The classical wave-model for the laminar, adiabatic combustion of a homogeneous, pre-mixed, gaseous flame is expanded and applied to the heterogeneous combustion of a pure solid propellant. The conservation equations for the gas, solid and Gibbsian surface states are solved to obtain an explicit expression for the steady-slate burning rate. The major problem of predicting the magnitude of the gas-phase heating rate, is resolved by relating the heating rate to the burning velocity of a simulated flame above the solid surface. The theory is readily modified to include energy loss corrections for the non-adiabatic processes of conduction, convection and radiation to the surroundings. The theory predicts a magnitude for the burning rate of pure ammonium perchlorate that is in good agreement with experiment