The relationship between plateau burning behavior and ammonium perchlorate particle size in HTPB-AP composite propellants

Abstract

The burning rates of series of bi- and trimodal AP-HTPB propellants have been incorporated into an empirical statistical model which describes a hypothetical pentamodal polydisperse AP pseudopropellant. This pseudopropellant is the linear summation of five unimodal polydisperse AP pseudopropellants. The five AP particle size distributions cover the range from 400 to 20 μm weight median diameter). The experimental propellants have been further subdivided into plateau burning and nonplateau burning propellants. The statistical model has been used to generate a plateau burning and a nonplateau burning pentamodal AP pseudopropellant. Plateau burning behavior in this propellant system (when compared to nonplateau burning behavior) appears to result from an enhanced burning rate at low pressures and a reduced burning rate at high pressures for the unimodal 20 μm AP pseudopropellant. It is speculated that such behavior is consistent with the presence of a binder melt on the burning surface. The larger AP unimodal pseudopropellants also show low burning rates and plateau burning behavior. Such behavior appears to be the result of variations in the importance of the AP monopropellant flame as a function of pressure and AP particle size. A melt layer on the AP particle surface during combustion may be the cause of the plateau burning behavior in the larger AP unimodal pseudopropellants. The temperature sensitivity of the unimodal pseudopropellants increases as a function of the AP particle size. Plateau burning behavior is maintained over the temperature range from 0° to 55°C.