Heat shield

In aeronautics, a heat shield is a protective layer on a spacecraft or ballistic missile that is designed to protect it from high temperatures, usually those that result from aerobraking during entry into a planet's atmosphere. It's also a design consideration for high-velocity aircraft.

Shape

H. Julian Allen of the National Advisory Committee for Aeronautics discovered in 1952 that a blunt "dish shape" makes the most effective heat shield. The shape increases drag and creates a shock wave ahead of the spacecraft that causes shock heating of the atmosphere, but deflects the heat away from the spacecraft. However the atmosphere between the heat shield and the shock wave is under very high pressure turning it from a gas to a very hot plasma. The heat from the plasma must be dissipated by the material of the heat shield.

Ablative Heat Shields

The simplest and cheapest type of heat shield is the ablative heat shield, which dissipates heat from the plasma by allowing its outer layers to vaporize.

All of the early spacecraft with the exception of the early Mercury capsules used ablative technologies to assist with the transition from high orbital speeds down to aerodynamic regimes where a spacecraft can be flown or parachuted to safety.

Such heat shields are used on virtually all expendable spacecraft and on many ballistic missiles, since it doesn't matter whether they can withstand a second reentry.

Reusable Heat Shields

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.

Heat Sinks

In some balistic missiles and the sub-orbital Mercury spacecraft, heat sinks were used to dissipate the plasma heat. However the technique required a considerable quantity of metal to be used, which greatly added to the mass. Consequently ablative or reusable shields are now nearly always used.

Some high-velocity aircraft, such as the SR-71 Blackbird and Concorde, have to deal with heating similar to that suffered by spacecraft but with lower intensity. Because such aircraft have pointed noses, the shock wave is not deflected and shock heating heats the aircraft. Generally they use their aluminium or titanium alloy, or occasionally stainless steel skins as heat sinks. In the case of Concorde the nose is permitted to reach a maximum operating temperature of 127 degrees C, typically 180 degrees warmer than the external air.

Active Cooling

Various advanced reusable spacecraft and hypersonic aircraft 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.

Temperatures

The most violent reentry temperatures (14,000 degrees C) 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 (170,700 km per hour). The heat shield, made from carbon-phenolic, made up around 50% of the probe's mass prior to entry.