A new approach to railgun analysis proposes local flux creation as the source of the EMF associated with armature motion. As the armature moves, the space behind it is continually filled with new flux, so the induced EMF exists in the immediate vicinity of the armature so that potentially damaging high fields exist along most of the length of the railgun.

There is a need to see how traditional codes arrive at their results, especially in view of the fact that the basic physics of induced EMFs is not established. The design team thus turned to multiphysics software to get some insight into the problem, and they found that the modeling results offer an understanding of previously unexplained phenomena commonly observed in railgun tests.

Figure 2: In-plane Current and Magnetic Streamlines in an extruded version of the armature and rails of a railgun.
With their multiphysics models, the research team made two discoveries. First, they demonstrated that the transmission-line equation applies to railguns. Second, with the model they also showed that local flux creation can have a profound effect on current distribution in and around the armature (Figure 2). Concerning the voltages at the railgun muzzle, the model shows that the localized back EMF produced a localized potential reduction in the vicinity of the armature, which explains the low muzzle voltage in systems with high rail-to-rail fields.

With a better idea of why the high currents are located where they are, engineers can create new designs to reduce hot spots that are a consequence of local flux creation. They will further address the difference between motional EMF and local flux as it bears on muzzle voltage. The 3D model can be expanded to look at the forces generated by the various currents as a complement to existing work. Until now, the team has been running "thought experiments" to check the plausibility of the new concepts; they are now in a better position to use the software to help engineer a better railgun.

This article is based on work done by Paul J. Cote, Mark Johnson, Krystyna Truszkowska, and Pat Vottis at the U.S. Army Research Engineering and Development Command, using software from COMSOL, Inc. For more information, visit http://info.hotims.com/10974-123.