Combustion characteristics of hypergolic solid fuel using NaBH₄-embedded polyethylene wax for hydrogen peroxide hybrid rocket

Hypergolic hybrid rocket motor (HRM) is characterized by its structural simplicity, high fuel regression rate and multiple start-up capability, providing a new option for future in-space propulsion tasks. A novel type of hypergolic solid fuel (HSF) consisting of polyethylene wax (PEW) and sodium borohydride (NaBH₄) is tested with 90 % hydrogen peroxide (H₂O₂) to validate the feasibility of hypergolic ignition and study the propellant's combustion performance. Minimum ignition delay times (IDTs) of 8 ms were observed in oxidizer drop-on-solid tests. A series of 4 motor-firing tests were conducted, with steady-state combustion efficiency reaching 97.9 %, following a positive correlation with motor chamber pressure. An internal ballistic reconstruction technique was implemented to get instantaneous datapoints of fuel regression rates r˙ and oxidizer mass flux G_ox. A power-form correlation of r˙ = 0.4551G_ox^0.4268 [mm/s] was obtained, exhibiting the fuel's high r˙ manner and regression behaviours similar with paraffin wax (PW). The results provide insights into combustion characteristics and practical applications of the HSF.