Assessment of imidazole-based ionic liquids as dual-mode spacecraft propellants

Imidazole-based ionic liquids are investigated in terms of dual-mode chemical monopropellant and electrospray rocket propulsion capabilities. A literature review of ionic liquid physical properties is conducted to determine an initial, representative set of ionic liquids that shows favorable physical properties for both modes, followed by numerical and analytical performance simulations. The ionic liquids 1-butyl-3-methylimidazolium dicyanamide, 1-butyl-3-methylimidazolium nitrate, and 1-ethyl-3-methylimidazolium ethyl sulfate meet or exceed the storability properties of hydrazine, and their electrochemical properties indicate that they may be capable of electrospray emission in the purely ionic regime. These liquids are projected to have 13-23% reduced monopropellant propulsion performance in comparison to hydrazine due to the prediction of solid carbon formation in the exhaust. The use of these ionic liquids as a fuel component in a binary monopropellant mixture with hydroxylammonium nitrate shows a 1-4% improved specific impulse over some "green" monopropellants. Also, this avoids volatility issues and reduces the number of electrospray emitters by 18-27% and the power required by 9-16%, with oxidizing ionic liquid fuels providing the greatest savings. A fully oxygen-balanced ionic liquid will exceed the state-of-the-art performance in both modes but will require advances in hardware technology in both modes to achieve minimum functionality.