Combustion mechanisms of nitramine composite propellants

The combustion mechanisms of nitramine composite propellants were investigated to determine the parameters that control the burning rate in combustion waves. The nitramines studied were RDX (cyclotrimethylene trinitramine) and HMX (cyclotetramethylene tetranitramine), which have similar chemical compositions, but different reaction schemes. Since RDX and HMX composite propellants consist of crystalline nitramines and a polymeric binder, the physical structure of these propellants is heterogeneous. From experiments, it was found that the combustion waves of these propellants are homogeneous. In addition, the combustion waves exhibited a two-stage flame structure. The luminous flame produced above the burning surface approached the burning surface when the pressure was increased. However, the burning rates of both RDX and HMX propellants were not affected by the luminous flame reaction. In the preparation zone separating the luminous flame and the burning surface, the gas phase reaction primarily consist of the exothermic reduction reaction of NO to N 2 which is similar to the reaction in the dark zone of double-base propellants. From quenched burning tests, it was found that the nitramines and the binder melted at the burning surface and formed an energetic mixture. Thus, the decomposed gases ejected from the burning surface consisted of a homogeneous mixture of oxidizer and fuel species. The temperature profile measurements in the combustion waves revealed that there is a large heat release at the burning surface. This exothermic decomposition reaction at the surface plays a major role in controlling the burning rate of RDX and HMX composite propellants. The higher burning rate of RDX propellants, as compared to HMX propellants, is due to the larger heat release at their burning surface.