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Heat shield

In aeronautics, a heat shield is a protective layer on a spacecraft designed to protect it from the high temperatures, usually those that result from aerobraking during entry into a planet's atmosphere.

The simplest and cheapest type of heat shield is the ablative heat shield, which dissipates heat by allowing its outer layers to vaporize. Such heat shields are used on virtually all expendable spacecraft, since it doesn't matter whether they can withstand a second reentry in such cases.

When the reusable Space Shuttle system was designed, it was decided that a non-reusable heat shield would not be an efficient approach. Instead the Space Shuttle's underside was coated with thousands of ceramic tiles that were intended to be able to survive multiple reentries with only minor repairs between missions. However, the original design proved to be somewhat less robust that intended; the Shuttle suffered from frequent lost and damaged tiles, and ultimately the Space Shuttle Columbia was destroyed with all hands when a piece of insulating foam[?] from its external fuel tank fell off and damaged the heat shield on its left wing.

Various advanced reusable spacecraft designs have been proposed recently that employ heat shields made from temperature-resistant metal alloys, some of them including active cooling systems in which water or cryogenic fuel is circulated over or through them. Some high-velocity aircraft, such as the SR-71 Blackbird, have to deal with frictional heating similar to that suffered by spacecraft but with lower intensity; these aircraft use metallic skins for the most part.

The most violent reentry temperatures successfully survived by a spacecraft were those endured by the Jupiter atmospheric probe carried by the Galileo spacecraft, which entered the giant planet's atmosphere at 106,000 miles per hour.

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