Catalytic Decomposition and Burning of a Dual-Mode Ionic Liquid Propellant

The ionic liquid dual-mode propellant, mainly composed of hydroxylammonium nitrate (HAN), 1-ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO4]), and H2O, has shown good capabilities for both chemical and electrospray space propulsion. This work focuses on the chemical performance of the propellant by investigating its decomposition and burning processes. First, the thermogravimetric-Fourier transform infrared spectroscopy measurement is employed to identify different stages of the propellant decomposition, including water vaporization and decompositions of HAN and [Emim][EtSO4]. The effects of the Ir/Al2O3 catalyst, H2O mass fraction, and heating rates on the decomposition process are analyzed, and the gaseous products are measured. It is found that the decomposition of HAN and [Emim][EtSO4] is coupled with the oxidation of small intermediates. The kinetic parameters of overall catalytic decomposition/oxidation reactions at different stages are determined by fitting the thermogravimetric (TG) curves. Then, the burning of the propellant is organized in a well-designed optical-accessible catalytic bed with a controllable preheating device and a fixed flow rate of 2 mL/min. The ignition delay and burning evolution are determined from the time-resolved temperature sampling at four different axial positions. When the spatially averaged preheating temperature is 130-180 K, the ignition delay varies from 0 to 60 s, mainly controlled by the prior HAN decomposition rate. Finally, a one-dimensional model incorporating kinetic parameters from TG fitting is established to characterize the ignition process of the propellant, which quantitatively elucidates the effect of preheating on the ignition delay.