ACISDoorFailureInvestigation andMitigationProcedures WilliamA,Podgorski*, NellW,Tice*',andPaulP,Plucinsky" Abstract NASA's Chandra X-rayObservatory (formerly AXAF)waslaunched onJuly23,1999andiscurrently in orbitperforming scientific studies. Chandra isthethirdofNASA's GreatObservatories tobelaunched, following theHubble SpaceTelescope andtheCompton Gamma RayObservatory. Oneoffourprimary science instruments onChandra, andoneofonlytwofocalplaneinstruments, istheAdvanced CCD Imaging Spectrometer, orACIS.TheACISfocalplaneandOptical Blocking Filter(OBF)mustbe launched undervacuum, soatightlysealed, functioning doorandventing subsystem wereimplemented. Thedoorwasopened twoandone-half weeksafterlaunch(aftermostout-gassing ofcomposite materials) andallowed X-raystobeimaged bytheACISCCD'sinthefocalplane.Afailureofthisdoorto openon-orbit wouldhaveeliminated allACIScapabilities, severely degrading mission science. During thefinalpre-flight thermal-vacuum testofthefullyintegrated Chandra Observatory atTRW,theACIS doorfailedtoopenwhencommanded todoso.Thispaperdescribes theefforts, underconsiderable time pressure, byNASA,itscontractors andoutside reviewteamstoinvestigate thefailureandtodevelop modified hardware andprocedures whichwouldcorrecttheproblem. Ofinterest isthefactthattheroot causeofthetestfailurewasneverclearlyidentified despite massive effort.Weultimately focussed on hardware andprocedures designed tomitigate theeffectsofpotential, butunproven, failuremodes. We describe afrequent real-world engineering situation inwhichonemustproceed onthebestbasispossible intheabsence ofthecomplete setoffacts. Introduction TheAdvanced CCDImaging Spectrometer (ACIS)isoneofthefourprimary science instruments onthe Chandra (formerly AXAF)X-rayObservatory andoneofonlytwofocalplaneinstruments. Apictureofthe ACISinstrument isshowninFigure1.Inthisview,theX-rayswouldbeimaged through theX-raymirror, located farabovetheinstrument, anddirected downwards through thecollimator andintothedetector housing, wheretheCCDimaging chipsarelocated. TheACISdoorisshownintheopenposition inthe picture. Duetothepresence ofafragileopticalblocking filterovertheCCD'sandtheneedforextreme cleanliness, theACISfocalplanemustbekeptinanevacuated detector housing andlaunched under vacuum. Theblocking filteressentially "blocks" anyvisibleorultraviolet lightfromentering ACIS's aperture. Acoustic loading duringthelaunchenvironment duetopresence ofairinthedetector housing woulddestroy thedelicate filters.TheACISdoorsealsthedetector housing duringground operations leading uptolaunch. STARSYS TMparaffin (wax)actuators areusedascomponents inarotaryactuator, toopenandclosethedoorforground testingandon-orbit operations. TheACISdoormechanism, designed andbuiltbyLockheed Martin,wasfullyqualified andlifetested(on anengineering model)andtheflightunithadbeenactuated atleast23timesduringcomponent testing andduringsystem assembly andtestoperations. However, duringthefinalChandra observatory level thermal/vacuum testatTRW,thedoorfailedtoopenwhencommanded todoso.Thisfailureoccurred in Juneof1998,atatimewhentheChandra launchwasscheduled forearlyCY1999.Asimilarfailureof thedoortoopenon-orbit wouldeliminate theuseoftheACISinstrument andthereby greatlydegrade mission science capabilities. Needless tosay,thisfailureatsuchacriticaltimeintheChandra Integration andTestcyclewas extremely distressing andgenerated animmediate, overwhelming andultimately successful response fromacombined teamofNASAMSFC,contractor andindependent reviewteampersonnel. "Smithsonian Astrophysical Observatory, Cambridge, MA "Lockheed MartinSpaceSystems Company, Denver, CO Proceedings ofthe34_Aerospace Mechanisms Symposium, Goddard SpaceFlightCenter,May10-12,2000 15