Aluminum agglomeration in ammonium perchlorate/cyclotrimethylene trinitramine/aluminum/hydroxy-terminated polybutadiene propellant combustion

Aluminum combustion efficiency and agglomeration in 13 equal-composition AP/RDX/A1/HTPB propellants were experimentally investigated using a quenched particle collection bomb (QPCB) under pressures up to 6895 kN/m. Formulation parameters studied included RDX size, Al size, fine AP size, and AP coarse/fine weight ratio. The results indicate that formulation variables can affect the combustion efficiency through either propellant microstructure changes, or extended condensed phase reactions associated with significant fine AP fraction. Effects of formulation variables on combustion efficiency are in qualitative agreement with the trends predicted by Cohen's pocket model. The addition of RDX has an adverse effect on agglomerate size, possibly through formation of a molten layer that facilitates the retention and coalescence of AI particles on the burning surface. Agglomerate sizes are adequately characterized by two microstructure parameters from a simplified pocket model. A correlation for estimation of agglomerate size of AP/Nitramine/Al/HTPB propellent is tentatively proposed based on the data from various sources. Moreover, the combustion efficiency was found to be inversely proportional to the agglomerate size.