Combustion Mechanisms of HAN-Based Green Monopropellants

Monopropellants based on hydroxylammonium nitrate (HAN) are a low-toxic, high performance alternative to hydrazine in space propulsion systems. However, their combustion and the decomposition of energetic ionic liquids used in their formulations, such as HAN and2-hydroxyethylhydrazinium nitrate (HEHN), are not well understood. In addition, gelling such propellants may provide long-term stability of compositions with energetic and catalytic particles, but the effect of gelling on the combustion characteristics and mechanisms of HAN based propellants is unclear. The present work included thermoanalytical studies on HAN and HEHN decomposition as well as 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. It has been shown 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. The thermal decomposition temperature of HAN 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. 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.