Modeling the combustion of propellant sandwiches

A two-dimensional, steady-state model of a burning composite solid propellant is developed to study the characteristics of the combustion process. The solid composite is a periodic sandwich unit comprised of two oxidizer laminates (ammonium perchlorate, AP) separated by a fuel binder layer (hydroxyl-terminated polybutadiene, HTPB). Included in the model are essential features for simulating composite propellant combustion: (1) a free surface boundary, (2) gas- and condensed-phase heat release distributions based on simplified chemical kinetics, and (3) an implicit surface regression rate (unique burning rate) determined by coupled gas-solid energy/species transport analysis. Comparisons of the model with experimental observations focus on surface geometry, flame structure, and the burning rate for variations in pressure, particle size, binder width, and propellant formulation. Experimentally observed trends for typical composite propellants are replicated. For example, the relative protrusion/recession of oxidizer and binder, recognized as an important feature of propellant surface topography, is correctly predicted. The simulation demonstrates the relation between gas-phase heat release and the heat-feedback driving the solid-phase pyrolysis. This information is critical to predicting surface geometry and regression rate. Success was also achieved in predicting the experimental burning rate pressure sensitivity without the use of arbitrary non-integer reaction order. The model provides a framework for future studies with more complex kinetic mechanisms, transient phenomena, and three-dimensional particulate propellants.