Elucidating pressure dependency and combustion mechanism of micro-unit composite propellants

The interfacial control method is a promising strategy for regulating energy output and enhancing the combustion performance of solid propellants. This assembly technique enables direct contact between metal fuels and oxidizers, forming micro-units encapsulated in a binder (<i>e.g.</i>, Al@AP (Aluminum@ammonium perchlorate) and AP@Al structures), thereby reducing the heat and mass transfer distance between them. This study conducted a series of molecular dynamics simulations to investigate the combustion behavior of two typical micro-unit structures, focusing on heat transfer, mass diffusion, and reaction kinetics. Particular attention is given to the AP@Al configuration, examining the effects of the continuity and thickness of the coated Al layer. Two extreme pressure conditions, including condensed-phase combustion with a constant volume and vacuum conditions with varying volume, were thoroughly examined to elucidate the pressure dependency. Under condensed-phase conditions, the Al@AP configuration demonstrates favorable combustion performance, though with a relatively slower consumption rate of active Al due to a single reaction front. In contrast, the AP@Al structure achieves a burning rate 2.4 times faster, benefiting from a larger reaction area and a double reaction front. Under vacuum conditions, both structures exhibit similar energy output performance, yet the AP@Al structure maintains a faster Al consumption rate, indicating a lower pressure dependency. These numerical findings shed light on the combustion mechanisms of micro-unit composite propellants, underscoring the importance of the interfacial control strategy and paving the way for the rational design and development of next-generation solid propellants.