Review).Theseiterationsprovideaninteresting exampleoftheeffectsof'requirement creep'on mechanical design. Nonetheless, severalbasicrequirements andtheireffectsremainedrelativelyconstantthroughout the designprocessandareasfollows: •Spacecraft Orbit:625kmcircular, 74.1°inclination, 2°/dayprecession rate,12000eclipses. This environment setsolararraytemperature excursions to+/-100°C. •Spacecraft: 3axisstabilized, nadirpointing. •SABER Instrument Radiator FOV:Required acoldsideofthespacecraft toconstantly pointantisunward andlimitedsolarpanelwidthto122cmtoeliminate reflected sunlight ontotheradiator. •TIDIInstrument Telescope FOV:Additional requirements bythe4telescopes thatlimited the Solararraywidthto122cm. •Rotation An.qle:Arraysmustrotate0-100°,singleaxisafterdeployment (0°position). FortheCoDR, heldDecember 1994,a300Wmaximum orbitaverage power, 11.14meters2arraywas chosen. Consisting of2wings,eachwingconsisted oftwoaluminum honeycomb panelsangled witha 10°droop,usingsiliconBSFRcellswith0.15-mm CMXcover-glass. WithanOSCTaurus launchvehicle baseline, the950kgallowable spacecraft launchmassprovided plentyofavailable massforthearray.To reduce program costsafterthisreview, spacecraft designeffortswerereduced toalowleveltoallowtime forthescience instrument designs to'catchup'andfirmuptheirrequirements through continued design efforts. Duringthisdowntime,anexperienced arraydesign teamwaschosen, consisting oftheTIMED lead spacecraft mechanical engineer, astructures andkinematics analyst, theTIMED leadspacecraft thermal engineer, aseniordesigner, andajuniordesigner. Upthrough delivery oftheflightwingstothe spacecraft, noneoftheseteammembers worked fulltimeonthesolararrayduetobudgetandmanpower constraints. Theanalyst (at75%)andthejuniordesigner (100%fromEDRtoCDR)spentthemosttime. Bythespacecraft preliminary design review(PDR), heldFebruary 1997,significant changes weremade asrequirements firmedupandnewissuessurfaced. AsofJanuary 1997,TIMED wasco-manifested with theJASON spacecraft onaDelta7920-10 launch vehicle tominimize launch costs.Asaresult,the maximum TIMED spacecraft launchmasswasreduced to660kg.Massimmediately became asignificant issueforthesolararraydesign. Exacerbating themassissuewasthegrowth intherequired maximum orbitaverage powerfrom300Wto410W.Packaging constraints increased thenumber ofpanels per wingfromtwotofour,increasing theparasitic hinge,damper andrelease masses proportionally. In response tothesesignificant changes, the12.8m2baseline arrayatspacecraft PDRconsisted oftwo rotating wings,eachwithfourGrCE(graphite/cyanate ester)facesheets andaluminum honeycomb sandwich panels, GaAscells,eachwingwitha30°droop. Table1:Evolving PowerSystem Requirements Summary Program Power(BOL) CellType Milestone CoDR 1200Watts Silicon PDR 2240Watts GaAs S/AEDR 2412Watts GaAs CDR 2412Watts GaAs FlightHardware 2453Watts GaAsAllowable Mass, TotalKg. 90+Kg. 110Kg.Max 110Kg.Max 110K9.Max 8O.9Kg.CellArea (meters 2) 11.1 12.8 11.1 11.1 11.1Droop Angle 10_ 30_ 20u 20u 20u Deployment System andIndividual Mechanism TradeStudies Performed Afterspending approximately 30dayssearching available sources intheliterature, itbecame apparent thatthereweresixcompeting deployment system designs thatmightsatisfyourrequirements. These systems arebrieflysummarized belowwiththeirassociated advantages anddisadvantages: 214