Trajectory modification of a transonic spherical projectile under Hop-up mechanism

Improvement of shooting accuracy of air gun pellets is important in sport competitions which is questioned by shooting enthusiasts. Hence, the performance of a transonic spherical projectile as an air gun pellet with 4.5 mm-caliber under a mechanism known as Hop-up is numerically examined in the present study. Hop-up mechanism is resulted in a rotational motion of spherical projectile, so a Magnus Force is generated which prevents the altitude loss of the projectile caused by its weight. The motion of the projectile is assumed in four degrees of freedom including three translational motions and one transverse rotational motion. The projectile confronts the continuous variations of velocity due to the influence of the aerodynamic forces so experiences an unsteady flow. In order to numerical analysis of the problem, the 3-D compressible turbulent Navier-Stokes equations based on “Roe” scheme and dynamic equations of the projectile motion are solved in a coupled form as a fluid-structure interaction and in a moving computational grid. The results obtained from these studies show that the proper rotation of the projectile for a certain distance, can neutralize the altitude loss. It is also demonstrated that the momentum of the projectile is decreased by increasing its angular velocity.