Background TheX-38project isademonstration oftechnology fortheproposed International SpaceStation Crew ReturnVehicle. Theprojectconsists ofseveral atmospheric flightvehicles andanunmanned spacecraft. ThedesignoftheX-38vehicles usesaliftingbodyshapeoriginally developed bytheU.S.AirForceX-23 (SV-5)andX-24Aproject inthemid-1960's. TheX-38spacecraft islaunched bythespaceshuttle and deployed bytheRemote Manipulator System. TheDPSperforms thede-orbit burnandisjettisoned prior toentry.Thespacecraft enterstheatmosphere asanunpowered gliderlikethespaceshuttle orbiter. Whenthespacecraft becomes subsonic, itdeploys alarge,steerable parafoil foritsfinaldescent to landing. Thespacecraft landing gearutilizesskidsratherthanwheels. Earlyoninthespacecraft designcyclethefive-point attachment scheme described earlierwaschosen. Thisconfiguration isnearlystatically determinate, whichsimplifies loadscalculations andreduces the interaction between theorbiter structure andthespacecraft structure. Oncethisbasiclayoutwas determined, thenextbigdecision waswhether tolocatetheforward pinsonthespacecraft sideoronthe ASEcradle. Locating thepinsontheASEcradlesimplified thespacecraft design andlowered the spacecraft weight, butmadere-stowing thespacecraft inthepayload bayverydifficult. Inordertocarry thehighlaunch andlanding loadsthepinswouldhavehadtogointotighttolerance holesonthe spacecraft. Oncethepinswerepulledoutofthespacecraft, re-inserting themwouldhavebeenvirtually impossible. Locating thepinsonthespacecraft sidegreatlysimplified theon-orbit operations. Inaddition, thealready existing andproven PRLAcouldbeusedtoretainthepins. Theoriginal design concept usedatrunnion pinthatwasjettisoned pyrotechnically. Packaging this concept wasquitedifficult. Itquickly became apparent thattheoriginal pininterface location (onacabin ringframehalfway downthelengthofthevehicle) didnothavesufficient spaceorstrength forthe mechanism. Adecision wasmadetomovethetrunnion pininterfaces tothecabinforward bulkhead wheretherewasmorespaceavailable. Moving themechanism tothespacecraft nosealsohelped by reducing noseballast. Driving Requirements Thetrunnion mechanism mustbemechanically lockedinboththeextended andretracted positions. It mustbeabletocarryanultimate loadof200kN(45,000 Ibf).Toaccommodate vehicle deflections of +76.2mm(+3in)themechanism musthaveanextension distance of355.6mm(14in).Themechanism mustbeabletofunction between -73°C(-100°F) and66°C(150°F). Themechanism hasa3-mission life withacyclelifeof8cycles. Design Overview Thetrunnion pinretraction mechanism ispneumatically actuated withnitrogen gas.Thevehicle Attitude Control Subsystem provides thegassupplyrequired forthemechanism tooperate. AsingleN2lineruns fromtanksintherearofthespacecraft andbranches offtoconnect tothetworetraction mechanisms. Having asinglelinesynchronizes thetworetraction mechanisms bysupplying pressure toboth simultaneously. Thetworetraction mechanisms areattached totheforward bulkhead oftheV-201crew cabinasshowninFigure2.Eachmechanism isattached usingacombination ofshearpinsandbolts threaded intolocking inserts. Thetrunnion pinsprotrude through thenosestructure asshowninFigure3. Onceretracted, aThermal Protection System capontheendofthetrunnion pinconforms totheoriginal outermoldlineshapeofthevehicle (seeFigure4andFigure5).Eachtrunnion pinretraction mechanism isapproximately 152mm(6in)indiameter and965mm(38in)longintheextended position. Each mechanism weighs approximately 667N(150Ibf). 48