Modeling Calculations for HMX Composite Propellants

HMX (cyclotetramethyline tetranitramine) composite propellants burn at rates significantly lower than HMX monopropellant. To model the behavior of these propellants, a new model was developed within the framework of the Beckstead-Derr-Price (BDP) modeling approach. A time-averaging approach has been developed assuming that the propellant burns through alternate layers of binder and oxidizer at significantly different rates. The model has been compared in detail with experimental results from 17 HMX/HTPB propellants. Both the data and the model show that there is only a small dependence of rate on particle size. The model predicts that the rates of HMX/HTPB propellants will converge with increasing solids loadings, and that above -85% solids there is very little change in rate for varying formulations. The interpretation of the data using the model indicates three predominant mechanisms leading to the peculiar characteristics of HMX propellants. First, the HMX binder diffusion flame is an energy poor flame that robs energy from the products that would otherwise result from the monopropellant flame. Second, there appears to be a significant ignition delay time associated with large particles that impedes the overall rate. Third, the binder rate appears to be very significant. The model indicates that changing the rate using conventional catalysis approaches would be very difficult, since the rate is more dependent on binder decomposition characteristics than on the oxidizer.