Shallow Angle Water Entry of Ballistic Projectiles

The water-entry of ballistic projectiles is investigated using high-speed digital imaging to capture the subsurface cavity dynamics. Specially designed 0.22 caliber projectiles are fired into water at shallow angles to the free surface (5◦ to 15◦) at Mach numbers between 0.3 and 1.0. Redesigned projectile tip geometries allowed projectiles to successfully enter the water and travel large distances underwater, due to the subsurface vapor-cavity that forms after impact, dramatically decreasing drag on the projectile. Projectile dynamics, critical entry angle and cavity formation are discussed for various bullet geometries, and results show that successful water-entry is a function of tip shape and length-to-diameter ratio. The data conclusively show that bullets with lower length-to-diameter ratios tumble inside the vapor cavity, while higher length-to-diameter ratios can lean against the cavity walls inducing a planing force pushing them back inside the cavity and mitigating the tumbling behavior. Experimental cavity observations of vapor-cavity formation is compared to a modified version of Logvinovich’s [1] theoretical model, which includes an updated formulation of the model and an angle of attack correction. Despite the unsteady nature of this problem, this improved steady state model fits well with experimental data and serves as an accurate design tool for naval engineers. ∗Address all correspondence to this author. INTRODUCTION The designs for bullet geometry, velocities, and spin rates vary considerably for projectiles fired in the air versus the water. Typically projectiles launched from air into water are fired at high angles ( 90◦) to the free surface [2]. At shallow angles (5−15◦) standard ballistic projectiles do not enter the water, instead they ricochet off the surface or break into many pieces. Projectiles designed to enter the water at shallow angles are designed with blunt tips and large length to diameter ratios, which create a vaporous supercavity that originates at the tip of the bullet upon impact with the free surface. The vapor cavity greatly reduces the drag of the projectile by diminishing its frontal area and viscous interaction. These projectiles ride inside of the cavity using the sidewall as a stabilization mechanism [3]. Several experimental studies have looked at vertical airwater impact of high-speed projectiles [4, 5, 6]. One of the most complete published study to date was performed by [3], in which a fully developed underwater cavity is formed by firing projectiles underwater, avoiding the free surface interaction and thus creating optimal conditions for determining the mechanisms of underwater stability as well as nearly steady state vapor-cavity size estimates. Several full-scale, shallow-angle, air-to-water studies have been performed by different researcher groups but none have been published to date. Several theoretical cavity models have been developed as well. [1] and [7] have developed analytical models for cavity formation and cavity oscillations based partially on empirical data and mostly on control volume analysis; while others such as [8] have focused more on the projectile stability. [9] wrote an entire volume of work on the 1 Copyright c © 2009 by ASME