Coupling of leading edge flames in the combustion zone of composite solid propellants

The burning rate trends of both pure binder sandwiches as well as those containing fine AP-filled binder (matrix) are investigated at different values of the middle lamina thickness over an appreciable range of pressure. The crucial aspect of this study is the variation of the ratio of fine AP/binder in the middle lamina of the matrix sandwiches. The surface profiles of matrix sandwiches quenched by rapid depressurization have also been mapped for different the different conditions. It is found that, as the fine AP is added to the pure binder, the AP particles tend to dilute the fuel and increase the lamina thickness for maximum interaction between the leading edge flames (LEFs). This causes a reversal in the trend of the variation of the optimum thickness for maximum burning rate with increase in pressure when compared to the case of pure binder sandwiches reported earlier. For addition of small quantities of fine AP in the middle lamina, the burning rates of these sandwiches decrease slightly as a consequence. As the fine AP content is increased further, there is greater interaction between the LEFs due to the inward shift of the stoichiometric surface and the extension of the fuel rich sides of the LEFs over the matrix lamina, ultimately resulting in the establishment of the canopy premixed flame between them. This restores the optimum thickness trends with pressure as in the case of pure binder sandwich. The surface profiles and the burning rates can be used to numerically obtain an estimate of the gas phase heat release rate distribution, as reported previously for an oxidizer/binder interface,1 with modifications to apply to matrix sandwiches. This is applied to the surface profiles of a family of the matrix sandwiches with different matrix lamina thicknesses, obtained experimentally in this study. The computations aid further with comparative locations of the LEFs and the consequent temperature distributions, among the cases considered.