Combustion mechanism of thermoplastic HTPE propellants: effects of oxidizer particle size distribution

A critical gap exists in understanding the combustion mechanism of thermoplastic hydroxyl-terminated polyether (HTPE) propellants, particularly the effects of oxidizer particle size distribution. This study innovatively demonstrates that the solid-liquid transition of HTPE binder fundamentally shifts the ignition pathway to a novel condensed-phase mechanism driven by surface reactions and liquid-phase encapsulation. By partially replacing coarse oxidizers with fine particles, we reveal their dual role: accelerating pyrolysis and enhancing condensed-phase exothermicity, while inducing non-monotonic ignition delay through competition between gas release and heat accumulation. Combustion tests show concurrent increases in burning rate and pressure exponent; with 30% fine ammonium perchlorate (AP), the burning rate rises 34.92% to 10.2 mm/s and the pressure exponent increases from 0.337 to 0.452. Furthermore, fine oxidizers significantly promote aluminum agglomeration, yielding products with d₉₀ > 1300 μm. A combustion model is established, elucidating the unique coupling among binder transition, oxidizer decomposition, and Al agglomeration. This research broadens and refines the mechanistic understanding of solid propellant ignition and combustion. It provides both experimental evidence and a theoretical foundation for regulating the energy-release behavior of HTPE propellants.