| Plasma armature transaugmented and muzzle-fed railgun experiments were performed in the UTSI 2.4 m, 1 cm bore diameter railgun. These experiments were evaluated with a performance model that included the simple electromagnetic force equation for the particular railgun, viscous and ablation armature drag, and the compression of bore gas ahead of the projectile. This evaluation revealed that at least one of the force or drag models was incorrect. The simple electromagnetic force equations for a "real railgun", one with a finite rail thickness, was derived using the conservation of power at the breech. Strain energy due to the transverse component of current density in the rails, and losses from eddy current generation from field diffusion and armature motion were neglected in this derivation.;Three-dimensional (3-D) electromagnetic simulations were performed on a generic square bore railgun configuration with the codes MEGA and MAP3 to determine the seriousness of the losses not included in the derivation. These simulations showed that only a fraction of the force predicted by the simple electromagnetic force model was exerted on the armature. For example, a force of 510 N was predicted for the muzzle-fed railgun at steady-state while a calculated armature force of ;A modification to the simple electromagnetic force equation is proposed,;Evaluation of the transaugmented and muzzle-fed railgun experiments using the modified electromagnetic force equation in the performance model showed that one of the fluid mechanical drag models was still in error. |
