Mass and energy balance equations have been solved in order to simulate the behavior of a silicon-based ablative composite used as a thermal protection shield for a non-animated space capsule. A method to calculate some of the parameters in the energy balance equation, is proposed, taking advantage of combined thermal analysis techniques. Furthermore, degradation kinetics in the hypothesis of constant volume is used to solve the mass balance. The results of the computer simulation are compared with the experimental data, obtained using plasma are testing. The model can be used for both material selection and thickness calculation for thermal protection shields.