The gas phase flame structure of composite solid propellant sandwiches with two ammonium perchlorate (AP) laminae sandwiching a middle lamina containing a homogenized mixture of hydroxyl terminated poly-butadiene (HTPB) binder and fine AP particles is numerically simulated. Experimentally obtained surface profiles and burning rates are used to decouple the gas phase from the condensed phase. In the literature, three global gas phase reactions for (1) the AP mono-propellant flame, (2) the primary diffusion flame, and (3) the final diffusion flame between gaseous products of AP decomposition/deflagration and the binder have been considered. Alternatively, multi-step detailed/reduced reaction mechanisms have been adopted to represent all these flames together. In the present study, three different reaction mechanisms are evaluated: (1) global chemistry using symbolic species for the three flames, (2) global chemistry using a 12-species mechanism for the above three flames, and (3) a 72-step reaction mechanism with 39-species representing all the flames. The 12-species, 3-step global reaction mechanism provides a temperature field similar to the 72-step mechanism unlike the global reaction mechanism involving symbolic species. Further, the triple flame structure of the primary diffusion flame and the 'canopy' premixed flame over the middle lamina (for a self-deflagrating mixture of fine AP and binder) are well captured by the 12-species mechanism unlike the other two considered. © 2015 The Combustion Institute.