<P> Some high temperature metal alloys can withstand reentry heat; they simply get hot and re-radiate the absorbed heat . This technique, called "heat sink" thermal protection, was planned for the X-20 Dyna - Soar winged space vehicle . However, the amount of high - temperature metal required to protect a large vehicle like the Space Shuttle Orbiter would have been very heavy and entailed a severe penalty to the vehicle's performance . Similarly, ablative TPS would be heavy, possibly disturb vehicle aerodynamics as it burned off during reentry, and require significant maintenance to reapply after each mission . (Unfortunately, TPS tile, which was originally specified never to take debris strikes during launch, in practice also needed to be closely inspected and repaired after each landing, due to damage invariably incurred during ascent, even before new on - orbit inspection policies were established following the loss of Space Shuttle Columbia .) </P> <P> The TPS was a system of different protection types, not just silica tiles . They are in two basic categories: tile TPS and non-tile TPS . The main selection criteria used the lightest weight protection capable of handling the heat in a given area . However in some cases a heavier type was used if additional impact resistance was needed . The FIB blankets were primarily adopted for reduced maintenance, not for thermal or weight reasons . </P> <P> Much of the shuttle was covered with LI - 900 silica tiles, made from essentially very pure quartz sand . The insulation prevented heat transfer to the underlying orbiter aluminum skin and structure . These tiles were such poor heat conductors that one could hold one by the edges while it was still red hot . There were about 24,300 unique tiles individually fitted on the vehicle, for which the orbiter has been called "the flying brickyard". Researchers at University of Minnesota and Pennsylvania State University are performing the atomistic simulations to obtain accurate description of interactions between atomic and molecular oxygen with silica surfaces to develop better high - temperature oxidation - protection systems for leading edges on hypersonic vehicles . </P> <P> The tiles were not mechanically fastened to the vehicle, but glued . Since the brittle tiles could not flex with the underlying vehicle skin, they were glued to Nomex felt Strain Isolation Pads (SIPs) with RTV silicone adhesive, which were in turn glued to the orbiter skin . These isolated the tiles from the orbiter's structural deflections and expansions . </P>

How many heat tiles on a space shuttle