Flame Structure of Ammonium-Dinitramide-Based Liquid Propellant in a Small Thruster

Abstract

In this paper, the flame structure of a liquid propellant ammonium dinitramide–methanol aqueous solution in a small model thruster is investigated. The thruster is composed of an injector and a combustion chamber (including a catalyst bed) with optical windows, through which the combustion process in the atmosphere is inspected, and the temperature distribution along the flame is measured by the tunable diode laser absorption spectroscopy method. Obvious fluctuations in temperature and flickering yellowish flame downstream of the combustion chamber are observed, indicating that the combustion is not stable. The mean temperature distribution has a significant peak where a yellowish flame appears. To explain the experimental observations, a detailed gas-phase ammonium dinitramide–methanol reaction model is used to establish the flame structure of an ideal liquid propellant, which can be divided into several different regions, including the liquid phase, aerosol zone, and two flame zones. The decomposition of ammonium dinitramide and the evaporation of methanol take place in the liquid phase and aerosol zone, whereas the gas phase reactions of methanol and the decomposition products of ammonium dinitramide take place in the two flame zones. Based on the proposed theory of flame structure, it is seen that, in our experiment, the physical and chemical processes in the liquid phase and aerosol zone are completed upstream of the catalyst bed, and the unstable yellowish flame appearing in the combustion chamber is caused by competitive influence between the two flame zones.