Why does adding a poor thermal conductor increase propagation rate in solid propellants?

Solid propellant additives have a long history of modulating burning rate by introducing materials with high thermal diffusivities to better concentrate and transfer heat to nearby areas. However, recent studies have demonstrated a counterintuitive result in that additives with thermally insulating properties-notably SiO2 particles-can also enhance the propagation rate in solid propellants. In this work, high-speed microscopy and thermometry were performed on 3D printed solid propellant films containing both thermally conducting (graphite) and insulating (SiO2) particles to investigate the role of these additives on film propagation rate. It was found that addition of SiO2 particles increased the effective surface area of the reaction front through inhomogeneous heat transfer in the films, and that such corrugation of the reaction front area on the micrometer scale manifests itself as a global increase in the propagation rate on the macro scale. Graphite additive was observed to have a substantially lower burning surface area and propagation rate, suggesting that the effect of reaction front surface area is larger than the effect of thermal diffusivity for low-weight percent additives in solid propellants.