Observations of surface structure and regression behavior of solid propellant strands burning under narrow gap conditions

The combustion of solid propellants in gun applications occurs over a wide range of pressures and other time dependent physical conditions. In highly packed propellant charges with closely spaced propellant grain surfaces, the one-dimensional flame zone structures normally associated with homogeneous and nitramine propellants may be altered. This may affect the overall energy release rate and the burning rate through changes in heat transfer within the combustion wave structure. The objective of this study was to examine the combustion behavior of two closely-spaced propellant surfaces.; Strand burning rates were measured at constant pressures while maintaining two burning propellant surfaces at constant separation distances. A constant gap strand burner system was developed. Optical position sensing was used to control two motor-driven stages. Microthermocouples were employed to evaluate the condensed phase and near surface thermal profiles. Comparisons were made of the flame zone and burning surface features of a plastisol double-base and a nitramine propellant. Quartz probe gas sampling from the dark zone of the double-base propellant was demonstrated. A purged-probe method was developed to prevent probe blockage by combustion particulates.; The gap burning rates of the double-base propellant at surface separations of 1 mm were essentially unchanged from single strand rates at pressures from 0.6 to 2.7 MPa. Under the counterflow conditions of gap burning, the luminous flame zone cannot form above the burning surface; however, conclusions on the influence of the luminous flame on the burning rate require further evaluation of the radiative properties of the burning surfaces.; The burning surface and flame structure of the double-base propellant revealed complex, multi-dimensional structures at the lower pressures. Randomly distributed surface reaction wave formation and propagation within the burning surface was observed. The luminous flame exhibited distributed flamelets, which may be due to condensed phase surface residues and the low propellant burning rate.; Thermal profiles for the double-base propellant revealed test-to-test non-uniformities at the burning surface, and an apparent temperature-dependent thermal diffusivity in the condensed-phase preheat layer. The surface reaction wave structure of the propellant was concluded to be responsible for poor reproducibility in thermocouple tests.