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RamjetAramjet,sometimesreferredtoasaflyingstovepipeoranathodyd(anabbreviationofaerothermodynamicduct),isaformofairbreathingjetenginethatusestheengine'sforwardmotiontocompressincomingairwithoutanaxialcompressor.Ramjetscannotproducethrustatzeroairspeed;theycannotmoveanaircraftfromastandstill.Aramjetpoweredvehicle,therefore,requiresanassistedtake-offlikearocketassisttoaccelerateittoaspeedwhereitbeginstoproducethrust.RamjetsworkmostefficientlyatsupersonicspeedsaroundMach3(2,284mph;3,675km/h).ThistypeofenginecanoperateuptospeedsofMach6(2,041.7m/s;7,350km/h).Ramjetscanbeparticularlyusefulinapplicationsrequiringasmallandsimplemechanismforhigh-speeduse,suchasmissilesorartilleryshells.Weapondesignersarelookingtouseramjettechnologyinartilleryshellstogiveaddedrange;a120mmmortarshell,ifassistedbyaramjet,isthoughttobeabletoattainarangeof35km(22mi).Theyhavealsobeenusedsuccessfully,thoughnotefficiently,astipjetsontheendofhelicopterrotors.Ramjetsdifferfrompulsejets,whichuseanintermittentcombustion;ramjetsemployacontinuouscombustionprocess.Theyaresimilartoscramjets,asystemdesignedforhigherspeedsthatusesasupersonicairflowinitscombustionchamber.Whileascramjetworkswiththesametechnology,thecombustionprocessdiffersslightly,resultinginahighercruisespeed.EnginecycleTheBraytoncycleisathermodynamiccyclethatdescribestheworkingsofthegasturbineengine,thebasisoftheairbreathingjetengineandothers.ItisnamedafterGeorgeBrayton(1830–1892),theAmericanengineerwhodevelopedit,althoughitwasoriginallyproposedandpatentedbyEnglishmanJohnBarberin1791.ItisalsosometimesknownastheJoulecycleDesignAramjetisdesignedarounditsinlet.Anobjectmovingathighspeedthroughairgeneratesahighpressureregionupstream.Aramjetusesthishighpressureinfrontoftheenginetoforceairthroughthetube,whereitisheatedbycombustingsomeofitwithfuel.Itisthenpassedthroughanozzletoaccelerateittosupersonicspeeds.Thisaccelerationgivestheramjetforwardthrust.Aramjetissometimesreferredtoasa'flyingstovepipe',averysimpledevicecomprisinganairintake,acombustor,andanozzle.Normally,theonlymovingpartsarethosewithintheturbopump,whichpumpsthefueltothecombustorinaliquid-fuelramjet.Solid-fuelramjetsareevensimpler.Bywayofcomparison,aturbojetusesagasturbine-drivenfantocompresstheairfurther.Thisgivesgreatercompressionandefficiencyandfarmorepoweratlowspeeds,wheretherameffectisweak,butisalsomorecomplex,heavierandexpensive,andthetemperaturelimitsoftheturbinesectionlimitthetopspeedandthrustathighspeed.DiffuserRamjetstrytoexploittheveryhighdynamicpressurewithintheairapproachingtheintakelip.Anefficientintakewillrecovermuchofthefreestreamstagnationpressure,whichisusedtosupportthecombustionandexpansionprocessinthenozzle.Mostramjetsoperateatsupersonicflightspeedsanduseoneormoreconical(oroblique)shockwaves,terminatedbyastrongnormalshock,toslowdowntheairflowtoasubsonicvelocityattheexitoftheintake.Furtherdiffusionisthenrequiredtogettheairvelocitydowntoasuitablelevelforthecombustor.Subsonicintakesonramjetsarerelativelysimple.Subsonicramjetsdonotneedsuchasophisticatedinletsincetheairflowisalreadysubsonicandasimpleholeisusuallyused.Thiswouldalsoworkatslightlysupersonicspeeds,butastheairwillchokeattheinlet,thisisinefficient.Theinletisdivergent,toprovideaconstantinletspeedofMach0.5(170.15m/s;612.5km/h).CombustorAswithotherjetengines,thecombustor'sjobistocreatehotair,byburningafuelwiththeairatessentiallyconstantpressure.Theairflowthroughthejetengineisusuallyquitehigh,soshelteredcombustionzonesareproducedbyusing'flameholders'tostoptheflamesfromblowingout.Sincethereisnodownstreamturbine,aramjetcombustorcansafelyoperateatstoichiometricfuel:airratios,whichimpliesacombustorexitstagnationtemperatureoftheorderof2,400K(2,130°C;3,860°F)forkerosene.Normally,thecombustormustbecapableofoperatingoverawiderangeofthrottlesettings,forarangeofflightspeeds/altitudes.Usually,ashelteredpilotregionenablescombustiontocontinuewhenthevehicleintakeundergoeshighyaw/pitchduringturns.Otherflamestabilizationtechniquesmakeuseofflameholders,whichvaryindesignfromcombustorcanstosimpleflatplates,tosheltertheflameandimprovefuelmixing.Overfuellingthecombustorcancausethenormalshockwithinasupersonicintakesystemtobepushedforwardbeyondtheintakelip,resultinginasubstantialdropinengineairflowandnetthrust.NozzlesThepropellingnozzleisacriticalpartofaramjetdesign,sinceitacceleratesexhaustflowtoproducethrust.ForaramjetoperatingatasubsonicflightMachnumber,exhaustflowisacceleratedthroughaconvergingnozzle.ForasupersonicflightMachnumber,accelerationistypicallyachievedviaaconvergent-divergentnozzle.OneofthetwoBristolThorramjetenginesonaBristolBloodhoundmissilePerformanceandcontrolAlthoughramjetshavebeenrunasslowas45metrespersecond(160km/h),belowaboutMach0.5(170.15m/s;612.5km/h)theygivelittlethrustandarehighlyinefficientduetotheirlowpressureratios.Abovethisspeed,givensufficientinitialflightvelocity,aramjetwillbeself-sustaining.Indeed,unlessthevehicledragisextremelyhigh,theengine/airframecombinationwilltendtoacceleratetohigherandhigherflightspeeds,substantiallyincreasingtheairintaketem