rising4cm,releaselanyards actuatethequick-release locks,andvalvesinthecompliance mechanism opentoactuatethelatchessupporting thetranslating frame.Next,thetranslating framedrops110cm intothetunnelasthecompliance mechanism decelerates andcentersthegroundpanel,hoses,and cables.Shockabsorbers thendecelerate thetranslating frameatthebottomofthetunnel.Finally,blast doorsonthetunnelclosetoprotectthegroundpanelassembly andmechanisms. Passive Compliance Mechanism Thekeytotheoperation oftheumbilical systemistheself-centering, counterbalanced, sixdegree-offreedom passivecompliance mechanism thatsupports thegroundpanelassembly withhosesandcables duringhorizontal andverticaloperation (Figure3).Thecompliance mechanism mustsupporttheground panelandadjustformisalignment duringmating. Italsoprovides theaxialforcerequired tomatethe connectors. Finally,itmustdecelerate, center,andsupportthegroundpanel,hoses,andcablesduring thehigh-speed disconnect. Thepassivecompliance mechanism usesavarietyofspecially-designed and commercial off-the-shelf components toaccomplish thesetasks. PassiveCompliance Mechanism Requirements Someofthesignificant designrequirements forthepassivecompliance mechanism were: •Providesixdegree-of-freedom capability •Supportthe800kgpanelassembly inhorizontal andverticalmodes •Providepositivecentering tonominal position •Limitlateralcentering forcesto1kNtransferred tovehiclewhilealigning •Generate nominalaxialcentering forceof11kNtoassistmating •Providelatchactuation viaintegralvalves •Maximize accesstocenterofpanel(support atperimeter) •Providelateralcompliance +3cm •ProvideAxialcompliance +16cm,-4cm •Provideangularcompliance withinlimitsoflateralandaxialcompliance PassiveCompliance Mechanism Description Thecompliance mechanism usesasimplearrangement ofeightspring-centered strutstomaintain the groundpanelintheproperpositionformatingwhileallowing formisalignment andvehiclemotionsduring mating,enginebuildup,andlaunch. Thefouraxialstrutassemblies (Figure4)weredesigned specifically fortheX-33umbilical. Alarge(9-cm diameter) compression springisusedtogenerate thedesignpreloadof2.75kNonthestrutpistonto resiststrutcompression. Thespringratewasminimized inordertoprovidearelatively constant force throughout the16cmcompression stroke.Thisresulted inasignificant initialcompression of19cm required forassembly. (Thestoredenergyinthespringcreatedasignificant risktotechnicians ifproper assembly ordisassembly stepswerenotfollowed. Theengineering teamtookgreatcaretoensurethat thehardware andprocedures contained adequate safetywarnings.) Thesecondspring,nestedwithin thefirstforspacesavings, resistsstrutextension byreacting between thestrutrodandthepiston.This configuration provides aself-centering strutwithdifferent strokes, preloads, andspringratesforeach operating direction. Spherical rodendsallowforendrotationandstrutlengthadjustment. Anelastomeric stopisprovided forthecompression strokeintheunlikelyeventthatthemaximum strutcompression occurssimultaneously withthetranslating framemaximum deceleration. Airtransferportsinthepiston providealimiteddegreeofcushioning anddamping. Noadditional damping wasaddedtothesystem. Thestrutpiston,inadditiontoproviding thecentering function, actuates anintegralpneumatic control valvefortheretractlatches(discussed later). Thefourlateralstrutassemblies centerthegroundpanelwithrespecttothetranslating frame.Onepair ofstrutsprovides springcentering only;theotherpairofstrutsprovides springcentering forvertical operation andhasadditional pneumatic counterbalance capability tosupportthegroundpanel,hoses, andcablesforhorizontal operation. Thedesignofthelateralstrutusesaback-to-back pairof 346