relative to th e flap at the con nection p oints by very  small am ounts, whi ch are within the capability of the  high tempe rature bea ring. This al so cau ses a small l ateral motio n of the arc’s circul ar path relative to th e  skin at the h ull penetration,  which is accommod ated b y letting the seal mou nting ring float a small amo unt  parall el to the hull ski n. 

Redesign Co nstrai nts an d Issu es  Due to th e pre-existin g fuselage structure and fla p geometry, majo r stru ctural ele ments i nterfered with th e  most logi cal actuato r locations, whic h caused the al ready larg e loads o n the a ctuator to b e substantially  increa sed. B ecause of th e canted  angle of the  flap hinge axis an d the necessity of kee ping the a rc axi s  parall el to th e hing e axis, t he arc and its path of rotati on were canted relative to the fuselag e frame s. With  the a ctuato r attachme nt point of th e flap located  under some of the fu selage prima ry loa d paths, it was  not po ssible to locate the arc in an orientation that al lowed its lo wer en d to mat ch up with th e flap attach   point. Th e flap tea m was able  to m ake some  min or modifications an d re-orient th e assembly of the  support st ructure for the att ach point bea ring to move t he attach p oint slightly out board. An offset foot wa s  then de signed into the lower end of the arc to shift the attachm ent point of the actuator i nboa rd sufficie ntly  to con nect with the flap. T he offset resulted in la rge out of plan e bendi ng loa ds applied to t he arc, which  requi red the arc to b e desi gned with a g enerou s cro ss section. 

This foot be came the interface compo nent betwe en the metallic a ctuator a rc and the CSiC flap beari ng  plates. Th e actual i nterface was accomplish ed with two la rge diamete r, hol low core CSi C pins that   connect the metal foot to the CSiC bea ring pl ates th at ride on the  CSiC bearing of the flap attach p oint.  One of the two hole s on th e foot side was slightly slo tted towards t he othe r hole to allow fo r the differe nt  thermal expa nsion rates a nd for manuf acturi ng differences . Both CSiC pins we re fitted snugl y into the foot  holes with in dividually cut  to fit bushin gs mad e of PM1000 oxid e dispe rsion stabili zed hig h temperatu re  alloy from Plansee Aktien gesellsch aft of Austria. Th e bushing s eliminated con cerns of po ssible ch emical  incom patibilit y between the CSiC an d Inconel at el evated temperatures . The bushi ngs al so provided an  addition al break in th e condu ction  path and  limite d the si ze of the con duction conta ct patches, whi ch  proved im portant in redu cing the amo unt of heat co nducted into the foot by  the hot flap. Th in she ets of  woven 3M Corpo ration Nextel ce ramic fabri c were pla ced betwe en the  non-lo ad b earin g adja cent  surfa ces of th e beari ng plat es an d foot to  provide ad ditional thermal prote ction.  

In orde r to carry the large load s while exposed t o the very h igh tempe rature a nd hig hly oxidizing  environ ment,  the arc was fabricated from Inco nel 7 18 su per allo y and prote cted by a refra ctory metal  shell. The off set foot was also fabri cated from In conel 718, but wa s prote cted by a semi-rigid fabri c  blanket com posed of layers of woven Nextel ce ramic fabric, cera mic batting, an d Incon el foil. 

The portio n of the a rc exposed below the the rmal protectio n of the fu selag e lower skin could reach  surfa ce temp eratu res well in excess of the cap ability of Inco nel to  support any meanin gful lo ad. Mo st of  the heatin g resulte d from  radiatio n of h eat co nducted thro ugh th e thickne ss of the flap a nd reflected off  the lower fu selage th ermal tiles. Also, t he reci rculati ng flow i n the cavity betwe en the flap  and the lo wer  fusela ge skin  was suffici ent to potentially destroy t he Inco nel b y oxidation. Acco rdingly, the exposed   portion  of the  arc was protected  by TP S. Origin ally it wa s planne d to u se the same  blanket system  that  prote cted the  foot, but testing reve aled that unde r operation al con ditions it wou ld not maintai n suffici ent  stren gth to re sist a brasion as it pa ssed throug h the therm al seal. The ultimate concept was to wra p the  lower arc with a thick laye r of ce ramic batting insula tion that woul d be surrou nded by a thin shell m ade of  Hitemco R-512E silicide co ated Oremet Wah Chang C-103 colum bium alloy.  

The lo wer part of the  Inconel arc wa s necked down and the external cro ss section of the shell wa s  identical to the cross section of the upper p art of the arc so that the semi -rigid therm al seal would   maintain contact with the  arc a s the flap rotate d. Th e insulate d region  of the  lower arc was bou nded by  the location of the lowerm ost drive unit v -guide bearing wh en th e flap was fully retracte d, whi ch limite d the  flap to ap proximately ha lf extension  durin g the peak of re-e ntry heating. This met flight control   requi rements,  with the exception of possible very bri ef extension of the flap to a sufficient a ngle to expose   a very small  regi on of th e ba re In conel to thermal  radiation. A nalysi s sho wed that with a suffici ently  polished surf ace the Inco nel coul d ma nage the se limited exposu res. Ove rall, the pea k hea t load would  have gene rated a very large thermal gradi ent bet ween the lower e nd of the actuato r and the more   382