wereallowed. Tosimplify thegroundcontrolsystems andsoftware, noT-0signalfromthevehicle or groundwouldbeprovided. Thisrequired thedisconnect sequence tobeinitiated byliftoffmotionofthe vehicleonly.Finally,allsystems mustoperate onstoredenergy(springs, compressed gas,orgravity). ..Umbilical System Description TheX-33Umbilical system consists ofaflightpanelassembly andaground umbilical assembly. The nominal sizeofthegroundandflightconnector panelsis89cmby127cm.Thefluidandelectrical connectors arerigidlymounted tothepanels. Theflightpanelassemblies (Figure 2)attachtothevehicle aftthruststructure adjacent totheengines andconsistofastructural aluminum panelsupporting theflight halvesofthevarious connectors aswellasreceptacles forthepanelalignment andlockingmechanisms. Aslidingdooroneachassembly closesafterdisconnect toprotectthepanelandconnectors fromheating andcontamination duringflight.Theground umbilical assemblies (Figure 2)attachtotherotating launch mountonthepad.Theground assembly isaprotective tunnelstructure containing theground paneland mating, release, andretraction mechanisms. Theumbilical tunnelweldment supports andprotects the ground umbilical mechanisms. Blastdoorsonthetunnelclosetoprotecttheumbilical mechanisms from launchblastdamage. Atranslating framestructure withinthetunnelsupports theground panelonasix degree-of-freedom compliance mechanism andretracts intothetunnelduringthedisconnect. Fluidand electrical linesareroutedthrough thetranslating frameandtunneltoaninterface plateonthelaunch mount. Umbilical System Operation Theoperation oftheumbilical canbedivided intothreephases. Thefirstoperational phaseismatingof theumbilical inthehorizontal mode(orvertical modeasacontingency). Thesecond phaseistheuseof thevarious fluidandelectrical systems throughout processing, rotation andtanking operations. Thethird phaseistheactualdisconnect oftheumbilical following liftoff. Matin.qOperations Thematingoperation beginsafterthevehicle isattached tothelaunchmount. First,theflightdooris manually opened andpinnedtoallowaccesstotheflightpanel.Next,theblastdoorsareopened andthe translating frameassembly isextended usingpneumatic jacks.Asthejacksareextended, alignment pinsontheground panelengage theflightpanelwhilethecompliance mechanism supports theweightof thegroundpanel,hoses,andcablesandallowsthepanelstoalignformating. Asthejacksarefully extended, thetranslating framelatches intheextended position. Thejacksarethenretracted toallowthe latches tosupport thetranslating frame. Next,thealignment pinsareretracted inparallel tomateallthe fluidandelectrical connectors. Finally,quick-release locksarepneumatically engaged torigidlyconnect thepanels. Processin.q Operations Throughout horizontal, rotational, andvertical operations, thecompliance mechanism continues to support theground panel,hoses,andcables(withatotalmassofover800kg)tolimitloadstransferred tothevehicle. Thequickrelease locksprevent separation ofthepanelsduetofluidpressure forces between theconnectors (upto71kNtotalS. Disconnect Operations Following completion oftanking operations, thecryogenic linesaredrained, andpowerisremoved from theelectrical connectors inpreparation forlaunch. Several pneumatic linesandtheenvironmental control system remainfullyorpartially pressurized through disconnect. Thelaunchsequence beginsasthevehicleengines arestarted. Astheengines causethevehicle and launchmounttodeflectfromtheirnominal position, thecompliance mechanism absorbs relative motion (upto2.5cm)between thevehicle andthemount. Theumbilical mustremainconnected duringthese deflections toensurethecapability tode-tank intheeventofanabort.Aftertheengines areverified tobe atproperoperational levels,thevehicle initiates thepyrotechnic charges inthehold-down system to release thevehiclefromthepad.Thevehicle risesoffofthepadwiththeumbilicals stillconnected. After 344