Calculation of Fluid Dynamic Loads on a Projectile During Firing : Development of a CFD-modelling Approach

Abstract: The transition from inner to outer ballistics is a crucial part of the launch of a projectile from a recoilless rifle. Since a launch of the rifle is a rapid process and due to the extreme conditions in terms of accelerations and temperature, physical measurements are hard to achieve. To gain knowledge about the fluid dynamic loads that act on the projectile during a launch CFD can be a useful technique. In this work a CFD model of the launch process has been developed. Different methods to implement the most important parts of the launch process have been evaluated and compared. An unsteady RANS-model have been utilised in combination with a dynamic mesh to handle the motion of the projectile. In this work, a spin-stable type of projectile has been analysed. To force the projectile to spin, helical grooves are used inside the launch tube. If the projectile does not fill out and seal the grooves completely, propellant gas can leak through these grooves. In the model it has been evaluated if the leak flow has an impact on the flow field around the projectile and its stability. To simplify the model the grooves were approximated as a gap with constant thickness between the tube and the projectile. Two different methods to implement the propellant burning have been tested. In the first case a pressure curve known from measurements are implemented. In the second, the mass flow from the combustion is modelled. This work shows that it is possible to predict the behaviour of the flow during a launch with a CFD model. The leak flow was found to have a significant impact on the flow field in front of the projectile. However, it has also been found that the leakage only have a limited effect on the fluid dynamic forces that works on the projectile during the transition phase. From this work it has been concluded that CFD can be a useful complement to physical tests and it gives a deeper understanding about the flow when the projectile leaves the launch tube. It has also been concluded that the launch process is an extensive topic and contains many different disciplines; therefore more work is needed to refine the model.