Ammonium nitrate-magnesium propellant combustion and heat transfer mechanisms

Microthermocouples and fiber optics were used to investigate the mechanisms of heat transfer and combustion in magnesium-ammonium nitrate (AN) composite propellants. It was found that a liquid layer at 300 ± 30°C covers the surface of both the AN and binder regions during combustion. The magnesium burns near the propellant surface in a dual vapor and surface mode producing both fine smoke and large-scale MgO ash. With no magnesium, conductive heat feedback from the partially premixed AN/binder gas flame drives the combustion of the propellant. The temperature of this flame varies with space and time, ranging between 1000°C and 160()°C. The spatial and temporal variations of the local flame temperature correspond to the local concentration fluctuations of the gas mixture above the propellant, which, in turn, correlate with the macroscopic heterogeneity established by the coarse AN prills and the binder-fine-AN-fill region. Burning rate was found to increase significantly with increasing magnesium loading due to conductive and radiative heat feedback from burning magnesium particles that were retained near the propellant surface by large-scale ash and due to condensedphase heat release from magnesium oxidation.