Experimental evaluation of HTPB/paraffin fuel blends for hybrid rocket applications

Hybrid rockets have many advantages over pure solid or liquid propellant rockets, but low solid fuel regression rates and correspondingly low thrust have hindered their application to operational systems. Paraffin-based fuels regress significantly faster than traditional polymeric formulations, such as HTPB, and paraffin inclusion in HTPB represents a potential tool for performance augmentation in hybrid rockets. A survey of the available literature indicated disparities regarding the utility of this approach which are resolved herein. Fuel specimen consisting of plain HTPB; plain paraffin; and HTPB loaded with molten macrocrystalline paraffin wax (10–75%) or solid microcrystalline paraffin particles (10–60%) were manufactured and evaluated for their thermal decomposition and ballistic properties. Fuel samples were heated (10 K/min) in an argon atmosphere in simultaneous TGA/DTA experiments. The inclusion of macrocrystalline paraffin enhanced the low-temperature decomposition of HTPB, while the inclusion of microcrystalline paraffin had the opposite effect. The prepared fuel grains were burned in gaseous oxygen on one of two lab-scale hybrid rockets over a range of oxidizer mass fluxes (5–430 kg/m2-s) and pressures (0.5–1.0 MPa). The plain macrocrystalline paraffin fuel exhibited a 300% increase in regression rate over plain HTPB. However, none of the mixed-fuel formulations exhibited notable, if any, regression rate enhancement at the evaluated operating conditions. First principles modeling was completed for the combustion of plain HTPB, plain paraffin, and mixed-fuel systems comprised of HTPB containing molten liquid paraffin or solid paraffin particles. The combustion of mixed-fuel systems is dominated by the pyrolysis of HTPB which does not allow for the formation of a melt layer at the fuel surface, such that any enhancement is due to an increase in the vaporization rate of the fuel and not entrainment effects. This study was the first to concurrently evaluate the inclusion of both molten liquid paraffin and solid paraffin particles in HTPB and demonstrated a lack of performance augmentation with either strategy in two separate laboratories. The results presented herein resolve the disparities in the literature and indicate that paraffin inclusion in HTPB is not a viable means for tailoring the combustion behavior of hybrid rocket systems.