Railgun

A **railgun** or **railcannon** is an electromagnetic mass driver based on principles similar to those of the homopolar motor. A railgun uses a pair of parallel conductors, or rails, along which a sliding armature is accelerated by the electromagnetic effects of a current that flows down one rail, into the armature and then back along the other rail. They function on a similar principal to coilguns. However, railguns generally possess lower rates of fire and higher rates of overheating.

Railguns are weapon systems that use neither explosives nor propellant, but rather rely on electromagnetic forces to achieve a very high kinetic energy of a projectile. While explosive-powered military guns cannot readily achieve a muzzle velocity of more than about 2 km/s, railguns can readily exceed 3 km/s, and thus far exceed conventionally delivered munitions in range and destructive force. The absence of explosive propellants or warheads to store and handle, as well as the low cost of projectiles compared to conventional weaponry come as additional advantages. For this reason, railguns are commonly found on warships, and before the advent of pulsar weaponry, railguns were extremely commonplace aboard Jordic warships. Railguns also exist as small-arms platforms, however they are often somewhat bulkier than their cousin, the coilgun

In addition to military applications, large railgun installations are sometimes used on planetary facilities to launch payloads into low orbit from the surface. Often times, these kinds of installations are used on planets where extreme atmospheric conditions can cause a hazard to landing spacecraft, or planets where gravity is weak and only a small a small amount of force is required to enter orbit.

A type of railgun, called a launch rail, is used on some spacecraft carriers and stations to launch spacecraft at velocity in a short span, without the hazard of using ion engines within a hangar bay.

## Design

A railgun consists of two parallel metal rails (hence the name) connected to an electrical power supply. When a conductive projectile is inserted between the rails (at the end connected to the power supply), it completes the circuit. Electrons flow from the negative terminal of the power supply up the negative rail, across the projectile, and down the positive rail, back to the power supply.

This current makes the railgun behave as an electromagnet, creating a magnetic field inside the loop formed by the length of the rails up to the position of the armature. In accordance with the right-hand rule, the magnetic field circulates around each conductor. Since the current is in the opposite direction along each rail, the net magnetic field between the rails is directed at right angles to the plane formed by the central axes of the rails and the armature. In combination with the current in the armature, this produces a Lorentz force which accelerates the projectile along the rails, away from the power supply. There are also Lorentz forces acting on the rails and attempting to push them apart, but since the rails are mounted firmly, they cannot move.

By definition, if a current of one ampere flows in a pair of infinitely long parallel conductors that are separated by a distance of one meter, then the magnitude of the force on each meter of those conductors will be exactly 0.2 micro-newtons. Furthermore, in general, the force will be proportional to the square of the magnitude of the current and inversely proportional to the distance between the conductors. It also follows that, for railguns with projectile masses of a few kilograms and barrel lengths of a few meters, very large currents will be required to accelerate projectiles to velocities of the order of 1000 m/s.