The Space Shuttle thermal protection system (TPS) was the barrier that protected the Space Shuttle Orbiter during the extreme 1,650 °C (3,000 °F) heat of atmospheric reentry. A secondary goal was to protect from the heat and cold of space while in orbit.
Materials
The TPS covered essentially the entire orbiter surface, and consisted of seven different materials in varying locations based on amount of required heat protection:
Reinforced carbon–carbon (RCC), used in the nose cap, the chin area between the nose cap and nose landing gear doors, the arrowhead aft of the nose landing gear door, and the wing leading edges. Used where reentry temperature exceeded 1,260 °C (2,300 °F). High-temperature reusable surface insulation (HRSI) tiles, used on the orbiter underside. Made of coated LI-900 silica ceramics. Used where reentry temperature was below 1,260 °C. Fibrous refractory composite insulation (FRCI) tiles, used to provide improved strength, durability, resistance to coating cracking and weight reduction. Some HRSI tiles were replaced by this type. Flexible Insulation Blankets (FIB), a quilted, flexible blanket-like surface insulation. Used where reentry temperature was below 649 °C (1,200 °F). Low-temperature Reusable Surface Insulation (LRSI) tiles, formerly used on the upper fuselage, but were mostly replaced by FIB. Used in temperature ranges roughly similar to FIB. Toughened unipiece fibrous insulation (TUFI) tiles, a stronger, tougher tile which came into use in 1996. Used in high and low temperature areas. Felt reusable surface insulation (FRSI). White Nomex felt blankets on the upper payload bay doors, portions of the mid fuselage and aft fuselage sides, portions of the upper wing surface and a portion of the OMS/RCS pods. Used where temperatures stayed below 371 °C (700 °F). Each type of TPS had specific heat protection, impact resistance, and weight characteristics, which determined the locations where it was used and the amount used. The shuttle TPS had three key characteristics that distinguished it from the TPS used on previous spacecraft:
Reusable Previous spacecraft generally used ablative heat shields which burned off during reentry and so could not be reused. This insulation was robust and reliable, and the single-use nature was appropriate for a single-use vehicle. By contrast, the reusable shuttle required a reusable thermal protection system. Lightweight Previous ablative heat shields were very heavy. For example, the ablative heat shield on the Apollo Command Module comprised about 15% of the vehicle weight. The winged shuttle had much more surface area than previous spacecraft, so a lightweight TPS was crucial. Fragile The only known technology in the early 1970s with the required thermal and weight characteristics was also so fragile, due to the very low density, that one could easily crush a TPS tile by hand.
Purpose
The orbiter's aluminum structure could not withstand temperatures over 175 °C (347 °F) without structural failure. Aerodynamic heating during reentry would push the temperature well above this level in areas, so an effective insulator was needed.
Reentry heating Reentry heating differs from the normal atmospheric heating associated with jet aircraft, and this governed TPS design and characteristics. The skin of high-speed jet aircraft can also become hot, but this is from frictional heating due to atmospheric friction, similar to warming one's hands by rubbing them together. The orbiter reentered the atmosphere as a blunt body by having a very high (40°) angle of attack, with its broad lower surface facing the direction of flight. Over 80% of the heating the orbiter experiences during reentry is caused by compression of the air ahead of the hypersonic vehicle, in accordance with the basic thermodynamic relation between pressure and temperature. A hot shock wave was created in front of the vehicle, which deflected most of the heat and prevented the orbiter's surface from directly contacting the peak heat. Therefore, reentry heating was largely convective heat transfer between the shock wave and the orbiter's skin through superheated plasma. The key to a reusable shield against this type of heating is very low-density material, similar to how a thermos bottle inhibits convective heat transfer. 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. The 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 potentially incurred during ascent, even before new on-orbit inspection policies were established following the loss of Space Shuttle Columbia. However, the average replacement rate was still low, with Discovery for example still having about 18,000 of its 24,000 tiles being original at the end of its career.
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