The complexities of the flame structure above a composite propellant containing 86% ammonium perchlorate (AP) and 14% hydroxy-terminated-polybutadiene (HTPB) have been elucidated using a two-dimensional, detailed gas-phase kinetic mechanism diffusion flame model. The model utilizes a vorticity formulation of the transport equations, which essentially eliminates the pressure field calculation and speeds convergence. Mass and energy coupling between the condensed and gas phases are achieved through iteration with one-dimensional, premixed combustion models to dynamically update the inlet boundary conditions. The model uses a detailed gas-phase kinetic mechanism consisting of 37 species and 127 reactions. Numerical studies have been performed to examine the influence of particle size on the flame structure above the AP/HTPB propellant. Three different combustion zones, based on AP particle size, were predicted: the AP monopropellant limit, the diffusion flame region, and the premixed limit. The modeled flame structure changed dramatically with particle size and was found to be qualitatively similar to the Beckstead-Derr-Price Model. Mechanistic insights are presented to explain AP's unique ability to modify a propellant burning rate based on particle size alone. The premixed limit at which decreasing the size of the AP particles no longer influences the burning rate was also predicted. Results show promise in predicting formulistic effects using fundamental calculations. Published by Elsevier Inc. on behalf of The Combustion Institute.