Mechanisms of combustion instability in MEMS solid-propellant microthrusters under transient depressurization

The combustion response of MEMS solid-propellant microthrusters under transient depressurization governs thrust decay and thrust cut-off accuracy, yet the extinction mechanism remains unclear because microscale confined flow, gas–solid thermal-feedback mismatch and propellant mesoscale heterogeneity are strongly coupled. In this work, a mesoscale gas–solid coupled unsteady combustion model is developed for a PCB-based MEMS solid-propellant microthruster loaded with AP/HTPB composite propellant. The model incorporates heterogeneous propellant microstructure, compressible gas-phase flow, condensed-phase pyrolysis and dynamic burning-surface regression. An unsteady ignition-initialization correction is introduced to suppress the non-physical near-wall low-temperature region caused by mismatch between steady initial fields and moving boundaries, and a local material-property pre-mapping strategy is used to improve computational efficiency. Experimental validation and numerical simulations show that burning-rate decay during depressurization is not governed by pressure decrease alone, but results from weakened gas-phase heat release, insufficient interfacial thermal feedback and restricted condensed-phase pyrolysis. Although the gas-phase high-temperature zone exhibits thermal inertia, the main-reaction-zone heat-release rate continuously decreases, weakening effective feedback to the burning surface. Propellant heterogeneity redistributes local thermal feedback and induces localized high-temperature reaction zones, affecting transient burning-rate fluctuations. The results reveal a chain instability mechanism of gas-phase heat-release attenuation, interfacial-feedback weakening and pyrolysis restriction, clarifying the transition from stable combustion to extinction and supporting thrust-regulation and extinction-criterion development.