Effect of Initial Temperature on the Interior Ballistics of a 120-mm Mortar System

In this study, the effect of the initial temperature of a 120-mm mortar system on its interior ballistics was investigated using four different experiments: temperature-conditioned closed bomb firings for determining the temperature sensitivity 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 initial temperatures of –12 °C and greater, the mortar system and its subcomponents exhibited regular initial-temperature-dependent behavior, with increasing initial temperature causing monotonically increasing propellant burning rates and monotonically decreasing ignition delays, which produce monotonically increasing system pressures and pressure differentials in the flashtube, ignition cartridge, and IMS. However, some anomalous behavior was discovered for temperatures around –46 °C. At this initial temperature, the closed bomb firings indicated that brittle fracture of the M48 propellant granules used in the ignition cartridge occurs. This phenomenon explains the occurrence in the IMS firings of dramatically increased variation in pressure-time behavior and projectile muzzle velocity for charge 4 firings as compared to higher temperatures, as well as the occurrence of maximum tube pressures for –46 °C firings, being greater than those for 21 °C firings for charge 0. However, one of the –47 °C closed bomb firings does not exhibit evidence of grain fracture and yet produces a higher propellant burning rate than the –12 °C firings, suggesting that a fundamental change in reaction kinetics or flame structure is occurring at extremely low temperatures. This supposition is bolstered by evidence of a liquid layer existing on the surface of M48 propellant granules ejected from the ignition cartridge during the –46 °C firings—a phenomenon that does not occur at the higher initial temperatures and is not theorized to occur in double-base solid propellant combustion. Based on the flash tube experiments alone, the flash tube was determined to have a weak effect on the initial-temperature-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 significantly affect the performance of the mortar at low temperatures.

Understanding Interior Ballistic Processes in a Flash Tube

Vented flash tubes have often been used in the ignition train of medium and large caliber weapon systems. Despite their long history of ballistic usage, there are undesirable features associated with uneven venting of the combustion products. Pressure measurements at various locations from the flash tube have shown severe variations with time, which is associated with spatially nonuniform mass discharging rate from the vent holes. Measured pressure profiles in the flash tube show counterintuitive, nonmonotonic pressure distributions with the lowest pressure in the middle of the venting section of the flash tube. A model of the flash tube venting process was developed to explain these phenomena using modern, high-order numerical schemes. Source terms accounting for mass addition from the black powder pellets, mass loss through the vent holes, wall friction, differential area changes, and volume changes from surface regression of black powder pellets were fully coupled in the model. The numerical results of this model reproduced the severe pressure variations and nonmonotonic pressure profiles observed in experiments. In general, they are caused by gas dynamic effects from a slowly moving normal shock wave in the middle portion of the venting section of the flash tube. As the driving pressure from the burning black powder pellets changes, the location of the normal shock wave jumps from one vent hole set to another, producing pressure variations observed in experiments. The physical understanding gained from this model solution has provided guidance for achieving more uniform mass discharging rate by varying the vent hole sizes as a function of distance along the flash tube.