Experimental study of ignition and combustion of polyethylene-based hypergolic solid fuels in a lab-scale hybrid rocket motor

One of the primary challenges for hybrid rockets is achieving fast, simple, and reliable ignition. The current study investigates the use of hypergolic solid fuels (HSFs) as a practical solution to meet this goal. A polyethylene-based HSF embedded with sodium borohydride (NaBH₄) and manganese acetate tetrahydrate (Mn acetate) was tested in a lab-scale hypergolic hybrid rocket motor (HRM) using 90wt% rocket-grade hydrogen peroxide (RGHP). The heterogeneous reactions between RGHP and the embedded additives generated sufficient thermal energy and sparks for ignition. Experimental results showed that rapid ignition was achieved only in HSF grains with cut surfaces resulting in freshly exposed surfaces. At high oxidizer flow rates, unstable combustion behavior was observed, characterized by intermittent gas release and erratic motor vibrations. This behavior is hypothesized to result from oxidizer flooding and the explosion of hydrogen peroxide vapor near the fuel surface. Increasing the fuel port diameter and incorporating Mn acetate effectively mitigated this issue. Ignition delay times and fuel regression rates were measured and compared with existing literature. While ignition delay times showed no clear dependence on additive concentration or oxidizer mass flow rate, fuel regression rates increased with both. Using the empirical regression rate expression $\dot{r}=a{G}_o^n$, an exponential factor of $n=0.9$ was obtained, indicating that the fuel regression rate is highly dependent on mass flux, potentially driven by heterogeneous surface reactions and hydrogen generation. Surface profilometry further revealed spatial variations in fuel regression and highlighted the significant impact of liquid spray impingement. The experimental findings in this study demonstrate the potential of employing HSFs to achieve a simple, reliable ignition and enhanced fuel regression in HRMs.