Des.iqn Redundancy Asisthecasewithmostmechanisms orstructures, thedoormechanism drivetraincomponents connecting therotaryactuator tothedoor(driveshaft, linkages, etc.)werenotredundant. Rotation joints inalllinkages weresinglefaulttolerant duetotheinclusion offloating Torlonbushings. Adding asecond rotaryactuator wouldhavesignificantly complicated themechanism. Theparaffin actuators themselves contained dualsealing O-rings andredundant heaters. Therotaryactuator alsocontained redundant limit switches. Returnsprings insidetherotaryactuator weredesigned tolowoperating stresslevelstoavoid redundancy concerns. Duringthedesignphase,asingleparaffin actuator withredundant heaters was deemed tobeadequate foraone-time on-orbit actuation requirement. Theseactuators hadsignificant flightheritage withextremely highreliability, andrequired only28voltinputpowertooneoftheredundant heaters toperform asdesigned. Alloftheelectronics required topowertheactuators werefully redundant andmetthesinglefaulttolerance requirement. Paraffin actuator sheardiskrupture fromhighinternal pressure wasconsidered apotential singlepoint failureresulting inlossofmission. Duringoperation, theactuator reaches atemperature considerably abovetheparaffin melting point.Ifcasetemperature becomes toohighbeforeheatershutoff, internal pressure isrelieved bysheardiskrupture. STARSYS research datashowed adiskrupture threshold temperature of154°C;thedesignteamconcluded thatashut-off of135°Cleftacceptable marginfordoor operation, basedonourdevelopment testing. Toprotectagainst failure,redundant temperature sensors wereinstalled ontotheactuator caseandweremonitored autonomously bytheACISPowerSupply and Mechanism Controller (PSMC). Powerwasremoved iftheactuator everreached 133-135°C. The subject ofsheardiskrupture willbediscussed inmoredetaillaterinthepaper. Pre-Flight Testing Qualification Testina Pre-flight testingwasperformed perNASAprocedures tofullyqualifythemechanism forflight.These testsincluded a253cyclethermal/vacuum mechanism lifetestonaspecial life-test unit(CAMSIM), Oringstiction tests,acoustic tests,andrandom vibration tests.Therewerenosignificant issuesthatwere notaddressed inthefinalflightdesignforthedoormechanism. Oneoftheminorproblems addressed wastheburn-out ofoneofthelife-cycle testheaterelements. Therootcauseofthisfailurewasa process change inaMincoheaterthatledtoadelamination oftheheaterandsubsequent burn-out as paraffin worked itswayintotheheaterandfatigued theelement. Forflight,newheaters werefabricated withtheoriginal process andwerere-qualified andlife-cycle tested. Asecond problem occurred during thelife-cycle testdueto1Gaffectsandrapidcyclingofthemechanism andparaffin actuators. Convection insidethehotwax(onlypresent in1G)resulted inanon-uniform re-freezing oftheparaffin as itpassed through thephase-change temperature. Sincetheparaffin frozeatthebottomoftheactuator first,theactuator outputpinwasprevented fromfullyretracting. Thisproblem wasaddressed by procedural changes, anewretraction springwhichhadahigherspringconstant, andanadditional limit switchcapability whichwouldalertoperators iftheshaftwasnotfullyretracted afteraclosecycle.There wasaprocedure inplace,whichneverhadtobeused,tore-settheactuator shouldthisproblem repeat. Thesequalification testsshowed thatthemechanism wasreadyforflightbuildwithnomajordesign changes. FlightUnitTestinq TheflightunitDetector Assembly doorwasopened andclosedatLockheed Martinatleast12timesas partofthecomponent levelProtoflight Testprogram undervarious testconditions priortodelivery toMIT. Testing included preandpostenvironmental exposure performance tests,random vibration tests,and thermal vacuum tests.TheflightunitDetector Assembly wasdelivered toMITinearly1997andtheflight focalplaneandopticalblocking filterswereinstalled inAprilof1997.Thedoorwascycledopenand close11moretimesinsystem leveltestingasdescribed below. AnACISsystem levelThermal Vacuum testofalloftheflighthardware wasconducted atMITLincoln Labs.Thedoorwascycledtwotimesaspartofthelongandshortformfunctional testingofallACIS 19