COMBUSTION OF HYPERGOLIC HYBRID ROCKET FUEL IN HYDROGEN PEROXIDE SPRAY ENVIRONMENT

Hypergolic propellants are defined by their ability to ignite immediately upon contact, resulting in rapid exothermic reactions occurring without the need for an external ignition source. In hybrid rocket engines, this spontaneous reaction simplifies thruster management, reduces system mass, and enables engine re-ignition. The present research investigates hypergolic ignition using a propellant combination of high-density polyethylene (HDPE) fuel embedded with sodium borohydride (SBH) additive, with rocket-grade hydrogen peroxide (RGHP) as the oxidizer. This HDPE-SBH-RGHP system offers a “green” propellant option, which is a less toxic alternative to traditional, highly hazardous hypergolic propellants, enhancing operational safety and reducing production costs. Our experimental framework utilizes spray ignition tests designed to emulate the operational environment of a hybrid rocket motor. This approach provides foundational data on the interaction between multiple ignition points, the dynamics of flame propagation, and the fuel’s response to a continuous oxidizer supply. The primary focus is on the critical influence of pressure as a key condition governing the initiation and sustainment of hypergolic reactions. The investigation confirms that the initial pressure and the O2/N2 ratio are the primary factors determining the ignition threshold. While success is consistent above 5 bar (with 2 bar marking a critical 50% probability), ignition at atmospheric pressure was only achieved by increasing the O2 concentration to 35%. Furthermore, this high initial pressure not only enables ignition but also promotes superior flame stability, resulting in more robust heat release compared to lower-pressure cases. Other operational configurations were also explored, including sample exposure to humidity, sample orientation with respect to oxidizer flow direction, liquid accumulation on the fuel surface, and potential for re-ignition. It is concluded that the reliability of ignition in hypergolic solid fuels is a strong function of the initial conditions in the reactor, even for highly reactive propellant combinations.

This work is towards an M.Sc. degree under the supervision of Assoc. Prof. Joseph Lefkowitz, The Stephen B. Klein Faculty of Aerospace Engineering, Technion