Green bipropellant development – A study on the hypergolicity of imidazole thiocyanate ionic liquids with hydrogen peroxide in an automated drop test setup

Abstract

Conventional hypergolic propellant combinations, using hydrazine and its derivatives as fuels and dinitrogen tetroxide based oxidizers, have been applied in spacecraft for attitude and roll control systems for more than 50 years. However, due to their high toxicity and carcinogenic potential, investigations into alternative green propellants are an active field in current research efforts. Promising alternative propellant candidates are combinations of hydrogen peroxide and suitable hypergolic room-temperature ionic liquids. In this work, imidazole thiocyanate ionic liquids are tested in drop tests with hydrogen peroxide. For this purpose, a newly developed automated drop test setup was designed and implemented. As a result, 1-ethyl-3-methylimidazolium thiocyanate (EMIM SCN) and 1-butyl-3-methylimidazolium thiocyanate (BMIM SCN) turned out to be hypergolic with highly concentrated hydrogen peroxide (96.1%). The ignition delay time on average is 31.7 ms for EMIM SCN and 45 ms for BMIM SCN. Theoretical performance of the two ionic liquids was calculated with NASA CEA and compared to a conventional hypergolic propellant combination (monomethyl hydrazine/dinitrogen tetroxide). The specific impulse of the green propellants is nearly 5% lower, but the density specific impulse is increased by 10%. Furthermore, the ignition delay time was reduced by dissolving a catalytic additive, copper thiocyanate, in the EMIM SCN. The lowest average ignition delay time of 13.9 ms was achieved for EMIM SCN and 5 wt% of copper thiocyanate. For higher copper concentration the ignition delay time is not further reduced. The fuel with EMIM SCN and 5 wt% of copper thiocyanate and the pure EMIM SCN were further characterized by thermal and spectroscopic methods. Fluid properties like density, viscosity and surface tension were also determined in laboratory investigations.