PDE Tech.pdf

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CPTR 70Apr i l2001HardcopiesavailablefromCPIAonly.eproductionis notauthorizedexceptbyspecificpermission.

OVERVIEW OFPULSEDETONATION PROPULSION TECHNOLOGY

M .L .Coleman

Serving thePropuWon C o m m u n i t ylorOv er SO YBCR

C HEM IC AL PROPULSIONINFORMATIONA G E N C Y THE JOHNSHOPKINSUNIVERSITY

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ublicreportingburdenforthiscollectionofinformationsestimatedtoaverage ourperresponse,ncludingthetimeforreviewinginstructions,earchingexistingdatasources,gatheringndthe dataneeded,andcompletingandreviewingth ecollectionofinformation.endcommentsregardingthisburdenestimateoranyotheraspectofthiscollectionof information,includingforreducingthisburden,toWashingtonHeadquartersServices,DirectorateforinformationOperationsandReports,1215 JeffersonDavisHighway,Suite1204,Arlington,V A22202-4302,to theOfficeof ManagementandBudget,Paperwork ReductionProject(0704-0188), Washington,DC 20503,. A G E N C Y U SE O N LY(Leave Blank) R E POR T D A T EApril2001

R E POR T TY PEA ND D A T ESC OV E R E DTechnicalReport ,1942 to1999

TITLE A ND SUBTITLEofPulseDetonationPropulsionTechnology

A U T HO R(S )M .L.

FUNDING N U M B E R SC:SP0700-97-D-4004

P ERF O RM I N G O RG A N I ZA T I O NN A M E (S ) A N D ADDRESS(ES)eJo h n sHopkinsUniversity

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systemsbasedon thepulseddetonationcycleofferthepotentialtoprovideincreasedperformance whilesimultaneouslyengine weight ,cost,an dcomplexity,relativetoconventionalpropulsionsystemscurrentlyinservice.Th eincreased potentialisdu etothehigh thermodynamicefficiencyofdetonationcombustion. Th enearconstant volumeh eataddition ofth edetonationcycle,alongwith thelackofacompressioncyclelendtoth ehigh efficiency an dspecificimpulse,simplicity, dlow-costofpulsedetonationpropulsionsystems.ulsedetonationEngines(PDEs)h a v eth epotentialforoperationatspeedsfromstatictohigh-supersonic( M < 5 ) ,withcompetitive efficienciesenablingsupersonicoperationbeyondconventionalga senginetechnology.urrently,no singleenginecycleexiststhath as such abroadrangeofoperability. PulseDetonation Engines(P D RE s )h av ethepotentialtodrasticallyreducethecost ofupperstagean dorbit-transferpropulsionsystems,an dar eattractive forlunar an dplanetaryexploration,an dlandersan dexcursionvehiclesthatrequirethrottling forsoftlanding.reportdiscusses th ethermodynamicbasisofpulsedetonationpropulsiontechnologyan didentifiesth emajor technologyinitiativesunderwayinth eUnitedStates.

S U BJ ECT T E R M SDETONATIONS,DETONATIVE CO MBU S TI O N ,P D R E(PULSEDETONATIONR OC KE TPD E(PULSEDETONATIONENGINES),T H R U S T ,INTEGRATEDSYSTEMS, TACTICAL PROPULSIONS YS TE MS ,S U P E RS O N I CFL I G HT,RO CK E TENGINES,AIR B R E A T H IN G

15 . N U M B E R OF P A G ES 68

16 .RICE CO D E . SECURITY CLASSIF ICATION O F REP O RT

UNCLASSIFIED 18 . SECURITYCLA S S I F I CA T I O N

OFTHISP A G E UNCLASSIFIED

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UL StandardFrom 2 98 (Rev .2-89) Prescribedby ANSIStd.Z39-18 298-102


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C P T R 70 Ap r i l2001 Hardcopiesavailablefrom CPIAonly.eproduction isno tauthorizedexceptby specificpermission.

OVERVIEW O F PULSED E T O N A T I O N PROPULSIONT E C H N O L O G Y

M .L .Coleman

ServingthePropulsionCommunityforOver50Yeas

C H E M I C A LPROPULSIONINFORMAT ION A G E N C Y TH EJ O H N SHOPKINSU N IV E R S IT Y

W HIT IN G SCHOOL OFENGINEERINGC O L UM B I A ,M A R Y L A N D 2 1 0 4 4 - 3 2 0 4

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Th eChemicalPropulsionInformationAgency(CPIA)isaDoD InformationAnalysisCenteroperatedby Th eJohnsHopkinsUniversity,W hitingSchoolofEngineering,underDefenseSupplyCenterColumbus(DSCC)contractSPO700-97-D-4004.h eapplicableDoD Instruction is3 2 0 0 . 1 2 - R - 2 ,"CentersforAnalysisofScientifican dTechnicalInformation."Th eCPIA's mailingaddressisTh eJohnsHopkinsUniversity,ChemicalPropulsionInformationAgency,Attn:SecurityOffice,10630LittlePatuxentParkway,Suite2 0 2 ,Columbia,Maryland,2 1 0 4 4 -32 04.h eCPIAalsoprovidestechnicalan dadministrative supporttoth eJointArmy-Navy-NASA-AirForce( JANNAF)InteragencyPropulsionCommittee.Th eGovernmentAdministrativeManagerforCPIAisth eDefenseTechnicalInformationCenter(DTIC),CodeDTIC-AI,8 7 2 5 J o h nJ .KingmanRoad,Suite0944,Ft.Belvoir,Viriginia2 2 0 6 0 -6 2 1 8 .h eGovernmentTechnicalManager(ContractingOfficer'sTechnicalRepresentative) isM r.StuartBlashill,NavalAir W arfareCenterW eaponsDivision,Code477000D,ChinaLake,California9 3 5 5 5 - 6 1 0 0 .Alldataan dinformationhereinar ebelievedtobereliable;how ever ,no warrant,expressedorimplied,istobeconstruedas to th eaccuracyorth ecompletenessofth einformationpresented.Thisdocumentw aspreparedunderth esponsorshipofth eDefenseTechnicalInformationCenterandisavailableonlytoqualifiedusers.eitherth eU.S.governmentno ranypersonactingonbehalfofth eU.S.Governmentassumesany liabilityresultingfrom th eus eorpublicationofth einformationcontainedinthisdocumentorwarrantsthatsuchuse orpublicationwil lbe freefromprivatelyowne drights.llrightsreserved.h ispublicationoranypartthereof,m ay no tbe reproducedinany form wi thoutwrittenpermissionofth eChemicalPropulsionInformationAgency.


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P R E F A C E

ThisChemicalPropulsionTechnologyRevi ew continuesCPIA'srecurrentseriesoftechnicalsummariesan dstatusreportson topicspertainingtomissile,space,andgu npropulsion technology.h egeneralaimistocollect,analyze,an ddiscusstechnology advancementsinalanguageunderstoodbyabroadrangeofpropulsiontechnologists.ThisCPTR presentsatechnicalreviewofpulsedetonationenginean dpulsedetonationrocketenginesciencean dtechnology.eflagrationan ddetonationcombustionprocessesare comparedbriefly,detonationcombustionphysicsarereviewed,benefitsofdetonationcombustionatth esystemslevelarediscussed,and currenttechnologyeffortsarepresented.CPIAsolicitscommentsonth etechnologyreview effort,includingsuggestionsontopicsforfutureissues.ortechnicalcommentsor suggestions,contactM r.TomMoore,CPIA TechnicalServicesSupervisor,at410-992-9951 ,ext.207,orM r.M a r kColemanat410-992-9950,ext.2 1 0 .ndividualsemployedbyorganizationsthatsubscribetoCPIAservicesm ay requestpersonalcopiesofthisdocumentby contactingCPIAat410-992-7300,[email protected],orhttp://www.cpia. jhu.

in


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AB STR AC T

Propulsionsystemsbasedonthepulseddetonationcycleofferthepotentialtoprovideincreased performancewhilesimultaneouslyreducingengineweight,cost,an dcomplexity,relativetoconventionalpropulsionsystemscurrentlyinservice. Theseimprovementscanbetracedtothehighthermodynamicefficiencyofthenearlyconstant-volumecombustioncyclean dthelowentropyriseintheworkingfluidproducedby detonation. Th epulsedetonationcycleca nbeappliedtobothairbreathingandrocket-basedsystems,an dpulsedetonationenginesm ayrequirelesspackagingvolumethanconventionalpropulsionsystemsdu etotheirinherentsimplicity. Inaddit ion,airbreathingpulsedetonationenginesca npotentiallyoperateovera widerangeofflightM a c hnumbers(M =0to ) . Thesecharacteristicscombinetomakepulsedetonationpropulsionsystemspotentiallyattractivetoawiderangeofmilitaryan dcommercialmissions. Recentadvancementsinmeasurement,diagnostic,an dcontroltechnologiescoupledwithadvancementsincomputationalcombustiondynamicsandcomputersh a v ecreatedtheenvironmentwheredevelopmentofpracticalpulsedetonationpropulsionsystemsmaynowbenowpossible.Th enearconstant-volumeheatadditionprocessofthedetonationcycle,alongwiththelackofacompressioncyclelendtothetheoreticalhighefficiencyandspecificimpulse,simplicity,an dlow-costpotentialofpulsedetonationpropulsionsystems. PulseDetonationEngines(PDEs)h a v ethepotentialtooperatestaticallyan dacceleratefromlowsubsonicthroughhighsupersonicvelocities,withcompetitiveefficienciesenablingsupersonicoperationbeyondconventionalga sturbineenginetechnology. Currently,nosingleenginecycleexiststhath astheabilitytooperateoversuchabroadrangeofflightvelocity(Mach0to ) . PulseDetonationRocketEngines(PDREs)h a v ethepotentialtodrasticallyreducethecostofupperstagean dorbit-transfervehiclepropulsionsystems,an darealsoattractiveforlunarandplanetaryexplorationvehicles,planetarylandersan dexcursionvehiclesthatrequirethrottlingforsoftlanding,an dspacevehicleattitudecontrolsystems.DevelopmentofpracticalPDEsandPDR Eswillintroducemanynewcomponent,subsystem,an dsystem-leveldesignchallenges. Practicalsystemswillrequiredevelopmentoffastacting,flightweightpropellantvalves,advancedcombustioncontrolsystems,efficientinletsan dnozzles,an dsystemspecificcomponentintegrationdesignsolutions. Inaddition,operationalsystemsmustbedesignedtooperatewithpracticalfuelsan dpropellantcombinations,suchasJP-10/air,RP-1/02,an dH2/02.ThisreportreviewstheconventionalChapman-Jouguetdetonationtheory;conceptualairbreathingan drocket-basedpulsedetonationpropulsionsystemdesigns;andthegoalsan dobjectivesoftechnologydevelopmentprogramscurrentlyunderwayintheUnitedStates.

IV


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C O N T E N T S PREFACE A B S T R A C TvLISTOF FIGURESiiLISTOF T A B L E SiiG L O S S A R YiiiN O M E N C L A T UR Ex1 .0 INTRODUCTION2.0 DEFINITIONSRE LATE DTO C O M B US T I O NP H E N O M E N A

2 .1O M B U S T IO N2.2EFLAGRATION2.3HE M IC A LEXPLOSION2.4ETONATION3.0 C O M B U S T IO NO V E R V I E W3 .1EFLAGRATIONWAVESTRUCTURE3.2ETONATIONWAVESTRUCTURE

4.0 HISTORICAL O V E R V I E W4.1HAPMAN-JOUGETT H E O R Y4.2DEPROPULSION 0

5.0 CHARACTERISTICSOF C O M B US T I O NPROPULSIONCYCLES 16.0 PDE/PDRECYCLEOPERATION 57.0 CONCEPTUALPDE/PDRESYSTEM DESIGNS 7

7.1ULSEDETONATIONENGINES(PDEs) 77.2ULSEDETONATIONROCKETENGINES(PDREs) 08.0 SYSTEMM O D E L I N G 29.0 PULSEDETONATIONPROPULSIONSYSTEM APPLICATIONS 310.0 U.S.PULSEDETONATIONTECHNOLOGYINITIATIVES 4

10.1VERVIEW.... 410.2E P A R T M E N TOF DEFENSE 410.21AVY 410.22IR FOR CE 010.23EFENSEADVANCEDRESEARCH PROJECTSAGENCY 210.3ASACENTERS 211.0 PRACTICALENGINEERINGSSUES 712.0 SUMMARY813.0 A C KN O W L E D G E M E N T S 9

v


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REFERENCESC HE M IC A LPROPULSION TECHNOLOGYR E V I E W SISSUEDB YCPIAINITIALDISTRIBUTION 0

VI


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LISTO FFIGURES

F IGURE TITLE P A G E FigureFigure2 Figure3 Figure4Figure5 Figure6Figure7AFigure7B Figure8 Figure9Figure 0Figure1 Figure 2 Figure 3 Figure 4Figure 5 Figure 6Figure 7Figure 8

Propagationan dStructureofaDeflagrationCombustionW a v einaTubeFilledWithCombustibleGa sMixturePropagationan dStructureofaDetonationCombustionW a v einaTube FilledWithCombustibleGa sMixturePhysicalPropertiesofthe -DDetonationWaveStructureHydrogen-AirDetonationWaveStructureSootFoilTraceSchematicofaStationary -DCombustionWaveHugoniotCurvean dR esultingEntropyforEachPointontheCurvePressure-Specific VolumeCycleDiagram 2Temperature-EntropyCycleDiagram 2ThermodynamicEfficiencyofBraytonsobarican dHumphreyDetonationCyclesforStoichiometricHydrogen/Air 4ThermodynamicEfficiencyofBrayton,Humphrey,an dChapman-JouguetDetonationCyclesforStoichiometricHydrocarbon/Air 4PulsedDetonationCycleOperation 5ConceptualPulseDetonationEnginePDE)Schematic 8ConceptualPulseDetonationRocketEnginePDRE)Schematic1 VariationofIspWithCombustorRelaxationMode 3PDEOperationalEnvelope 5ON R PDEResearchRoadmap 6ExpectedTransitionsFromON R PD EResearch 0NASAGlennResearch CenterPD ETechnologyDevelopmentRoadmap3NASAGlennResearchCenterPulseDetonationEngineTechnologyProgram4

LISTO FTABLES

T A B L E TITLE P A G E Table QualitativeDifferenceB e t w e e nDetonationan dDeflagrationinGaseous Fuel/OxidizerMixture

vu


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GLOSSARYAFirForceAF BirForceBaseAFR LirForceResearchLaboratoryAFRL/PRF R LPropulsionDirectorateAFRL/PRSA A F R LPropulsionSciencesan dAdvancedConceptsDivision AFRL/PRST A F R LTurbineEngineDivisionCombustionSciencesBranch APRIdvancedprojectsResearchncorporatedAP SirbreathingPropulsionSubcommitteeAS IdroitSystemsIncorporatedASTPdvancedSpaceTransportationProgram CF DomputationalFluidDynamicsC-Jhapman-JougetCTaliforniaInstituteofTechnologyDARPA/TTO DefenseAdvancedResearchProjectsAgency/TacticalTechnologyOfficeDD TeflagrationtoDetonationTransitionDFRCrydenFlightResearchCenterDo DepartmentofDefenseGEeneralElectricGEAEeneralElectricAircraftEngine GH 2aseousHydrogenGR ClennResearch CenterIRADndependentResearch AndDevelopmentIVTANnstituteofHighTemperature,RussianAcademyofSciencesLaRCangleyResearchCenterL M T A SockheedMartinTacticalAircraftSystemsL oxiquidOxygenM EMicroElectro-mechanicalM SFCarshallSpaceFlightCenterM U R IultidisciplinaryUniversityResearchnitiativeM V L S O.V .LomonosovStateUniversityNASAationalAeronauticsan dSpaceAdministrationN A W CavalAirWarfareCenterNPSavalPostgraduateSchoolNR LavalResearchLaboratoryON RfficeofNavalResearch PDEulseDetonationEngine PDETulseDetonationEngineTechnologiesPDREulseDetonationRocketEnginePSUennsylvaniaStateUniversityRB CCocket-BasedCombinedCycleRevConevolutionaryConceptsinAeronauticsprogramR PAVevolutionaryPropulsionforAeronauticalVehiclesSAICcienceApplicationnternationalCorporationSBIRmallBusinessInnovativeResearch SUtanfordUniversityTR LechnologyReadinessLevelUCSDniversityofCaliforniaSa nDiegoUFniversityofFloridaUSnitedStatesUTRCnitedTechnologiesResearchCorporationUTSIniversityofTullahomaSpacenstituteZ NDel'dovich,v onNeumann&Dring

Vlll


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N O M E N C L A T U R E

c SonicVelocityCP ConstantPressureHeatCapacityCv ConstantVolumeH e a tCapacityy G a m m a ,Rat ioofSpecificHeats(c p/cv)h EnthalpyH2 HydrogenK CoefficientofThermalDiffusivitykcal KilocalorieM MachNumberm /s MetersPe rSecond Mole 6.022x1023M oleculesinaMolemsec Millisecond02 OxygenP Pressure AP ChangeinPressurePc InitialPressureP i GasPressuremmediatelyBehindDetonationCombustionWaveq HeatAddedtoSystem P DensityR SpecificGasConstantT ReactionRateT TemperatureTb TemperatureofBurnedGas T0 InitialTemperatureT u TemperatureofUnburnedGa sT, TemperatureofGasImmediatelyBehindDetonationCombustionW a v e u ReactantVelocityv D Chapman-JougetDetonationVelocityoftheCombustibleGasMixtureVdet DetonationVelocity

IX


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1 .0 INTRODUCTIONPulseDetonationEngines(PDEs)an dPulseDetonationR o c k e tEngines(PDREs)detonatecombustiblepropellantmixturestoproducehigh chamberpressuresan dthrust. PracticalPDEan dPDREdesignsm ayincludemultipledetonationchamberstoobtainhighaggregateoperatingfrequenciesan dquasi-steadythrust. Currentcombustionan dsystemmodelspredictveryhigh propulsionefficienciesforPD EandPDR Edevicesan dgoodthrustcharacteristicsfromthelowsubsonictothehighsupersonicflightregimes. Potentialperformanceadvantagesoverconstant-pressurecombustiondevicesincludelowerspecificfuelconsumption,h igherspecificimpulse,an dh igherthrust-to-weightcharacteristics.Detonationsenableveryrapidmaterialan denergyconversion. Thisrapidmaterialconversionrate,orburningrate,doesnotallowenoughtimeforthelocalexpansionofthecombustionproductstooccur. Therefore,thedetonationprocessisthermodynamicallyclosertoaconstantvolumeprocessthantheconstantpressureprocesstypicalofconventionaldeflagration-basedpropulsionsystems.Th eh igherthermodynamicefficiencyofthenearlyconstantvolumecombustionprocess(detonation)isdirectlytraceabletothelowerentropyriseintheworkingfluid,w h e ncomparedtoth econstantpressure(deflagration)combustionprocess.h ehigh-pressureratiosassociatedwithdetonationcombustionmayeliminatetheneedforexpensive,high-pressure feedsystems,therebyreducingpropulsionsystemweight,complexity,cost,an dpackagingvolume. Inaddition,thepulseddetonationcycleca noperateoverawiderangeofflightM a c hnumberswithouttheassistanceofboosterstages,andca nbeappliedtoawiderangeofmilitary,civil,an dcommercialmissionsan dsystems.Pulsedetonationpropulsiontechnologyh asreceivedconsiderableattentioninth eUnitedStates(U.S.)overthepastdecade. Renewedinterestinpulseddetonationforpropulsionapplicationsisdu etothepotentialforsubstantialimprovementinperformance,an dsubstantialreductionsinpropulsionsystemweight,complexity,costandspecificfuelconsumption,relativetopropulsionsystemscurrentlyinservice.Practicalapplicationofpulseddetonationforpropulsionrequirestheabilitytocoupleanncreaseinthermalefficiencytoanincreaseinpropulsionefficiency. Beforedetonationscanbeusedeffectivelyinacontrolledmannerforpropulsionapplications,thephysicsbehindreliablyestablishingadetonationanditspropagationalongacombustoraxismustbeunderstood.NumerousagencieswithintheU.S.Government,academia,an dindustryarecurrentlypursuinginitiativestocharacterizefundamentaldetonationphysicsforselectpropellantcombinations,demonstratesingleandmulti-cycledetonations,developcriticalcomponentsforprototypesystems,an dacquiretestdatatovalidateperformancemodels.Thisreportprovidesanoverviewofcombustionphysicsan dhistoricaldetonationresearch,describesthethermodynamicbasisforPD Ean dPDREperformance,an dsummarizesthePDEan dPDREtechnologydevelopmenteffortscurrentlyunderwayintheU.S. Inaddition,thisreportreviewsPDEan dPDREenginecycleoperationan dreviewsconceptualPDEan dPDREsystemdesigns.Specificengineeringissuesandtechnologyareasrequiringfurtherstudyarealsohighlighted.2.0 DEFINITIONSRE LATE DTOC O M B U S T IO NP H E N O M E N AEstablishedusageofcertaintermsrelatedtocombustionphenomenaca nbemisleading. Beforeproceedingwiththetopicofpulseddetonationpropulsiontechnologyitisusefultoreviewapplicableterminology.


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2 .1 C O M B U S T IO NCombustionisdefinedhereasanexothermicchemicalreactionbetweenafuelan danoxidizerthatonceinitiatedca nsustainitselfaslongastheingredientsarepresentintheproperproportionsan dthermaldiffusionlimitsar enotexceeded. However,notallcombustioneventsarethesame. Th evelocityatwhichacombustionwavepropagatesthrough propellantmixtureisanaccuratemeasureofthestrengthorviolenceoftheevent. Combustionwavevelocityisdependenton severalfactorsincludingmixturecomposition,pressure,temperature,an dthegeometryofthevolumewherethecombustionoccurs. Ingeneral,acombustionwaveisconsideredadeflagration,althoughthedetonationwaveisanotherclassofthecombustionwave1 .Therearetwomechanismswherebyenergyrequiredforactivationofachemicalreactionca nbe transferredfromthereactedmaterialtotheunreactedmaterialthermalradiationan ddiffusionan dshockpressureforces(mechanicalshockan dcompression).hermalradiationan ddiffusionarethemechanismsthatpropagatechemicalreactionsindeflagrationsan dexplosions. Materialsurroundinganinitialchemicaleactionswarmedaboveitsdecompositiontemperaturetosustainth ereaction. Mechanicalshockan dcompressionarethemechanismsthatinitiatechemicalreactionsindetonations. Compressionforcesimposedonunreactedmaterialbyasupersonicdetonationwavecausesrapidheatingan dsubsequentcombustionofthereactantstosustainthereaction-2.2.2 DEFLAGRATION Deflagrationisthecommoncombustionphenomenaassociatedwithcurrentflightpropulsionsystemssuch asramjets,turbojets,androckets. Adeflagrationisarapidchemicalreactioninwhichtheheatoutputissufficienttoenablethereactiontoproceedan dbeacceleratedwithoutinputofheatfromanothersource. Deflagrationisasurfacephenomenonwiththereactionproductsflowingawayfromtheunreactedmaterialalongthesurfaceatasubsonicvelocity.Thermalenergyreleaseisthemechanismthatsustainsthereaction1-2'3.h edeflagrationflamespeedisafunctionofpressure,temperature,an dturbulenceofreactants,an dthepermissiblerangeiscorrectlypredictedbymanyclassiclaminarandturbulentflametheories4'5. Th edeflagrationflamespeedisdependentonthechemicalcomposition,massdiffusionrates,an dthermaltransferratesofthereactants. Typicalflamespeedsforadeflagrationcombustionwavear e o30m /s6- '. Th eeffectofadeflagrationunderconfinementisanexplosion. Confinementofthereactionincreasespressure,ateofreaction,an dtemperature,an dm aycausetransitiontodetonation3.Though deflagrationsthemostcommoncombustionprocess,tistheoreticallynotthemostefficientthermodynamicpathforcombustiontooccurbecausetheentropyoftheresultinggasesismaximized,whichreducestheamountofenergyavailabletodousefulwork.2.3 C HE M IC A LEXPLOSION Achemicalexplosionisachemicaleactionorchangeofstatethatiseffectedinaveryshortperiodoftimean dgeneratesalargevolumeofhightemperaturegas. Th eexothermicreactionrateincreasesexponentiallywiththesubsequentincreaseintemperaturean dpressure. Anexplosionproducesashockwaveinthesurroundingmedium. Eventhoughtheexplosionispowerfuland occursveryfast,thecombustioneventitselfoccursasadeflagrationwaveasittravelsthroughtheunbumedreactants. Th ethermalenergyreleasedduringthereactionsustainsthereaction. Inadeflagrationthecombustioneactionprocessan dshockwavepropagationprocessproceedinan uncoupledmanner1 -2-5-6-7.


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2.4 DETONATIONAdetonationisaviolentchemicalreactionthatproceedsthroughthereactedmaterialtowardtheunreactedmaterialatsupersonicvelocity. Th esupersoniccombustioneventpropagatesathigh velocitiesan dproducesarapidan dviolentcombustionofthereactantsdu etothestrongshockwaveleadingthedetonation. Inadetonationthecombustioneactionan dshockwavepropagationproceedinatotallycoupledan dmutuallysupportingmanner. Th eshockimposedontheunreactedmaterialbythesupersoniccombustionwavecausesarapidheatingan dsubsequentcombustionofthereactantstosustainthereaction. Th ereactionssustainedthroughthepropagationofthereactioncoupledtotheshockwave,an disdescribedbytheChapman-Jouguettheorydiscussedinthefollowingsections1,2- 3- 4- 5- 7.3.0 COMBUSTIONO V E R V I E W 3 .1 DE FLAGRATIONW A V ESTRUCTURE Th eclassicmethodusedtoanalyzecombustionwavesisalongtubefilledwitha combustiblemixtureofgases. Anignitionsourceisusedtoinitiatecombustion. Figures an d provideacomparisonofsteadystatedeflagrationan ddetonationcombustionwavepropertiesasthecombustionwavespropagaterelativetothereactants. InFig. ,massflowofthecombustiblega smixtureisflowingfromlefttorighttowardsadeflagrationcombustionzone. Figure showsthevariationofga stemperaturean dconcentrationofreactantsacrossthecombustionzone.

Flow o fheatan d chaincarriers f rom an e l ementofth e react ion zone

>xFigure1. Propagationan dStructure o f aDeflagration Combustion WaveinaTubeFilledwith CombustibleGas Mixture5

Betweentheboundariesuan dbachemicalreactionoccursandmoleculesofreactantsdiffuseinthedirectionutob,an dmoleculesoftheproductsofcombustiondiffuseinthedirectionbtou. T uisthetemperatureoftheunburnedga san dTbisthetemperatureoftheburnedgas. Asca nbe


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seenfromthefigure,thedeflagrationflamefrontgraduallyraisesthetemperatureoftheunbumedga sfromT ytoT,beforeth eonsetofthechemicalreaction. Th erateofriseoftemperatureforone-dimensionalheatflowisgivenbyKd2T/d x2,whereKisthecoefficientofthermaldiffusivity.Th epositivevalueofthesecondderivativebetweenT uandT - ,ndicatesthatthega smixturereceivesbyconductionmoreheatfromthehottergasdownstreamthanitlosestothecoolerga supstream. T,markstheinflectionpointinthecurvebeyondwhichthesecondderivativeisnegative,ndicatingthatafterT,hega slosesmoreheattotheupstreamga sthanitreceivesfromthedownstreamga s5.Th epressurevariesslightlyacrossthedeflagrationflamefrontshowninFig. du etotheconfinementofthetube. Ifthedeflagrationwereoccurringintheopenatmosphere,thepressurewouldequalizeimmediatelyan dthepressureofthereactantswouldequalthepressureofthecombustionproducts. Th econfinementimposedonthedeflagrationflamefrontbythetubeinFig.1esultsinaslightexpansion,educingthepressureofthecombustionproducts.3.2 DETONATIONW A V ESTRUCTUREDetonationcombustionw a v epropertiesareshownnFig. foradetonationpropagatinginatubefilledwithacombustiblega smixture. Th etubeisclosedaton eendan dopenattheotherend.Th edetonationisinitiatednearthecloseden dofthetubeandispropagatingtowardtheopenend.

RarefactionWave s

vonNeumannPressureSpike

Closed(Ignition) End

WaveFrontPropagatingatVde,

P0 T0

Chapman-JoagnetPlaneUnburnedFuel-AirMixture

Figure2. Propagat ionand Structure ofaDetonationCombustionWavein aTubeFilled With Combust ibleG a sMixture*


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Th edetonationwaveca nbemodeledasastrongshock thatrapidlycompressesthereactantstoinitiatecombustion,an dathinflamefrontinwhichheatadditionoccurs. Th eshockfrontmovesatthedetonationvelocity,V de t,elativetothega san ddramaticallyincreasesthetemperatureand pressureofthega sfrominitialvaluesT0an dP0toT,an dPvTh eregionofunburnedga simmediatelybehindtheshockisastablehigh-pressureregionknownasthevonNeumannspike.Thisregionepresentstheignitiondelay,an dits widthisdictatedbychemicalkineticsofthega smixture. Oncethechemicaleactionsinitiatedheatisaddedtotheflowcausingthetemperaturetoincreasean dthepressuretodecrease. Th ewidthoftheheatadditionregionisdeterminedbythetimetocompletethecombustioneactions8 . Atthispointth eburnedga sisatstate2 ,whichcorrespondstoChapman-Jouguetconditionsfora self-sustainingdetonation. Th etemperature,pressure,an ddensityofthega satstate2aresignificantlygreaterthanatstate .Th epressurean ddensityinthestabledetonationwave(P 2 ,p2)aresignificantlylowerthaninthevonNeumannregionbetweentheshockfrontan dthechemicalreactionzone. However,thedetonationwavetemperature(T 2 )justbehindth eflameregionissignificantlyhigherthaninthevonNeumannspike. Inclosedtubedetonations,anexpansionregionexistsbehindtheheatadditionregion. Rarefactionwavesemanatefromtheclosedendtoensurethatthenormalvelocityofthega satthewalliszero. Asaresultoftheexpansion,mostoftheburnedga sinthedetonationtubeisatpressureP3 ,whichissignificantlylowerthanthepressurejustbehindthedetonationwave8.Inadetonationthecombustionreactionan dshockwavepropagateinacoupledan dmutuallysupportingmanner. Zel'dovich1940)9,v onNeumann1942)10,an dDring(1943)11believedthedetonationwavecouldbeviewedasthreedistinctregionswhosewidthsaredependentontheequivalenceratiosan dkineticsofthega smixtureinwhichthedetonationwaveispropagating.Figure3showsthethermodynamicpropertiesintheregionsofthecommonlynamedZ N D detonationwavestructure.

ShockWave

Figure3 . Physical Propertiesof th e1 -D Detonation Wave structure'Thefirstregion,theshockwave,h asawidthofjustafewAngstroms,yetdeliversatremendousamountofenergyintotheunburnedreactants.h isenergyinputresultsinimmediatean ddramaticincreasesinpressure,density,an dtemperature. Th edramaticincreaseinthethermodynamicpropertiesofthega smixtureincreasesthechemicalreactionratesan dacceleratestheenergyreleasephaseofthewavestructure. Th edeflagrationregionconsistsoftwozonesthatdescribe


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thethermodynamicsofcombustingthereactants. Th efirst,whichisknownasthenductionzone,h asarelativelyshortwidthinwhichthechemicalreactionisbeginningbutisnotyetimpactingthethermodynamicpropertiesofthega smixture. Th enductionzonetransitionstotheReactionzonewhenthereactionatebeginstoincreaseexponentially,drivingtemperaturesupandstabilizingpressurean ddensitytotheirfinalequilibriumvalue. Th etotalwidthofthethreezonesisontheorderofon ecentimeter,an deachzoneisdependentonthenexttosustainthedetonationwave4,7.Althoughtheone-dimensionalZ NDmodelh asworkedwellforapproximatingdetonationwavestructure,inactualitythedetonationwaveh asacomplexthree-dimensionalstructure. Thethree-dimensionalstructureistheresultoftransverseshockwavesthatpropagatelaterallybehindtheleadingnormalshockwave. Th eintersectionofthetransversewaveswiththeleadingnormalshockwaveresultsinocalizedhigh-pressure,high-temperatureregionsknownastriplepoints. Th eextremehighheatingthatoccursatthesepointsgreatlyacceleratesthelocalreactionatesan densuresthattheheatreleaseregioniscloselycoupledtotheleadingnormalshockwave. Th erapidoscillationofthetriplepointsacrosstheleadingshockwavepromotesthestabilityofthedetonationwaveandresultsinthecharacteristicfishscale"patternscommonlyseeninsootfoiltraces. Sootfoiltracesaretypicallyobtainedbyplacingthinsheetsoftinorstainlesssteeltreatedwithfinesootparticlesintorecessedareasinsideofexperimentaldetonationtubes. Th edepthoftheecessinsidethedetonationtubeisequaltothethicknessofthesoot-treatedmetalsothatthedetonationwaveseesaconstant-areacross-sectionasittraversesthelengthofthetube.Detonationpressureimbedsthesootintothetinorstainlesssteelfoil"eavinga"footprint"ofthedetonationwavestructureasthedetonationwavetraversesthetube. Anexampleofahydrogen-ai rdetonationwavestructureobtainedusingthesootfoiltracetechniqueisprovidedinFig.4.

13

ID

"1niinrTTimtTTTTrrrijrnn&SS-D|m]

Figure 4.ydrogen-Air Detonation WaveStructureSootFoilTrace


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AqualitativecomparisonbetweenadeflagrationandadetonationisgiveninTable orgaseousfuelsandoxidizers7.h ereferenceframeforthisanalysisistheone-dimensionalstationarycombustionwaveshowninFig. . Th epropertiesusedtodescribetheeventincluderatiosofreactantvelocity(u),sonicvelocity(c) ,density(p) ,temperature(7) ,an dpressure(p). Subscriptsdenotetheburnedan dunburnedsideofthecombustionwave. Th eresultsshowninTable depictthedramaticdifferencesbetweendeflagrationan ddetonationcombustionevents.

TABLE1 .ualitative DifferenceBetween Detonationan dDeflagrationin GaseousFuel/Oxidizer Mixture7R eacted/UnreactedPropertyRatiosW a v eVelocityRatio u^lc^ Deflagration0.0001-0.03 Detonation5-10ReactantVelocityRatio U2/Ui 4-6acceleration) 0.4-0.7deceleration)PressureRatio PdP\ ~0.98slightexpansion) 13-55(compression) TemperatureRatio T2IT, 4-16heataddition) 8-21heataddition)DensityRatio P2/P1 0.06-0.25 1 .7-2.6(h ighercompression) |R elativeReact ionRateRatio T,/T2 1 ~200

Stat ionaryombustionWave (Unburned)

u, l "ft.Tl.pl(Burned)

W////////S///////////)/////////////}////////2!T2tp2Figure 5.chematicofStationary1 -D Combustion W a v e[R-06]4.0ISTORICALO V E R V I E W 4.1HAPMAN-JOUGET T H E O R Y

Th efirstrecognizeddetonationwasdiscoveredan dlaterpatentedbyA.Nobelin1864. Nobelan dh isfatherinventedamercuryfulminateignitorthatinitiatedadetonationinanitroglycerinecharge,an dlaterperfectedtheprocesswiththeinventionofdynamite1 2 . Inthe1870's,otherresearchers begantorelateth estrengthofanexplosiontohowitwasinitiatedan ditspropagationvelocity. Atthispointintimeleadingresearchershypothesizedthatdetonationswereinitiatedbysomeformofmechanicalshock,an dthatth eshockwasthemechanismthatsustainedthedetonation. B y1880,researchersconcludedfromnumeroustestsofdifferentfuelsan doxidizersatdifferentequivalence ratiosthatdetonationvelocityisuniformandonlydependentonthefuelan ditsmixtureratios1 3 .In1883researchersdemonstratedthat,undertherightconditions,deflagrationswouldtransitionintoadetonationwave. Thisnewdiscoveryledtoexperimentalproofthatthedetonationprocessca nbeviewedasarapidadiabaticreactionwhoseenergyreleasedrivesthedetonationwave,an dthesuppositionthatthereexistsainherentrelationbetweenthechemistryofthereactants,theconditionsinwhichtheyareignited,an dthedetonationpropertiestheyexhibit1 3 .


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Earlydiscoveriesan dideaspromptedcontinuedresearchan danalysis,an dby1890researcherswereabletoshowthedetonationpressureasafunctionofdetonationvelocityan dthereactantsheatofreaction. UsingtheRankinetheory,V.A.Michelsonwasabletoshowthattherearetwopossiblesolutionsforcombustion. Michelsonwasthefirsttoconcludethatreactantsatdifferentconditionswillburnnaturallyaroundtwodistinctconditions. H ealsonotedthattherew asa convergenceofpressuresattheupperpoint,correlatingwiththedetonationprocess1 4.Th ecombinedworkofD.L.Chapmanan dE.Jouguetconfirmedtheworkofearlyresearcherswhileworkingindependentlyduringtheate1890'san dearly1900's. Publishingin1899,Chapmanstatedthatthereexistsaminimumvelocityinwhichadetonationcanoccur,an ditisthermodynamicallytiedtothepropertiesoftheburnedga s1 5. Jouguetworkedfrom1901-1905an destablishedtherelationthatthedetonationwavevelocityisequaltothesoundvelocityoftheburnedga sinwhichtpropagates1 6. H everifiedthisresultbycomparingcomputedresultswiththeexperimentalresultsofseveralofh ispredecessors. J.L.CrussardvalidatedtheChapman-Jouguet(C-J)theoryin1907byrelatingthetwospecificcombustionpressurepointsontheHugoniotcurve(pressure-specificvolumeadiabat)17. C- Jtheoryisrecognizedastherelationshipbetweenvelocitiesofcombustionwaveprocessesan dthepressuresatwhichtheyoccur. C-J theorypostulatesthattherearetworegionsatwhichcombustionprocessca noccur.Assumingsteady,one-dimensionalflowintheconstant-areacombustorshowninFig. ,withno externalheataddedorrejected,negligibleinterdiffusioneffects,andnoviscouseffects,'theHugoniotrelationshipca nbederivedfromtheconservationequations. B yanalyzingthedetonationinthisform,tca nbeviewedasasupersonicshockwavewithcalculablepropertiesinfrontofand behindthewave.Th egoverningequationsofathermodynamicprocessca nbederivedfromthebasicconservationequations:

Continuityequation: dx (1 )

Conservationofmomentum: du d(pu) dp pp. +// K =-dxxx (2 )Conservationofenergy: PH (

2Y l d *+-dx 2_ ^ )\

=-{a ) t-v2cond'x^lcond' (3 )W h e r eenthalpy(h )an dheatadded(q )tothesystemaredefinedby :

h= CpT+ hq= h?-h

(4 )(5 )

q = -XdTdx (6)


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Th eHugoniotcurveisdefinedby therelationshipbetweenenthalpy( / ? ) ,pressurep) ,anddensity(p)ofthegasesinacombustionevent.h erelationshipbetweenh,p,an dpisdirectlyrelatedtothevariouscombustionconditions.h efourthan dfinalequationforderivingtheHugoniotrelationisbasedontheassumptionthatthegasesinboththeburnedan dunburnedregionsbehavelikeaperfectgas.h eperfectga slawisdefinedforbothegions,whereRisthespecificgasconstantforthereactants.

Perfectga slaw: p = pRT (7) Withintegration,substitutionan dmanipulationofintermediateequations,thefollowingequationsca nbederived:

7 y-\ Pi Ei --(p2-ii+A P i.fh=-(P2-Pii-2p (8 )(9) Equat ions(8 )an d(9 )areformsoftheR ankine-relation,butareformallynamedforHugoniotwhoderivedtheman dfitthevariouscombustionconditionstoth epressurev s.specificvolume1/p) curve. Equation8 )providestherelationbetweenheatadditionandthegasesinitialan dfinalpressuresanddensities.Th eHugoniotcurveshowninFig.6describesthedifferentconditionsatwhichcombustioncan

occur.hesecombustionconditionsincludevariousstrengthsofdeflagrationsan ddetonations,dependentuponthepressurean dspecificvolumeconditionsatwhichtheeventisoccurring.

I PhysicallyRealizableCombustionRegions

-"y*

/p MinimumEntropyStateFigure6. HugoniotCurveand ResultingEn tropyfo rE a c h Pointon theCurve4

Th eC- Jpoints,describedearlier,areth eboundariesforstrongan dweakcombustionevents. Th eC-Jpointsarethetwophysicalsolutionstoth eHugoniotrelationfortheconstantareageometryshowninFig. . Th esolutionsaredefinedby theintersectionoftheRayleighlinewiththeHugoniotcurve. Th eRayleighlineconditionisathermodynamiclimitation. Forconstantarea combustiontubes,themaximumflowvelocityislimitedtothesonicvelocityoftheburnedgas


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( thermalchoking). Althoughth edetonationwavepropagatesan dconsumesthereactantsatsupersonicvelocity,theflowofthecombustionproducts"away"fromthedetonationwaveislimitedtoM a c h ,elativetoth ewave. Thisison eofthemajorconditionsthatdeterminethelocationoftheupperan dlowerC- Jpoints.SincetheRayleighlineisastraightlinewithanegativeslopepassingthroughtheoriginoftheHugoniotcurve(PointA) ,theacceptableen dstatesaredividedintotwodistinctbranches:theupperdetonationbranchan dthelowerdeflagrationbranch.4.2 PDEPROPULSIONTh eobjectiveofanypropulsionsystemistominimizetheentropyriseintheworkingfluid. Figure 6showsthatforagivenspecifiedinitialcondition, detonationresultsinthelowestpossibleentropystateontheHugoniotcurve. PDEan dPDREperformancegainsoverconventionalengine cyclesca nberealizediftheentropygainshowninFig.6ca nbeachievedinpracticalenginedesigns,andiftransientoperationalissuesareappropriatelyaddressed.Detonationcombustionsanefficientmeansofburningpropellantmixturestoreleasethechemicalenergycontent. Th everyrapidenergyconversionassociatedwithdetonationcombustionca nea dtomorecompactan defficientpropulsionsystemdesignsrelativetoconventionalsystemspresentlyinservice. Researchersh a v erecognizedtheperformancepotentialofdetonationcombustionforover75 years. However,pulsedetonationpropulsiontechnologyh asbeenslowtomaturedu etothedifficultiesinvolvedwithrapidlyan dreliablyinjecting,mixing,an dignitinggaseousan dliquidpropellants,controllingtransitiontodetonation,an dexhaustingcombustionproductsonatimescalesuchthattheentireprocessca nberepeatedinmilliseconds18 - 19- 20.Designsforintermittentflowpropulsiondevicesw e r eenvisionedasearlyastheturnofthe20thcentury. Germanscientistsdevelopedmanyintermittentflowjet-propulsionenginedesignsbeginningasearlyas1900butw e r eneversuccessfulindevelopingatrueconstant-volumepropulsiondevice. Germandesignsincluded"explosioncycle"an d"pulsejet"engines,bothofwhichar esignificantlylessefficientthanatrueconstant-volumepropulsiondevice. Performanceoftheexplosioncyclean dpulsejetenginesislimitedbytheslowreactionrate,whichimitsoperatingfrequency,pressurerise,an dspecificimpulse. Inmanyoftheearlydesignsthefrequencyofoperationwasdependentonthetimingofthemixturean dignitionarrangement. Theseenginesdidnotnecessarilyoperateatthenaturalfrequencyofthesystem,resultingintheirclassificationas non-resonator-engines. Otherdesignswereclassifiedasresonatortypeswhereintheoperatingfrequencywastunedtotheacousticresonancesofthecombustionchamber21. Th eV -1Buzz-Bomb"whichenteredservicein1944isanexampleofaresonatortypepulsejet. Continuedeffortsledtodevelopmentan dtestofvalvelessenginedesignsthatoperatedatveryhigh frequencies21 .Itisunclearfromtheliteraturewhythetechnologyfailedtoreceivecontinueddevelopmentattention,giventhedemonstratedperformancelevelsan dthesimplistic,lightweightdesignofthepropulsiondevices.Th eU.S.Navy,OfficeofNavalResearch (ONR )nitiatedProjectSquidshortlyafterWorldWarIItoinvestigatetheperformanceofpulsejetenginedesignsformilitaryan dcommercialsystems7.Th eworkwasinitiatedwithpropulsionassetsan dinformationcapturedfromtheGermansattheconclusionofthewar. Thesepulsejetengineswereshownnottobedetonationenginessincetheircombustionprocessesoccurredwithsubsonicflamespeedinaresonantcavity. Th eU.S.effortwaseventuallyterminatedsinceitwasdeterminedthatthepropulsionsystemdidnoth a v eahigh overallefficiency7.B ythemid-1980'sU.S.esearcherswereabletodemonstratehigher-performancewithsustaineddetonationsatmoderateoperatingfrequencies. Thesemoderneffortsh a v eledtorenewedinterestindevelopingPDEan dPDREtechnologydu etothepotentialforimprovementinthermodynamicefficiencyandperformance,elativetoexistingsystems,an dtheemergenceofmanypotentialmilitaryand commercialsystemapplications.

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Dr .SchmuelEidelmanintroducedtherecentrevivalofpulsedetonationengineresearch by demonstratinganexperimentalpulsedcombustiondevicein19862 2 . Thisworkw asconductedattheNavalPostgraduateSchoolan dwassponsoredbytheOfficeofNavalResearch. DuringthisstudyfundamentallynewelementswereintroducedthatdistinguishedtheNP Sconceptfrompreviouswork. Thiseffortwasthefirstsuccessfuldemonstrationofa self-aspiratingpulsedcombustiondevice. Inaddition,theoperatingfrequencywassynchronizedwiththatofth efuelmixtureinjectionbytimingthefuelvalveopeningan dsparkignition,thusestablishingthefeasibilityofintermittentinjection. Initialworkconsistedofestablishingsingledetonationsina chambercontainingethylenean dai rusinganethylene-oxygenpre-detonator. Additionalworkresultedindemonstrationofrepetitivedetonations. Periodicfuelnjectionwithinthenaturallyaspirated chamberresultedinamaximumoperatingfrequencyof25H z. Th especificimpulseestimatedusingthepressure-t imehistoryan dtheamountoffuelconsumedangedfrom1000-1400seconds2 2 . SubsequentanalysisofthisworkbyKailasanathsuggeststhevelocitiesoftheobserveddetonationwavesweresignificantlybelowtheC- Jdetonationvelocitiesforthereportedmixtures,indicatingthatafullydevelopeddetonationwavewasnotformed8. However,th eresultsofEidelman,Helman,an dShreeveledtocontinuedresearch withthisconceptan dinfluencedmodernPDEan dPDREdevelopmentefforts1 8.AbriefhistoryofthedevelopmentofdetonationtheoryisprovidedinReference7. AnoverviewofearlydetonationwaveresearchsprovidedinReference 3 . Adetailedoverviewofthestatusofexperimentalan dtheoreticalresearch onpulsedetonationenginesisalsoprovidedinreference 8 .Kailasanathreviewsearlyattemptstous edetonationsforpropulsionan ddiscussesthepossiblereasonsforsuccessor failureoftheseexperimentalworks. Inaddition,Kailasanath reviewsrecentexperimentalwork,drawsobservationsfromtheresults,an ddiscussespossibleimplicationsoftheseresultsonfuturePDEdevelopmentefforts.Th eDepartmentofDefense(DoD)an dNASAar epresentlysponsoringfundamentalresearch an dexploratorydevelopmentprogramsthatwillestablishabasisfordemonstrationofprototypePDEan dPDREsystems. U.S.industryfirmsandacademiaarecontributingtothegovernment-sponsoredprograms,andtheU.S.ndustryisalsosupportingdevelopmentofthetechnologywithcorporatefunds. ActivepulsedetonationpropulsionprogramsareunderwayinFrance,Canada,Russia,Belgium,an dsrael,an dJapan,Norway,China,an dPolandareperformingPDE-relatedwork. AdetaileddiscussionofU.S.technologyprogramsisprovidedinSection10.0ofthisreport.5.0 CHARACTERISTICSOFC O M B U S T IO NPROPULSIONCYCLES Th emotivationforpursuingdevelopmentofpulsedetonationpropulsiontechnology,asmentionedpreviously,residesintheinherentthermalefficiencyadvantageassociatedwiththedetonationcycle.h ehighthermodynamicefficiencyofthedetonationcycleistraceabletothelowentropyriseintheworkingfluid. Abriefcomparisonofdeflagrationan ddetonationcombustioncycle efficienciesisprovidedinthefollowingparagraphs.Deflagrationcombustioninconventionalairbreathingan drocketenginesoccursundernearlyconstantpressureconditions. Deflagrationeactionspropagateatrelativelylowflamespeeds. Th eflamespeedisgovernedby thelaminarorturbulentdiffusionofunburnedgasesaheadoftheflamean dburnedgasesbehindtheflame. Typicalwavespeedsforadeflagrationcombustionw a v erangefrom -3 0m /s7. Deflagrationsproducesmalldecreasesinpressureandca nbemodeledas nearlyisobaric,orconstantpressure,processes8 -2. Propellantfeedsystemsforthesepropulsiondevicesarerequiredtodeliverthefuelan doxidizerstothecombustionchamberatelevatedpressuresinordertoachievedesiredthrustlevels.PDEsan dPDREsrelyonperiodic,cyclicaldetonationoffuel/airand fuel/oxidizermixturestoproducethrust. Adetonationisasupersoniccombustionwavethattypicallypropagatesata few

1 1


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thousandmeterspe rsecondrelativetoanunburnedfuel-airmixture. Detonationisamuchmoredynamican dviolentphenomenonthandeflagrationan dproduceslargeoverpressures. A detonationwavecompressesthereactants,ncreasingtheirpressure,density,andtemperature.Detonationsca nbemodeledassupersonicshockwavesthatinitiatean darecloselycoupledtoathinf lamefrontofthecombustionegion. Due tothehigh-speednatureofadetonationwave,detonationcloselyapproximatesaconstantvolumecombustionprocess8 -2324.Figure7providesacomparisonofthepressure-specificvolumeFig.7A),an dtemperature-entropy(Fig.7B )characteristicsoftheBraytonandHumphreycycles. Th eidealBraytonan dHumphreycyclesaresimilarinthatbothus eisentropiccompressionan dexpansionprocessestotransferworktoan dfromthesystem. Th eBraytoncyclerepresentstheconstantpressureheatadditionofdeflagrationcombustion. Th eHumphreycyclerepresentstheconstantvolumeheatadditionofthedetonationcombustionprocess. TheBraytoncycle(0-1-4-5-0)consistsoftwoconstantpressureprocesses(1-4an d5-0)andtwoisentropicprocesses(0-1an d4-5). Th eHumphreycycleissimilar,exceptthattheconstantpressurecombustionprocessoftheBraytoncycle,1-4),isreplacedbyaconstantvolumeheatadditionprocess(1-2). Th etotalareaundertheHumphreyP- v curveisgreaterthanthetotalareaundertheBraytonP-vcurve,indicatingagreateravailabilityofusefulworkfromtheHumphreycycle8 -2'2.

e2

1

BzaytoQ V Humphrey 0-1-4-5-00-1-2-3-0,

0 3 5

Brayton 0-1-4-5-0Humphrey-1-2-3-0

Specific Volume(v ) En t r o py (s)

Figure7A . Pressure-Specific VolumeCycleDiagram8igure7B . Temperature-Entropy Cyc leDiagram 8 Th eefficienciesoftheconstantpressureBraytoncyclean dtheconstantvolumeHumphreycycleca nbecomputedfromthepressure-volumeandtemperature-entropydiagramsshownnFigs.7Aan d7B .h eefficiencyofacycleisdefinedastheusefulworkoutputdividedbythetotalheatenergyinput2 4.Th eefficiencyoftheBraytoncycledependsonlyonthetemperaturechangeduringeitherofthetwoisentropiccompressionorexpansionprocesses(i.e.,To/T1 T4/T5):

T'RAYTONT (8 )

12


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Th eefficiencyoftheHumphreycycleisgivenas :

1 1 HUMPHREY -i-y [TJ-l

Ii-l (9 )

Th eefficiencyoftheHumphreycycledependsnotonlyonth eisentropiccompressiontemperatureratio,To/T1,butalsoontheratioofspecificheats,y,an dthetemperaturechangedu etotheconstantvolumecombustioni.e.,thedetonationtemperatureratioT2/T1).Th edifferencebetweentheBraytonandHumphreycycleefficienciesisthefollowingTo/T1 multiplier:

rrI\r-1ii-1 ( 1 0 )

Th evalueofthisexpressionisalwayslessthanon efordetonationcombustion. Asaresult,theefficiencyofaHum ph r e y(detonation)cycleisgreaterthanth eefficiencyoftheBrayton(deflagration)cycle. Foradditionalthermodynamiccycleanalysis,se ereferences8 ,23,and24.Reference8alsoprovidesadetaileddescriptionofdetonationphysicsan ddetonationwavemodeling.AcomparisonofBraytonan dHumphreycycleefficienciesca nbemadeusingarepresentativedetonationcombustionprocessan dapproximatingtheratioofspecificheats(y)asaconstantthroughoutthecycle. An equilibriumchemistrycalculationforstoichiometrichydrogen/airatatmosphericconditionsyieldsadetonationtemperatureratio /T,of10.2an dspecificheatratiosof1.4intheunburnedga san d1.16intheburnedgas. Th eHumphreydetonationcycleefficiencyca nbecalculatedforaveragecycley'sof .4 an d1.16,whichrepresenttheupperan dlowerlimitsofthevaryingspecificheatratio.

1 3


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Figure8providesthecalculationofcyclethermalefficiencyasafunctionofcompressionatio,PT/PQ.Th eactualdetonationcycleefficiencyliessomewherebetweenthetwolimitingspecificheatcurves(y=1.4an dy=1.16). Atacompressionratioof6,forexample,theconstantvolumeprocessoffersa30to50%mprovementinthermalcycleefficiencyovertheconstantpressurecycle8'2.

CompressionRatioFigure8 . Thermodynamic E ffic iencyofBray tonIsobaricand HumphreyDetonat ion Cycles fo rStoichiometric Hydrogen/Air8

Aconstantpressureenginecycleiscomparedtoaconstantvolumean datrueC-JdetonationcycleinFig.9. F orpurposesofcomparison,theonlyprocessthatisdifferentinthethreecyclesisthemethodofenergyconversionorheataddition.50f 40 nrr 1 I1 i >>i 111t " ^^^ f:onst.Pres.(Eff.=27%) ::onst.Vol.(Eff.=47%) ;DetonationEff.=49%) ;!Hr L I i-Lt1 1 \V i \*

TTT" 1 1 1 Li i

302010 0 0 1000 2000 3000 4000 5000 6000

SpecificVolume(cm3/gm)Figure 9. Thermodynamic E ff ic iencyofBray ton ,Humphrey ,and Chapman-JouguetDetonationCyclesFo rStoichiometric Hydrocarbon/Air25

Th eamountofheataddedisalsokeptthesameat50kcal/mole(avaluetypicalofhydrocarbonfuels)forthethreecycles. Inallcases,thefuel-airmixtureinitiallycompressesadiabaticallyfrom

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to3atm.beforeheataddition. Afterheataddition,theproductsofcombustionareexpandedadiabaticallyto atm. Finally,thesystemisreturnedtoitsinitialstate. Sinceallprocessesexceptheatadditionarethesame,therelativethermodynamicefficiencyofthethreecombustionprocessesca nbeobtainedbycomparingtheareasenclosedby th ecurves. Th ethermodynamicefficienciesforthethreecyclesare:27%forconstantpressure,47%forconstantvolume,an d49%ordetonation2S . Thus,thethermodynamicefficiencyofthedetonationcycleisclosetothatoftheconstantvolumecycle,an dsignificantlybetterthanthatofth econstantpressurecycle. Amajorchallengeinthedevelopmentofpulsedetonationpropulsionsystemsisattainingthishigherpotentialefficiencyinpracticalpropulsiondevices. APDEorPDREmustpossessahighpropulsionefficiencytobenefitfromthehigh thermodynamicefficiencyoftheconstantvolumecombustioncycle2 5 .Althoughtheconstantvolumecycleshowsasignificantefficiencyadvantageinbothcases,thiscomparisoncannotbetakenasthecorrectquantitativesystemcomparisonbecausepulsedetonationpropulsiondevicesoperateinapulsedtransientmode. However,thiscomparisondoesindicatethaton ebeginswithamuchmoreefficientcycletodeveloppulsedetonationpropulsiontechnology1 9.6.0 PDE/PDRECYCLEOPERATION Figure 0showspictoriallytheeventsoccurringinasingledetonationcycleforatubewithon een dclosedandon een dopen. Th edetonationisinitiatedneartheclosedend.h ecyclebeginswiththeemptychambershowninFig. 0A .Continuingclockwise,acombustiblefuel-airorfuel-oxidizermixtureisinjectedatthevalveden d(closedend)ofthetubeatpressureP - ,an dtemperatureT,nFig. 0B . P,andT,ar edeterminedbyflightconditionsan dinletdesigncharacteristicsforairbreathingengines,an dthechamberinletpressurem aybeaslowas200ps iforPDREsystems.

RaratacbonB

Figure10 . Pulsed Detonat ionCyc leOperation 8

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Continuingclockwise,thefuelan doxidizervalvesareclosed(Fig. 0C )onceafullpropellantloadh asbeeninjected,an dthecombustorexitremainsopen. Valvetimingensuresthatthepropellantmixturean dthedetonationwavereachthecombustorexitsimultaneouslytopreventanyunburned gasesfromescapingan dloweringtheoperationalefficiency.Oncethefuelan doxidizervalvesareclosed,angnitionsourceinitiatesadeflagrationneartheclosedendofthechamberthatquicklytransitionstodetonation(Fig. 0D). Anexpansionzoneiscreatedbetweenthecloseden dofthechamberan dthedetonationwavesincethevelocitymustbezeroattheclosedend. Rarefactionwavesaregeneratedattheclosedendofthechamberan dproceedtowardsopenendofthechamber. Th erarefactionsoriginateattheclosedendan dsatisfytheconstraintofzeroaxia lfluidvelocitynormaltothewall. Th estrengthoftheexpansionregionisafunctionofy,theratioofspecificheatsoftheburntgases. Th eratioofspecificheatsofthecombustionproductsdeterminestheaxialvelocityoftheburnedgasesbehindthedetonationwave,whichmustbedeceleratedtosatisfythecloseden dboundarycondition.Oncethedetonationsinitiatedthedetonationwavepropagatestowardstheopenen dofthechamber(Fig. 0E). Ideally,thedetonationwavewillproceedattheC-J detonationvelocityofthemixture,VD. Theregioninfrontofthedetonationwillcontaintheunburnedmixtureatstate .Th eburnedmixtureimmediatelybehindthedetonationwavewillbeatsignificantlyh ighertemperatureandpressure,state2 ,asdiscussedpreviouslyinSection . Th eburnedmixturenearthecloseden dofthechamberwillbeatstate3 ,muchlowerintemperaturean dpressurethanstate2du etotheexpansionregionse eFig. ) .Asthedetonationwaveexitsthechamber,thechamberwillcontaincombustionproductsatelevatedtemperaturesandpressures(Fig. 0F). ConditionsalongthelengthofthechamberrangefromT3andP3attheclosedendtoP2an dT2attheopenend. Th eaxia lvelocityofthecombustionproductsvariesfromzeroatthecloseden dtosupersonicvaluesoutsidethechamberexit.As thedetonationwaveexitsthechamber,apressuredifferentialexistsattheopenend. Thispressuredifferencecreatesaseriesofrarefactionwavesthatpropagatebackintothechamberhelpingtoexpelthecombustionproducts. Th erarefactionwavestravelintothechamberatthespeedofsoundofthecombustionproducts.Aftertheprimarycombustionproductsareexpelledfromthechamber,theremainingga swithinthechamberisatapressurenearP3 (Fig. 0G). Th eunsteadyblowdownexpansion)processischaracterizedbyaseriesofcompressionan drarefactionwavesthatarealternatelycreatedan dreflectedandacceleratetheburnedga stowardstheopenen dofthechamber. Th eblowdownprocessisself-aspirating,an dtheflowfieldatthecombustorexitalternatesbetweenoutflowand inflow. Th epressureandtemperatureeventuallydecaytoambientlevelsan dtheexhaustvelocitydecaystozero8.Oncethepressurewithinthechamberdropstoappropriatelevels,thechamberisrechargedwithafresh fuel-airorfuel-oxidizerchamberloadanddetonationisinitiatedonceagain. Th ecyclefrequencyforapulseddetonationpropulsionsystemisth einverseofthetimerequiredtocompleteafulldetonationcycle: T CYCLE=TDET0NATI0N+TB L 0 W D 0 W N(EXPANSI0N)+TFILL8. PDEsan dPDREm ayalsorequireactivepurgingoftheresidualcombustionproductsfromthechamberpriortorefillingthechamberinordertoavoidprematureignitionofthefreshpropellantcharge.Asmentionedpreviously,manyexperimentaleffortsarepresentlyunderwaytocharacterizeoptimumcombustorgeometry,ignitionocation,fueldetonationproperties,gnitiondelay,an ddeflagration-to-detonationtransitionpropertiesusingsingle-chambertestapparatuswithcycletimes

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rangingfromowtomoderatefrequencies26,27,28. OthertechnologyprogramsareproceedingwithdevelopmentandtestingofPDEan dmulti-chamberPDREcomponentsan dsubsystems2 9'3'3.Engineswithmultiplecombustionchamberswillmakeuseoffastactingpropellantmeteringvalvestosequentiallyloadthechamberswithpropellanttoincreaseaggregateoperatingfrequencies. Th eobjectivesofcomponentan dsubsystemdevelopmentprogramsincludedemonstrationofoperation,performance,an dthrottlingofflight-scalecomponents. AdditionaldiscussionofcurrenttechnologydevelopmenteffortsisprovidedinSection 0ofthisreport. TechnologicalchallengesassociatedwithimplementationofthepulsedetonationcycleinanoperationalsystemarebrieflydiscussedinSection 1 .7.0ONCEPTUALPDE/PDRESYSTEMDESIGNS7. 1ULSEDETONATIONENGINES(PDEs)Pulsedetonationenginetechnologyisstillinanearlystageofdevelopment. Severaltechnicalchallengesmustbesuccessfullyaddressedbeforeoperableenginesbecomeareality. PDEscyclicallydetonateonboardfuelan datmosphericairmixturestogeneratethrust. M a j o rPDEsubsystemsincludeinlets,detonationchambers,an dnozzles. PracticalPDEsm ayincludeseveraldetonationchambersfedbyacommonnletan dexhaust ingthroughacommonnozzleflowpath. Inadditiontothesemajorsubsystems,PDEsrequirepressurizedfuelstoragean dfeedsystems,fuel/airinjectionsystems,anddetonationinitiationsystems. PDEpropellantinjectionsystemswillincludehigh-speedfuelan dairmeteringvalvestocyclicallyloaddetonationchamberswithfreshpropellantchargesatthebeginningofeachcombustioncycle.Detonationpressureforcesactingon theclosedvalved)en dofthePDEdetonationchambers(thrustwalls)convertchemicalenergyintokineticenergy. PDEswillrequireauxiliarypowersystemsfordetonationnitiationan dflowcontrol,an dmayincludepowerextractionsystemsforcertainapplications.Rapidandreliableinitiationofdetonationisoneofthemajorchallengesthatmustbeaddressed beforeoperationalPDEsbecomeareality.h eabilitytorapidlyan dreliablyinitiatedetonationswithpracticalfuelsan dinitiationenergylevelsiscriticaltosuccessfuldevelopmentofPDEs,as veryhighoperatingfrequenciesan drepeatableignitiont imesarefundamentalengineoperatingrequirements. Initiatorunits,orpre-detonators,m aybeemployedtoensurereliable,epeatabledetonationan dalsominimizeengineweight,packagingvolume,an ddetonationcycletime. Inaddition,eliablefuel/airmixingtechniquesar erequiredtoensurepropellantmixturesinthemaindetonationchambersarewithinthedetonabilitylimitsofth eselectedpropellantcombination.Initiatorunitsmakeus eofonboardfuel,high-densityoxidizer,an dai rtoestablish deflagrationsthatrapidlytransitiontodetonation. Initiatorunitcomponentsincludeonboardoxidizerstoragetank,oxidizer,fuel,an dairfeedsystem components,an dsmalldiameterdetonationinitiationchambers. Operationally,nitiatorunitsworkasfollows:adeflagrationisinitiatedinasmall-diameterdetonationchamberfilledwithanoxidizer/fuel/airmixture,thehigh-speeddeflagrationrapidlytransitionstodetonation,minimizingtransitionength,andfinally,thedetonationcombustionwavetransitionsfromthesmal ldiameterchamberintothemaindetonationchamberfilledwiththeless-detonablefuel/airmixture. Oncedetonationisestablishedinthemaindetonationchamberthedetonationtransitionsthelength ofthechamberadiabaticallycompressingan dcombustingthereactants. Thehigh-pressure ratioofthedetonationcombustioneactionimpartsthrusttothethrustwall,therebytransferringchemicalenergytokineticenergy.Developmentofdirectinitiationmethodsm ayreducesystem weight,cost,andcomplexityevenfurtherthrougheliminationofhigh-densityoxidizerstorage,feed,an dmixingsystems. However,directinitiationofthefuel/airmixturemustbeaccomplished withpracticalenergy-inputlevels,whichmayrequiredevelopmentofnewhigh-energydensityfuelswithexceptionaldetonabilitycharacteristicswhenthefuel-spraydropletsaremixedwithair.irectinitiationofdetonationh as beendiscussedforPDEapplications. However,extremelyhighenergylevels( thousandsofjoules)

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arerequiredfordirectignitionofliquidfuel-airmixtures. Therefore,nitiatorunitsemployingtheDDTprocessaretheleadingtechnologyoptionfornear-termenginedevelopments. Fuel/oxygeninitiatorsh a v everyshortDDTlengths,shortignitiondelays,an dareveryreliable,an darethereforeveryattractiveforinitiationapplications.Additionally,designofPDEinletsan dnozzlesthatoperateefficientlyoverarangeofflightconditionsarerequiredforsuccessful,practicalapplicationofPDEs. PDEdesignsan dconfigurationsmayvarywidelydependingupondesignsolutionsadoptedtoovercomethese technicalchallenges.Fuelselectionisdrivenby specificmissionapplications. High-speedaircraftapplicationsmayfavorhydrogenfuel,whereasvolumean dweightlimitationsofmissilesystemsm ayfavorh igherdensityliquidfuels. PerformanceoptimizationdrivesPDEdesignstoveryhighoperatingfrequenciesbecausethrustscaleswithfrequency3 4. Ith asbeenestimatedthatinorderforthePDEcycletobe competitivewithconventionalturbojet/turboramjetsystems,theywillberequiredtooperateinthe75to100H zrangewithnearstoichiometricfuel/airmixtures3 5 .h isrepresentsacycletimeofapproximately 0msec. A 0msec,detonationcyclerequiresfastactingpropellantvalvestofill/refilldetonationchamberswithfreshpropellantchargeswithina5msec,timespan,allowingapproximately2msec,fordetonationwaveformationan dpropagationalongthelengthofthedetonationchamber,an dapproximately3msec,forexpansionofrarefactionwavesan dchamberpurging35.Aschematicofaconceptual,singledetonationchamberPDEisprovidedinFig. 1 . Abriefdescriptionofthevariouscomponentsan dtheiroperationisprovidedinthefollowingparagraphs.

OxldizerSource[Distribution!

Manifold

[Distribution. >Manifold

Detonat ion Chamber Nozz le

FadSourceFigure1 1 .onceptualPulseDetonationEngine(PDE)Schematic32 Th econceptualPDEsystemshownincludes: )ai rinlet; )fuelsource;3 )oxidizersource-4) fuel/air/oxidizerdistributionmanifolds; )uel/air/oxidizermixingsection;6)nlet/detonationchamberinterface;7)nitiatorunit; )detonationchamber;9)detonationchamber/nozzle interface;and 0)nozzle. Inaddition,enginepoweran dcontrolsystemcomponentswillalsoberequired.

Powerextractioncomponentsm ayberequiredforsomeengineapplications. However,du etotheearlystageofPD Etechnologydevelopment,powerextractiontechniquesarenotwellformulated.

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DesignanddevelopmentofPDEinletsthatoperateoverarangeofflightconditionsintroducenon-trivialengineeringchallenges. PDEinletsneedtobeisolatedinsomemannerfromthenon-steadycombustionprocessinordertomaintainhighoperatingefficiencies. Unsteadyinlets,wheresomeofthecombustionproductsfromthecombustioncycleareexpelledthroughtheopeninlet,h a v eseenimiteduse,mostnotablywithV -1pulsejetengines32. M o r erecently,researchersh a v e evaluatedunsteadyinletswithpulsedetonationdevices36" 3 8. Severalunsteady,valveless,steady,an dmixedcompressionnletdesignsh a v ebeenproposedan danalyzed,3" 40butdetaileddesign,development,an dintegrationofefficientPDEinletsremainsacriticalenablingtechnology.Inletflowmustbeseparatedfromthecyclicaldetonationprocesses. Bussingbrieflydiscussesaerodynamican dmechanicalmethodsthathavebeenemployedorproposedtoisolatethedetonationprocessfrominletflowinReference32. Inaddition,Bussingintroducesarotaryvalve solutionthatca nbeusedwithamultiplecombustorPDEconceptinReference41. Th erotaryvalveservestoisolatethesteadyoperationofth einletan dfuelsupplysystemsfromtheunsteadycombustionprocesses. Th erotaryvalveconceptcyclicallyrechargessomeofthedetonationchamberswithfuelan dairwhiledetonationsoccurinotherchambers,allowingtheinletan dfuelsystemstooperateinasteadystatemode. Othermethodsproposedtoreduceoreliminateflowperturbationsintheinletflowincludeincorporationofhighbypassratioinlets,oversizedinletswithfast-actingbypassvalves,an dpressure-activatedflappervalves.Akeyissueinthepulsedetonationengineconceptisthedesignofthemaindetonationchamber.Th edetonationchambergeometrycontributestoth eoverallpropulsionefficiencyoftheenginean ddeterminesthedurationofthedetonationcycle. Th edetonationchambermustbestructurallydesignedtowithstandthepressuresan dtemperaturesofthecyclicdetonationprocess,an dthedesignmustalsoincludeamechanism fortransferringthrustloadstoth evehicle. R efuelingan dpurgingstrategiesmayalsoinfluencethedesignofthedetonationchamber. Becausetheoxidant(air)sobtainedfromtheexternalflowfield,theoverallpropulsionefficiencyoftheengineisdependentupontheinteractionofthesurroundingflowwiththeinternalflowdynamicsoftheinletanddetonationchamber32,33,36.Th ePD Einitiatorunitcreatesself-sustainingdetonationsthattravelintothemaindetonationchambertoinitiatedetonationsinprimaryfuel/airmixtures. Asdiscussedabove,theinitiatorunitcombinesonboardfuelan doxidizerwithatmosphericairtoestablishdeflagrationsthatrapidlytransitiontodetonation3 2 . Multi-chamberPDEsm ayrequiremultipleinitiatorunitstomaintainhigh aggregatedetonationcyclefrequencies. Initiatorunits,orpre-detonators,havebeeninvestigatedinanumberofstudies22-4,43.PDEnozzledesignan dintegrationntroduceschallengesnotpresentinconventionalsteadystatecombustionengines. PDEnozzledesignan ddevelopmentremainsacriticalenablingtechnology.Th epurposeofanozzleistoextractkineticenergyfromthermalenergybyexpandingtheflowofhotcombustionproductsthroughachokedpoint,therebycausingtheflowofhotga stoachievesupersonicflowvelocities. Th enozzlemaintainsbackpressurewithinthedetonationchamber,therebyensuringdetonationofpropellantmixturesatelevatedpressurestomaintainengineoperatingefficiency. Th echallengeforpulsedcombustionnozzledesignersistomaintainchokedflowatthenozzlethroatwithunsteadycombustionprocessesoccurringupstream ofthenozzle.UnsteadynozzleinletpressurerequiresthatthenozzleoperateoveralargeAPrange. Unsteadynozzleflowsubjectsnozzlestounsteadythermalan dmechanicalloads,whichcomplicatedesign.Delavalconverging-diverging)nozzlesm ay resultinunwantedreflectionofdetonationwavesback intothedetonationchamber. Reflectedshocksca nperturbcombustorflowan dimparthigh mechanicalloadingonthenozzleitself. Nozzlelessdetonationchambers,chamberswithsimple divergingnozzles,oraerospikenozzlesmighteliminatethisconcernan dm ayprovidehigheroverallperformance.

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OneoftheattractivepotentialbenefitsofPDEsisthelackofacompressioncycle. Th ehigh pressureratiosattainablewithdetonationcombustioneliminatetheneedforfuelsan doxidizerstobedeliveredtothedetonationchamber(s)atveryhighpressures. Consequently,simplepressure-fedsystemsca nbeemployed,equiringstorageofonboardfuelatmoderatepressuresonly.Similarly,onboardoxidantusedinconjunctionwithinitiatorunitsm aybestoredinrequired quantitiesatmoderatepressurelevels.Fuel/air/oxidizerdistributionmanifoldsdeliverthevariouspropeliantstothedetonationchamberatslightlyelevatedpressure. High-speedpropellantvalvesmeterproperproportionsofthesepropeliantsintothedetonationchambertoproduceuniformdetonablemixtures. Dedicatedmixingschemesm ayberequiredtoensureuniformmixingofpropeliantssothateachnitiatorunitand m amdetonationchamberpropellantchargeisreliablywithinthedetonabilitylimitsofthespecificmixture. Th epositive-pressuredistributionmanifoldsworkinconjunctionwithhigh-speedpropellantinjectionvalvesan dengine/ignitorcontrolsystemcomponents.FuelselectionscriticalforPDEoperationbecauseignitioncharacteristicsh a v eadramaticimpactonthedetonationprocess. Fuelca nbestoredinseveraldifferentformsbutmustbedeliveredtothefuelmanifoldasagas,iquid,and/orsolidofsufficientlysmalldroplet/particlesizetopermittheformationofstabledetonation. Solidpaniculateorliquiddropletsmustbeverysmalltoensurethatthedroplet/particlecombustiontimeiscompatiblewiththedetonationtimescale32.EachofthePDEsubsystemsmentionedmustmeetcertaincost,weight,an dvolumerequirementsinordertobeofpracticaluse. Inaddition,activesubsystemsmustbeabletooperatewithreasonablepowerrequirements. Throttlingcapabilitym aybenecessaryforsomePDEapplicationssuchasaircraftpropulsion,planetaryexcursionvehicles,etc. PDEthrottlingm aybeachievedby 'varyingpropellantvalveactuationratesan dinitiatorfrequency,orby othermeansdependingupontheengineconfiguration.7.2 PULSEDETONATIONROCKE TENGINESPDREs)SimilartoPDEs,PDREsproducethrustby cyclicallydetonatingpropeliantswithinadetonationchamberan dexhaustingthecombustionproductsthroughanozzle. However,unlikePDEs PDR Esmustcarryalloftheoxidizernecessarytocompletetheirspecificmissionsonboard,andPDREsmayh a v evacuumstartan drestartrequirementsformanyapplications. PDREswillincorporatepropellantstorage,feed,an dinjectionsystemcomponents,on eormoredetonationchambersignitionsystems,detonationchamber/nozzleinterfacehardware,nozzles,an denginecontrolsystemcomponents. SimilartoPDEs,numerousPDREconfigurationsmayevolvedependinguponengineeringdesignsolutionsadoptedtoovercometechnicalchallenges.OncepropeliantsareinjectedintoPDREdetonationchambers,thefast-actingpropellantinjectionvalvesclosetosealthedetonationchamberan ddetonationisinitiated. Th edetonationwavepassesthroughthedetonationchamberatsupersonicvelocities,gnitingthepropeliantsan delevatingtheupstreampressuretoseveraltimes(6-12times)thatoftheinitialfillpressure30Detonationwaveresidencetimewithinthedetonationchamberisontheorderof -3 msecdependingonthethermodynamicconditionsofthepropeliantsan dthedetonationchambergeometry Oncethedetonationwaveexitsthechamber, seriesofrarefactionwavespropagatefromtheopenendofthechambertowardstheclosed(valved)end,helpingtoexpelresidualcombustionproductsan dreducethepressurewithinthechamber. Oncethechamberpressuredropstoaspecifiedlevel,thechamberpurgean drefilloperationsca nbeinitiated.Likeconventional,steadycombustionrocketengines,PRDEsrequirepressurizationofthedetonationcombustion)chamber(s)toobtainhighperformance. However,du etothehigh pressureratiosassociatedwithdetonationcombustion,PDREdetonationchamberfillpressuresare2 0


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muchlowerthanchamberpressuresassociatedwithconventionalrocketengines30. PDREsca nemployavarietyofthermodynamiccyclestopowerturbopumps,suchasga sgenerator,stagedcombustion,orexpandercycles,orincorporatepressurizedfeedsystems30. PDREoperatingpressuresmaybeontheorderof100-200psia,ascomparedtoseveralhundredpsiaforconventionalopencycleenginesandseveralthousandpsiaforconventionalclosedcycleengines.Th eNASAMarsha l lSpaceFlightCenteriscurrentlysponsoringtwoPDREresearchan ddevelopmentefforts. M S F Ch asawardedcontractstoPratt&WhitneyAerosciencesan dtheUnitedTechnologiesResearchCenter(UTRC)todevelopan ddemonstratealternativesystemdesigns. Pratt&WhitneyAerosciencesh asdevelopeda conceptwheremultipledetonationchambersexhaustthroughacommonnozzleflowpath30. Thischamber/nozzlearrangementprovidesthenecessarybackpressuretomaintaindesiredchamberfillpressuresan dobtainadequate performancelevels. UTRCisdevelopingasingledetonationchamberPDREthatincorporatesanozzlewithanaerodynamicthroat40. Th evariableboundarylayercreatedbytheaerodynamicthroatensuresmaintenanceofadequatebackpressurewithinthedetonationchamberandallowsthenozzletooperateinaquasi-steadymode,therebyensuringadequateoverallengineperformance. BothoftheM SFC-sponsoredPDREdevelopmentenginesrequirewell-contourednozzlestoensurethatthenozzleflowsremainattachedtothenozzlewalls.Th emultiple-chamberPDREconceptshowninFig. 2 isprovidedbyPratt&WhitneyAerosciences(formerlyAdroitSystems,nc.)2 3 . Thisconceptincludessixdetonationchamberscoupledtoacommonfeedsysteman dnozzleassembly. Operationally,thedetonationchambersarefiredinaphasedmannerallowingthefeedsysteman dmanifoldstooperateinasteadystatemanner. Allofthedetonationchambercombustionproductsareexhaustedthroughthesinglecommonnozzle.

FromFuel FromOxidizerTanksanks

FeedSystem Cyc le Hardware {PressureFed ,GasGenerato r . Expander,orStag ed Combust ion)

0Fualan aO i d d t e o rTuroopump(t)Ga sGanwalor/ ISIValves an d DuosPreciseRow

Metering Valves(AllowSteadyManifoldRow)

DetonationCombustors

Nozzfe/CombustorInterface

Fuel/OxkfizerManifolds

DetonationInitiationSystemandControls

HeatTransfer/CoolingSystem WellContoured Nozz l e

Figure2. Concep tua lPulseDetonat ion Rocket EnginePDRE ) Schemat i c 44InadditiontothemajorPDREcomponentsan dsubsystemsdiscussed,PDREsm ay alsorequiregimbalmounts,thrustvectorcontrolactuators,thermalprotectionsystems,an dpowerconditioningsystems24. PDREdesignsneedtoincorporatemethodstomaintainchokedflowatthenozzlethroat

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tomaintainengine-operatingefficiency,an dthenozzledesignsneedtoaccommodatethecyclicthermalandmechanicaloadstomeetserviceliferequirements. MultiplechamberPDREsexhaustingintoasinglecommonnozzlefaceth eaddit ionaldesignchallengeofmitigatingtube-to-tubeinterferencetoachieveoperabledesignloadenvironments. Currenttechnologyeffortssuggestnozzledesignan doperationrequiresadditionaldevelopmenteffort.OtherPDREconfigurationsarepossible. PDREdesignsan dconfigurationsareinfluencedbyspecificmissionequirementsan dengineeringsolutionsadoptedtoaddressfundamentaltechnicalan doperabilityissues.8.0 SYSTEM MODELING Althoughsignificantprogressh asbeenmadeinth eU.S.egardingpulsedetonationsciencean dtechnologydevelopment,systeman dperformancemodelingrequiresaddit ionaleffort. VariouscomputationalstudiesindicateawidevariationinpredictedPDEperformancewithestimatesrangingfrom1100secondsto8000secondsofspecificimpulseforstoichiometrichydrogen-air25.Pulsedetonationsystemmodelingoffersnewanalyticalan dcomputationalchallengesnotpresentinconstantpressurecombustionmodeling,an dverylittleexperimentaldataonsystemperformanceh asbeeneportedintheopenliterature. Therefore,thereissignificantuncertaintyintheactualperformanceofevenanidealizedlaboratory-scalePDE. PDE/PDREresultsfromcomputationalstudiesar estronglydependentonthefidelityofthephysicalmodelonwhichtheequationsarebased,numericalresolution,nitialconditionsassumedfordetonationinitiation,specificgeometrysimulated,an dboundaryconditions25- 45 .Numerousmodelsofvaryingcomplexityh a v ebeenemployed. Whilenoneofthemodelsh a v eattemptedtorepresenttheunsteady,multi-phase,reactive, Dflowsinanengine,manycapturesomeaspectoftheessentialphysicsinaDor2 Dgeometry2 5 .Oneoftheprimarygoalsofcurrentpulsedetonationtechnologyinitiativesistheadvancementand validationofsystemmodelingtools. Manycurrentmodelsdo notfullyaccountforvalvelosses,mixinglosses,an dother"realengine"effects. Calculationoftheoreticalcycleefficiencyrequirespredictionofdetonationwavestructure,an dtheresultinghead-endpressure-t imehistory,whichsdependentuponcombustorgeometry. Th egeometryinfluencestheevacuationan drefillingtimesaswellasthepressurehistorywhilethedetonationwavetraversesth echamber. Considerationoflossesan dcombustorgeometrym ayresultinconsiderabledifferencesinthecalculationofpulse detonationcycleefficiencies. Varyingassumpt ionsan dboundaryconditionsusedintheproblem formulationresultinvaryingperformancepredictions.

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AnexampleofperformancemodelvariationisprovidedinFig.13,whichshowsthevariationnPDEspecificimpulseforinstantaneousversusgradualrelaxationofcombustiongasesoncethedetonationw a v eexitsachamberofthesamegeometry1 9.

..--->-Wl.i.i.- --

I4000VeryFastRelaxat ionVerySlow Relaxat ion

0.001 0.002 0.003

9 .0

time(s)Figure13 . VariationofIsp With CombustorRelaxation Mode19

PULSEDETONATIONPROPULSIONSYSTEM APPLICATIONSFlightvehiclemainpropulsionsystemscanusuallybecategorizedintooneofthefollowinggeneralcategories:airbreathing,rocket,combined-cycle,dual-mode,orhybrid. Manypotentialapplicationsexistforairbreathingan drocketpulsedetonationpropulsiondevices. Ifsuccessfullydeveloped,PDEscouldbeusedtopowertacticalaircraft,air-launchedandship-launchedmissiles,unmanned aerialvehicles,an da widerangeofstand-offmunitions. PDREscouldbeusedtopowerspacelaunchvehicleupperstages,orbittransfervehicles,excursionvehiclesan dplanetarylanders.PDREsm ay alsobeusedforspacecraftattitudecontrol,satellitestation eeping,an dsatellitemaneuver ingpropulsion.Combined-cyclesystemsplacecomponentsofdifferingenginecyclesinthesameflowpath. APDEplacedinth esameflowpathwithaconventionalrocketisanexampleofacombined-cyclesystemthatmightbeusedtopowerahigh-speed,long-rangemissile. Thistypeofsystemwouldrequirelesspackagingvolumethanaconventionaltwo-stagel iquidfueledrocketintendedforthesamemission. APDE/ramjet /scramjetcombined-cyclesystemmightbeusedtopowerahypersonicflightvehicle. Th ePDEwouldbeusedasthelow-speedacceleratoran dhandoverpoweredflighttotheramjetafterachievingaflightvelocityinexcessofM a c h 3 .Avariationofthecombined-cyclesystemistosharehardwarefortwodifferingenginecyclesinthesameflowpath. Thisvariationofthecombinedcycleistermeddual-mode. Dual-modeapplicationsforPD Ean dPDREsystemsarenotcurrentlywelldefined.TherearemanypotentialapplicationsforPDEsinhybridsystems. Hybridsar edefinedhereasanycombinationofaPDEwithturbomachinery. Inthehybridmode,aPD Eca nbeusedinplaceofhigh-pressurecompressorstages,combustionchambers,high-pressureturbinestages,and afterburners(o raugmentors). Foragivenai rflow,aPDEwouldprovideanapproximate2-foldincreaseinoverallpressureratioonatime-averagedbasisdu etothedetonationwavecompressionprocess. PDEsusedforthrustaugmentationwilllikelyimproveperformancean dreducefuelrequirementsrelativetocurrentaugmentorconfigurations.

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Pulsedetonationenginesca nalsobeconsideredorpropulsionsystemsusingcombinationcycles,i.e.henworor engineyclesrese dutootnteract.nxamplef DE combinationcyclesuseofaPDEasaductburner.ikewise,oraccessospaceapplications,PDEsca nbemountedinenginebaysseparatefromthevehicleundersidescramjetflowpath.10.0.S.PULSEDETONATIONT E C HN O L O G YINITIATIVES10.1V E R V I E W Recentadvancesinhigh-frequencypulsedetonationpropulsiontechnologybya numberofdifferentorganizationshaveresultedinrenewednterestindevelopmentofthetechnologyfora broadrangeofpropulsionapplications1 8 -20'22-- 24'26 -49. Th eDoDan dNASAarecurrentlysponsoringanumberoffundamentalresearchan dexploratorydevelopmentactivitiestoadvancethestate-of-the-art1 9'44 "49 .Academiaan dindustryarecontributingheavilytoeachofthegovernmentinitiativeswiththeobjectivepropulsiontechnologiestargetedtowardshigh-speedmissile,aircraft,accesstospace,an dspaceapplications. NASA'sprimarytechnologyapplicationareasincludesubsonican dsupersoniccommercialaviation,accesstospace,an dspaceexplorationmissions49. Ifsuccessfullydeveloped,th eDo DcouldapplyPDEtechnologytoabroadrangeofapplicationareas,including tacticalaircraftpropulsion,missilepropulsion,spaceaunchvehicleupperstagepropulsion,advancedspacevehiclepropulsion,an dspacecraftattitudecontrol,maneuvering,an dstationkeepingpropulsion.RecentPDEtechnologyadvancesh a v eledtotheformationoftwoU.S.ndustryteamsthatare participatinginmanyofthegovernment-sponsoredinitiatives. Th eindustryteamsarealsopursuing independenttechnologydevelopmentactivitiesdu etothehighpotentialpayoffforcommercial,military,an daerospaceapplicationsofpulsedetonationtechnologyshouldthetechnologybe successfullydeveloped. Th eindustryteamsh a v eformedtopulltogethertherangeofpropulsion,flightdynamics,an dsystemsexpertisetomovethetechnologyforwardquickly. Atpresent,th e'industryteamingarrangementsarePratt&WhitneyAerosciences/TheBoeingCompany/Pratt&Whitney/UnitedTechnologiesResearchCenter( UT R C ) ,an dGeneralElectric/ScienceApplicationsInternationalCorporationSAIC)/AdvancedProjectsResearchncorporated(APRI) . Commercialindustryobjectivesincluderesolutionofexistingtechnicalchal lengesan ddevelopmentan ddemonstrationofflightweightairbreathing,ocket,an dhybridpulsedetonationpropulsionsystemsthroughcorporateandgovernment-sponsoredtechnologyprograms.DoD an dNASAarepresentlysponsoringtheoreticalan dexperimentalresearchrelatingtoallaspectsofpulsedetonationsciencean dtechnologydevelopment. CurrentactivitiessupportPDE,PDRE,combined-cycle,an dhybridpropulsionsystemdevelopmentobjectives. CurrentlyfundedU.S.Governmentinitiativesan dtheirobjectivesarediscussedbelow.10.2 DE PARTM E NTOF DEFENSE1 0 .2 1 N A V YSuccessfuldevelopmentofPDEtechnologycouldresultinitsadaptationtomanyNavalweapons systems. Navyshipboardan daircraftmissilesystemsarenecessarilyvolumean dweightlimited.Currently,mosttacticalmissilesemploysolidrocketmotorsdu etotheirsimplicityan dhigh-speedcapability,buttheyh a v elimitedrange. Turbojetsan dturbofansareusedformissilesrequiringlongerrangeorheavierpayloadsdu etotheirhighspecificimpulse,butthesesystemsbecomeexpensiveforhighM a c h numbermissions(M =2-3). Forlongrangeathigherspeeds(M =2-4),ramjetsan dductedrocketsh a v ebeendeveloped. However,theyrequiresolidrocketboosterstoacceleratethemtoramjettakeoverspeed,whichincreasestheircost,complexity,an dpropulsion systemvolume. Combinedcycleengines,suchasturbo-rocketsan dturbo-ramjetshavealsobeen2 4


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consideredformissionsrequiringwiderangesinoperatingspeed,buttheyarealsomorecomplexan dmoreexpensive19-26.ApplicationofthepulseddetonationcyclepotentiallyoffersNavysystemsincreasedrange,stealth,andreliabilityforsystemsintheM a c h0-3operatingrange,whilesimultaneouslyofferingreductionsinsize,vulnerability,an dcost. Inaddition,multi-chamberPDEsmayenablefluidicthrustvectoring,therebyeliminatingtheneedforheavy,high-dragaerodynamiccontrolsurfaces. Th eNavym aywanttocontinuetheus eofhydrocarbonfuels,suchasJP-10,hydrocarbonblends,orhighdensitystrainedhydrocarbonfuelstosatisfyvolumean dsafetyconstraints. Asaconsequence,Navyresearch includesfundamentaldetonationstudiesofliquidfuelsan dair,whichprovidesasignificantchallengebecauseofthedifficultyassociatedwithnitiatingan dsustainingdetonationswithliquidfuel-airpropellantcombinations19.PD Eperformance estimatesan doperationalrangearecomparedtoothercandidatepropulsiontechnologiesinFig. 4.

602FlightMachNumber 14 16Figure4 . PDE Operat iona lEnvelope50Th edatainFig. 4h asbeenprovidedbyPratt&WhitneyAerosciences,andsuggeststhatairbreathingpulsedetonationenginesoperatingwithhydrocarbonan dhydrogenfuelsca noutperformothercandidatepropulsiontechnologiesaboveMach . Th edataalsosuggeststhatPDEsca nproviderelativelysteadyperformanceovertheentireM =0to5operationalrange.Th eOfficeofNavalResearchONR)issponsoringfundamentalresearchandexploratorydevelopmentofairbreathingpulsedetonationenginetechnology. Th eON R exploratorydevelopmentinitiativesincludea MultidisciplinaryUniversityResearchnitiative( M U R I )withsix participatingUniversit ies,nternationalscientificresearch initiatives,andscientificresearch initiativesattheNavalPostgraduateSchool,NavalAirWarfareCenter,andNavalResearchLaboratory. FromtheseactivitiesON R h asorganizedanintegratedesearchteamtodevelopanunderstandingoffundamentaldetonationscienceforpropulsionapplicationsan drequirementsforPD Edevelopment.h eprimaryobjectiveoftheNavyprogramistoadvancethesciencean d

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technologysufficientlytodevelopaviable,moreefficient,owercostalternativepropulsionsourcethaniscurrentlyavailablefornavalapplications19- 26.ON R sponsoreda seriesofworkshopsduringthemidtolate-1990stoexplorethescientifican dtechnicalissuesassociatedwithPD Etechnologydevelopment,an ddevelopan dprioritizescientificapproachestosolvingthem195. Basedonthefindingsan drecommendationsofth eworkshopparticipants,ON R initiatedtheM U R Inearly1999toaddressthetechnologydevelopmentissuesidentifiedbytheworkshopparticipants. Th eM U R Ih asbeenplannedasa3-yearplus2-yearfollow-oneffort19.Th eON R M U R Iteamsinclude: )PennsylvaniaStateUniversityPropulsionEngineeringResearch Center,CaliforniaInstituteofTechnology,andPrincetonUniversity;an d2 )UniversityofCalifornia atSa nDiego,StanfordUniversity,andUniversityofFlorida20. Th eUniversityteamsareaugmentedwithth esupportofresearch staffan dtestfacilitiesoftheNavalPostgraduateSchoolheNavalResearch Laboratory,an dtheNavalAirWarfareCenter1950. Th eON Rresearch effortincludes someinternationalparticipation19-50. Figure 5 outlinestheON R researchoadmapan dtheresponsibilitiesoftheparticipatinginstitutions.

International:MVLSU => M.V.LomonosovStateUniversity,Moscow IVTAN> InstituteofHigh Temperature , Russian Academy ofcience

.NIT IAT OR VII. om put e r Simulation andCycle Analysis

IV.Multi-Cycle Operat ionL|SU| T|

PSU ,ULTIPLE mmATOR DETONATIOtl CHAMBERS III.nlet-Combustor-Nozzle Performance L>U F

f 'IVI .ynamicsandControl UCSDPSU t V. iagnosticsand Sensors

t C isu

II.nject ion,Mix ing andInitiation PSU UCSD]HI jVTAN |CT

|.undamental Detonation StudiesNPS I NAWC|MVLSU

UCSD||CT||P U 1 [|S U1 UF ADROIT ]

C T = a lTech PSU =en nState Univers ityPU =* r inceton UniversitySU => tanford UniversityUCSD>Univers ity o fCal i fornia , San Diego UF =*niversityofFioridsN A W C =>NavaiAirW arfareCenterNP SavalPostgraduateSchoolNR L =>avalResearch LaboratorySBIR=>ADROIT ,UF

Figure15 . ONRPDE ResearchRoad MapSO

Th eON R research initiativesareintendedtoadvancePDEsciencean dtechnologyan dleaddirectlytoexploratorydevelopmentofapracticalPDE. Th ecurrentresearch includesfundamentaldetonationstudieswithselectpropellantcombinations,nvestigationofpropellantinjection,mixingan ddetonationnitiationcharacteristics,componentdesignandperformanceanalysis,component

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integrationstudies,diagnostic,sensor,an dcontrolrequirements,nvestigationofmulti-cycleoperation,an dadvancementofperformancepredictionan dcycleanalysiscapabilities50.Industryh asdemonstratedmulti-tube,multi-cycleoperationoflaboratory-scalePDEhardwarewithgaseouspropellants30,51.h eON R M U R Ian dcoreresearchprogramseekstosupplementindustry'stechnologyadvancementswithdevelopmentofabroadscientificdatabasetosupportfurtherexploratoryan dadvanceddevelopmenteffortswithpracticalfuels. AnONR-fundedresearchengineh asbeendevelopedan disinoperationattheNava lPostgraduateSchooltosupporttheNavyM U R Ian dcorePDEresearchprogram47 . Detonationexperimentsh a v ebeenperformedwiththeON R research enginetodeterminethedetonabilitylimitsofJP-10/air. Inadditiontoth eengineeringissuesregardingvalvedesign,thermalmanagement,an dmechanicalloading,th eON R research programisinvestigatingnumerousscientificissuesthatmustbe understoodbeforea practicalflightweightengineca nbedemonstrated9.Th esevenprimaryareasofresearchcurrentlybeingaddressedbythecombinedM U R Ian dcoreresearcheffortsareshowninFig. 5 . Figure 5alsoshowstheUniversityteamsan dgovernmentlaboratories participatinginthevariousinitiatives. Thereisagreatdealofoverlapandsynergism amongtheresearchactivitiesbeingperformedunderthevariousM U R Ian dcoreresearch initiatives.lnitiative-1objectivesinclude,butarenotlimitedto,developmentofasoundunderstandingofthecomplexphysical,chemical,andthermodynamicphenomenaassociatedwithgaseousan dliquid phaseinjection,mixingan dignition,factorsthatinfluencerapiddevelopmentofplanardetonationwaves,an dth eroleoftransversewavesinthedetonationprocess. Thesetheoreticalan dexperimentalstudieswillhelptoestablish minimumgnitionenergyrequirementsforpracticalfuel/aircombinations. Recentpublicationsdiscussingtheprogressoffundamentaldetonationstudiesinsupportoflnitiative-1objectivesareprovidedinReferences52-60.lnitiative-2research effortsareintendedtodevelopanunderstandingofpropellantinjection,mixing,an ddetonationinitiationrequirements. EfficientPDEoperationwillequirerapidinjectionan datomizationofrelativelylargeamountsofliquidfuelpe rdetonationcycle. Rapidatomizationisrequiredtoensurefastan dreliabledetonationinitiation. Researchersparticipatinginnitiative-2activitiesarecurrentlyinvestigatingatomizationtechniques,detonationnitiationsensitivitytovariationsinfueldropletsizes,ocaldegreeofmixing,turbulence,nitialpropellantmixturetemperature,an dotherparameters. Inaddition,methodsofcontrollingdetonationfrequencyan ddeflagration-to-detonationtransitiontimesarealsounderinvestigation. RecentpublicationsoftheON R M U R Iinitiative-2effortsareprovidedinReferences61-67. Importantobjectivesoflnitiative-2activitiesincludedeterminationofmixingaccuracycontrolrequirements,detonationan dDDT controlschemes,an dminimizationofDD Ttimean ddistancerequirements.lnitiative-3effortsincludeinvestigationofefficientmethodsofintegratingmixedcompressionsupersonicinlets,combustors,andhigh-performancenozzles. Th eunsteadybehaviorofsupersonicinletdiffuserflowsh asbeenaconcerninthedevelopmentofairbreathingPDEsdu etoundesirable longitudinalpressureoscillationscausedby thecycliccombustionprocess68 . Th einletexitplaneofmultiple-chamberPDEswithacommoninletwillexperiencenon-uniformpressurefieldsarisingfromoperationofth ePDEdetonationtubevalves. Th eflowareaoftheinlet/combustorinterfacechangeswithtimeastheairinletvalvesforthemultipledetonationchamberscycleopenan dclose.Oscillationsinbackpressurewillcausetheterminalshocktooscillateaboutitsmeanposition.Extremeoscillationsinbackpressuresintroducethepotentialforhammershockan dunstartingoftheinlet. Therefore,theinletdiffusermustprovideastabilitymarginsufficientforaccommodatingperturbationsoftheshocksystem.Asignificantamountofinformationrelevanttoinitiative-3objectivesisreportedintheliterature.Researchershavereportedextensivelyonexperimentalan dnumericalnvestigationsofdiffuserflowwithpressureoscillationstounderstandtheeffectofcombustioninstabilitiesondiffusersaswellastheeffectofnaturaloscillationsonthecombustionchambers69'78. Thesestudiesh a v efocused

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primarilyonramjetswithuniform,time-dependentvariationofbackpressure,an dconsideredonlysimplenormalshocksinconvergent-divergentnozzles. Hsiehan dYangh a v eanalyzedmixedcompression,supersonicinletflowwithconsiderationofcompressionprocessesupstreamoftheterminalshock6S. Hsiehan dYangalsoinvestigatedtheresponseoftheshocksystemtovariousdisturbances,analyzedchangesinflowcharacteristicsdu etoshockoscillations,examinedinfluencesofthecompression/expansionprocessesupstreamoftheterminalnormalshock,an dexaminedth einfluencesoftheviscousboundarylayeran dflowseparationsdownstreamofthenormalshock.Previoustheoreticalstudiesh a v eanalyzedtheflowattheexitofasupercriticalinletofamultiple-chamberPDEan dconcludedthatthetimeavailabletotransferairbetweenadjacenttubeswhilevalvesarecyclingismuches sthanthetimerequiredtoformhammershockconditions35. Theseresultssuggestthattheconceptofaninletplenumsupplyingairtomultipledetonationchambersh asthepotentialtobecomeapracticalsolutionforPDEinlets.Th emostrecentON R M U R Iresearch onthissubjectisreportedinReference79. M ullagirian dSegalexperimentallysimulatedtheoperationofexternal/internalcompression,two-dimensionalsupersonicinletby varyingtheflowareafrom32-83%blockagewithexcitationfrequenciesranging from15-50H z. Th eexperimentalresultsindicatedthatthemagnitudeofpressureoscillationsincreasedwithincreasingblockagean ddecreasedwithincreasingexcitationfrequency. However,theinletstartedan dremainedstartedovertheentirerangeoftestconditions. Theseresultssupportearlieranalyticalstudiesthatshockdisplacementamplitudesareinverselydependanton backpressureexcitationfrequency.lnitiative-4research objectivesincludecharacterizationofthedynamiccouplingbetweendetonationchambersonmultiplechamberPD Econfigurations. Agreatdealofpriorresearch asfocusedon fundamentaldetonationstudiesan dcyclicoperationofsingle-chamberdevices. Practicalenginesm ayincorporatemultipledetonationchambersintegratedwithacommoninletan dnozzletoobtain highaggregateoperatingfrequenciesan dincreasetime-averagedthrust.ResearchersatPennsylvaniaStateUniversity,theNava lPostgraduateSchool,an dtheCaliforniaInstituteofTechnologyarecurrentlyconductingtheoreticalande

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