During machine gun or machine cannon firing the gun barrel can warm up to very high temperatures. Depending on the firing cycle, especially for small calibres, the barrel can be fired red-hot. In this case, the machine gun firing must be immediately stopped for cooling. A theoretical calculation to predict the heating of the gun barrel would be desirable to design the weapon according to its future mission demands. For this purpose an interior ballistics model was developed using Prandtl's boundary layer equations to solve the unsteady, compressible and turbulent boundary layer development inside of a gun barrel from the breech to the projectile. The solution for the wall shear stress uses the Reynolds analogy to determine the heat flux into the barrel wall produced by the temperature gradient, which is directed from the hot propellant gases to the colder tube wall. To get the barrel's temperature distribution during firing, the heat conduction equation must be solved inside of the gun tube wall. The rapid fire calculation must take into account the unsteadiness of two completely different flow conditions which appear during the repetitively firing: a) one is present during the projectile acceleration inside of the gun tube, and b) the other develops after the projectile leaves the muzzle up to the next firing. A numerically based time step procedure models the rapid fire behaviour. The numerical results are compared with bore temperature measurements in a 20-mm-caliber machine cannon at several measuring ports placed along the cannon's barrel.
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