Experimental investigations on the direct current ignition characteristics of hydroxylammonium nitrate-based ionic liquid propellant

The applications of non-toxic hydroxylamine nitrate (HAN) and imidazole-based ionic liquid propellants exhibit excellent merits in both high-thrust chemical mode and high-specific impulse electric propulsion mode. This dual-mode propulsion is promising to improve the mission flexibility. In this paper, we investigate the direct current (DC) electrical ignition characteristics of the dual-mode propellant (50 wt% HAN, 40 wt% [Emim][EtSO₄], and 10 wt% H₂O) in an optically-accessible quartz reactor under various voltages and electrode spacings. Results indicate that both increasing the voltage and minimizing the electrode spacing can reduce the ignition delay time, yielding an optimized delay of ∼2.15 s. The voltage threshold for successful ignition varies from 100 V to 140 V with a change of electrode spacing from 4 mm to 8 mm. Moreover, the bubble evolution captured by a high-speed camera before the ignition reveals that the multi-component liquid undergoes different stages during the decomposition process. It is noted that the growth rate of bubble formation can be regulated, and the mild process is crucial for achieving successful ignition. Meanwhile, simultaneous thermogravimetry (TG), differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) analyses of various mixtures suggest that the copper ions act as catalysts to accelerate the HAN decomposition. The surface analysis also exhibit good erosion resistance of copper electrodes. These findings demonstrate the superiority of copper electrodes during electrolytic decomposition. The possible ignition mechanism of the ionic liquid propellant via electrolytic decomposition is also proposed. Electrochemical/chemical decompositions of water and HAN are firstly initiated to produce light oxidizing gases, which subsequently react with imidazole-based substances to activate the redox reaction for heat accumulation and further self-ignition.