In-bore stiffness considerations for electromagnetic projectile armatures and boreriders

Experience with conventional tank cannons has shown that interaction between a projectile and gun bore determines the projectile muzzle exit conditions which greatly influences the subsequent out-of-bore flight and terminal accuracy. Significant effort has been devoted to examining this phenomenon and has led to the development of models capable of predicting the in-bore dynamic characteristics of projectile/tube interactions. These models have shown that the relative stiffness of the projectile boreriding surfaces can be a deterministic factor of the resulting in-bore motion. The armature of an EM projectile must serve a dual role, namely, provide a current path across the rails while also providing lateral support during the launch. This presents a conflict for the EM projectile designer who must ensure an armature stiff enough to maintain rail contact, yet have sufficient "spring" to absorb lateral disturbances imparted by the gun barrel. This paper reports the results of an in-bore dynamic analysis which examined the effects of varying armature and borerider stiffness. Analysis runs were conducted with a SLEKE II projectile model launched from the Task B railgun. The analysis found that maintaining equivalent contact stiffnesses for the boreriders of a saddleback sabot, such as the SLEKE II, minimizes the difference in transverse acceleration between the two contacts, thus reducing projectile in-bore balloting.