A Model of Composite Propellant Combustion Including Surface Heterogeneity and Heat Generation

Abstract

The surface heterogeneity of a composite propellant is incorporated in a model of the propellant combustion process. This process is pictured as the sum of fuel pyrolysis, oxidizer decomposition, heterogeneous chemical reaction between the fuel and decomposed oxidizer in small fissures surrounding individual oxidizer particles, and gas phase combustion of all final decomposition products. Expressions for the burning rate and the rate of heat generation at the propellant surface and in the gas phase flame are formulated, explicitly including the oxidizer particle size distribution. Expressions for the mean, one-dimensional, propellant surface and flame temperatures are derived assuming planar regions of heat generation. A collected set of implicit, algebraic equations is solved numerically for the propellant burning rate, surface (and flame) temperatures for a variety of physical parameters. The burning rate is found to depend strongly on the oxidizer particle ignition delay at low pressures, and upon the position of the external flame at high pressures. The effect of the heterogeneous reaction on the burning rate is strongest at intermediate pressures. The results agree quite well with experimental data on the effect of pressure and oxidizer particle size on composite propellant burning rates, surface temperatures, and surface structure.