A computational model of composite solid-propellant combustion that combines three important properties is described. First, the model is heterogeneous, in that it incorporates a multilevel flame structure similar to the Beckstead–Derr–Price proposition. Second, it is nonsteady, and its main purpose is to predict pressure-coupled frequency response. Third, it is fully nonlinear, where thermal capacitance calculations come from a computational solution of the transient heat-conduction equation separately in the oxidizer and binder. Calculations show a frequency response that varies significantly with oxidizer particle diameter and mean pressure. Some observed trends are that the magnitude of the real part of RP tends to diminish with higher mean pressures and that propellants with very coarse oxidizer particles tend to develop two peaks, one for the oxidizer and one for the binder.