Numerical simulation of the transient process of monopropellant rocket engines

Abstract

Liquid monopropellant rocket engines are widely used in space propulsion and are developing in the direction of non-toxic and pollution-free propellants. The use of non-toxic monopropellants with a low decomposition rate makes the start-up and shutdown processes of the engines longer, and the gas inlet assumptions that were applicable to the calculation of hydrazine monopropellant engines cannot be used. Based on the steady-state calculation model of hydroxyl ammonium nitrate (HAN)-based monopropellant engine, we propose a combined model that considers decomposition of liquid monopropellant and non-equilibrium heat transfer between fluid and solid catalyst bed, and uses this model and the gas inlet hypothesis to calculate the working process of an HAN-based monopropellant rocket engine of 60 N thrust. The calculated results are compared with the test curve, and the results show that the decomposition of liquid monopropellant plays a key role in the starting and stopping processes. When the gas inlet assumption is adopted without considering the decomposition of the liquid monopropellant, the calculated start-up time is much shorter than the hot-firing test, and there is no tail-off section when shutting down. The pressure-rise and pressure-decay curve calculated by the combined model agree well with the test curve, and the maximum deviation during the steady state is less than 10%. For HAN-based monopropellant engines, the pressure rises slowly at the first start because the temperature of catalyst bed is lower, which results in a lower rate of decomposition. When restarted shortly after a shutdown, the pressure rises rapidly due to the high temperature of the catalyst bed and the high rate of decomposition. The HAN-based monopropellant can penetrate a few millimeters downstream of the injector, and there is a significant tail when the engine is shut down, which is caused by the continued decomposition of the remaining propellant after the shutdown.