Railguns

Rather than duplicate work already done by others, here is some information on railguns from the Center for Electromechanics at The University of Texas at Austin. The CEM site doesn't go into the basics of how railguns work, but it does show some pictures of railguns and armatures and discusses some of the capabilities of railguns.

The Basics

Railguns are very simple devices with only one moving part, the projectile, also called an armature. Railguns consist of the following parts:

The Rails

These give the gun its name. They are generally made out of thick copper bars, but can be any conductive material. These rails run parallel to each other with a gap between them for the armature. They must be capable of conducting very high currents (MAmp and higher) and must be braced as the electromagnetic forces on them are tremendous.

The Armature

So-called because it is the moving portion of an electric device, the armature is the projectile. It closes the loop between the rails and is forced along the barrel (the space between the rails) by tremendous electromagnetic forces. These can either be solid pieces of conductive metal or they can be a conductive sabot with some kind of dart or sub-caliber penetrator.

The Power Supply

Railguns require tremendous currents to generate the necessary magnetic fields. The voltage required will vary with the resistance of the railgun, which is usually fairly low. The power supply must be able to supply a burst of very high current. Typically, the current will be in the millions of Amperes, while the duration will be only tens of milliseconds. Ideally, the current will ramp up very quickly and stay at its maximum level until just before the armature leaves the barrel. At that point, the current should ramp down to zero just as the armature exits the barrel. If there is still current when the circuit is opened, there will be some arcing from the barrel to the armature and across the barrel. If the current ramps down early, then the armature will be slowed by barrel friction without the benefit of force for the last portion of it's in-barrel flight.

Following is a rather amatuerish diagram of a railgun:

• The red box is a power supply (a battery, homopolar generator, capacitor, etc...). The power supply must be able to generate very high curent pulses at low voltage.
• The yellow lines represent buswork. In a high-power railgun, these will be rigid bars well secured against the tremendous electromagnetic forces. The buswork carries the current to the rails.
• The gold bars are the two rails. They will generally be encased in a barrel which must be strong enough to hold them together. The electromagnetic forces from the currents will tend to force them apart.
• The current (white arrows) flows around the circuit in a counter-clockwise direction. Any current which flows in a wire will generate a magnetic field which wraps around the wire. The direction of the magnetic field lines is in the direction your fingers would curl if the thumb of your right hand were pointing in the direction of the current (the "right hand rule"). In order to get magnetic fields strong enough to fire a projectile at useful velocities, very high currents are required.
• The blue circles represent a strong magnetic field between the bars. In this case, the field lines will be coming out of the screen between the rails The fields from the two rails add between the rails, creating the intense magnetic field required. Outside the rails, the magnetic fields from the two currents will tend to cancel out.
• The silver bar represents the armature and is free to slide back and forth between the rails. The current crosses from one rail to the other through the armature. This closes the circuit and gives the magnetic field something to act on.
• The red arrow represents the electromagnetic force acting on the armature. Any current which flows in an external magnetic field will be acted on by an elctromagnetic force. This force will be perpendicular to the current and the magnetic field and proportional to both. This force propels the armature along the barrel. Note that this same force will be acting to push the bars apart, which is why the barrel must be very strong.

Discarding Sabot

Recently, the topic if discarding sabot rounds in railguns has been debated on the GURPSnet mailing list. While it may seem like a metal conductor would make an ideal armature and projectile, this is not necessarily the case. In order to get lower resistances and lower forces and higher heat dissapation, the whole gun and armature should be as big as possible. A large conducting area (larger projectile and barrel) will get you more current and less resistive heating as well as being stronger to take the electromagnetic forces. Conversely, in order to penetrate armor, a smaller projectile allows you to concentrate the force of impact in a smaller area, doing more damage. In addition, materials which make good penetrators are not necessarily good conductors. Therefore, an armor-piercing penetrator for a railgun would most likely consiste of a copper or aluminum sabot surrounding a hardened steel or tungsten carbide penetrator. Having an oversized barrel would also allow you to fire larger diameter explosive or other projectiles.