Combustion instability - The relationship between acoustic and nonacoustic instability.

Abstract

An analysis is presented in which the instability, known as nonacoustic or L* instability, in the combustion of solid propellants is re-examined by means of an acoustic approach, i.e., by solving the wave equation for the cavity. It is found that nonacoustic instability is, in reality, a zeroth longitudinal acoustic mode for the motor, an observation not previously made. This conclusion allows the application of equations and data developed for acoustic instability to understand nonacoustic instability. The analysis was further developed by making use of the Denison-Baum combustion model to describe the combustion, after gaining assurance that this model could adequately describe existing acoustic and nonacoustic instability data. In this manner, it was possible to explain many experimental observations with considerable clarity. Finally, in the course of this analysis, a quantitative comparison has been made between the Denison-Baum combustion model and existing instability data in a rather complete manner for several propellants.