Studies on aluminum agglomeration and combustion in catalyzed composite propellants

Composite propellants are tested using the quench particle collection bomb (QPCB) for the pressure ranging from $2$ to $8$ MPa to estimate the particle size distribution of aluminum agglomerates from quenched combustion residues emerged out from the burning surface. The major ingredients included in the propellants are ammonium perchlorate (AP), aluminum (Al), hydroxyl-terminated polybutadiene (HTPB), and toluene diisocyanate (TDI). Five propellant compositions are considered in this study; two of them are mixed with catalysts. Propellant formulation variables like the coarse AP/fine AP ratio, total solid loadings, catalyst percentage, and aluminum content are varied to assess their effects on the aluminum agglomeration process at different pressures. Unburnt aluminum in agglomerates is continuously getting combusted as they move out from the propellant burning surface. Large agglomerates comprise both $\mathrm{Al}_2\mathrm{O}_3$ and unburnt aluminum. The majority of agglomerates are spherical in shape, and the sizes vary from $31$ to $115$ $\mu$ m for non-catalyzed propellants and from $28$ to $136$ $\mu$ m for catalyzed propellants over the tested pressure conditions. These results can give further insight into the aluminum agglomeration process of catalyzed and non-catalyzed propellants and also affect the choice of the propellant ingredient percentage aimed at reducing aluminum agglomeration, which causes two-phase flow losses of thrust and slag accumulation in full-scale solid rocket motors.