Condensed Combustion Products Characteristics of HTPB/AP/Al Propellants under Solid Rocket Motor Conditions

Condensed combustion products (CCPs) generated during the combustion of aluminized propellants can reflect invaluable information about the combustion mechanisms of propellants. CCPs of hydroxyl-terminated polybutadiene/ammonium perchlorate/aluminum (HTPB/AP/Al) propellants were collected using an experimental apparatus capable of controlling pressure fluctuations within 0.3 MPa, and their microscopic morphologies, particle size distributions, and chemical compositions were characterized using a scanning electron microscope (SEM), laser particle size analyzer, energy disperse spectroscopy (EDS), X-ray diffraction (XRD) and complexometric titration. The results showed that the size of CCPs presented a bimodal distribution, with modes at ~5 µm and ~100 µm; particles less than 2 µm were spherical, with smooth surfaces. The main components of CCPs were C, AlN, AlCl3, Al2O3, Fe2O3 and Al, with Al2O3 being the most abundant. The combustion efficiency of aluminum increased by 3.27% when the size of virgin aluminum particles decreased from 23 µm to 13 µm, but the content of catocene (a burning-rate catalyst) and fine AP (1 µm) had little effect on combustion efficiency. Higher combustion efficiencies and smaller agglomeration sizes can be achieved at higher pressures, due to the positive correlation between pressure and the driving forces for aluminum particles exciting the burning surface.