Measurement of the burning surface temperatures of propellant compositions by infra-red emission

Abstract

The temperatures of extremely thin surface layers of burning solid propellant compositions have been determined by measurement of their infra-red radiation. Specific wavelengths, at which the flame gas emission is low, have been selected for the measurements. The path length of the radiating gaseous flame through which the surface is viewed has been reduced using a specially designed burner so that adequate correction for gas emission can be made. The surface temperatures of ammonium perchlorate compositions have been examined in some detail and the effect of the magnitude of the absorption coefficient of the material on the observed temperature has been demonstrated with this system. For example, at wavelengths at which the absorption coefficient is low, the surface temperatures of very weak fuel ammonium perchlorate powder mixtures appear to fall somewhat as the burning rate increases but in spectral regions of suitably high absorption coefficient the surface temperatures remain virtually constant, irrespective of the burning rate. The surface temperature for such mixtures burning at atmospheric pressure is 495° ± 15°C. Practical propellant compositions, based on ammonium perchlorate and containing rather more than stoichiometric proportions of fuel, give the same temperature provided that the hot gas emission can be successfully eliminated, but observed temperatures are in some instances raised (to a maximum of about 560°C) by gas interference. It is concluded that the process taking place at the burning ammonium perchlorate surface is an equilibrium sublimation process and that the rate of consumption of the solid is controlled by subsequent gas phase reactions among the dissociation products. The average surface temperature of double-base propellants increases with increase of burning rate, although it is likely that the liquid nitric ester component vaporizes at constant temperature. The retention of carbonaceous products near the burning surface of some compositions makes measurements of surface temperature inaccurate and has so far prevented the determination of the precise temperature dependence of the nitrocellulose decomposition. Double-base compositions have surface temperatures ranging from 300° to 400°C at atmospheric pressure. Binder-fuels currently used in composite propellants have surface temperatures close to 500°C when burning in a diffusion flame with oxygen.