The Impact of Cation Structures on Hypergolicity of Thiocyanate Ionic Liquids with Hydrogen Peroxide

Ionic liquids are promising candidates for future fuels in space propulsion. In this work, six ionic liquids with thiocyanate anions are proposed as new fuel candidates with hydrogen peroxide as an oxidizer. The influence of the cation is studied by pyridinium- and pyrrolidinium-based thiocyanate ionic liquids with various alkyl chains. Characterization of the fuels comprises, inter alia, thermal analysis, viscosity, and density measurement. Calculations of the theoretical performance of the new hypergolic propellants were performed with the NASA Chemical Equilibrium with Applications for comparison with conventional ones. All presented hypergolic propellant combinations show significantly increased theoretical density specific impulses compared to the currently used hypergolic propellant combination monomethylhydrazine/dinitrogen tetroxide. Hypergolic ignition behavior of the propellant combination was evaluated by a drop test setup, which was filmed by a high-speed camera. 1-Ethylpyridinium thiocyanate ([EPy][SCN]) shows the shortest ignition delay time of 26.8 ms and the lowest viscosity of 27.5 cP and possesses a high density as well as high thermal stability. With this substance, not only a promising candidate for future orbital propulsion systems is presented, but also, new insights into the role of the cationic structure of ionic liquids in hypergolic propellant combinations with hydrogen peroxide are gained within this study.