Combustion of aqueous HAN/methanol propellants at high pressures

Abstract

“Green” monopropellants based on hydroxylammonium nitrate (HAN) are of interest for replacing highly toxic hydrazine in space propulsion systems and for other applications. However, their combustion mechanisms at high pressures are not well understood. In the present paper, decomposition of HAN at pressures up to 15 MPa was studied with differential scanning calorimetry, while combustion of an aqueous HAN/methanol solution (70.1 wt% HAN and 14.9 wt% methanol) was studied in a strand burner at pressures up to 30 MPa. The experiments in both setups were conducted in a nitrogen environment. It has been shown that the decomposition temperature of HAN significantly decreases with increasing pressure from atmospheric to 2 MPa and remains virtually constant with further increasing pressure to 15 MPa. The decrease with pressure is explained by suppressing evaporation of HNO3 and H2O formed during the decomposition process. The strand burner experiments have revealed an increase in the linear burning rate by over 50% at 12−14 MPa, explained by reaching the critical pressure of the liquid. Other observed pressure dependencies of the burning rate include a decrease at 9–12 MPa, a plateau at 14–19 MPa, a decrease at 20 MPa, and an increase at 22–30 MPa.