Comparison of simplified models for nitramine propellant combustion

Although 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX) and Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) are very similar molecularly and their burning rates as a function of pressure are nearly identical, it is well known that they differ significantly in temperature sensitivity, especially at low pressures. To understand these differences better, three simple models were applied to HMX and RDX combustion. Both the Denison-Baum-Williams and Li-Williams-Margolis models have previously been calibrated for use with RDX. However, the RDX calibration of the Ward-Son-Brewster model was developed in the present work. All three models were compared with relevant measured data including: burning rate, flame stand-off/thickness, combustion stability, and temperature sensitivity. It was shown that all models are capable of accurately determining the burning rate of HMX and RDX as a function of pressure at the baseline initial temperature, but only two of the models are capable of capturing the variation in temperature sensitivity for both HMX and RDX, and only one model can replicate all the other measured characteristics within experimental uncertainty. Analysis using this model suggests that the surface reaction of RDX is much less exothermic than HMX and that there is a shifting between the gas phase and surface reaction dominance with pressure for HMX. This explains why the temperature sensitivity for RDX is nearly flat for low pressures while the temperature sensitivity for HMX increases significantly as the pressure decreases. Importantly, these trends are achieved without adding significant model complexity or having parameters change with pressure or initial temperature.