Surface layer dynamics in the pyrolysis of ammonium perchlorate-hydrocarbon binder propellants

Mixtures of ammonium perchlorate particles and polybutadiene binder appear to exhibit preferential accumulation of one or the other component on the pyrolyzing surface over a range of rocket-operating pressures. At low pressures, the surface layer is enriched with the oxidizer, and at high pressures, it is enriched with the binder. The degree of accumulation of the oxidizer is found to be higher for smaller particle size. These experimental data can be explained by the difference in activation energies for pyrolysis of the oxidizer and the binder. The particle size effect is explained by consideration of nearly identical surface temperatures for the oxidizer and binder in mixtures with fine particles, but different temperatures for those with coarse particles. The results obtained are important to the explanation of the mechanism of plateau burning rate exhibited by certain composite propellants.