Railguns, which propel a projectile using electromagnetic forces instead of chemical explosions, promise to revolutionize projectile launchers. Such guns have been built and operated successfully on a test basis, but several problems are holding them back from usage in the field. To solve these problems, researchers must understand the inner workings of these weapons. Several groups are conducting research taking different approaches to how the electromagnetic fields within these guns operate. One group, whose proposals differ from traditional thought, is using COMSOL Multiphysics software to illustrate the validity of their views.

Conventional guns have reached their inherent limitations. The limits of gas expansion prohibit launching an unassisted projectile to velocities exceeding 1.5 km/sec with ranges of more than 50 miles. In contrast, railguns are expected to achieve double the muzzle velocities and greater firing ranges with less drift.

Figure 1: Geometry of a Railgun. The armature (projectile) exits towards the right.

To understand the problems associated with developing field-ready railguns, one must first understand some of the basic principles. A power supply creates a voltage across two parallel conductive rails, and a conductive projectile (here called the armature) touches each rail to complete the circuit path (Figure 1). A voltage pulse creates a very high current, and the resulting magnetic field accelerates the projectile along the rails and then out the muzzle. Typical peak currents in large systems can exceed 1,000 kA.

This high current, however, creates problems, especially along the rails. In particular, the rails are prone to considerable erosion due to the high heat generated by the current and also the propulsion of the armature. Another source of rail damage is the transition of the armature conductive interface from a molten layer to a high-temperature plasma-brush interface. Railguns today require that the rails be replaced frequently, which limits their effective use as standard weapons.

It is clear that considerable research is required to find the best materials and design for effective railguns. Yet, here comes the key area of dispute: how, exactly, are the electromagnetic fields generated, and what is their distribution? What is the exact effect of the back EMFs on the railgun? Where are the locations of maximum current density and heating? Why do the erosion patterns look the way they do? The answers to these questions are critical to designing a reusable railgun, and multiphysics modeling software is helping to find the answers.

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, Click Here .