wouldbuildovertime,startingoutatalowvaluewhenthedoorwasinitiallyclosedandreachingan asymptoteatsomelatertime.Tominimizethepotentialstictionbuild-up,itwasdecidedtoopentheflight dooraslateaspossibleintheintegrationprocess. AnotherstictionmitigationtechniquewasdevelopedbyLockheedMartinintheirtestingusingengineering modelsofthedoormechanism. Itwasdetermined thatthermalcyclingofthedetectorhousingwould lowerstictionlevels.MitigationtestingontheCAMSIMtestunitshowedthatarapidtemperature cycleto 25°CwouldvirtuallyeliminateanyO-ringadhesionwhichwaspresentpriortothecycle.Twotestswere preformedafteraroomtemperature dwellwith1atmosphere ofpressureonthedoorfor21and172 hours.Usingapredictionbasedonpreviouso-ringadhesiontesting,onewouldpredictsealadhesionof 0.6and1.2N-m(5.8and10.5in-lb)ofadhesionrespectively. Afterthemitigationthermalcycle describedabovewasperformed, only0.1N-m(0.7in-lb)ofadhesionweremeasuredinbothcases. Inpreparation forflight,theChandraFlightOperations Team(FOT)developedaprocedurewhichwas essentiallyidenticaltothemultipleheatingcyclegroundprocedureandmodifiedthetelemetry decommuation softwareanddisplaypagestoutilizethenewinformation availableonthedoor mechanism. Afullend-to-end testofthenewdoorprocedurewasexecutedonSeptember 27,1998from theChandraOperations ControlCenterinCambridge, MA,controllingtheObservatory atTRW.TheISIM wasinstalledontheObservatory, whichwasinambientconditions. Theflightprocedureutilizedalibrary ofcommandsequenceswhichcontainedtheactuatorheateroncommand,afixedtimedelay,andthe actuatorheateroffcommand. Thesesequenceswerebuiltwithtimedelaysof50sto200sin5s intervalsandadditionally 250sand300sintervals.Theoperatorcouldthenselectthecommand sequencewithanappropriate timeintervalbasedonthestartingtemperature oftheactuator,the spacecraftbusvoltage,andtheresultsofthepreviousheatingcycle.OnSeptember 27,1998thedoor openednominallyinfiveheatingcycles.Theheatingcyclesusedwereof70,85,100,110,and250sec. andreachedmaximumactuatortemperatures of61.2,68.9,74.1,76.6,and99.9°C.Thespacecraftbus voltagewas31.5VandtheinputvoltageatthePSMCwas30.9V. Thefinalpre-flightdooropeningtookplaceonMay10,1999,beforeChandrawasintegratedwiththe orbiterColumbia.Theopeningwentsmoothly,afterequalizingthepressurewiththeVGSE,withalldata perfectlynominal.Theinstrument wascommanded fromtheACISEGSEatKSC.TheObservatory was inambient conditions intheVertical Assembly Building. Thedoorwasopened with4heating cyclesof 90,100,110,and180secdurations andtheactuator reached maximum temperatures of63,68,72,and 100C.Thespacecraft busvoltage was30V. On-Orbit DoorOpening TheChandra X-rayObservatory waslaunched fromKSConJuly23,1999intheorbiterColumbia. On August 8,1999theACISdoorwasopened asplanned, 16daysafterlaunch. Thedooropening sequence isshowninFigure4.Thesameprocedure wasusedtoopenthedoorinflightashadbeen usedonthegroundtestatTRW.Theonlydifference wasthetimingoftheindividual heating pulses. The doorwasopened with5heating cyclesof90,110,125,140and200secandtheactuator reached a maximum temperature of56.1,68.9, 79.2,87.0,and110.3°C. Thespacecraft busvoltage was31.0V andtheinputvoltage atthePSMCwas30.4V.Actuator temperature, dooranglesandlimitswitch transitions wereallasexpected andareshowninthefigure.TheChandra X-rayObservatory wasnow readyforplanned calibration observations andscientific studies. Thestiction mitigation technique had notbeenrequired. Lessons Learned Ifthereisonelessontobelearned, itistoalways"testasyouplantofly"andnotpushthelimitsofa mechanism tofacilitate schedule, logistics ortoreduceotherpotential impacts tothetestprogram. Onorbit,itwasalways planned toopenthedoorwiththecamera bodyandO-ringat25°C.Duringtesting, if 27