Hypergolic ignition characterization of a non-toxic hybrid rocket propellant

Hybrid rockets are those that use fuel in the solid state and oxidizer in the fluid state. They are safe, cheap and simple to design, but they are notoriously difficult to ignite. Hypergolic ignition, where propellants undergo rapid exothermic reaction upon contact, without the need for a separate ignition source, is being investigated as a potential solution to this ignition problem. This presentation reviews recent work encompassing a comprehensive ignition characterization of a non-toxic hypergolic hybrid rocket propellant consisting of rocket grade hydrogen peroxide (RGHP) as oxidizer, high-density polyethylene (HDPE) as fuel and Sodium borohydride (SBH) as the energetic additive embedded in HDPE. In phase 1 of the work, ignition was characterized as a function of thermo-chemical, fluid dynamic and environmental parameters in a drop-test configuration. We found, for the first time, ignition sensitivity to droplet impact velocity and dimension. This interesting trend was investigated in detail in the second phase, which isolated the sensitivity of ignition delay to the contribution from minimum post-impact droplet thickness. A mathematical model was derived which found excellent agreement with experimental data, thereby highlighting the importance of considering droplet properties in modelling efforts related to hypergolic ignition. Finally, in phase 3, sprays of oxidizers were used to ignite the fuel samples, emulating real rocket motor oxidizer feed conditions. Results demonstrated the importance of air pressure and oxidizer infusion volume on flammability. The results of this work can lead to better hybrid rocket design in addition to increasing our understanding of hypergolic ignition.