Mixed HTPB/Paraffin Fuels and Metallic Additives for Hybrid Rocket Applications
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Hybrid rockets have distinct advantages over their pure solid or liquid propellant counterparts, but their widespread application has been hindered by characteristically low regression rates and combustion efficiencies. A comprehensive review of hybrid rocket enhancement strategies is given within, with special emphasis on metallic additives and mixed-fuel (HTPB/paraffin) systems. Several metallic additives (micro-aluminum, micro-magnesium, micro-titanium, micro-zirconium, nano-aluminum, nano-boron, and magnesium-coated nano-boron) were selected as potential candidates for hybrid rocket applications and characterized by applicable microscopy techniques. The regression rates and combustion efficiencies of plain HTPB and HTPB loaded with each additive at various concentrations (10%, 20% and 30% by mass) burning in GOX were evaluated at moderate oxidizer mas fluxes (10-150 kg/m^2-s) and pressures (<125 psia). In general, the inclusion of any of the metallic additives led to a reduction in the regression rate. The one exception to this trend was the formulation containing 10% micro-zirconium which yielded a moderate (10-20%) increase in regression rate. The observed trends were more prevalent at higher oxidizer mass fluxes and higher additive loadings. The reductions in regression rate were attributed to heat transfer blocking effects derived from accumulation of mass on the fuel surface layer. These phenomena were especially prevalent in highly loaded fuel formulations containing nano-aluminum or boron which exhibited unstable combustion and periodic surface layer shedding. Zirconium appears to be the best metallic additive available since it can yield the highest theoretical density specific impulse under the lowest O/F operation ratio without resulting in substantial decreases in the fuel regression rate or mass flux. Combustion efficiency data of all fuel formulations were well correlated to the fuel residence time, and high combustion efficiencies (>95%) were achievable when a satisfactory residence time (~75 ms) was realized. The inclusion of molten paraffin in HTPB at concentrations of 10-75% as a regression rate enhancement strategy was evaluated under similar conditions. The plain paraffin fuel exhibited a 300% increase in regression rate in comparison to plain HTPB, but none of the mixed-fuel systems showed signs of regression rate enhancement at the tested operating conditions.
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