Doctoral Dissertations

Date of Award

5-1995

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Engineering Science

Major Professor

Dennis R. Keefer

Committee Members

Roger Crawford, Monty Smith, Remi Engels

Abstract

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, were 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-170 N was found from the 3-D simulation with a stationary armature. This showed that the simple electromagnetic force model could not be used to predict railgun performance for the muzzle-fed railgun

A modification to the simple electromagnetic force equation is proposed,F =± ½ α L’I2;1; +β M; I1; I2; ,where the positive sign gives the force for the transaugmented railgun and the negative sign gives the force for the muzzle-fed railgun. The parameters α and β represent the fraction of force exerted on the armature from the inner and outer rails respectively. 3-D simulations are required to find values for α and β.

Evaluation of the transaugmented and muzzle-fed railgun experiments using the modified electromagnetic force equation in the performance model showed that one of the drag models was still in error. Further evaluation of the experiments, and published results of 3-D MHD simulations suggested two possibilities that could explain the further deficit in performance. The value of Cf; (the viscous drag coefficient found from turbulent pipe flow) could be in error by an order of magnitude due to the increased velocity gradient at the rail surface produced by the J x B forces in the plasma armature. Another possibility is that material trapped in the "cold" region between the armature and projectile (which was not included in the performance model) reached a mass an order of magnitude greater than the mass of the armature itself.

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