This paper reports the results of numerical studies carried out for a periodic sandwich propellant geometry with two-dimensional unsteady gas and condensed phase. A non-planar regressing surface along with a kinetic model of two reaction steps in the gas phase was used. This paper discusses the importance of lateral momentum equation to the combustion of sandwich propellants. It demonstrates that the instabilities in sandwich combustion reported in literature are due to neglect of lateral momentum equation and incorrect boundary conditions at the regressing surface. It demonstrates that neglect of momentum equations will lead to a different result from the one obtained considering the momentum equations for sandwich propellants. © 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.