Internal ballistics

Internal ballistics (also interior ballistics), a subfield of ballistics, is the study of the propulsion of a projectile. Internal ballistics (also interior ballistics), a subfield of ballistics, is the study of the propulsion of a projectile. In guns, internal ballistics covers the time from the propellant's ignition until the projectile exits the gun barrel. The study of internal ballistics is important to designers and users of firearms of all types, from small-bore rifles and pistols, to high-tech artillery. For rocket-propelled projectiles, internal ballistics covers the period during which a rocket motor is providing thrust. Hatcher breaks the duration of interior ballistics into three parts: There are many processes that are significant. The source of energy is the burning propellant. It generates hot gases that raise the chamber pressure. That pressure pushes on the base of the projectile, and causes the projectile to accelerate. The chamber pressure depends on many factors. The amount of propellant that has burned, the temperature of the gases, and the volume of the chamber. The burn rate of the propellant depends not only on the chemical make up, but also on the shape of the propellant grains. The temperature depends not only on the energy released, but also the heat lost to the sides of the barrel and chamber. The volume of the chamber is continuously changing: as the propellant burns, there is more volume for the gas to occupy. As the projectile travels down the barrel, the volume behind the projectile also increases. There are still other effects. Some energy is lost in deforming the projectile and causing it to spin. There are also frictional losses between the projectile and the barrel. The projectile, as it travels down the barrel, compresses the air in front of it, which adds resistance to its forward motion. Models have been developed for these processes. These processes affect the gun design. The breech and the barrel must resist the high-pressure gases without damage. Although the pressure initially rises to a high value, the pressure starts dropping when the projectile has traveled some distance down the barrel. Consequently, the muzzle end of the barrel does not need to be as strong as the chamber end. There are five general equations used in interior ballistics: Prior to the mid-1800s, before the development of electronics and the necessary mathematics, (see Euler), and material science to fully understand pressure vessel design, internal ballistics did not have a lot of detailed objective information. Barrels and actions would simply be built strong enough to survive a known overload (Proof test), and muzzle velocity change could be surmised from the distance the projectile traveled. In the 1800s test barrels began to be instrumented. Holes were drilled in the barrel, 'crusher gauges' using copper pellets were attached, the gun was fired, and the pressure was measured indirectly by how much the copper pellet was deformed. But the measurement only indicated the maximum pressure that was reached at that point in the barrel. By the 1960s, piezoelectric strain gauges were used. They allow instantaneous pressures to be measured and did not need a pressure port drilled into the barrel. More recently, using advanced telemetry and acceleration-hardened sensors, instrumented projectiles were developed by the Army Research Laboratory that could measure the pressure at the base of the projectile and its acceleration.