FROM PRESSURE MEASUREMENTS TO THE GAS TEMPERATURE AND PROJECTILE VELOCITY ESTIMATION

Measurements of interior ballistics parameters are particularly difficult, mostly due to the short time duration between the solid propellant ignition and the ejection of the projectile from the barrel but also due to the violent nature of the event. The evaluation of the parameters that characterise the gas generation inside a weapon’s combustion chamber is challenging the ballistic community due to the intensity and the reduced time in which the event occurs. Estimation of the gas temperature evolution and its velocity inside the barrel poses enough problems considering today’s means of measurement. This work is proposing an indirect measurement technique to determine the temperature and velocity of the combustion gases during the internal ballistic cycle. Because the complete cycle is shorter than a few milliseconds and the temperatures and pressures are very high, no existing temperature sensor is capable to record that fast or withstand the violent nature of the event without being destroyed. The only sensors that are robust enough and able to record one of the internal ballistics parameters are the pressure sensors. Taking into consideration the resulting geometry of the assembly pressure measurement ports – pressure sensor, it can be compared with a Helmholtz resonator. After initial percussion, the total pressure rises inside the chamber making the projectile advance inside the barrel. The fluid velocity will make the air volume inside the 2.5 mm diameter of the tube, that connects the inner bore of the cannon with the pressure sensor, oscillate. The assembly, pressure port and pressure sensor create a Helmholtz resonator cavity. The Helmholtz resonance frequencies are directly proportional to the local speed of sound, which depends on the γ, the specific heats ratios, T the absolute temperature and R the specific gas constant. The velocity of the fluid that passes tangentially to the pressure ports situated in the different locations of the cannon, cannot be higher than the velocity of the projectile at the same location. Being able to measure the temperature and velocity of the gases during the internal ballistic cycle will result in a better understanding of propellant combustion which will lead to better internal ballistic codes. The long-term effect is that more energy will be harnessed from the same amount of burned propellant. The propellant will burn much cleaner which will reduce its environmental footprint.