Response of a burning heterogeneous propellant to small pressure disturbances

A numerical code for the simulation of heterogeneous propellant burning is used to examine the response to the pressure field of an incident acoustic wave. It is shown that there are two natural response functions, one defined by the perturbation mass flux beyond the combustion field, the other by the perturbation energy flux beyond the combustion field. The first of these plays an essential role in L* instability (a coupled propellant–rocket-chamber phenomenon), and the second in whether the acoustic wave is amplified on reflection (a phenomenon exclusive to the propellant). We show, for several choices of parameter values and propellant morphology, that the two responses are qualitatively similar, albeit they differ in magnitude by modest amounts. We show that in some cases each response displays a single maximum or peak as a function of frequency, in other cases multiple peaks are obtained. A tentative hypothesis is proposed as a predictor of multiple peaks.