Thermal Protection

means of ensuring normal temperatures in devices and equipment operating in situations in which their surfaces are subject to significant heat fluxes. Thermal protection is used extensively in aviation and rocketry to protect aircraft and spacecraft from aerodynamic heating when passing through the dense layers of the atmosphere and to protect the combustion chambers and nozzles of air-breathing jet and rocket engines.

A distinction is made between active and passive methods of thermal protection. In the active methods, a gaseous or liquid coolant is fed to the surface being protected and absorbs a significant proportion of the heat incident on the surface. Several types of thermal protection are distinguished, depending on the method by which the coolant is fed to the surface being protected.

In regenerative cooling, the coolant is passed through a narrow channel, or sleeve, along the side of the protected surface that is closer to the incident heat flux. This method of protection is used in permanent power-engineering installations and the combustion chambers and nozzles of liquid-propellant rocket engines.

In transpiration cooling, a gaseous coolant is fed through a slit in the protected surface to the outer, “hot” side, thus blocking the effect of the high-temperature external medium. As the stream of coolant mixes with the hot gas, however, its obstructing effect is reduced. Therefore, a system of sequentially arranged slits is used to protect large surfaces. This method is used in aviation for thermal protection of the combustion chambers and nozzles of air-breathing jet engines, in which the outside air is used as the coolant. Film cooling is similar to transpiration cooling, but in this case a liquid coolant is fed through the slit in the protected surface and forms a film on the surface. As it flows along the surface, the film of liquid is evaporated and atomized. In this method, the heat incident on the surface is absorbed by the heating and evaporation of the film of liquid coolant and by the subsequent heating of its vapors. Film cooling is used to protect the combustion chambers and nozzles of liquid-propellant rocket engines.

In porous cooling, a gaseous or liquid coolant is fed through the protected surface itself, which is made porous or perforated for this purpose. Porous cooling is used where increased heat fluxes are incident on the surface and the preceding methods are inadequate.

In the passive methods of thermal protection, the effect of the stream of heat is absorbed by a specially designed outer shell or special coatings applied to the basic structural component. Several types of passive thermal protection techniques are distinguished according to the way in which the heat flux is handled.

In heat-absorbing components, or heat sinks, the heat striking the surface is absorbed by a fairly thick shell. The effectiveness of this method depends on the specific heat of the material used for the heat-absorbing elements; beryllium is the most effective.

Radiation thermal protection utilizes an outer shell made of a material that retains adequate mechanical strength at high temperatures. In this case almost the entire heat flow incident on the surface of the material is radiated into surrounding space. Heat transfer into the protected element is minimized by placing a layer of light heat-insulating material between the outer high-temperature shell and the basic structural component. This method can be used only for the thermal protection of outer surfaces, where the radiation from the heated surface can pass freely to an external space.

Thermal protection by means of ablation coatings has become the most common method in rocket technology. In this method, the protected element is coated with a layer of a special material. Under the action of a heat flux, part of the material is destroyed by processes of fusion, evaporation, and sublimation and by chemical reactions. In this case most of the incident heat is consumed in the various physicochemical transformations. The comparatively cold gaseous products of the destruction of the thermal protection material produce a supplementary obstructing effect as they are blown off into the surrounding medium. This type of shielding is used to protect the nose cones of ballistic missiles and spacecraft entering the dense layers of the atmosphere at high speed against aerodynamic heating and to protect the combustion chambers and nozzles of rocket motors, especially solid-fuel motors, where the use of other methods of protection is difficult. This method is more reliable than the active methods of thermal protection.

Most of the ablation thermal protection coatings used in practice have quite complex compositions, with at least two constituents—a filler and a binder. The filler absorbs a fairly large quantity of heat during the process of destruction caused by physicochemical conversions, and the binder imparts to the material as a whole sufficiently high mechanical and thermophysical properties. Fiber glass reinforced plastics and other types of plastics based on organic and organosilicon binders are examples of ablation thermal protection coatings.