Perspirable Skin: A Multifuctional Material System for Self-Cooling

Utilizing the negative thermal expansion property of ZrW₂O₈, Perspirable skin, an autonomous cooling system, was designed for externally heated surfaces of reentry vehicles such as Space Shuttle. In this material system, several ZrW₂O₈ pegs are shrink-fit into the Reinforced Carbon-Carbon composite (RCC) skin. At working conditions, a gap or interference forms between the two materials and the compressed gas within the vehicle can blow out through the gap. The atmospheric air, rather than contributing to the frictional heat between air and skin, is mixed with the cold compressed air over the surface. Due to the nonlinear thermoelastic properties within the operating temperatures, Finite Element Analysis was used to design the geometry of the ZrW₂O₈ pegs and the best fiber arrangement of the RCC for this material system. The geometry of the peg with a larger radius at the top surface and a smaller radius at the bottom part ensures the secure contact of two materials at the working condition. The design with the frustum of a cone shaped top offered a better and shorter route for the coolant gas to be passed. The best type of RCC is found to be 3D woven orthogonal.