THE thermal protection system (TPS) of a reentry vehicle is one of the key components of its design. The material used for the

TPS can be classified into two main categories: ablative materials, as in the one used on the Apollo missions, and nonablative materials, such as the ceramic tiles used on the space shuttle. The former can also be divided into two subcategories: charring (also know as pyrolyzing) and noncharring ablators. The theory behind the use of ablators is quite simple: the energy absorbed by the removal of material from the surface is not used to heat the TPS, thus keeping the vehicle at a reasonably “cold” temperature. In the case of charring ablators, the main ablative material is a resin which fills the pores of a carbon matrix. Although the matrix may ablate, it usually does not, thus preserving the original geometry of the aerodynamic surface during reentry.