Green bipropellant: Performance evaluation of hypergolic ionic liquid-biofuel with hydrogen peroxide

The development of environmentally-friendly propellants, as an alternative to hydrazines, has been growing interest within the space community. Hence, the present study was focused on developing a low-toxicity hypergolic liquid propellant based on ionic liquid-biofuel fuel and hydrogen peroxide (H2O2) oxidizer for cost effective and sustainable space exploration. The new fuel blend of ionic liquid, 1-ethyl-3-methyl imidazolium cyanoborohydride ([EMIM][BH3CN]), ethanol and copper promoter ([monocyanotrihydroborate tetrakis(imidazole) copper(II)][cyanotrihydroborate], Cu-P1), reported for the first time. A concentration of 13 wt% Cu-P1 in [EMIM][BH3CN]:Ethanol (50:50 wt%) blend exhibited an average ignition delay time (IDT) of 4.1 ms with 95 wt % H2O2. Considering the required performance of fuel for rocket engine fire testing, 9 wt% Cu-P1 in [EMIM] [BH3CN]:Ethanol (50:50 wt%) was studied further; it is named as 'ILethCu01'. The ILethCu01 fuel exhibits a low viscosity of 3.85 mPa.s and favourable density of 0.90 g/cm(3). Theoretical specific impulse and density specific impulse of fuels were 317 s and 403 s.g/cm(3), respectively with 95 wt% H2O2 under vacuum conditions. Considering the fuel's merits, the performance evaluation of ILethCu01/95 wt% H2O2 was studied in a 50 N liquid rocket engine. Twenty-one hot firing tests were performed, varying total fuel and oxidizer mass from 2.8 to 5.5 g/s and 11.3 to 17.2 g/s, respectively. Steady-state combustion was achieved with a maximum combustion efficiency of 96%. The combustion instability was less than 5%; indicating the propellant burns relatively stably. Moreover, the fuel was stored for more than six months without chemical deterioration, confirmed by using Raman spectroscopy. These findings demonstrate that ILethCu01/95 wt% H2O2 is one of the best alternatives for replacing conventional hydrazine-based hypergolic propulsion systems.