Space planes require high-performance heat-resistant materials which can withstand ultrahigh temperatures and extremely large temperature gradients. To meet these needs, functionally gradient materials (FGMs) were proposed about 10 years ago in Japan. Figure 1 shows a conceptual diagram of functionally gradient materials, taking into account the relaxation of thermal stress. For the surface that contacts high-temperature gases at thousands of degrees, ceramics are used to provide adequate heat resistance. For the surface that provides cooling, metallic materials are used to furnish the necessary thermal conductivity and mechanical strength. In addition, the composition of these materials is formulated to provide optimum distribution of composition, structure, and porosity to effectively relax thermal stress. Since fiscal 1987, an RD Phase I was carried out from 1987 to 1989, and Phase II from 1990 to 1991.