COMBUSTION CHARACTERISTICS OF HYDROXYLAMMONIUM NITRATE - WATER GELS

Abstract

Linear burn rates of 80 wt.% hydroxylammonium nitrate (HAN) gels were characterized between 0.1 and 3.1 MPa. The burn rate of a HAN aqueous solution is relatively slow when the boiling temperature of the water content is lower than the decomposition temperature of HAN at lower pressures. However, the burn rate abruptly surges when the pressure exceeds a threshold value, at which the boiling temperature is higher than the decomposition temperature. As a result, additional fuels and stabilizers had to be added in HAN-based liquid monopropellants to improve the burning characteristics. Fumed silica adsorbs water in a HAN aqueous solution through the formation of hydrogen bonds between the silica particles and water molecules and is effective on stabilizing the burn rate in the high pressure range. Hydrophilic fumed silica (CAB-O-SIL M5) was added to 80 wt.% HAN aqueous solutions to produce the HAN gel. Burn rate behavior of the gelled HAN solution was experimentally studied via strand burner tests at pressures between 0.1-3.1 MPa, which is the operating pressure range relevant to space thrusters, using a pressure chamber with observation windows. It was found that gelation of HAN aqueous solutions using fumed silica effectively eliminated the burn rate jump between 1.1-1.6 MPa that typically occurs in HAN aqueous solutions.