This paper presents current work on improving the accuracy of solid composite propellant modeling to include catalytic additives and using it to develop a better understanding of the working mechanism of burning rate enhancement. To this end, isolating the physical mechanism where the catalyst impacts the composite propellant burning rate was studied. To accomplish this goal, a parametric study was conducted by systematically varying different aspects of the burning that are covered by the simple model. The results from the model were compared to experimental data collected from propellants formulated in the authors' laboratory both with and without catalytic nanoparticle additives, namely TiO_2. Some variations of the TiO_2 additive tested included powder that was produced by spray-drying with and without heat treating as well as titania that was premixed into the binder before making the ammonium perchlorate (AP)/binder mixture. The propellants are compared to two ammonium perchlorate and hydroxyl-terminated polybutadiene (HTPB) baselines with 80% and 85% solids loading. Advancements on the Beckstead-Derr-Price (BDP) model to incorporate catalytic additives were accurately demonstrated using the technique outlined in this paper. This study determined that the catalyst primarily impacted the condensed phase by increasing the reaction rate of the condensed-phase AP; this conclusion was based on the fact that only changes in the condensed-phase AP reaction rate produced the pressure dependence and absolute magnitude of the increased burning rates due to the TiO_2-based additive that were seen in the data. In contrast, an unrealistically high increase in the pressure dependence was found if the binder kinetics were modified to match the observed burning rates, and changes in the primary flame kinetics only varied the slope of the burning rate curve; these results are not supported by the experimental data. Furthermore, an empirical constant (Ω_c) was found to model the effect of the additives on the AP reaction rate in the form of a burning-rate-magnitude modifier. Typically, an increase of around 50 to 60% in the reaction rate was observed for the use of nano-titania in an 85% AP propellant.
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