Solid propellants: The combustion of particles of metal ingredients

Abstract The specific impulse of normal solid propellants can be increased by the inclusion of powders of certain light metals such as aluminium but, in practice, the calculated improvement is not realized. It is probable that the main reason for this loss in performance is failure of the condensed phase of the products to reach velocity equilibrium during expansion. This is a function of the particle size of the solid combustion products. Also under certain conditions incomplete combustion of the metal can contribute to the loss of efficiency. Hence the understanding of the factors influencing the combustion of the metal in the atmospheres characteristic of propellant combustion is very important. The combustion of metal particles at atmospheric pressure was followed experimentally by introducing the particles into a premixed flame. For observations at higher pressures, propellants of known compositions and containing only a small concentration of metal powder (to avoid coalescence of molten metal on the propellant burning surface) were employed. The combustion of a particle ocurs in two stages-an ignition delay and then burning. The effects of particle composition, temperature, pressure and gas composition on these two stages indicate that different mechanisms are involved. The ignition delay is determined essentially by the external rate of heating of the particles, while the burning time depends more on the concentration of oxidant in the atmosphere. The burning mechanism of an aluminium particle at 1000 lb/in2 and above appears to be similar to that operating in hydrocarbon droplet combustion which seems to indicate that, at least at high pressures, the presence of a condensed product, namely alumina, does not drastically alter the diffusion mechanism in the gas phase. The results suggested that the basic requirements for efficient combustion of an aluminized propellant are (a) that the aluminium particles entering the gas phase should be as small as possible, and (b) that the propellant-matrix temperature should be sufficiently high to ensure early ignition of the particle. High operating pressures (say >1000 lb/in2) also favour rapid combustion of aluminium.