Influences of particle size and content of HMX on burning characteristics of HMX-based propellant

Nitramine composite propellants have received significant attention in the field of solid rocket and gun propulsion. Cyclotetramethylene tetranitramine (HMX) is a nitramine compound that is well known as an energetic material. The burning characteristics of HMX-based composite propellants have been investigated, but the experimental data collected thus far is not sufficient to reveal the combustion mechanism. In this study, the burning characteristics of HMX-based propellants were measured using three HMX samples, each with different particle sizes, and six propellant formulations of each sample with HMX content varying from 55% to 80%. Then, the relationship of HMX particle size and content with the burning rate characteristics was revealed. The burning rate decreased and the pressure deflagration limit increased as the HMX content decreased. The pressure exponent increased as the particle diameter increased and the HMX content decreased. The propellant prepared with small HMX particles underwent steady-state combustion, and the burning surface was nearly flat. On the other hand, the burning surface of the propellant containing coarse HMX particles was very rough and coarse particles protruded from the burning surface. The protruding coarse HMX particles generated remarkable flashing flames. The flashing flames and rugged burning surface caused strong heterogeneous combustion in the vicinity of the burning surface. The burning rates at high pressure were barely dependent on the particle diameter, but those at low pressure decreased with increasing particle diameter. It was found that the HMX content, particle diameter, and pressure were the dominant factors in the assessment of the effects of particle size.