Materials used for the outward dissipation of extremely high heats by mass removal. Their most common use is as an external heat shield to protect supersonic aerospace vehicles from an excessive buildup of heat caused by air friction at the surface. The ablative material must have a low thermal conductivity in order that the heat may remain concentrated in the thin surface layer. As the surface of the ablator melts or sublimes, it is wiped away by the frictional forces that simulta neously heat newly exposed surfaces. The heat is carried off with the material removed. The less material that is lost, the more efficient is the ablative
material.
The ablative material, in addition to low thermal conductivity, should have a high thermal capacity in the solid, liquid, and gaseous states a high heat of fusion and evaporation; and a high heat of dissociation of its vapors. The ablative agent, or ablator, is usually a carbonaceous organic compound, such as a plastic. As the dissociation products are lost as liquid or vapor, the char is held in place by the refractory filler fibers, still giving a measure of heat resistance. The effective life of an ablative is short, calculated in seconds per millimeter of thickness for the distance traveled in the atmosphere. Single ablative materials seldom have all of the desirable factors, and thus composites are used. Phenolic or epoxy resins are reinforced with asbestos fabric, carbonized cloth, or refractory fibers such as asbestos,fused silica, and glasses. The refractory iibers are incorporated not only for mechanical strength but have a function in the ablative process, and surface-active agents may be added to speed the rate of evaporation. Ablative paint for protecting woodwork, may be organic silicones which convert to silica at temperatures above 2000oF (1O93oC).Pyromark, of Tempil Corp., is a paint of this type. Metals can resist temperatures higher than their melting point by convection cooling, or thermal cooling, which is heat protection by heat exchange with a coolant. Thus, tungsten can be arc—melted in a copper kettle which is cooled by circulating water. The container metal must have high thermal conductivity, and the heat must be quickly carried away and stored or dissipated. When convection cooling is difficult or not possible,cooling may be accomplished by a heat sink. Heat-sink cooling depends on the heat absorption capability of the structural material itself or backed up by another material of higher heat absorption. Copper, beryllium, graphite, and beryllium oxide have been used. A heat-sink material should have high thermal conductivity, high specific heat and melting point, and for aerodynamic applications, a low specific gravity.
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