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AIAA 2001-3598Status of Army Pintle Technology forControllable Thrust PropulsionS. BurroughsU.S. Arm y Aviation and Missile CommandBedstone Arsenal, AL

37th AIAA/ASME/SAE/ASEE JointPropulsion Conference and Exhibit8-11 July 2001Salt Lake City, Utah

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( c )2 0 0 1 A m e r i c a n I n s t i tu t e o f A e r o n a u t ic s & A s t r o n a u t i c s o r P u b l is h e d w i th P e r m i s s i o n o f A u t h o r ( s ) a n d / o r A u t h o r ( s ) ' S p o n s o r i n g O r g a n i z a t i o n .UNCLASSIFIED

2001-3598STATUS OF A R M Y PINTLE TECHNOLOGYFO R CONTROLLABLE THRUST PROPULSION

Susan. L. BurroughsU . S . Army Av iation and Missile CommandPropulsion and Structures DirectorateRedstone Arsenal, AL

ABSTRACTTh e U . S . Army Aviation and Missile Command(AMCOM) is developing pintle technology forcontrollable thrust propulsion. The technology programis investigating several technical areas: modeling andsimulation, pintle motor design and performanceprediction, pintle and nozzle design, materials testing,actuation and control mechanisms, and ammoniumnitrate propellants. The program heavily leverages theU . S . Army's Small Business Innovative Research(SBIR) program through multiple on-going efforts.These efforts have produced modeling and simulationtools currently being used at AMCOM, as well as pintlemotor hardware for static testing. A twelve-inchdiameter heavywall pintle motor has been successfullytested at AMCOM, providing a controlled boost-sustainthrust profile. Recent controlled thrust tests have beensuccessfully completed on a seven-inch diameterheavywall pintle motor. Another SBIR program iscurrently developing a miniaturized, low cost actuationand control system for pintle motors. These varioustechnology areas are being focused for futuregeneration U . S . Army tactical m issiles.

INTRODUCTIONControllable thrust propulsion technology is beingexplored by government and industry as a solution tothe propulsion requirements for the next generation ofU . S . Army missiles. Pintle controlled solid propulsionis one approach to achieving this goal. Using pintletechnology, a conventional solid propellant rocketmotor can provide variable thrust levels, providing thecapability to decrease missile flight time to target, or toincrease maximum range capability.To maximize the controllability of thrust with a pintlemotor, a propellant with a high burn rate exponent isdesirable. Th e governing equations fo r rocketpropulsion show that a high burn rate exponentRelease C: "This material is declared a work of theU . S . Government and is not subject to copyrightprotection in the United States."

propellant will provide a higher change in chamberpressure than a lower burn rate exponent propellantwith the same change in throat area. This fact, alongwith the desire to minimize propulsion systemsensitivity, has directed propellant selection towardsammonium nitrate based propellants that typically havehigh burn rate exponents in the range of 0.7 to 0 . 9 .One of the first SBIR efforts in pintle technology wasthe Axial Pintle Motor ( A P M ) program, which involveddevelopment of a twelve-inch diameter heavywallpintle controlled motor.1 Th e scope of the effort was todevelop a fully functional, reusable heavywall pintlemotor for the development and testing of pintle andnozzle materials, propellants, and pintle controlhardware. Th e SBIR program encompassed the design,fabrication, and testing of the pintle motor. Thisprogram has been successfully completed.A recent SBIR program was the Variable Thrust Motor( V T M ) program, a two-fold program that encompasseddevelopment of a modeling and simulation tool, as wellas a low cost tactical pintle motor test bed in a seven-inch diameter configuration.2 Th e test bed was requiredto be of a tactical size fo r tactical missile applications,and would be .used for validation of the modeling tool.Th e SBIR program encompassed the design,fabrication, and multiple static firings of the test bed forvalidation of the design as well as the modeling andsimulation tool. This program is nearing completion.Another two-fold SBIR program that also involvesdevelopment of a modeling and simulation tool as wellas a test bed for validation is ongoing.3 The modelingand simulation tool has been developed, and design andfabrication of the heavywall pintle motor is underway.An SBIR program just underway involves developmentof actuation and control technology. Requirementsinclude high performance, low cost, and low weight fora system that will provide fo r movement of the pintleupon command, to include the control software andelectronics to interface with the rocket motor. Theprogram includes the requirement to test the system in a

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pintle controlled rocket motor fo r final validation. Asthis program is just underway, no detailed informationis available for publication.Other work at AMCOM includes pintle motor designand analysis, as well as materials testing. Materialstesting has focused on lightweight materials for thepintle. Design and analysis efforts include flightsimulations to determine the effects of pintle motordesigns on missile performance.4 Th e focus has beenon thrust management optimization schemes and effectson time of flight and range.

AXIAL PINTLE MOT ORAPM DESIGNIndustrial Solid Propulsion, with Aerojet assubcontractor, conducted the APM program. The APMis shown in Figure 1. It consists of a motor chamber 12inches in diameter and 20 inches in length.Programmatic decisions drove the selection of ahydraulic actuation system for the pintle.1 Th e systemis housed in the actuator and nozzle section. Hydraulicfluid for the actuation system is housed externally in ahydraulic power cart. Multiple burst disk assemblieswere used in the motor in the event of an over-pressurecondition. Th e propellant grain tested in the APM is aClass 1.3 reduced smoke propellant cast in a hardphenolic sleeve, and cartridge-loaded into the chamber.The grain tested was a seventy pound charge in an end-burner configuration. Ignition is accomplished with a"bag" igniter consisting of BKNO3 pellets. The APMcontrol system consists of a controller, control software,and required cabling.

Figure 1. Axial Pintle Motor

TEST RESULTSResults from the first static firing are shown in Figures2 and 3. Figure 2 shows pressure versus time, with aplot of pintle position overlaid fo r comparisonpurposes. Figure 3 gives the thrust-time trace.Comparisons of the actual pressure and the predictedpressure show deviations in performance during boththe boost and sustain phases. As seen in Figure 2, at2.25 seconds the pintle begins move in towards thethroat. Note that zero on the figure corresponds to zeropintle position, which corresponds to the minimumthroat area achievable with the hardware as built. Th edesign is such that the pintle cannot fully block thethroat. At 3 seconds the pintle reached the zeroposition, and was held there until approximately 5seconds, during which time the pressure decreased fromthe commanded 1750 psi to just under 1000 psi. This isindicative of throat erosion that the pintle could notcompensate for. Post test inspection of the hardware infact revealed that the nozzle throat experiencedexcessive erosion. Figure 2 shows that the pintle an dcontrol system compensated for the throat erosion untilthe physical limits of the hardware were reached at 3seconds. At this time there was not sufficient stroke toobtain the throat area to hold 1750 psi.At 5 seconds the correct pressure ramp wa s obtained bythe pintle, as seen in Figure 2. At this time the pintlebegins to move from the zero position and begins tocorrectly control motor pressure again. The 500 psisustain pressure is reached at the correct time. Notethat at 5.5 seconds the pintle beings to move towardsthe throat, indicating that the nozzle is continuing toerode significantly. The pintle successfullycompensates for the erosion until 11 seconds into thetest, at which time the pintle is at zero position. As thenozzle continues to erode and no pintle stroke is left,the pressure decreases below the commanded 500 psi.The motor provided boost and sustain thrust levels of1700 lbf and 800 lb f. These thrust levels represent a2.1:1 thrust turndown ratio, with a total impulse of15,140 lbrsec and a duration of 15 seconds.Despite the anomalies noted above, the test wa sconsidered a success. The overall design of the motorwas successfully validated. Th e control of motorchamber pressure to a predetermined duty cycle wasaccomplished within the limits of the hardware.Although unexpected, the extensive nozzle throaterosion provided a scenario that allowed the pintlemotor to demonstrate the capabilities of the controlsystem. The test demonstrated that the pintle motor canadequately compensate for nozzle erosion if the erosion

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2000

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Figure. 2 ^ Pressure-Pintle Posit ion: AP M Test

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Figure 3. Thrust-Time: AP M Test

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rate is known in advance. The system demonstrated itscapability for use as a reusable test bed.The APM has been turned over to AMCOM fo r furtherwork. Near-term efforts with the APM include a testplanned with an aluminized propellant grain atAMCOM facilities. Other planned efforts includemultiple tests with increased mass flow rates.

VARIABLE THRUST MOTORVTM DESIGNColorado Engineering Analysis, with Aerojet assubcontractor, conducted the VTM program. The VTMdesign is shown in Figure 4. It was configured toprovide durability, reusability, and quick assembly andpost-test disassembly. Th e test bed was also designedfor evaluation of pintle actuation and control concepts,new propellants, pintle materials, and pintle designs.The motor's external diameter is seven inches, chosenfor applicability to future tactical missiles. Th e motorconsists of a motor chamber into which a cartridgegrain is loaded, a forward mounted actuator assembly,an aft nozzle closure assembly, and the pintle with itsguide and insulation sleeve. The pintle shaft has adynamic seal at the forward end of the motor. The shaftruns the length of the motor chamber, insulated fromthe hot gases by a sleeve, and is supported at the aft endby a pintle guide. Multiple burst diaphragms are usedfor pressure release in the event of an over-pressurecondition. An electromechanical actuator and electroniccontroller provide pintle actuation and control. Th epropellant grain consisted of an ammonium nitratebased minimum smoke propellant in a 24 pound chargecase-bonded to a cartridge sleeve. The igniterassembly, not shown in Figure 4, consists of anexternally mounted igniter with a small propellantcharge for ignition.

Figure 4. Variable Thrust Motor

The VTM program included the development of amodeling and simulation tool for pintle motorperformance prediction. Predictions of pressure, pintleposition, thrust, and efficiencies were required. Th esoftware combines computational fluid dynamics withinternal ballistics for a complete package forperformance prediction.2*5TEST RESULTSFigure 5 shows the results from one of the VTM tests.Th e pressure profile is shown with pintle positionoverlaid for comparison purposes. Th e boost pressureof 3000 psi was attained with a rapid ignition transient.The sustain pressure of 1400 psi was held constantthroughout the sustain phase of operation until motorburnout occurred at approximately 23 seconds. Th esteady (constant) pintle position during sustain indicateslittle- effect of burn surface variations, erosion, orthermal growth. Figure 6 shows the thrust-time tracefrom the test. A boost thrust of 1500 lbf was attained,followed by a sustain thrust of 110 lb f. Thrustturndown ratio was 13.6:1 for the test. Both Figures 5and 6 show calculated values for pressure and thrustfrom the modeling and simulation tool developed. Th eextremely close agreement between the calculatedvalues and actual values indicate the fidelity of themodeling and simulation tool for performanceprediction.Th e VTM test provided a total impulse of 5485 Ibrsec.Figure 7 shows the calculated versus measured totalimpulse and specific impulse for the test. The d eliveredsea level specific impulse in Figure 7 can be comparedto the average delivered specific impulse of 224 lbrsec/lbm calculated from the total impulse of 5485 lbrsec.Another VTM has been successfully testeddemonstrating a boost thrust of 1350 lbf followed by asustain thrust of 200 lbf. Thrust turndown ratio was 7:1for this test. Total impulse was 5381 Ibf-sec/lbm withan average delivered specific impulse of 223 Ibf-sec/lbm. For the two tests discussed, specific impulseand thrust efficiencies on the order of 90 to 92.5% forthe boost phase and 89 to 91% for the sustain phasewere achieved.The testing portion of this program has beensuccessfully completed. Final work is ongoing in thevalidation of the modeling and simulation tool. Th ehardware is being turned over to AMCOM for furthertesting.

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MODELING AND SIMULATION TOOLSIn addition to the extensive performance predictiondiscussed in the Variable Thrust Motor section, otherwork has been done in the area of modeling andsimulation.5'6'7 Th e modeling and simulation tooldeveloped by Colorado Engineering Analysis combinescomputational fluid dynamics analysis capabilities withinternal ballistics prediction capabilities. Th e pintleimbedded internal to the combustion chamber interruptsthe normal gas flow paths seen in conventional solidrocket motors. Effects of an imbedded pintle on gasflow, to include Mach number, pressure, andtemperature were investigated with the modeling andsimulation tool.Figure 8 shows the Mach number contour plot for theAPM pintle and nozzle configuration at various pintlestroke positions relative to the nozzle throat. Figure 8shows the 0.5 inch and the 1.0 inch pintle positions andthe resulting change in Mach number contours.

5 10 15 20 25Time (sec)

Figure 7. Isp-Total Impulse: VTM Test

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Figure 8. Mach N u mber Contour Plots

Figure 9 shows the 1.5 inch and the 2.0 inch pintlepositions and the resulting change in Mach numbercontours.American Institute of Aeronautics and Astronautics

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This tool is an effective means of examining the effectsof the pintle on gas flow. All stroke positions can beevaluated and pressure and thrust values obtained. Th etool provides a visual means of evaluating any nozzleflow separations, an important consideration foroptimizing motor performance. Pressure contours aswell as temperature contours can also be obtained forany condition. The direction of gas flow can bepredicted by the code and displayed visually. This isuseful fo r determining areas experiencing flow turning.

presented in the figure the nozzle design is the same.The pintle size varies, however, as does the position.Both cases were designed to produce the same motorchamber pressure. The resulting effect on the nozzleflowfield can be clearly seen. In the second case theflow turns and the separated zone at the end of thepintle is more pronounced, as is the series of weakshocks down the length of the nozzle.

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Figure 10. Mach Number Contour PlotsFigure 9. Mach N um b e r Contour Plots

A second modeling and simulation tool is beingdeveloped by CFD Research Corporation under anotherSBIR contract managed by AMCOM.3'8 Work isongoing in the development of the tool. Testing will beconducted with a seven inch diameter heavywall pintlemotor designed by CFD Research Corporation in FY 01to provide data for validation of the software.Figure 10 shows the results of an analysis of theflowfield in the seven inch diameter pintle motor nozzleusing the second code being developed. In both cases

AMCOM is making use of both modeling andsimulation tools for pintle motor design efforts as wellas performance prediction.9 One of the tools has beensuccessfully validated, while the second is expected tobe completed and validated in the FY 02 timeframe.Other modeling and simulation efforts at AMCOM arefocusing on the system level in terms of missileperformance. Missile flight simulations are beingconducted using three degree-of-freedom (DOF)trajectory simulations. Various pintle motor designs arebeing evaluated with the focus on thrust managementschemes. Maximizing missile range is a keyconsideration in the flight trajectory studies.

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MATERIALS TESTINGThe APM and VTM tests discussed earlier made use ofrefractory metal alloys for pintle materials. Thesematerials experienced minimal erosion, but weight is anissue with their use. Materials testing at AMCOM hasfocused on lightweight, high temperature materials forpintle applications. Testing of a carbon/silica carbide(C/SiC) pintle provided by Snecma, France wasrecently conducted at AMCOM in a controllable thrustmotor designed by Aerojet.10 Th e C/SiC pintle wastested in a non-axial, heavywall pintle motor with anammonium nitrate based propellant. The pintleweighed 5.2 grams, an 82% weight reduction incomparison with a rhenium-molybdenum pintle. Th epropellant grain was an end-burner configurationcartridge loaded into the motor cham ber. Ignition wasprovided by a BKNO 3 and black powder "bag" igniter.Figure 11 shows the motor on the test stand.

Figure 11. Non-Axial Pintle MotorFigure 12 shows the thrust-time profile for the test. Th emotor provided a boost-sustain-coast-boost thrustprofile.

CSiC Pintle Test

Th e C/SiC pintle experienced essentially no erosionduring the test. Pre and post-fire weights show anincrease of 0.1 grams, which is attributed to materialbuild-up on the pintle that was not fully removed.There were no dimensional changes between pre andpost-fire examination.Th e results of this test make the C/SiC material a viablecandidate for a lightweight, high temperature materialin a pintle application. Additional testing is planned inthe VTM configuration in an extended duration testwith higher mass flow rates. This test should provideadditional performance data for the C/SiC material.

CONCLUSIONSTh e various SBIR programs, along with in-houseefforts at AMCOM, represent a multi-pronged approachto developing pintle technology for controllable thrustpropulsion. Tw o heavywall, reusable pintle motorshave been successfully developed and tested and are atAMCOM fo r further testing. Controllable thrust wassuccessfully demonstrated in both the 12 inch diameterand the 7 inch diameter pintle motors. Two modelingand simulation tools have been developed for pintlemotor design and performance prediction. Validationof one tool is essentially complete, with work ongoingwith a second. Materials testing at AMCOM with aC/SiC pintle have demonstrated the material to be aviable candidate for a lightweight, high temperaturematerial. Reductions in weight of 82% weredemonstrated with this material over refractory metalalloys. Extensive work is beginning on actuation andcontrol technology to interface with the pintletechnology being demonstrated. Systems work isongoing in motor design and missile trajectorysimulations to identify the best methods of thrustmanagement implementation to maximize missileperformance. These multiple technology areas arebeing focused for controllable thrust propulsion.

10 12 14 16Time, seconds

Figure 12. Thrust-Time: Non-Axial Pintle MotorAmerican Institute of Aeronautics and Astronautics

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REFERENCES

1. Burroughs, Susan L., Rosenfield, Gary C, Lynch,Michael D., Wong, Kent J., McClellan, James A.,Turner, Tom W., "An Axial Pintle Motor forThrust Control of Tactical Rocket Motors," 1999JANNAF Rocket Nozzle TechnologySubcommittee Meeting, Salt Lake City, Utah.

2. Burroughs, Susan L., Luke, Gary D., Lynch,Michael D, Wong, Kent J., "Controllable ThrustPropulsion Using Pintle Technology," 2001 AIAAMissile Sciences Conference, Monterey,California.3. Ostrander, Mark J, Bergmans, John L., Thomas,Matt E., Burroughs, Susan L., "Pintle MotorChallenges for Tactical Missiles," 2000 AIAAJoint Propulsion Conference, Huntsville, Alabama.4. Maykut, Albert R., Burroughs, Susan L., "ATrajectory Study and Performance Analysis ofPintle Rocket Motor Designs," 2001 JANNAFRocket Nozzle Technology SubcommitteeMeeting, Cocoa Beach, Florida.5. Prozan, Robert J., Luke, Gary D., Burroughs,Susan L., "Effects of Pintle Size and Geometry onPerformance of Pintle Rocket Motors: SBIR PhaseI," 1998JANNAF Joint Propulsion Meeting, 1998JANNAF Propulsion Meeting, Cleveland, Ohio.6. Prozan, Robert J., Luke, Gary D., Burroughs,Susan L., "Developments in the Automated PintleDesign Code," 1999 JANNAF Rocket NozzleTechnology Subcommittee Meeting, Salt LakeCity, Utah.7. Luke, Gary D., Prozan, Robert J., Burroughs,Susan L., "Validation of Pintle Design Code UsingCold Flow and Hot Fire Static Test Data," 1999JANNAF Rocket Nozzle TechnologySubcommittee Meeting, Salt Lake City, Utah.8. Ostrander, Mark J., Burroughs, Susan L.,"Performance Analysis of Pintle Controlled RocketMotors Using the Axial Pintle Motor Design(APMOD) Software," 1999 JANNAF RocketNozzle Technology Subcommittee Meeting, SaltLake City, Utah.9. Densmore, Barry D., Burroughs, Susan L.,Ostrander, Mark J., "Component Design andAnalysis for a Pintle Controlled Motor," 2001

JANNAF Rocket Nozzle TechnologySubcommittee Meeting, Cocoa Beach, Florida.10. Burroughs, Susan L., McClellan, James A., Lynch,Michael D., Wong, Kent J., 'Testing ofCarbon/Silicon Carbide in a Controllable Thrust

Pintle Motor," 2001 JANNAF Rocket NozzleTechnology Subcommittee Meeting, Cocoa Beach,Florida.

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