HAN-based green propellant, application, and its combustion mechanism

Hydroxylammonium nitrate (HAN)-based solutions have been investigated as candidates for a green monopropellant. However, their high burning rate characteristics have hampered the application. In order to elucidate the mechanism of extremely high burning rates of some HAN-based solutions, the combustion characteristics of HAN-based aqueous solutions are studied. It is found that the role of the two-phase region is very important, and intense boiling of water owing to the superheat mechanism is responsible for the high burning rate. Hydrodynamic instability is taken into account, and the dependence of the instability on pressure is estimated. It is found that the instability is strongly affected by the Markstein number. The HAN-based monopropellant is applied to thrusters of a small-sized experimental mockup of a supersonic aircraft and used in the first free-fall test. Their operation is found to be successful and is described in detail.