Reducing the Ignition Delay of Hypergolic Hybrid Rocket Fuels

Paraffin-based fuels incorporating solid amine boranes are investigated to identify formulations suitable for use as hypergolic hybrid rocket propellants. Their ignition delays are measured following contact with 90% white fuming nitric acid droplets. The mechanical properties of the paraffin binder and of the melt layer it produces are modified using an alpha-olefin polymer. The tests are carried out at atmospheric pressure, with visible flame light emission as well as OH* chemiluminescence recorded. The ignition delays measured from both signals are nearly identical, confirming that OH* emission begins at the same time as the visible light emission from the boron-containing additive mixed with the fuel. Identification of the different steps of the combustion process is done with a high-speed schlieren imaging technique. Consistently shorter ignition delays are obtained by increasing the proportion of polymerized alpha olefin. The effect of this addition on the viscosity of the melt layer produced in the fuel blend upon contact with the acid is inferred from rheological measurements realized on unburned samples. The effect of alpha-olefin addition on the theoretical thermochemical performance of the fuel is also computed. The results obtained confirm that polymer-based additives can be used to control mechanical and rheological properties in a way that lowers the ignition delays of hypergolic fuel systems based on paraffin, as required for use in space propulsion applications.