Theinfluenceofmaintaining aschedule,minimizing costsandacceptingsomelevelofriskarealways presentduringaproject.Manyofthelessonslearnedcouldhavebeenrealizedifthedevelopment effort hadnotyieldedtothesepressures.Althoughitisinevitablethatthesefactorswillalwaysinfluencethe development process,themechanism designermustensurethateveryaspectoftheirdevelopment processisvalidatedbytheory,analysis,andtest. Conclusions Disposition ofShutter FailureInvestiqation Aftertheextensive testeffort,aSIRTFprojectpeerreviewwasheldwherethefollowing conclusions and recommendation werepresented byGSFCtoapanelofindependent experts: •Failuretoopenatzerocurrent isduetoresidual magnetism intheshuttermechanism actuator •Thecauseoftheresidual magnetism isacombination oftwofactors: oAnimprovement inrotortostatortabalignment whentheshutter wasreassembled justprior toinstallation oTherevised shutter opensequence insoftware v3.20augmenting previously unseen characteristics •Thesensitivity ofshutter release current toitsmechanical alignment was,notpreviously well understood andunintended magnetic effectswererevealed following thesoftware change fromv3.13 tov3.20. •Shutter characterization testssinceMay15thhaveindicated stableperformance -nochanges in performance wereobserved preandpostCTAvibration testing. •Residual magnetic forcesarehighlysensitive torotortostatoralignment andgaps •Residual magnetism canreadilybeoffsetbyasmallcurrent flowinreverse polarity tothedirection of actuation •TheFlightMechanism hasno"defect" andisoperating nominally withcurrent understanding of mechanism behavior andresidual magnetism. •IthasbeenshownthatSideAshuttercanrequire aslightreverse current toopen.SideBhasalways opened atzerocurrent. •Theshuttercanbereliably closedandopened withv3.21commands. •Intheeventofafailureoftheprimary driveelectronics, asimple resistor "softcross-strap" can provide areliable secondary current sourcetoopentheshutterfromtheredundant driveelectronics. •Asimplemeans toprovide asmallreverse current flow,evenintheeventofprimary electronics failurehasbeendeveloped •Thecaseofanopencoilwinding hasbeenstudied anddetermined tobenon-credible •FlightshutterSideBhasbeenshowntobecompletely failsafe inalltestandoperational scenarios Recommendations •Continue useoftheflightshutter onbothSidesAandBwhereBistheprimary sideandAisthe secondary side •Implement theSoftCross-Strap Circuittoprovide abackup release meansforbothactuators Theindependent reviewpanelconcurred withalloftheconclusions andrecommendations thatGSFC proposed. Theyagreed withtheproposed testingandaddedthataminilifetestbeperformed toensure thereliability ofthemechanism. GSFCwasprepared tofollowthrough withtheproposed cross-strapping modification andnecessary testing. Unfortunately, duetoprogrammatic concerns, SIRTF project management deemed therewouldbenofurthershutteroperations andtheproposed softcrossstrapping modification wouldnotbeimplemented. Anoperational scheme willbeusedwhichwillallowcalibration of thedetectors without usingtheshutter mechanism. TheIRACshutter isaprimeexample ofhowsecondary effects orsubtlechanges inthedesign, assembly, andtestingofaflightmechanism canmaskorrevealundesired behavior. Asstatedabove,the ultimate conclusion oftheinvestigative teamisthattheshutter isbehaving asoneshould expect the designtobehave. Itisbehaving nominally asbuilt,butnotasintended. Its"anomalous" behavior is,in NASA/C Pm2002-211506 25