....f7 AFourPanel,Rotating SolarArrayWingfortheTIMED Spacecraft Mechanism Design, Development andTestResults Steven R.Vernon* andDavidF.Persons" Abstract TheTIMED (Thermosphere, Ionosphere, Mesosphere, Energetics andDynamics) spacecraft isthefirst science mission sponsored bytheNASAOfficeofSpaceSciences intheSunEarthConnections program andisbeingdeveloped bytheJohns Hopkins University Applied Physics Laboratory (JHU/APL). Requirements forthefield-of-view (FOV)oftheinstruments, pointing accuracy, jitterrequirements, the requirement forananti-sun, coldsideofthespacecraft, andsunanglevariations duringtheyearplaced severelimitations onthedesignofthesolararray.Presented isatimeline ofsystem concept evolution to illustrate themanydesign iterations required asthedesignteamreacted tochanges inkeyspacecraft andprogram requirements. Alsogivenareseveral ofthemechanism tradesoffstudies performed, along withadetailed description ofthefinaldesign. Themechanical ground support equipment, analysis (dynamics andstress), mechanism materials andfinishes, andflightqualification testingarecovered in detail.Flightarraydeployment testresultsarepresented andthe"lessons learned" duringtheengineering modeldevelopment andtestingprogram aresummarized. Introduction TheApplied Physics Lab(APL)oftheJohnsHopkins University hasalongandsuccessful historyof spacecraft, sensor andmechanism designforawidevarietyofspacemissions andsponsors. Sincethe early1960's manygovernment agencies suchastheNavy,AirForce, BMDO andNASA have approached APLforconsultation andsolutions toproblems ofnational interest. TheAPLsystem design philosophy isprobably beststatedasonetailored toeachindividual mission. APLspecializes inuniqueand"oneofakind"typesofmissions whereafastresponse isrequired. One constant throughout everymission APLhasundertaken istoestablish acloseassociation between the leadengineers andthehardware theyareresponsible forproducing. Theleadengineers areempowered tomaketechnical aswellasschedule decisions inacloseteamenvironment wheretheyareexposed to therequirements andchallenges facingtheprogram officeandtheothersubsystem engineers. Thelead engineer ofasubsystem isexpected tobeinvolved intheprojectfrom"cradle tograve". Formal reviews arealsoheldandareofahighquality, utilizing outside andinternal reviewers. TheAPLpanelmembers areoftenthemostcriticalanddifficultreviewers thetechnical leadengineers face. FortheTIMED mission, thetwodecisions thathadthemostimpact uponthesolararraydesignwere madeatthesystem engineering level.Thefirstdecision wastoallowunlimited powersystem growthto occurinordertomeetthescience instrument requirements forcontinuous operation (24hoursperday,7 daysaweek)fromthestartofin-orbitinitialoperations untiltheconclusion ofthemission. Theorbitand theprecession ratechosen created asolararrayandassociated powersystem thatonthesurface appears farinexcess ofthetypicalspacecraft ofthissizeandclass.Thesecond decision wastocomanifest TIMED withasecond spacecraft tominimize launchcosts.Thisdecision created animmediate massproblem forthearraythatinfluenced thedesignthroughout thedevelopment effort. Program Requirements Evolution FortheTIMED solararray,evolving program andsystem requirements drovethreemajordesign iterations in18months between theCoDR(Conceptual Design Review) andtheCDR(Critical Design "TheJohnsHopkins University Applied Physics Laboratory, Laurel,MD Proceedings ofthe35thAerospace Mechanisms Symposium, AmesResearch Center,May9-11,2001 213