—MichaelBelfiore
Rutansolvedtherocketenginesideoftheequationfirst,evenbeforehehadfoundsomeonetoactuallyfinanceconstruction.BurtRutanlookedinitiallyatusingaliquid-fueledrocketenginethesameaswasusedbyNasatolaunchtheApollospacemissions,buthewasconcernedaboutthecomplexityofthoseenginesandtheinherentdangersinvolved.AnotheroptionwastouseasolidfuelrocketengineofthetypeNASAusestolaunchthespaceshuttles.Theseenginesgeneratelessboostthantheirliquid-fueledcounterparts,buttheyaresafertousebecausetheirfuelandoxidizerareinjectedduringthemanufacturingprocessmakinglauncheseasier.Theonlypotentialdisadvantageofsolid-fueledrocketenginesisoncetheyarelit,theycannotbeshutdownorthrottledback.Theenginehastoburnthroughallitsavail-ablefuelwhichmightbeanissueifaproblemariseselsewhereinthespacecraft.
Thethirdalternativewastouseahybridrocketengine.Hybridsuseasolidfuelandeitheraliquidorgaseousoxidizer.Whentheoxidizerflows,therocketenginegeneratesthrust.Iftheflowoftheoxidizeristurnedoff,theengineshutsdown.Hybridsarelessefficientthaneitherpureliquid-fueledorsolid-fueledrocketenginesbutthefacthybridscanbethrottledbackwasahugepositive.Afterlookingatafewalternatives,Rutandecidedtogowithahybridrocketengineforhisnewspaceshipdesign.
"IknewthatIcouldn'tdoenvelopeextensionairplanetestingwithasolidbecauseit'sdangerousifyoucan'tshutitdown.It'swaytoodangerousifyoucan'tshutitdownifyou'veneverflowntheairplanetoacertainMachnumber."
—BurtRutan
FundingforthedevelopmentprojectultimatelycamefromMicrosoftcofounderandbillionairePaulAllenwhohadgrownupduringthe1960s,theheadydaysofNASA'sGemini,MercuryandApolloprograms.AllenwasaspaceenthusiastanditseemedlogicaltohimthatfundinganX-Prize-classspacecraftwouldbeagoodstarttowardsmakingspacetravelmoreroutineandaffordable.AllenwasconfidentRutan'sorganizationhadagoodchanceofactuallypullingitoff.Afterseveralmonthsofdiscussions,AllensignedontoprovideRutanwithatleast$20millioninfundinginMarch2001.