Gas-phase chemistry of re-entry.

R of missiles is often accompanied by the hightemperature oxidation of the missile cover. The temperature at the surface of the missile is controlled by the ablation temperature of the heat shield. However, the temperature is higher in the boundary layer and at the stagnation point and may rise to 7000°K or more. Cooling occurs in the wake by diffusion and turbulent mixing until ambient temperature is reached; even in the far wake, however, chemically active species can be present. In a re-entry vehicle it is desired to have minimum heatshield weight and low observability. Carbon, graphite, carbon-phenolic, graphite-phenolic, and silica-phenolic result in low heat-shield weights, but have very high ablation temperatures. On the other hand, some low-temperature heat shields are more difficult to detect, but the weight required is considered excessive. Although there are many types of possible coatings, we shall restrict our discussion to materials that contain carbon. The purpose of this discussion is not to review comprehensively all possible reactions, but rather to examine the broad outline of typical combustions in terms of classes of reactions. Addition polymers such as polyformaldehyde and polytetrafluoroethylene are typical low-temperature ablators. They decompose at about 250°-650°C by "unzipping" to fragments of low molecular weight, principally their monomers. These fragments then enter the boundary layer and are oxidized. Typical high-temperature ablators are pyrolytic graphite and carbonand graphite-phenolic resins. The latter char at temperatures above 250°C with the release of gaseous material, but the heat-shield surface temperature may exceed 3000° K during peak heating. In the following sections we shall first discuss some general considerations; these are followed by considerations of cleanair chemistry, graphite combustion, hydrocarbon combustion, and fluorocarbon combustion. For simplicity in all of these systems, the effects of impurities will be ignored. However, it should be realized that trace impurities can greatly alter the chemistry. For example, only parts per million of alkali metals can markedly enhance the electron concentration and thus increase the possibility of detection by radar. The chemistry of sodium in re-entry has been discussed by Bortner.