Effect of Solid Propellant Compressibility on Combustion Instability

The recent theory of the interaction of sound with a burning propellant surface is extended to include density fluctuations of the solid propellant. Both compressibility and thermal expansion are treated, but the latter is found to be less important than the former. For reasonable values of the compressibility, it is shown that the acoustic response, Real (μ̃1/ε̃), of the surface is reduced by as much as 0.1 in a broad range of frequency around several Kc. A simple low-frequency approximation for the burning surface response is also discussed.