RE-ENTRY MATERIALS
The major requirement for a re-entry material is that it protect the vehicle from the aerodynamic heating associated with re-entry. A relatively short time ago, the term re-entry brought to mind a ballistic type trajectory and its characteristic heat pulse. Today, however, a variety of thermal re-entry conditions are common due to the various types of vehicles currently of interest. Extremes of re-entry heating-time conditions vary from the relatively low heat flux-long time pulse associated with glide and shallow orbital re-entries through high flux-short time ballistic re-entry to the very high heat flux-moderate time pulse associated with super orbital re-entry. Materials for providing thermal protection, therefore, vary considerably depending upon the type of reentry heat-time pulse. From a functional standpoint, thermal protection materials may be divided into two types; absorptive and radiative. The absorptive material dissipates heat by storage (heat sink) or mass transfer and chemical change (ablation, transpiration or sublimation).