Effect of initial temperature on the interior ballistics of a 120-mm mortar system

The effect of the initial temperature (T_i) on the interior ballistics of a 120-mm mortar system was investigated using four different experiments: temperature-conditioned closed bomb firings for determining the temperature sensitivity (σp) of the ignition cartridge's M48 double-base propellant and instrumented firings of temperature-conditioned flash tubes, ignition cartridges, and an instrumented mortar simulator (IMS). The results of these experiments reveal that for Ti ≥ 12°C, the mortar system and its sub-components exhibited regular T_i-dependent behavior, with increasing T_i causing monotonically-increasing propellant burning rates, which produce monotonically-increasing system pressures and pressure differentials in the flashtube, ignition cartridge, and IMS. However, some anomalous behavior was discovered for T_i around 46°C. Brittle fracture of the M48 propellant granules occurs at extremely low T_i. This phenomenon explains the dramatically increased variation in P-t behavior and projectile muzzle velocity at 46°C for Charge 4 firings as compared to higher T_i. Test data suggest a fundamental change in reaction kinetics occurring at extremely low T_i. This supposition is bolstered by evidence of a liquid layer existing on the surface of M48 propellant granules ejected from the ignition cartridge during T_i = 46°C firings - a phenomenon that does not occur at the higher T_i. Based on the flash tube experiments alone, the flash tube was determined to have a weak effect on the T_i-dependent behavior of the mortar system; however, IMS testing with two different flash tube configurations revealed significant differences in longitudinal pressure wave amplitude and projectile muzzle velocity in Charge 4 firings between the two configurations at -46°C, suggesting that the uniformity of combustion product discharge from the flash tube could improve the performance of the mortar at low temperatures.