Hypergolic Hybrid Rocket Engine Ignition and Relights with Mixed Oxides of Nitrogen

Combined with a common fuel binder, solid hypergols can simplify the overall complexity of hybrid rocket engines, as the fuel grain can be ignited and reignited without any external power source or external fluid. Also, with the hypergolic additive embedded in the binder, the flame zone could be placed at the surface of the grain itself, thereby providing heat to the grain to maximize regression rate and promote combustion sustainability. The objective of this study was to demonstrate hypergolic ignition and successive relights of a 2-in motor grain configuration, with a paraffin-based fuel and MON-3 (3 wt % nitric oxide in nitrogen tetroxide) as the oxidizer. With sodium amide and potassium bis(trimethylsilyl)amide as solid hypergolic additives, the study focused on the influence of the additive type, loading, location, and format on the grain ignition delay and combustion sustainability. Hypergolic ignition was achieved with grain configurations composed of a front segment with 90 wt % additive and the main grain with loadings of additive from 40 down to 0 wt %. Two-s single burn tests provided regression rate estimates for the different grain combinations. Grain ignition delays varied between similar to 20 and similar to 250ms, depending on the grain configuration, with C* efficiencies between 64% and 98%.