Developing a thermal protection system (TPS), often called a heat shield, was one of the larger challenges facing scientists and engineers as they began seriously looking at lifting-reentry vehicles during the 1950s. In selecting a concept, researchers had to consider the physical, mechanical, chemical, and economic characteristics of the material and the vehicle [1]. Initially, almost all heat-shield research centered on various ablative and radiative metallic materials, and with one exception, it was not until well into the Space Shuttle conceptual development cycle that scientists and engineers seriously considered other materials.All of the manned orbital space capsules used ablative heat shields, but during the 1950s and early 1960s, most scientists and engineers assumed any advanced TPS would be metallic. One of the larger problems that researchers uncovered was that most metals tended to rapidly oxidize when exposed to high temperatures. The oxidation quickly compromised the material's strength and thermal properties. As a result, researchers expended a great deal of effort trying to find a long-lasting coating that would provide adequate protection for the metal, not affect its mechanical properties, and not have a negative impact on the desired emittance properties. By 1964, researchers concluded there was no "ideal" coating for any given material because each application had different needs. Nevertheless, they had made significant progress toward understanding specific silicide and aluminide coatings for
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