPMMA fuels. The averageregression rates were within 10%of results from similar motors.The LHM also reproduces a non-acoustic pressure oscillationsoriginally observed by ThiokolCorporation. These results haveestablished the operation of theLHM at MSFC. The LHM will beuseful tool for fuel screeningstudies and investigations of theeffect of motor geometry onunsteady combustion phenomena.

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APPROACHA complete facility for

testing the LHM was establishedat MSFC during this project. Thebasic motor hardware was acquiredby MSFC from Thiokol Corp., Utah.The system was installed in theMSFC East Test Area, Building4583. Specific work accomplishedincluded the adaptation of aexisting gaseous oxygen feedsystem, modification of the motorto accept additional pressureinstrumentation, and theinstallation of instrumentationand controls.

W

Jiardwate and LatianA diagram of the motor isshown in Fig. 1. Cartridge

loaded fuel grains are installedin a steel grain holder. Eachgrain is 2.5 in. long, has a0.820 in. diameter port, and a1.375 in. outer diameter. ThePMMA (C,H,O,) grains were machinedfrom cast material and pressed

W into phenolic sleeves. HTPBvacuum cast in the phenolicsleeves. Oxygen is introduced inthe forward end of the motor anddiffused in a forward closurecontaining a diffuser screen.The aft closure contains agraphite insert that holds atungsten nozzle. An alternateaft closure is available thatallows an extended mixing volumebehind the last grain. Variousgrain holders can be combined toallow aggregate grain lengthsfrom 5 to 2 4 in. to be tested.

grains ( C 1 . 0 3 H 1 0 . 1 1 N 0 . 1 3 ~ ~ 0 . 1 7 8 ) were

Instrumentation, shown inFig. 2, supplied data on themotor pressure, temperature, andoxygen flow rate. Forward, aft,and differential (forward minusaft) pressure gages measured thepressures inside the chamber.Eight thermocouples measured thesurface temperatures of theforward and aft closures. Thepressure and temperature of the

L/

incoming oxygen was measuredupstream of a venturi todetermine the oxidizer mass flowrate. Data were recorded on adigital system (0-50 Hz) and onan analogue system (0-6,500 Hz).The analogue data were digitizedfor FFT analysis. A precisionbalanced was used to weigh eachgrain before and after each test.Video cameras monitored the motorand plume during each firing.

Motor controls allowed thestarting and stopping of themotor. The oxygen flow iscontrolled by a valve upstream ofa metering venturi. Ten secondsprior to ignition, the oxygenflow is initiated. A currentapplied to a squib in the forwardclosure ignites the motor. Twoseconds after ignition, theoxygen flow is terminated andnitrogen is flowed through themotor.

Table 1 shows the testmatrix for the PMMA regressionrate study. Eight experimentswere performed for Gox flux ratesfrom 0.047 to 0.378 Ibm/sec,in2.The motor contained f o u r fuelsegments combining to form a10.0-in. long bore. The long aftclosure was used with a 0.306in. diameter throat in thenozzle.

Figure 3 shows theconfigurations used for the HTPBexperiments. Again, four fuelgrains were employed with a 0.306in. diameter nozzle. The Goxflux used for all HTPB tests was0.378 lbm/sec.in2. Only the aftclosure volume was changed forthese tests. The short mixingsection provided a L* of 16 in.while the long aft mixing sectionhas an L* of 85 in.

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experimentally determined total-. mass flow rate

Data from the experimentswere analyzed to evaluate theactual oxidizer-to-fuel ratio, & o c = ribx + mr ( 5 )fuel regression rate, and acombustion aualitv factor. The The theoretical characteristic

velocity for the fuels andmass flow rate of the fuel issimply the change in mass of the oxidizer used were calculatedgrains measured before and after using a thermochemistry code.

The characteristic velocity forthe test divided by the testaction time equilibrium combustion is afunction of equivalence ratio,

mr = hAt

The mass flow rate of each grainwas first determined, thencombined to determine an averageregression rate for each test.The mass flow rate of oxyger. tothe motor is determined from theexpression

( 2 )W where the upstream pressure and

temperature are measured duringthe test. An effective averageregression rate of the fuel wasdetermined using

At 2 At ( 3 )

pressure, and reactantcomposition

C*rheo = f(@, Pc, React) ( 6 )This allows the computation of aquality factor

C * Wqc. = __C*theo ( 7 )

This factor has been used in thepast in injector design,propellant evaluation, and in thestudy of irregular combustion.[ 6 1

FcmlLlsThe results of the

experiments show the overallperformance of the LHM. Figure 4is a plot (from digital data) ofthe head end chamber pressure asa function of time. This sampleresult is for an HTPB fuel and a0.378 lbm/sec.in? Gox rate.

This expression assumes that allregression occurs over the bore Figure5 has plots of theand does not account for burn rate of individual grains

~resression on the exDOSed ends of for the PMM?. experiments. Thethe forward and aft-grains. The burn rate for each segment isexperimental characteristic plotted by test. Segment 1velocity of the motor is defined represents the grain nearest thehead end of the motor and seament. -4 the grain nearest the aft endof the motor.

Figure6 shows test-averageP M M A regression rates as a4 )This is determined from an function of actual oxygen flux.average chamber pressure, the The solid line is a least-squaresmeasured throat diameter, and the curve fit of the test data. Thetwo dashed lines represent datav

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f i t s f o r PMMA f u e l s f ro m p r e v i o u si n v e s t i g a t i o n s [ 5 1 .W F i g u r e 7 s h o w s t h ep r e s s ur e - t i m e c u r v e s f o r t h e HTPBs t u d i e s . F i g u r e 7a sho ws t h er e s u l t s f o r a s h o r t a f t c l o s u r e

w h i l e 6 b i s f o r t h e l o n g a f tc l o s u r e . F i g u r e s 8a and 8b showw a t e r f a l l p l o t s of t h e p r e s s u r e -t i m e t r a c e s for t h e s h o r t a ndl o n g a f t c l o s u r e . The h o r i z o n t a la x i s r e p r e s e n t s f r eq u en c y ( 0 t o5 0 0 0 H z ) , t h e v e r t i c a l a x i sr e p r e s e n t s p r e s s u r e ( p s i ) , a ndt h e d e pt h a x i s r e p r e s e n t s t i m e( A t i s 0 . 2 0 s e c . )

F i g u r e 9 s h o ws t h et h e o r e t i c a l c h a r a c t e r i s t i cv e l o c i t y f o r t h e PMMA a n d HTPBf u e l s a s a f u n c t i o n o fe q u i v a l e n c e r a t i o . The a c t u a lv a l u e s f r om t h e e x p e ri m e n ts a r ep l o t t e d on t h i s g ra ph a s w e l l .The s t o i c h e m e t r i c o x i d i z e r - t o -f u e l r a t i o f o r PMMA i s 1 . 9 2 a n dHTPB i s 3 . 0 8 .

l2Lxmm2-v

F i g u r e 6 shows t h a t t h ePMMA f u e l r e g r e ss i o n i s l i n e a rw i t h Gox f l o w r a t e u n d e r t h ec o n d i t i o n s t e s t e d . A l e a s t -s q u a re s d a t a f i t p r od uc ed

t = 0 .044 GOX'.' ( i n / s e c ) ( 8 )T he t wo d a s h e d c u r v e s r e p r e s e n tc o r r e l a t i o n s f ro m p r e v i o u ss t u d i e s . The u p pe r c u r v e f o rm o t o r s t e s t e d a t 50 p s i a a nd t h el o we r c u r ve f o r t h o s e a t 3 0 0p s i a . The r an g e o f p r e s s u r e se x p e r i e n c e d d u r i n g t h e L H Mt e s t i n g were from 50 t o 315 p s i ga n d f a l l be tw ee n t h e s e tw oc u r v e s . T h e LHM d a t a a r e f r om 5t o 1 0 p e r ce n t d i f f e r e n t t h a n t h er e f e r e n c e d w ork.

T h e b u r n i n g r a t ed i s t r i b u t i o n p l o t s shown i n F i g . 5 show s i g n i f i c a n t v a r i a t i o n i nt h e b u r n i ng r a t e a s a f u n c t i o n of'4

g r a i n s eg me nt . Th e d i s t r i b u t i o np a t t e r n i s c o n s i s t e n t w i t h t h ef o r w a r d g r a i n s h a v i n g t h eg r e a t e s t b u rn i ng r a t e s . T h i s i si n d i c a t i v e o f o n e known b i a s i nt h e d a t a r e d u c t i o n t e c h n i q u e .T he m et ho d f o r d e t e r m i n i n gb u r n i n g r a t e a ssu me s t h a t t h em as s l o s t d u r i n g t h e t e s t a l lcomes from t h e b o r e . I n s p e c t i o no f t h e g r a i n s a f t e r t h e t e s t ss how s t h a t r e g r e s s i o n o c c u r s o nt h e e x po s ed f a c e s o f t h e f o r wa r da nd a f t g r a i n s . Not a c c o u n t i n gf o r t h i s w i l l b i a s t h e s eg me ntb u r n i n g r a t e s a bo ve t h e a c t u a lv a l u e s a n d t h e r e f o r e w i l l b i a st h e m o to r a v e r a g e ab ov e t h e t r u ev a l u e .

T h e a g re e me n t of t h e d a t aw i t h p a s t r e s u l t s i n d i c a t e s t h a tt h e LHM s y s t em an d a n a l y s i st e c h n i q u e s a r e r e a s o n a b l e .

T h e H T P B e x p e r i m e n t s s h o wt h a t t h e a f t c l o s u r e i n f l u e n c e st h e m a g n i tu d e o f p r e s s u r eo s c i l l a t i o n s i n t h e chamber. Witht h e s h o r t m ix in g s e c t i o n , t h em o t o r o p e r a t e d w i t h a meanc h a m b e r p r e s su r e o f 4 6 0 p s i a w i thf l u c t u a t i o n s h a v i n g p ea k- to -p ea ka m p l i t u d e s u p t o 1 0 0 p s i a s showni n F i g u r e 7 a. T he l o n g m i x in gc ha mb er p r o d u c e d mo re s t a b l ep r e s s u r e c u r ve s t h a t h ad pe a k- to -p ea k o s c i l l a t i o n s o f b elo w 30 p s ia r o u n d t h e l o c a l m e a n .

The s p e c t r a l d a t a show t h a tt h e s h o r t a f t c l o s u r e e x h i b i t sl ow -f re qu en cy a c t i v i t y . Thep ow er s p e c t r a b el o w 1 0 0 H Z a r emuch mo re a c t i v e f o r t h e s h o r ta f t c l o s u r e . B ot h e x p e ri m e n tse x h i b i t some o s c i l l a t i o n s a t 2500Hz. The s h o r t c l o s u r e t e s t h a s al a r g e r a m p l i t u d e . The l o w e rf r e q u e n c y o s c i l l a t i o n s a r ei n d i c a t i v e o f n o n - a c o u s t i co s c i l l a t i o n i n t h e c h a m b e rp r e s s u r e .

O n e p r o j e c t e d c a u s e i sv o r t e x s h e d d i n g a s i l l u s t r a t e d i n

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Fig. 10. This hypothesesproposes that a flow separationfrom the fuel rich portion of theboundary layer forms an eddy ringin the recirculation zone of theaft fuel grain step. Fuelpyrolyzed from the aft face ofthe grain further contributes tothe fuel-rich eddy. This fuel-rich eddy then intermittentlymixes with the core flow ofoxygen that is passing throughthe center of the motor resultingin an intermittent combustion.The geometry of the short mixingsection allows more continuousdiffusion of the fuel andoxidizer. This is only ahypothesis at this point.

W

c* R e s uThe characteristic velocity

results in Fig. 9illustrate therelative efficiencies of thetests conducted. The theoreticalvalue of C*, as described byBarrere [61 "depends only on thecharacteristics of the reactionused, particularly on itscombustion temperature and itsmolecular weight. It is afundamental parameter which givesthe energy available aftercombustion and which can be usedto compare different reactionsindependently of chamberpressure. The theoreticalcurves illustrate that the PMMAand HTPB fuels when reacted withoxygen have very similar C*behaviors. This means that for agiven equivalence ratio theenergy available after completecombustion is almost the same.The experimental results showthat the PMMA fuels run at lowerequivalence ratios than the HTPBfuels. This means lowerpotential (theoretical) C* valuesfor PMMA. The dependance ofregression rate on Gox flowcontrols the actual equivalenceratio.

W

Comparing the theoreticaland experimental values gives aquality factor, qC., f o r the PMMA

46

tests of 60 to 85%, while theHTPB tests have quality factorsfrom 8 5 % to 95%. The qualityfactor is lowered by incompletecombustion o r heat transfer fromthe combustion chamber. Largescatter was observed in the C*values for PMMA. This can beattributed to the propagation ofthe uncertainty in themeasurement of the throatdiameter of the different motornozzles. Less scattering wasseen in the HTPB tests, but thesetests were all run with the samenozzle. The HTPB (long)configuration produced higherquality factors than the shortindicating more completecombustion for the long aftclosure.

Several tests arerecommended for the furtherinvestigation of the pressureoscillation phenomenon. Thefirst recommended series is aninvestigation of PMMA fuel usingthe variable aft mixing sectiongeometry. These tests are todetermine the effect of fuelcombustion characteristics on theoscillations. The second seriesis shown in Fig. 11. These testsare to determine the effect ofincreased mixing through the useof a phenolic baffle aft of thefuel grains. The third series isshown in Fig. 12. This test willlook specifically at the vortexshedding concept and determinethe effect of complete removal ofareas for eddy formation. Thisis accomplished by machining theaft fuel grain so that it can bebutted to the carbon insert.Thus the entire aft volume willbe removed.

The LHM reproduces fuelregression rates for PMMApropellants cited from previouswork. It also produces acombustion oscillation for HTPB

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fuel when fired with a short aftclosure. The oscillation ismostly attenuated with a longermixing section in the aftclosure. The instabilitymechanism is not related tooxidizer vaporization since theoxidizer was introduced as a gas.The spectral content of theoscillation was mostly below 100Hz suggesting a non-acousticsource.

v -This work was performed

under support through the NASASumer Faculty Fellowship Programat MSFC and from ThiokolCorporation IR&D funding to theUniversity of Alabama inHuntsville, Propulsion ResearchCenter.

1. Personal communication, Mr.Terry Boardman, ThiokolCorporation, June, 1991.

2. Sutton, G.P., UM E d & m e n L % , 6th

edition, ISBN 0-471-52938-9,1992.

3. Wooldridge, C.E., Marxman,G.A., and Kier, R.J.,Investigation of CombustionInstability in HybridRockets, Final Report, NASACR-66812, N69-37571, SRI,1969.

4 . Guthrie, D.M., and Wolf, R.S.,on-Acoustic Instability inHybrid Rocket Motors, AIAA20th Joint PropulsionConference, Orlando, FL, July16-18, 2990.

5. Personal conversation onunpublished data, A.L.Holtzman, United TechnologiesCorporation, Chemical SystemsDivision, August 6, 1991.

6. Barrere, M., et. al., BQkeLP r o ~ u L a i o n , ElsevierPublishing Company, New York,1960, Library of Congress CardNO. 59-8941.

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.

Opllonal Al l Clonure

~ n p h i t . 1 JGmln R.Ulning Ws*h.r N o ~ I w T h r 0 . 1- Acquired From Thlokol, Utah.Modified for use at MSFC. eed System.Instrumentation. nstalled In Cell 103, Bldg. 4583F i g u r e 1. LHM Motor Configuration.

StaticTemperature

SurfaceTemperature(4 plaoes)1SurfaceTemperature(4 places)

BttEadHigh FreguencyHcadERLHigh Frequency PressureStatic Pressure Static Pressure

F i g u r e 2 . LWM Instrumentation.

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U

W

Gox = 0.380 Ibm/secin2D = 0.816 inL = l O i nD , = 0.306 in

Short mixing section Long m ixing section

L * = 1 6 in L' = 85 inF i g u r e 3. HTPB C o n f i g u r a t i o n s .

HTPB, short aft closure

n m s (sac)

F i g u r e 4 . E x a m p l e P r e s s u r e - T i m e T r a c e .

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.Gox Line

Segment1Segment2Segme"f3Seg m ent 4

TestNumber 1 1 5 5 13 6 1 2U0.370

U0.189

u0.095

UGor(desired) 0.047

Figure 5. PMMA Regression Rate Distribution.

.t

,001

t =0.044 0xO.l

Go= obnvucld)

Figure 6 . PMMa Pegression Rate Results.

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Short mixing section Long mixing section

.- ..

F i g u r e 7 . X TPB P l e a s u r e O s c i l l a t i o n R e a u l t s

WSHORT MIXING

5

LONG MIXING

C.. 50011

f ig . 8b.

Figure 8 . HTPB Waterfa l l P l o t s .

..

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U

W

,0.0 0.2 0.4 0.6 0.8 1 O 1.2 1. 4 1.6 1. 6 0Equlvalsnce Rat lo

Figure 9. Experimental and Theoretical C* for PMMA and HTPB.

. Flow separation from fuel rich portion of boundary layetforms an eddy ring in the recirculation zone of the aft fuel grainstep. This collection is further contributed to by fuelvaporization from the en d of the aft fuel grain.Oxidizer Diffusion\ +L

Figure 10. Projected Cause of Oscillations.

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Experiment to determine the effectiveness of baffles at mixingexcess fuel and central oxidizer core flowW

Short Mixing SectionFuel Grains

Phenolic BaffleFigure 11. Aft B a f f l e Experiment.

W Experimen t to determine effectiveness of elimination ofentire aft expansion volume

I I

/End grain is machined to fit'carbon insertFigure 12. Butted End Grain Experiment.

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(Ibmlsec in2)

0.047

0.095

0.189

0.378

CHEMICAL COMPOSITION:C5H802

CONDITIONS:0.047 s Gox 5 0.378 Ibm/sec.in2

0 4 fuel segments

Long aft mixing section0.306 in. diameter throat

NumberTested

2

2

2

2

Table 1. PMMA T e s t Matr ix .

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