Ignition-delay measurement for drop test with hypergolic propellants: Reactive fuels and hydrogen peroxide

Green hypergolic propellants have been extensively studied. In the present study, we attempted to improve the measurement technique for the ignition delay in hypergolic reactions by using a drop-test apparatus. The evaluation method for the hypergolicity is important because an increase in the diversification of green hypergolic combinations is inevitable. Hydrogen peroxide (90 wt%) was used as an oxidizer, and six different reactive fuels were prepared. Sodium borohydride was utilized as an ignition source for the fuels. Optical and acoustic sensors were employed to reliably measure the ignition delay. The proposed measurement technique used in this study accurately reflected the physical phenomena of the hypergolic interactions. The technical limitations in the conventional measurement technique using a high-speed camera were experimentally investigated. Liquid-phase reactions were dominant in the hypergolic reactions of the glyme-based reactive fuels. Gas-phase reactions significantly influenced the hypergolic reactions of most of the amine-based reactive fuels. The pyridine-based reactive fuel had a longer delay time for the gas-phase reaction process than the liquid-phase reaction process. Recognizing the rate-determining step of the hypergolic ignition is necessary for not only designing a rocket injector but also reducing combustion instability, e.g., reactive stream separation.