Evolution of structural damage of solid composite propellants under slow heating and effect on combustion characteristics

Composite propellants are used to fabricate solid motors for space vehicles. The fundamental factor that affects motor safety is the structural damage in the propellant owing to slow heating in an abnormal storage or service environment. Hence, their thermal damage and combustion characteristics should be studied comprehensively. In this study, we investigated the combustion behaviours of hydroxyl-terminated polybutadiene/ammonium perchlorate/aluminium powder (HTPB/AP/Al) and hydroxyl-terminated block copolyether/AP/Al (HTPE/AP/AI) propellants under slow heating conditions. The pore structure was observed using scanning electron microscopy and micro-computerised tomography. Their weight loss behaviour, gaseous products, and pore structure evolution were analysed using thermogravimetric analysis, mass spectrometry, and Fourier-transform infrared spectroscopy. The influence of the pore structure on the combustion behaviour was studied by establishing the relationship between combustion and energy release rates. The weight loss rates of HTPB/AP/Al and HTPE/AP/Al before ignition were 34.5% and 16.1%, respectively. Upon heating, the HTPB binder decomposed to form pore channels, through which the gaseous products produced from the decomposition of AP were released, which increased the structural damage in this propellant. In contrast, upon heating, the HTPE binder liquefied and formed a coating on the AP particles, which slowed the structural damage in this propellant. Moreover, the calculated reaction intensity of HTPB/AP/Al was 4.16 times that of HTPE/AP/Al, indicating that larger pore channels increase the burning surface for combustion.