Theoretical performance evaluation of hypergolic ionic liquid fuels with storable oxidizers†
Hypergolic ionic liquids (HILs) have gained significant importance in recent years as green space propellants. It is mainly due to their attractive physicochemical properties, such as high density, negligible vapor pressure and wide liquid range. More importantly, they show low ignition delay with storable oxidizers viz. white fuming nitric acid (WFNA), red fuming nitric acid (RFNA), dinitrogen tetroxide (N2O4) and hydrogen peroxide (H2O2). Therefore, investigation into the performance parameters of HILs, such as specific impulse (Isp), density and vacuum-specific impulse (ρIsp & Ivac), and characteristics velocity (C*), has received enormous attention. In the present study, 14 HILs have been selected for evaluation of their performance parameters using the NASA-CEC-71 program. Amongst them, three HILs have been developed in the laboratory by the authors. The effects of oxidizers, oxidizer to fuel ratio, heat of formation, hydrogen content of the HIL, and the chamber pressure parameters were studied in detail. The performance of HILs was also compared with that of a proven fuel, UDMH. The HILs 1-ethyl-3-methyl imidazolium cyanoborohydride, [EMIM][BH3CN], and 1,3-diallyl imidazolium cyanoborohydride, [diAIM][BH3CN], were particularly promising with all the oxidizers. The H2O2 seems to be a good potential oxidizer as it exhibited a higher value of ρIsp for most of the HILs than the UDMH. A propellant based on high-density HIL as a fuel and H2O2 as an oxidizer can be classified as a green propellant. Thus, the investigation helps in designing or selecting the best combination of HIL and oxidizer for rocket applications.