Heat shield

(Redirected from Ablative 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 is also a design consideration for high-velocity aircraft.

1 Shape
2 Ablative Heat Shields
3 Reusable Heat Shields
4 Passive Cooling
5 Active Cooling
6 Temperatures
7 See Also

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 (suborbital) 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 does not matter whether they can withstand a second reentry.

Astronaut Andrew S. W. Thomas takes a close look at the heat shield underneath the Space Shuttle Atlantis in the Orbiter Processing Facility at Kennedy Space Center (KSC).

Reusable Heat Shields

When the reusable Space Shuttle system was designed, it was decided that a non-reusable heat

Passive Cooling

In some ballistic 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 material, adding greatly to the mass. Consequently ablative or reusable shields are now far more common.

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. Shockwaves can attach to the pointed nose and heat the aircraft through wave drag. Generally heat is conducted through the aluminium or titanium alloy, or occasionally stainless steel skins. 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.