How a launch vehicle works - Encyclopedia Britannica

A launch vehicle is a good illustration of Newton's third law of motion, “For every action, there is an equal and opposite reaction. launchvehicle TableofContents launchvehicle IntroductionOriginsV-2EarlyU.S.launchvehiclesEarlySovietlaunchvehiclesSoundingrocketsLaunchvehiclesoftheworldUnitedStatesRussiaandUkraineEuropeChinaJapanIndiaIsraelIranHowalaunchvehicleworksStagesUpperstagesFuelPayloadprotectionNavigation,guidance,andcontrolReliabilityLaunchingintoouterspaceLaunchbasesCommerciallaunchindustryThequestforreusabilityBeyondrockets FastFacts 2-MinSummary RelatedContent Media Images More MoreArticlesOnThisTopic AdditionalReading Contributors ArticleHistory Howalaunchvehicleworks AlaunchvehicleisagoodillustrationofNewton’sthirdlawofmotion,“Foreveryaction,thereisanequalandoppositereaction.”(Foradetailedexplanation,seerocket.)Inthecaseofalaunchvehicle,the“action”istheflowouttherearofthevehicleofexhaustgasesproducedbythecombustionofthevehicle’sfuelinitsrocketengine,andthe“reaction”isthepressure,calledthrust,appliedtotheinternalstructureofthelaunchvehiclethatpushesitinthedirectionoppositetotheexhaustflow.Unlikejetengines,whichoperateonthesameaction-reactionprinciplebutobtaintheoxygenneededforburningtheirfuelfromtheatmosphere,rocketscarrywiththemtheirownoxidizingagent.Inthatway,theycanoperateinthevacuumbeyondtheatmosphere. Theprimarygoaloflaunchvehicledesignersistomaximizethevehicle’sweight-liftingcapabilitywhileatthesametimeprovidinganadequatelevelofreliabilityatanacceptablecost.Achievingabalanceamongthesethreefactorsischallenging.InorderforthelaunchvehicletoliftoffofEarth,itsupwardthrustmustbegreaterthanthecombinedweightofitsspacecraftpayload,thevehicle’spropellants,anditsstructure.Thisputsapremiumonmakingthevehicle’smechanicalstructure,fueltanks,androcketenginesaslightaspossiblebutstrongenoughtowithstandtheforcesandstressesassociatedwithrapidaccelerationthrougharesistantatmosphere.Mostoften,propellantmakesup80percentormoreofthetotalweightofalaunchvehicle–spacecraftcombinationpriortolaunch. BritannicaQuiz Space:FactorFiction? MarsandtheMilkyWayaremorethanjustcandybars!Seehowmuchmoreyouknowaboutspacewiththisquiz. Stages Abasicapproachtolaunchvehicledesign,firstsuggestedbyKonstantinTsiolkovsky,istodividethevehicleinto“stages.”Thefirststageistheheaviestpartofthevehicleandhasthelargestrocketengines,thelargestfuelandoxidizertanks,andthehighestthrust;itstaskistoimparttheinitialthrustneededtoovercomeEarth’sgravityandthustoliftthetotalweightofthevehicleanditspayloadoffofEarth.Whenthefirst-stagepropellantsareusedup,thatstageisdetachedfromtheremainingpartsofthelaunchvehicleandfallsbacktoEarth,eitherintotheoceanorontosparselypopulatedterritory.Withtheweightofthefirststagegone,asecondstage,withitsownrocketenginesandpropellants,continuestoacceleratethevehicle.Mostexpendablelaunchvehiclesinusetodayhaveonlytwoorthreestages,butinthepastuptofivestages,eachlighterthanitspredecessor,wereneededtoattainorbitalvelocity.Whenanupperstagehascompleteditsmission,iteitherfallsbacktoEarth’ssurface,entersorbititself,or,mostfrequently,disintegratesandevaporatesasitencountersatmosphericheatingonitsfallbacktowardEarth. Aparticularlaunchvehiclecanbeconfiguredinseveraldifferentways,dependingonitsmissionandtheweightofthespacecrafttobelaunched.Thisreconfigurationcanbedonebyaddingavaryingnumberofstrap-onboosters,usuallysolidrocketmotors,tothevehicle’sfirststageorbyusingdifferentupperstages. Inprinciple,aspacelaunchercouldreachEarthorbitusingonlyonestage,andinfacttherehavebeenseveralattemptstodevelopareusable“singlestagetoorbit”vehicle.Allattemptshavefailed,however;thepropulsionandmaterialstechnologiesneededtomakeasingle-stagevehiclelightandpowerfulenoughtoachieveorbitalvelocitywhilecarryingameaningfulpayloadhavenotbeendeveloped. Upperstages Alllaunchvehiclesemploymorethanonestagetoacceleratespacecrafttoorbitalvelocity.Sincethefirstorbitallaunch(Sputnik),in1957,therehavebeenmanydifferentupperstages.Mostareusedaspartofonlyonetypeoflaunchvehicle.Theevolutionoftheseupperstagesisdrivenbyadesiretointroducemoremoderntechnologythatwillincreasetheoverallliftcapabilityofthelaunchvehicle,loweritscosts,andincreaseitsreliability—oracombinationofthesefactors.Smallimprovementsinupperstagescanproducesignificantgainsinlaunchvehicleperformance,sincethesestagesoperateonlyafterthefirststagehasacceleratedthevehicletoahighspeedthroughthethickestpartsoftheatmosphere. Severalupperstageshavebeenusedwithmorethanonefamilyoflaunchvehicle.Forexample,theAgenaupperstagewasfirstdevelopedintheUnitedStatesaspartofitsinitialreconnaissancesatelliteprogram.TheAgenaupperstageofaThor-AgenalaunchvehiclepropelledtheCoronaspacecraftintoorbit,stayedattachedtoit,andprovidedpowerandpointingforthespacecraft’soperation.Agenausedhypergolicpropellant;itwasalsocombinedwiththeAtlasandTitanfirststagesonanumberofsubsequentmissions.LaterversionsofAgenawereabletorestarttheirengineinorbit,carriedothernationalsecuritypayloads,sentRangerandLunarOrbiterspacecrafttotheMoonandMarinerspacecrafttoVenusandMars,andservedasthetargetvehicleforrendezvousbytheGeminitwo-manspacecraft.UseoftheAgenaupperstageextendedthroughthemid-1980s.UpperstageAgena,thetargetvehiclefortheGemini12rendezvousanddocking,launchedtwohoursbeforetheGeminispacecraft,onNov.11,1966.NASA AnotherU.S.upperstage,usedwiththeAtlasandTitanlaunchvehicles,isCentaur.ThiswasthefirstU.S.rocketstagetoemploycryogenicpropellant.ThefirstuseoftheAtlas-CentaurlaunchvehiclewastosendSurveyorspacecrafttotheMoonin1966and1967;itflewmanysubsequentmissionsatopanAtlasfirststage.WhencombinedwithpowerfulversionsoftheTitanlaunchvehicle,CentauralsohasbeenusedtosendvariousspacecrafttoMarsandtheouterplanetsandtolaunchvariousheavynationalsecuritypayloads. Variousupperstagesusingsolidpropellantswereusedtocarrypayloadsfromthespaceshuttle’slowEarthorbittohigherorbits.Therewereplanstocarrytheliquid-fueledCentaurontheshuttletolaunchplanetaryspacecraft,butthoseplanswerecanceledafterthe1986Challengeraccidentbecauseofsafetyconcerns.Solid-propellantupperstageshavealsobeenusedwiththeDeltaandTitanlaunchvehicles. SovietandRussianlaunchvehicleshaveusedavarietyofupperstages;mosthaveusedconventionalkeroseneasfuel.Morerecentlytwoupperstages,theBlockDMusingcryogenicpropellantandthemorepopularBrizMusinghypergolicpropellant,havebeendevelopedfortheProtonlauncher.TherehasbeenaconstantevolutionofupperstagesusedwiththeSoyuzlauncher;in1999upperstageswithrestartablerocketenginesenteredservice. TheESAusedacryogenicupperstageforitsAriane1–4launchers.InitialversionsoftheAriane5usedhypergolicpropellantinitsupperstage,thoughanewcryogenicupperstagewasintroducedin2006.JapanandIndiausecryogenicpropellantsintheupperstagesoftheirmostpowerfullaunchvehicles,theH-IIAandtheGSLV,respectively. Fuel Thefuelusedtopowerrocketscanbedividedintotwobroadcategories:liquidandsolid.Liquidfuelscanrangefromawidelyavailablesubstancesuchasordinarykerosene,whichcanbeusedatgroundtemperature,toliquidhydrogen,whichmustbemaintainedattheextremelylowtemperatureof20°K(−253°C,or−423°F).Liquidhydrogeniscalledacryogenicfuel.Anothertypeofliquidfuel,calledhypergolic,ignitesspontaneouslyoncontactwithanoxidizer;suchfuelsareusuallysomeformofhydrazine.Hypergolicfuelsareextremelytoxicandthusdifficulttohandle.However,becauseoftheirreliableignitionandtheirabilitytoberestarted,theyareusedinthefirstorsecondstagesofsomerocketsandinotherapplicationssuchasorbitalmaneuveringmotors.DuringtheApolloprogramtheywereusedtoliftthecrewcompartmentofthelunarmoduleoffoftheMoon’ssurface. Inordertoburn,liquidrocketfuelmustbemixedinthecombustionchamberofarocketenginewithanoxygen-richsubstance,calledanoxidizer.Theoxidizerusuallyusedwithbothkeroseneandliquidhydrogenisliquidoxygen.Oxygenmustbekeptatatemperaturelessthan−183°C(−298°F)inordertoremaininaliquidstate.Theoxidizerusedwithhypergolicfuelisusuallynitrogentetroxideornitricacid.Likehypergolicfuel,theoxidizersareextremelytoxicsubstancesandsoaredifficulttohandle. Liquid-fuelrocketenginesarecomplexmachines.Inordertoreachmaximumefficiency,bothfuelandoxidizermustbepumpedintotheengine’scombustionchamberathighrates,underhighpressure,andinsuitablemixtures.Thefuelpumpsaredrivenbyaturbinepoweredbytheburningofasmallproportionofthefuel.Therearevariousapproachestopoweringtheturbomachineryofarocketengine,butallrequirehigh-performancemechanismsandareoneofthemajorpotentialsourcesoflaunchvehiclefailure.Aftercombustion,theresultingexhaustgasexitsthroughanozzlewithashapethatacceleratesittoahighvelocity.Spaceshuttlemainengineflow.EncyclopædiaBritannica,Inc. Solid-propellantrocketmotorsaresimpleindesign,inmanywaysresemblinglargefireworks.Theyconsistofacasingfilledwitharubberymixtureofsolidcompounds(bothfuelandoxidizer)thatburnatarapidrateafterignition.Thefuelisusuallysomeorganicmaterialorpowderedaluminum;theoxidizerismostoftenammoniumperchlorate.Thesearemixedtogetherandarecuredwithabindertoformtherocketpropellant.Solidrocketmotorsaremostoftenusedasstrap-onstotheliquid-fueledfirststageofalaunchvehicletoprovideadditionalthrustduringliftoffandthefirstfewminutesofflight.(However,theUnitedStateshasbegundevelopmentofanewlaunchvehiclenamedAres-1thatwillusealargesolidrocketmotorasitsfirststage.)Unlikesomerocketenginesusingliquidfuels,whichcanbeturnedoffafterignition,solidrocketmotorsonceignitedburntheirfueluntilitisexhausted.Theexhaustfromtheburningofthefuelemergesthroughanozzleatthebottomoftherocketcasing,andthatnozzleshapesandacceleratestheexhausttoprovidethereactiveforwardthrust.Cutawayofalargesolidrocketmotor.Thistypeofmotor,usedontheU.S.spaceshuttle,consistsoffoursegmentsandanozzleassemblythatarematedatthelaunchsite.The"factoryjoints"showninthediagramarecase-segmentjointsassembledbeforepropellantcasting;"fieldjoints"areassembledsubsequently.Theshuttlemotorsarerecoveredatsea,refurbished,andreused.EncyclopædiaBritannica,Inc. Payloadprotection Thespacecraftthatalaunchvehiclecarriesintospaceisalmostalwaysattachedtothetopofthevehicle.Duringthetransitoftheatmosphere,thepayloadisprotectedbysomesortoffairing,oftenmadeoflightweightcompositematerial.Oncethelaunchvehicleisbeyondthedensestpartoftheatmosphere,thisfairingisshed.Afterthespacecraftreachesinitialorbitalvelocity,itmaybedetachedfromthelaunchvehicle’sfinalupperstagetobeginitsmission.Alternatively,ifthespacecraftisintendedtobeplacedinotherthanalowEarthorbit,theupper-stagerocketenginemaybeshutdownforaperiodoftimeasthespacecraftpayloadcoastsinorbit.ThentheengineisrestartedtoimparttheadditionalvelocityneededtomovethepayloadtoahigherEarthorbitortoinjectitintoatrajectorythatwillcarryitdeeperintospace. Navigation,guidance,andcontrol Inorderforalaunchvehicletoplaceaspacecraftintheintendedorbit,itmusthavenavigation,guidance,andcontrolcapabilities.Navigationisneededtodeterminethevehicle’sposition,velocity,andorientationatanypointinitstrajectory.Asthesevariablesaremeasured,thevehicle’sguidancesystemdetermineswhatcoursecorrectionsareneededtosteerthevehicletoitsdesiredtarget.Controlsystemsareusedtoimplementtheguidancecommandsthroughmovementsofthevehicle’srocketenginesorchangesinthedirectionofthevehicle’sthrust.Navigation,guidance,andcontrolformostlaunchvehiclesareachievedbyacombinationofcomplexsoftware,computers,andotherhardwaredevices. Reliability Alaunchvehiclethuscomprisesoneormorerocketengines;fuelforthoseenginescarriedinfueltanks;guidance,navigation,andcontrolsystems;apayload;andastructurehousingalloftheseelements,towhichextraenginesmaybeattachedforaddedlift.Therearealsovariousattachmentsbetweenthelaunchvehicleanditslaunchpadandassociatedstructures.Anexpendablelaunchvehiclehasonlyoneopportunitytoperformitsmissionsuccessfully,soallofitselementsmustbedesignedandmanufacturedpreciselyandforveryhighoperationalreliability.Also,asnotedabove,launchvehiclesaredesignedtobeaslightaspossible,inordertomaximizetheirpayloadliftingcapability.Asaresult,everypartinalaunchvehicleoperatesclosetoitsbreakingpointduringalaunch,asthevehicleundergoesthestressesassociatedwithacceleratingpastthespeedofsoundandtransitingtheatmosphereandasitsrocketenginesoperateunderextremesofpressure,temperature,shock,andvibration. Theendresultisthatlaunchingaspacecraftintoouterspaceremainsanextremelydifficultundertakingandthatlaunchfailuresareafactoflifeforthoseseekingaccesstospace.Manyspacelaunches,particularlythosecarryingcommercialspacecraft,areinsuredagainstfailure,sincetheyoftenrepresentaninvestmentofmorethanahundredmilliondollars. Launchvehiclesthatcarrypeopleintospaceare“humanrated.”Thismeansthattheyusecomponentsofthehighestpossiblereliability,haveredundancyincriticalsystems,undergomoretestingpriortolaunchthandoesalaunchvehiclecarryinganautomatedspacecraft,andcontainsystemsthatwarnofimpendingproblemssothatacrewmightbeabletoescapeanaccident.Therehasbeenonlyonefailureofalaunchvehicleatliftoffthatresultedincrewfatalities;thiswastheexplosionoftheChallenger,onJanuary28,1986,whichkilledallsevenastronautsaboard. Launchingintoouterspace Althoughtheydifferinmanydetailsforvariousvehiclesandatdifferentlaunchbases,thestepsneededtopreparealaunchvehicleanditsspacecraftpayloadforlaunchare,ingeneral,similar. Mostoften,thedifferentstagesandotherelementsofalaunchvehiclearemanufacturedseparatelyandtransportedtothelaunchbaseforassembly.Thatassemblycantakeplaceeitherinafacilityawayfromthelaunchpadoronthelaunchpaditself.Theadvantageofaseparateassemblybuildingisthatmanyofthestepsneededtopreparethevehicleforlaunch,includingassemblyandthencheckoutoftheintegratedvehicle,canbeperformedinaclosedenvironment.Thisalsomeansthatthelaunchpadisavailableforotherusesduringtheassemblyandcheckoutperiod. Launchvehicleassemblyandcheckoutarecarriedouteitherverticallyorhorizontally.Verticalassemblyrequiresafacilitytallenoughtoshelterthewholevehicleandspacecraft.Thevariouscomponentsare“stacked,”startingwiththefirststageandoftenendingwiththeattachmentofthespacecrafttothelaunchvehicle.(Sometimesthespacecraftandthelaunchvehiclearematedonlyatthelaunchpad.)Strap-onsolidrocketengines,iftheyaretobeused,areattachedtothecorefirststage.Horizontalassemblyiscarriedoutonanend-to-endbasisanddoesnotrequireahighbuildingwithvehicleaccessatmultiplelevels.Afterassembly,asmuchtestingaspossibleisconductedontheintegratedvehicletocheckitsreadinessforlaunchbeforeitistransportedtothelaunchpad.spaceshuttleAtlantisInterioroftheVehicleAssemblyBuilding(VAB)attheKennedySpaceCenter,Florida,U.S.SuspendedwithinisthespaceshuttleAtlantisbeforeitisjoinedwithitsexternaltankandsolid-fuelrocketboostersforflight.Oneofthelargestbuildingsintheworld,theVABwasbuilttoaccommodatetheassemblyoftheApollo-SaturnVvehicles.Itcontainsnearly3.7millioncubicmetres(about130millioncubicfeet)ofspace.KenThornsley/NASA Onceitreachesthelaunchpad,thevehicleisattachedtoalaunchtower,whichcontainsthevariousumbilicalconnectionsandaccesspointsneededtocompletethecheckoutprocessandtomonitorthevehicle’sfinalreadinessforlaunch.Ifthevehiclehasbeenassembledhorizontally,itmustberaisedintoaverticalpositionasitreachesthelaunchpad.Oftenthelaunchpadincludessomesortofsheltertoprotectthelaunchvehicleandspacecraftfromtheelementsuntilclosetothetimeforlaunchandtoallowtechnicianstocontinuethecheckoutprocess.Thelaunchvehicleisheldonthepadbysomeformofattachmentdevice. Ifthelaunchvehicleisassembledonthelaunchpad,alloftheabovestepsareconductedthere.Assemblyandcheckoutcantakeseveralmonths,andduringthisperiodthelaunchpadcannotbeusedforotherpurposes. Asthetimeforlaunchapproaches,acountdownisinitiated.Countdowntimecanrangefromhourstodays.Duringthecountdown,variousfinalstepsarecarriedoutatspecifictimestomakethevehiclereadyforlaunch.Ifthevehicleusesliquidpropellants,theyareloadedinthehoursbeforelaunch,afterbeingstoredintanksnearthelaunchpad.Cryogenicpropellantsaredifficulttomaintaininaliquidstate;theytendtobecomegaseousand“boiloff”ofthevehicle.Therefore,theyareloadedintothevehicle’sfueltanksasclosetothetimeoflaunchaspossibleandmustbeconstantlytoppedofftoensurethatthefuelandoxidizertanksarefull.Somehoursbeforeascheduledlaunch,thestructurethathasbeenprotectingthevehicleisrotatedawayfromitandthelaunchtower. Launchbasesmusthaveaccesstoup-to-dateweatherinformation.Thereareusuallypresetruleswithrespecttowhatweatherconditionsareacceptableforaspacelaunch,includingwindsatthelaunchsiteandaloft,visibility(formonitoringthevehicleduringthefirstfewminutesofflight),andtemperature.Theseconditionsvaryamonglaunchsitesandfordifferentlaunchvehicles.Forexample,crewedspacecraftarelaunchedfromRussiansitesduringmuchmoresevereweatherconditionsthanthosedeemedacceptableforthelaunchofaU.S.spaceshuttlefromitsFloridalaunchbase. Inthelastfewminutesofthecountdown,afinalcheckismadetoensurethatthevehicleandspacecraftarereadyforlaunchandthatallotherconditionsareina“go”status.Allumbilicalconnectionsbetweenthelaunchtowerandthevehiclearedetached.Liquid-fueledrocketenginesareusuallyallowedtofireforafewsecondsbeforethevehicleiscommittedtolaunch;arapidcomputercheckisperformed,andtheenginescanbeshutdownifthereareanyindicationsofaproblem.Oncesolidrocketenginesareignited,thevehicleiscommittedtolaunch.Whenthemomentoflauncharrives,thedevicesholdingthevehicletothelaunchpadareexplosivelydetached,andthevehiclebeginsitsliftoff. Launchpadshavetrenchesforchannelingexhaustflamesawayfromthevehicle,andfrequentlylargevolumesofwaterareinjectedintotheflames.Thisisdonetominimizedamagetovehicleandlaunchpadfromtheheatandsonicvibrationsassociatedwithliftoff. Associatedwitheachlaunchbaseisalaunchrangewithfacilitiesfortrackingandcloselymonitoringthelaunchvehiclethroughallstagesofitsmission.Arangesafetyofficermakessurethatnoaspectsofthevehicle’sperformancecouldposeathreattopublicsafetyordestroyproperty.Ifsuchaconditionarose,theofficerwouldbeabletocommandthelaunchvehicletodestroyitself.