Assessment of combustion characteristics and mechanism of a HAN-based liquid monopropellant

Abstract

The combustion characteristics and reaction mechanism of a HAN-based liquid propellant (HANGLY26, consisting of 60% HAN, 14% glycine and 26% water by weight) were investigated. Combustion tests of liquid strands formed in test tubes were performed in an optically accessible strand burner. Thermocouples were also installed in the strand to measure the temperature distribution of the reaction zone. The burning rate of HANGLY26 exhibited four burning rate regimes for pressures ranging from 1.5 to 14.5 MPa. No luminous flame was observed in any combustion tests. The product temperatures were found to be at the water boiling points at the test pressures below 8.8 MPa. For pressures above 8.8 MPa, the product temperatures were found to be lower than the boiling points of water. Slope break points for burning rate were found to coincide with those of concentration curve versus pressure. The reaction mechanism changes between adjacent burning rate regimes dictate the slope break phenomenon observed in burning rates. Major species found from the recovered liquid residues are nitrogen, nitric oxide, carbon dioxide, and formaldehyde. Two threshold temperatures of 473 and 673 K were found in the pyrolysis study of the liquid propellant. The decomposition mechanism of glycine was also studied. INTRODUCTION Hydroxylammonium nitrate (HAN)-based liquid monopropellants have been considered as potential candidates for the next-generation space propulsion applications because of their high density, low toxicity, and low freezing point''. In these propellants, HAN acts as the oxidizing component and fuel-rich components are added to achieve higher energy release H-N-O-H* NO Figure 1. Chemical structure of HAN. and higher flame temperature. Water is added to adjust the properties, such as viscosity and flame temperature. The chemical structure of HAN is shown in Figure 1. HAN itself can be considered as a monopropellant. The combustion characteristics and the reaction mechanism of HAN have been extensively studies by various researchers. Vosen studied the apparent burning rate of aqueous HAN solution with different concentrations. He found that the apparent burning rate first decreased with increasing pressure, then remained constant once the pressure exceed a threshold value. Reaction kinetics of HAN and HAN-based liquid propellants (LP) have been studied by various researchers. Lee and Thynell and Thynell and Kim studied the thermal decomposition of solid HAN and HAN-water solutions and found that HAN decomposition consists of several stages: 1) proton transfer and subsequent reactions occurring while water evaporates as the sample heats up. As the critical concentration of HAN is reached, the reaction rate is greatly accelerated, forming a pool of highly reactive HONO and HNO species; 2) evolution of gaseous Hs H H / ,O +H H H