Comparison of laser- and pressure-driven thrust response of HMX

The laser- and pressure-driven thrust response functions of HMX (cyclotetramethylene tetranitramine) at pressures near 1 atm were measured during laser-supported combustion at mean CO₂ laser heat fluxes of 35 and 60 W/cm². During laser-driven testing, the laser flux was modulated at ±15 W/cm² at driving frequencies ranging from 4 to 250 Hz. Pressure-driven thrust measurements were made with a 10% peak-to-peak variation about the mean chamber pressure at driving frequencies ranging from 4 to 100 Hz. Measured steady-state temperature profiles and numerical predictions of the temperature profiles in the gas-phase reaction zone were used to estimate steady and unsteady heat feedback, respectively. Trends in the experimental results are consistent with a condensed-phase response to the unsteady heat fluxes and with results in the literature from similar experiments. Differences in trends with changing mean heat flux between the laser- and pressure-driven results are explained using the unsteady heat feedback estimates. The experimental results are compared to an analytical model, which used one-step reactions in the condensed phase and gas phase, and numerical modeling results that used detailed gas-phase chemistry. The agreement between the analytical model and the data was found to be better than that between the numerical modeling results and the data.