Thermal Degradation of a Char-Forming Plastic During Hypsersonic Flight

A detailed theoretical aerothermostructural analysis is presented of the problem of determining the rate of ablation of a typical thermosetting hydrocarbon plastic tha t is subjected to the severe heat transfer rates encountered at the stagnation point of a hypersonic re-entry vehicle. The thermal degradation of the plastic is treated as a chemical reaction in depth by means of a chemical kinetic equation for the pyrolysis process. The analysis includes a mathematical description of the flow of the chemically reacting gases through the porous char. Hypersonic boundary layer mass transfer effects including gas phase combustion are considered in the overall energy balance. Numerical results are obtained for the reaction zone thickness, the rate of gasification of the plastic, the pressure drop through the char, and the temperature distribution within the material. The thermal stresses and the mechanical stresses due to the gas pressure drop through the porous char are calculated, and the mechanism by which the char layer builds up and collapses with time is demonstrated theoretically for the first t ime. Finally, correlated results are obtained for the maximum surface temperature, the period of spallation, and the maximum char thickness as a function of the stagnation pressure and stagnation enthalpy for the entire hypersonic suborbital flight regime.