An investigation of balloting of projectiles launched from a two-stage light-gas gun

Projectiles launched at hypervelocities at the ballistic ranges of Arnold Enginering Development Center are subject to extreme loads of over 100,00 g's and occasionally fail. The ultimate material strength, in practice, limits the maximum speed to which the projectile can be launched and still remain intact. It is thought projectiles ballot, or rock back and forth, within the bore and that this could be a source of projectile failure. Balloting is a little understood dynamic, and many potential mechanisms may be responsible for this pitching motion. A more thorough understanding of the dynamics of projectile balloting could lead to improved projectile designs and operational procedures to reduce the possibility of model failure without degrading the overall performance of the gas gun. This proposal addresses the need for an analytical tool to explore the nature of balloting. Under Phase I, a simple conceptual model will be developed to investigate a broad range of potential mechanisms, including barrel misalignment, side wall friction, unbalanced axial pressure loads, and unbalanced-load pressure loads. Mechanisms identified as likely candidates for balloting will then be further analyzed using two- and three-dimensional finite elements to assess the magnitude of the balloting-induced stresses. Finally, two concepts for experiments will be studied: a laser-based angle sensor to measure in situ projectile balloting and a shock tube experiment to measure aerodynamic moments on a fixed body within the tube. Numerical simulations and experimental studies will be conducted in Phase II, and the actual prediction system, including hardware and software, will be developed.