Effect of acoustic environment on the burning rate of solid propellants

Abstract

During unstable combustion of solid propellants, the burning surface is exposed to severe oscillations in the adjoining gas environment, and large changes in burning rate of the propellant result. By using specially designed burners, it was found possible to determine, by interrupting burning, the effect on the propellant burning rate. Because of the standingwave nature of the oscillatory behavior, it was possible to correlate the change in burning rate at each point on the propellant surface with the nature of the acoustic environment. It was observed that burning rate is decreased by acoustic pressure and increased by acoustic velocity (double-base propellants), the two effects being of comparable magnitude. The fluctuations in equilibrium pressure during unstable rocket motor and vented vessel firings were found to be readily explainable in terms of the burning-rate effects obtained from the interrupted burning tests, provided the acoustic behavior during the firings was simple enough to analyze. However, a singular class of behavior was observed under certain conditions; dramatic increases in burning rate occurred at characteristi c times during burning when the frequency of the unstable tangential mode (first mode) was an integral multiple of the frequency of an unstable axial mode.