A HiLo interior ballistic model based on the hypothesis of gases adiabatic transformation in the high pressure chamber

Weapons built on high-low pressure (HiLo) principle involve the existence of two chambers between which a gas flow occurs. The usual approach in the mathematical models developed for such systems is to consider that the gas flow is accompanied by an isothermal transformation of gas in the high pressure chamber. The present paper deals with the development of a lumped interior ballistic model built on more realistic assumptions like an adiabatic transformation of the gases in the high pressure chamber, an isentropic flow of real gases through the nozzles, a variable burning surface of the propellant grains and the occurrence of heat losses at the level of vent holes lateral surfaces. The experimental results obtained with a modified closed vessel were used in order to validate the proposed model. The pressure time variations in both chambers were accurately predicted by the proposed lumped model. The lumped model application on a 40 mm grenade launcher gives results consistent to the experimental measurements regarding the grenade muzzle velocity.