were id entified, but thei r develop ment would  hav e exce eded the scop e of the availa ble time an d  resources  of this  projec t. 

Insufficie nt understan ding of the impl ications of the extreme op erating enviro nment on the actuator design  and the absence of ade quate detail of t he actuator system ea rly enoug h in the vehicle conceptual de sign  process lead  to an un workabl e actuato r design that was corre cted only throug h a compl ete redesign of  the system l ate in the project sch edule. What would have bee n a very chall engin g design unde r any   circum stances became all the mo re diffi cult d ue to the necessity of accomm odating the a dvanced state of   maturity in the rest of the vehicle de sign and operating unde r a compresse d developme nt sche dule. In   retrospect, the actuato r conceptual de sign sh ould h ave received h igher priority durin g the ov erall vehi cle  layout and particul arly in concert with t he flap a nd aft fuselag e config uratio n. As it turne d out, only the  adoptio n of a n unusual, complex, and “chall engin g to im pleme nt” desig n and extraordina ry effort by the   team memb ers along with a gre at dea l of good fo rtune dete rmined the differen ce between failure o r  success of th e flap actu ator sy stem . Preventing the actuato r redesign from be comin g a pa cing item fo r  the proj ect schedul e with such a delaye d start requi red the expe nditure of substantially more re sources  than had b een expecte d for this sy stem. 

It is ea sy to  criticize the  absence of emphasis on thi s system  in th e early proje ct phases in hindsight, but  the reality is that at the time, t he team lacked the exp erience necessary to reali ze the challe nges ahe ad in  this sy stem. Duri ng the e arly stage s of the proje ct, there was very littl e rece nt experien ce developing reentry vehicle s anywh ere in the indust ry, let alone small re-entry  vehicles with active cont rol su rfaces.  Since the tim e of the proje ct, the autho rs have been involved in an d observe d a numbe r of ot her, si milar  proje cts in th e ea rly pha ses of develo pment a nd many  of the m seem to also l ack a sufficie nt early  empha sis on unde rstanding the issu es of the control surface actuator system s. It must be und ersto od that  in a small re -entry vehicle with co ntrol surfaces m ounted on thin airfo il section s, the actuat ors of tho se  surfa ces will be expo sed to environme ntal condition s unlike anyt hing fou nd in lowe r speed aircraft and   they can not be isolat ed from those condition s as ef fectively as woul d be po ssible in la rger hyperso nic  vehicle s. The actuato r developme nt risks si mply cannot be e stimated by analo gy to similar si zed  airpla nes or t o the spa ce shuttle, despit e the natural t ende ncy to d o so.  

Ackn owledgements 

The autho rs wish to ackno wledge the te am membe rs that contrib uted to the syst em describe d in this  paper: NASA Johnson Space Center (JSC) Engi neering Directorate (Syste m Integration, Actuator Mount  and Linkage, Therm al Seal s), Lockheed Martin Spac e Operations v ia NASA/JS C Science, Engineering,  Analysis, & T est co ntract (System Integration, Act uator Mou nt and  Linka ge, The rmal Seal s), Moog Inc.,  Motion Syste ms Divisi on (Actuator Drive Unit), O ceaneeri ng Th ermal Systems,  (The rmal Se al), MAN  Tech nologie AG Space T echnology Divi sion (B ody Flap). 

Referen ces 

1. Hagen, Jeffre y D. “Actuator Installation, V- 201 Body F lap” SGK51 35537 9 – NAS A/JSC Engin eerin g  Drawing Co ntrol Ce nter.  2. Ecke rt, A. & Steinach er, A. “X-38 Body Flap Assembl y and Integrat ion Pro cedure” TET-MA N-11-AIP1013 – MA N Tech nologie.  3. Layer, C. “E. M. Actuator Assembly Mod el 176E11 1” C52002 – Mo og Inc., Motio n Systems Di vision.  4. Curry, Don ald M. & Handri ck, Kari n E. “Dy nami c and Static High T emperature Resi stant Ce ramic  Seals for X38  Re-e ntry Vehicle ” 53rd Internatio nal Ast ronautical Congress – IAC-02-1.3.0 1.  5. Curry, Don ald M. et al. “Furthe r Investi gation s of Co ntrol Surfa ce Seals for the X-38 Re-e ntry Vehicle”  AIAA-2001-3 628 – Ameri can Institute of  Aerona utics and Astrona utics.  390