Combined Rocket and Airbreathing Propulsion Systems for Space-Launch Applications

JOURNALOFPROPULSIONANDPOWERVol.14,No.5,September–October1998

CombinedRocketandAirbreathingPropulsionSystems

forSpace-LaunchApplications

RussellDaines*

BrighamYoungUniversity,Provo,Utah84602

and

CorinSegal†

UniversityofFlorida,Gainesville,Florida32611

Areviewofrocket-airbreathingcombined-cyclepropulsionsystemsforEarth-to-orbitapplicationsis

presented.Rocket-basedcombined-cycle(RBCC)enginestakeadvantageofthesynergisticinteractions

betweentherocketandtheairbreathingelementsoftheengineandtheuseofhigh-speci󰀷cimpulsecycles

toyieldamission-averagedspeci󰀷cimpulsethatishigherthanall-rockettechnologycanprovide.An

overviewofthemultimodeoperationisgiven,alongwithareviewofbothexperimentalandmodeling

workthathasbeendoneonthisclassofengines.Selectedissuesinvolvedwiththeseenginesarediscussed.

Theseincludeengine/vehicleintegration,󰀸ow-pathdesignformultimodeoperation,fuelselection,mixing

enhancementandafterburninginrocket-ejectormode,thermalchoking,and󰀸ameholding.RBCCpro-

pulsionisbecomingrecognizedasapromisingtechnologyforachievingasigni󰀷cantreductioninthecost

ofdeliveringpayloadtoorbit.

IntroductionCURRAN1pointsoutthatnosingleexistingclassofen-

ginesprovidesahighenoughthrust-to-weightratioto

support󰁏ightoveranextendedMachnumberrangeandpos-

sessesahighspeci󰁎cimpulseovertheentirerange.Individual

airbreathingcyclesoperateoveronlyaportionoftheMach

numberrange,asshowninFig.1.Rocketsprovidethethrust-

to-weightrationecessaryandcanoperateovertheentireMach

numberrange,butatarelativelylowspeci󰁎cimpulse.

Figure1showsthespeedregimeswherevariouspropulsion

cyclesarepreferablefromaspeci󰁎cimpulsestandpoint,with

gasturbine-basedcyclessuperioruptoaroundM=2–3.Ram-

jetmodeismostef󰁎cientfromthereuptoaboutM=6,and

scramjetcyclesaresuperiorintheupperMachnumberregime.

Theuseoftankedoxidizerinrocketcycle,whichcausesthe

lowspeci󰁎cimpulseforthecycle,makesitwell-suitedtoop-

eratefor󰁎nalinsertionintoorbitfortransatmospheric󰁏ight.

Inspiteofthelowspeci󰁎cimpulse,therocketcycleisalso

advantageousasaninitialboosterbecauseofitshighthrust-

to-weightratio,withanincreaseinspeci󰁎cimpulsepossible

whenusedinrocket-ejectormode.

Historically,multiple-stagedvehicleshavebeendesignedto

operatewithasingletypeofpropulsionsystemforeachstage.

Stagesareoptimizedfordifferentaltitude/Machnumberre-

gimesinthetrajectory,increasingtheoverallsystemspeci󰁎c

impulse.Asanexample,NASA’srecentlyproposedhypersonic

aircraftdemonstrator,Hyper-X,willusesubsonicaircraftpro-

pulsionasthe󰁎rststage,followedbyasecondstageprovided

byaPegasus(󰁎rststage)rocket,withthescramjet-basedre-

searchvehicleasthethirdstage.2Thislimited-rangeacceler-

atorbeginsitsautonomous󰁏ightatM>5.

ReceivedSept.2,1997;revisionreceivedMay8,1998;acceptedforpublicationMay29,1998.CopyrightQ1998bytheAmericanInstituteofAeronauticsandAstronautics,Inc.Allrightsreserved.*AssistantProfessor,MechanicalEngineeringDepartment.E-mail:dainesr@et.byu.edu.MemberAIAA.†AssistantProfessor,DepartmentofAerospaceEngineering,Me-chanicsandEngineeringScience.E-mail:cor@aero.u󰁏.edu.Member

AIAA.Thelowspeci󰁎cimpulseofrocket-basedvehiclesalong

withtheuseofexpendablestagesresultsinahighcostper

poundtodeliverpayloadtoorbit.Amajorgoaldrivingcurrent

spacepropulsionresearchistosigni󰁎cantlydecreasethecost

ofaccesstospace.Therearecurrentlyeffortsunderwayto

developreusablelaunchvehiclesthatpromisetodecrease

long-termcostsascomparedtothetraditionalexpendable

stagedvehicles.Onewaytoavoidexpendablestagingand

makeuseofmoreef󰁎cientenginecyclesduringpartofthe

ascenttoorbitisbyusingtwoormoreseparatepropulsion

systemsonthevehiclethatoperateindependently.Theseare

referredtoascombinationpropulsionsystems(CPS).Anex-

ampleofthisistherocket–ramjet,whichusesarocketbooster

toachievetheinitialaccelerationtospeedscapableofsustain-

ingramjetoperation.3Atthatspeed,theengineswitchesto

ramjetoperationfortheremainderofthe󰁏ight.Althoughthe

useofCPSavoidspropulsionsystemintegrationissues,itre-

quirescarryingatleastonepropulsionsystemthatisnotac-

tivelyparticipatinginpropellingthevehicle.

Anotherwaytousehigh-ef󰁎ciencyairbreathingcyclesdur-

ingascentinareusablesystemisthroughtheuseofcombined-

cyclepropulsion(CCP)systems.Combined-cyclepropulsion

systemscanbebroadlydividedintotwocategories:airbreath-

ingcombined-cyclesthatcouldincludeturbojetorturbofan

cycles,andcombined-cyclesystemsthatincludearocketsub-

system.ExamplesofairbreathingCCPsystemsarethedual-

modecombustionramjet,whichoperatesinbothramjetand

scramjetmodes,4andtheturbine-basedcombined-cycleen-

gine,whichusesaturbine-basedcycleforlow-speed󰁏ight

alongwithramjetandscramjetmodes.5Airbreathingcom-

bined-cycleenginesareintendedprimarilyformissionsin-

volvinghigh-speedcruiseintheatmosphere,butarenotcan-