A model for solid propellant combustion

A model has been developed to describe the combustion characteristics of composite solid propellants. The model is based on several new concepts. First, the oxidizer and binder have different surface temperatures rather than a single averaged temperature that has previously been assumed. Second, the overall burn rate is calculated from a time averaged approach rather than the conventional space averaging used in most propellant combustion models. A key contribution in the time averaging approach is use of an ignition delay time for the oxidizer. Third, a generalized flame standoff distance has been developed based on a modified Burke-Schumann diffusion flame analysis. The analysis accounts for variable oxidizer/fuel (O/F) ratio that differing oxidizer size fractions can experience. The analysis shows that the primary diffusion flame can extend over either the oxidizer or the fuel depending on the local O/F ratio. Calculated results for a series of HMX composite propellants show several key trends and excellent agreement when compared to experimental data. The propellants vary in oxidizer particle size and concentration over a range of pressures and temperatures. The model indicates that the fuel binder has a more significant influence than previously thought.