<P> The tile TPS was an area of concern during shuttle development, mainly concerning adhesion reliability . Some engineers thought a failure mode could exist whereby one tile could detach, and resulting aerodynamic pressure would create a "zipper effect" stripping off other tiles . Whether during ascent or reentry, the result would be disastrous . </P> <P> Another problem was ice or other debris impacting the tiles during ascent . This had never been fully and thoroughly solved, as the debris had never been eliminated, and the tiles remained susceptible to damage from it . NASA's final strategy for mitigating this problem was to aggressively inspect for, assess, and address any damage that may occur, while on orbit and before reentry, in addition to on the ground between flights . </P> <P> These concerns were sufficiently great that NASA did significant work developing an emergency - use tile repair kit which the STS - 1 crew could use before deorbiting . By December 1979, prototypes and early procedures were completed, most of which involved equipping the astronauts with a special in - space repair kit and a jet pack called the Manned Maneuvering Unit, or MMU, developed by Martin Marietta . </P> <P> Another element was a maneuverable work platform which would secure an MMU - propelled spacewalking astronaut to the fragile tiles beneath the orbiter . The concept used electrically - controlled adhesive cups which would lock the work platform into position on the featureless tile surface . About one year before the 1981 STS - 1 launch, NASA decided the repair capability was not worth the additional risk and training, so discontinued development . There were unresolved problems with the repair tools and techniques; also further tests indicated the tiles were unlikely to come off . The first shuttle mission did suffer several tile losses, but they were in non-critical areas, and no "zipper effect" occurred . </P>

What are the heat tiles on the space shuttle made of