Reentry protection for radioisotope heat sources

A new reentry protection material has been developed which acts as a thermal switch. It is a composite material, consisting of a ceramic foam uniformly impregnated with a metal which coats the ceramic foam structure. The composite is a thermal conductor [k ≅ 5 Btu/(ft h °F)] below the melting point of the metal impregnant and switches to an insulator [k ≅ 0.1 Btu/(ft h °F)] above its melting point. This switching effect is irreversible.Materials investigated to date are silica, alumina, and zirconia foams with silver and copper as the metal impregnants. These combinations yield a thermal switch which is activated at the melting point of the impregnants, i.e., 1762°F for silver and 1982°F for copper. Other metals can be utilized to increase or decrease the switching temperature.These composite materials have specific application to radioisotope fueled space power systems. For this application, the material selected would be placed around the fuel capsule, allowing the heat to pass through with only a small temperature differential incurred. When exposed to a reentry heat pulse, the material would switch to an insulator, thereby allowing intact and safe reentry of the capsule.Thermal conductivity testing and plasma jet testing have been performed and indicate that the composite material is an effective reentry protection material.