Ballistics of Supercavitating Projectiles

Abstract

An ordinary gun projectile will, after entry into water, be subject to an extremely high drag and will stop after 50 – 100 calibers of travel through the medium. To avoid such a response, supercavitation must take place. By a specially shaped projectile nose, the flow of water around the projectile may escape contact with the surface of the projectile, except at the nose tip. Under these conditions, the projectile is said to be supercavitating. In such a state, the projectile is unstable, but, the stability is maintained by the oscillation caused by the interaction between the rear part of the projectile and the cavity wall. In order to keep the amplitude of this oscillation as small as possible, the size of the cavity should be just slightly larger than the projectile. The cavity size is, however, determined by the size and shape of the nose tip, by the velocity of the projectile, and by the pressure in the ambient water (i.e. the water depth), through Garabedian’s equations. These parameters will also determine the duration of the cavity which determines how far the projectile will travel before the supercavitation ceases, and the projectile stops. The paper outlines the theory for the size, shape, and duration of the cavity. It discusses also the conditions for designing an optimal design of a supercavitating projectile. Special emphasize is put on how to determine the drag on the projectile by observing the size of the bubble.