temperature critical ele ments of the d rive unit at th e uppe r end. This g radient was achieve d prim arily by  constructing t he arc an d foot from ampl e amou nts of  Inconel, which acted a s a very effective heat sin k  such that the brief heatin g perio d wa s insufficie nt to achieve a nything clo se to te mperature e quilibriu m. 

The flap and actuato r toget her comp rised a system with indetermi nate load path s, so the loa ds appli ed to   each com ponent were det ermin ed by applying the flight load to a  finite element model of the com plete  system. Anal ysis of the a ctuator compo nents ultim ately requi red a very high fidelity finite element mod el  of the comple te arc an d foot assem bly and too k considerable computation tim e. Applying t he flap d riving  load tan gent to, and throu gh a curved beam rathe r than the  original st raight beam caused an in crease in  the total load. The incre ase resulted fro m the load vector b eing close r to paral lel to the flap  and thu s  ‘loosi ng’ the effectivene ss of that portio n of the load vector. (T he actual EMA l oad ve ctor was d etermi ned  by the pre ssure an gle of th e pinion gear teeth on the ‘ arc’ sector gear teeth.) T he extra load a pplied in the  direction p arallel to the fl ap in creased the ra dial lo ads felt by th e flap hin ge beari ngs, exa cerbating th e  alrea dy serious friction p roblem of the CSiC bea rings. Moving the d rive unit lo cation farthe r away from the  flap cau sed the load ve ctor to be mo re parall el to the  flap and worsen the l oad conditions, but moving the   drive unit cl oser to the fla p reduced the length of the arc that co uld be in sulat ed and thu s reduced the   amount of fla p exten sion available d uring the peak re-entry heatin g. Lo cating th e drive  unit attach p oints  far from ele ments of the fu selage st ructure a pplied l oads that ex ceeded the capability of the stru cture, but  locatin g the a ttach point s close to the structu re required cle arance cut-o uts that  wea kened th e stru cture  unacceptably . After and e xtensive peri od of iterative desig n and  analysis, a compromise lo cation for the  drive unit wa s dete rmine d that kept th e radi al load s just within the limits of the flap hing e bea rings,  allowed ju st enoug h room f or an insulate d section  of arc long enough to a chieve  the ne cessary flap travel ,  and kept the stru ctural loads just withi n accepta ble limits witho ut causi ng any uncorrecta ble interferen ces  with the existing stru ctures. Fortu natel y the ex isting stru cture was able to accomm odate a wo rkabl e  solution. 

Although recirculating flo w of hot ga s betwe en the flap and t he lower fuselag e skin caused oxidatio n  probl ems for actuato r com pone nts, t he contri bution of convectio n to the overall heating was fairly mino r.  Most of th e heating i n the act uator re sulted f rom radiation f rom the hot structure  flap and lower skin  thermal  tiles, and from conductio n throu gh the fo ot. Wi th the  large ma sses an d thick sections of mo st of  the pa rts, a nd the b rief duration  of ma ximum heatin g expo sure, the total h eat input to the  parts was of  more impo rtance than particular surface or environment  tem peratu res. The complex nature  of th e  actuato r design and its o peratin g envi ronment, alo ng wi th the tran sient natu re of the the rmal state,   requi red a se parate high-fidelity finite element mod el for thermal a nalysi s. Beca use m any po rtions of the   actuato r reached te mperatures that ca used signifi cant wea kening of the  mat erials, an d because the   thermal  cond uction  between parts was largely  dependent on the contact pressu res b etween them, it was  importa nt to maintain a n iterative loo p betwee n the th ermal and stress an alysis. Each of the fi nite eleme nt  model s required different simplificatio ns, but both were de rived d irectly from t he detaile d design model s  by direct computer mo del tran slation to achieve sufficient fidelity. 

In orde r to minimize the  extra load  introdu ced into the flap (especially la unch vib ration  load s) and to  minimize the risk of jammi ng, the ar c needed to be very stiff. Achieving suffi cient stiffness was difficult in a  curve d, end-sup porte d beam with  a high len gth to  cross section ratio  (a good p ortion of which was  substantially necke d down) and which was subjected to a large t ange ntial loa d. Additionall y, the resultin g  high stresse s had  to be carried at tem peratu res that coul d exce ed 5 38°C (1000°F ) degrees in some  locatio ns. Th e result wa s an integrally machi ned b eam com posed of thick se ction s and d ense intern al  webbing to minimize the  length of un supporte d spans, especially in the regi on wh ere the  guide rollers  applie d concentrated l oads. (The rollers we re slightl y crowned to minimize fri ction cau sed by slipping a s  the tape red rollers conta cted th e angled surfa ce with a radiu s that vari ed along  their l ength, which  reduced the  size of the conta ct pat ches.) It wa s also ne cessary that the bea ring surface s of the   compl eted arc maintain a precise and consta nt cro ss-section a nd have a very smooth surface to en sure  proper pa ssage th rough the rollers when under lo ad. In order to maintain suffici ent st rength (roug hly 80%   of room  temp eratu re in the worst areas) and oxidatio n resistance at the elevat ed temp eratu res, In conel  718 supe r alloy was chosen. Fabri cating  the large, comple x, pre cision pa rts from  very hard a nd difficult to   machi ne mat erial turned o ut to be quite time co nsuming and caused extens ive, unanti cipate d delays. 

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