Aerospace programs use advanced heat shield systems to protect spacecraft from the severe heating encountered during hypersonic flight through planetary atmospheres. During entry into a planetary atmosphere, frictional forces subject spacecraft vehicles to extreme thermal conditions by raising the vehicle temperature to levels that are destructive to the outer shell of the spacecraft. Thus, the vehicle's outer shell is typically covered with thermal protection materials that are designed to withstand these extreme thermal conditions and provide insulation to protect the vehicle's outer shell from high temperatures.
Aerospace programs such as NASA are currently developing the Thermal Protection Systems (TPS) needed for exploration missions involving planetary aerocapture and entry. Both reusable and ablative TPS have been developed to protect spacecraft. Reusable TPS have typically been used when reentry conditions are relatively mild, such as for space shuttles. In contrast, ablative TPS materials have been used on planetary entry probes where high heating rates are generated and heat loads are dissipated through phase change and mass loss. Most ablative TPS materials are reinforced composites employing organic resins that produce gaseous products and protective char. In non-oxidizing atmospheres, the resin decomposition reactions are endothermic (vaporization, sublimation) and have an important impact on the net energy to the surface. The gases produced are heated as they percolate toward the surface thus transferring some energy from the solid to the gas.
Future aerospace missions to the inner and outer planets will be more demanding and require improved TPS. The current state-of-the-art for ablative insulators is phenolic impregnated carbon ablator (PICA) which is prepared by impregnating a carbon fiber preform with a thermosetting phenolic/formaldehyde resin SC-1008 originally manufactured by Monsanto. The preparation of PICA is described in U.S. Pat. Nos. 5,536,562, 5,672,389, and 6,955,853. PICA has a char yield of around 55%. PICA generally has better properties relative to many other available ablative TPS material, since the phenolic/formaldehyde resin form a gel which produces a uniform distribution of the resin within the fiber preform. The final ablative material is obtained by drying the gel by evaporating the solvent under vacuum or at elevated temperatures.
Low-density aerogel materials are widely considered to be the best solid insulators available. Aerogels function as insulators primarily by minimizing conduction (low structural density results in tortuous path for energy transfer through the solid framework), convection (large pore volumes and very small pore sizes result in minimal convection), and radiation (IR absorbing or scattering dopants are readily dispersed throughout the aerogel matrix). Aerogels can be used in a broad range of applications, including: heating and cooling insulation, acoustics insulation, electronic dielectrics, aerospace, energy storage and production, and filtration. Aerogel materials display many other interesting acoustic, optical, mechanical, and chemical properties that make them abundantly useful.
However, aerogels can be extremely brittle and difficult to handle.
A need therefore exists for the development of reinforced aerogel materials which are flexible, durable and easy to handle; which have favorable performance as ablative TPS materials; and which have favorable ablative properties such as high char yields.