The role of HMX particle size in the combustion and agglomeration of HTPB-based propellant

Cyclotetramethylene tetranitramine (HMX) is usually added to solid propellants, but the inclusion of HMX significantly affects the propellant combustion characteristics, especially by yielding more condensed combustion products (CCPs). The effect of HMX size on aluminum agglomeration and CCPs of hydroxyl-terminated polybutadiene (HTPB)-based propellant is evaluated experimentally in this paper. Using a thermogravimetry–differential scanning calorimeter, laser ignition setup, and high-pressure combustion bomb, we have examined the propellants' thermal reactivity, ignition behavior, combustion characteristics, agglomeration, and CCPs with 10–200 μm virgin HMX particles. The results indicate that HMX in propellant suppresses the pyrolysis reaction of ammonium perchlorate. Meanwhile, the exothermic peak temperature of 200 μm HMX increases by 17.8 °C compared to 10 μm HMX. With the increase of HMX size from 10 μm to 200 μm, the combustion intensity of the propellant increases, and the ignition delay time of aluminum particles decreases from 580.6 ms to 498.3 ms. Increasing HMX size leads to an increased burning rate, while the pressure exponent decreases from 0.34 to 0.23. Including HMX in propellants also promotes aluminum agglomeration during propellant combustion. With HMX size expanding from 10 μm to 200 μm, the mean agglomerate size in the CCPs decreases from 221.9 μm to 124.5 μm, and the fraction of aluminum involved in agglomeration decreases from 0.206 to 0.104. The unburned aluminum content in agglomerated particles increases monotonically with increasing agglomerates size. Overall, HMX size has a significant impact on the ignition, combustion, and agglomeration characteristics of aluminized propellant. The conclusions of the paper can be used as a guidance for optimizing the propellant formulation