STUDIES ON THE COMBUSTION OF HAN/METHANOL/WATER PROPELLANTS AND DECOMPOSITION OF HAN AND HEHN

Green monopropellants based on hydroxylammonium nitrate (HAN) are of interest for replacing hydrazine in space propulsion systems. However, their combustion and the decomposition of energetic ionic liquids used in their formulations, such as HAN and 2-hydroxyethylhydrazinium nitrate (HEHN), are not well understood. Here we summarize the results of our thermoanalytical studies on HAN and HEHN decomposition as well as of strand burner experiments with aqueous HAN/methanol propellants. The thermoanalytical studies involved thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), mass spectroscopy, and Fourier transform infrared spectroscopy. For thermal decomposition of HAN, the apparent activation energy was 62.2 ± 3.7 kJ/mol based on TGA and 57.5 ± 3.5 kJ/mol based on DSC data. The use of an iridium/rhodium catalyst decreased the temperature of full decomposition by approximately 60° C. The thermal decomposition temperature decreased by about 50° C with increasing pressure to 2 MPa, but remained virtually constant with further increasing pressure, apparently because of suppressed evaporation of nitric acid. It has been shown for the first time that the decomposition of HEHN has two stages. The apparent activation energies are 113.7 ± 1.7 kJ/mol at the first stage and 123.6 ± 2.5 kJ/mol at the second stage. In the strand burner experiments with HAN/methanol/water propellants, different pressure dependencies of the linear burning rate were determined over the pressure range of 9-30 MPa. An increase in the burning rate by over 50% was observed at 12-14 MPa and explained by supercritical fluid effects. Gelling the propellant suppressed these variations, leading to a steady increase in burning rate over the range of 4-30 MPa.