4.Thewheelassemblies withtheproximity sensing capability builtintothestructure ofthewheelhada massof20grams. Astheelectronic design matured, threeadditional wheel-mounted electronic components wererequired toimprove theresonant performance ofthedriving circuit.Thethree component masses are1.2gramsintotal,buttherequired addition ofasmallcircuitboard,supports, andcapacitive coupling acrosstherotating interface tothewheeladded18grams. Thiswasa90% increase inthewheelmassanditismultiplied timesfourfortheNanorover system. 5.Theelectronic circuitdesign wasverysensitive tothecomponents thatcouldbefoundthatwould operate attheextreme coldtemperature. Someofthecomponents required additional support parts toperform theneeded function. Aselectronic partswereidentified thatwouldfunction asrequired at thetemperature extremes, thecomponent countgrew. 6.Thequantity ofmemory required forasystem isafunction oftheamount ofsoftware coderequired to perform thesystem functions. Asthesystem designmatured, thememory quantity requirement grew accordingly. Thiswasverysignificant because thememory chips,eveninsilicononly,areverylarge. Theradiation hardmemory siliconis1/3thesizeoftheCentral Processing Unit.Whenanother bank ofmemory chipswereadded,eightadditional chipswererequired. 7.Thebiggest vulnerability tomassincrease isintheelectronics system. Whileanyindividual component isextremely smallmass(especially withsiliconchipcomponents), theadditional circuit boardarearequired tomountthecomponent androutethecircuitry isthedrivingmassincrease. As theelectronic circuitdesign matured forthereasons statedinnumbers 5and6,therequired board areagrewsignificantly. Theuseofchip-on-board packaging technology reduced theelectronic assembly massfrom1.4grams persquare centimeter forstandard packaging to0.5gramsper squarecentimeter. Theincrease inareaoftheelectronic assembly duetodesignmaturity resulted in a108%increase initsmass. 8.Another factoraffecting thefinalsystem massisthemachining tolerances onthevarious components. Whileatypical component masswillmatchtheCADmodels byaboutonetotwo percent, thetolerances usedonextremely smallpartscannotexpectthesameresult.Thetolerances ontheLoupemechanism components are0.05millimeters. Thistolerance represents 10%ofthe basicdimension. Ontypicalparts,thetolerances arelessthan1%ofthebasicdimension. This situation resultsinagreater variation ofthefinalmachined component fromthedesignvalue.Inone particular caseontheNanorover, thematerial usedisBeryllium. Duetothehighcostofthematerial, itsbrittlenature, andtominimize theriskofscrapping thepart,themachinist madethepartsatthe maximum material condition ofthetolerances ontheentirepart.Theresulting component mass arrived20%higherthanthedesignvalue. 9.Theuseoffasteners mustbeconsidered earlyenough toobtainthedesired size.Often,therequired sizeforafastener fromdesignloadconsiderations isnotastandard sizeinthe;smallfasteners. The useoflargerthanneeded fasteners isverytaxingonthesystem massforsmallassemblies. Asan example, theuseofa#2fastener (theNanorover usedEnglish fasteners) overa#0fastener isa massincrease of105%.Goingtothenextstandard sizesofa#4overa#2,themassincrease is 70%.Atthelargerscales, a#10fastener hasamassthatis34%greater thana#8fastener, forthe samelength. Themetricgroupoffasteners exhibits asimilartrend.Fastener changes asdesign maturity occursinasmallsystem designcancausesignificant massincreases overthesametypes ofchanges inlargerhardware. Conclusions Thedesign ofsmallspacecraft systems orcomponents requires additional attention toissuesthat normally donotsignificantly affectlargerdevices andassemblies. Thematurity ofaflightdesignalmost always results inmassincreases foranysystem. Smallsystems aremoresensitive tothematurity changes thatoccurinthenormaldesigncyclethanlargersystems. Tobeginwith,acertain percentage ofaverysmallmassisnotverymuchinabsolute value.Additionally, thegrowthofcomponent masses asthedetaildesignprogresses isoftenasignificant percentage ofthe initialvalue. NASA/CP--2002-211506 113