Railgun

A railgun is an electrically powered electromagnetic projectile launcher based on similar principles to the homopolar motor. A railgun comprises a pair of parallel conducting 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.

The armature may be an integral part of the projectile, but it may also be configured to accelerate a separate, electrically isolated or non-conducting projectile. Solid, metallic sliding conductors are often the preferred form of railgun armature but "plasma" or "hybrid" armatures can also be used. A plasma armature is formed by an arc of ionised gas that is used to push a solid, non-conducting payload in a similar manner to the propellant gas pressure in a conventional gun. A hybrid armature uses a pair of "plasma" contacts to interface a metallic armature to the gun rails. Solid armatures may also "transition" into hybrid armatures, typically after a particular velocity threshold is exceeded.

In its simplest (and most commonly used) form, the railgun differs from a traditional homopolar motor in that no use is made of additional field coils (or permanent magnets). This configuration is thus a self-excited linear homopolar motor formed by a single loop of current.

A relatively common variant of this configuration is the augmented railgun in which the driving current is channelled through additional pairs of parallel conductors, arranged to increase ("augment") the magnetic field experienced by the moving armature. In electric motor terminology, augmented railguns are usually series-wound configurations.

A railgun requires a pulsed, direct current power supply. For potential military applications, railguns are usually of interest because they can achieve much greater muzzle velocities than guns powered by conventional chemical propellants. Increased muzzle velocities can convey the benefits of increased firing ranges while, in terms of target effects, increased terminal velocities can allow the use of kinetic energy rounds as replacements for explosive shells.

Thus typical military railgun designs aim for muzzle velocities in the range of 2000 - 3500 m/s with muzzle energies of 5 - 50 MJ. For single loop railguns, these mission requirements require launch currents of a few million amperes, so a typical railgun power supply might be designed to deliver a launch current of 5 MA for a few milliseconds. As the magnetic field strengths required for such launches will typically be approximately 10 T, most contemporary railgun designs are effectively "air-cored", i.e. they do not use ferromagnetic materials such as iron to enhance the magnetic flux.

It may be noted that railgun velocities generally fall within the range of those achievable by two stage light gas guns; however, the latter are generally only considered to be suitable for laboratory use while railguns are judged to offer some potential prospects for development as military weapons. In some hypervelocity research projects, projectiles are "pre-injected" into railguns, to avoid the need for a standing start, and both two stage light gas guns and conventional powder guns have been used for this role.

In principle, if railgun power supply technology can be developed to provide compact, reliable and lightweight units, then the total system volume and mass needed to accommodate such a power supply and its primary fuel can become less than the required total volume and mass for a mission equivalent quantity of conventional propellants and explosive ammunition. Such a development would then convey a further military advantage in that the elimination of explosives from any military weapons platform will decrease its vulnerability to enemy fire.

Railguns have long existed as experimental technology but the mass, size and cost of the required power supplies have prevented railguns from becoming practical military weapons. However, in recent years, significant efforts have been made towards their development as feasible military technology. For example, in the late 2000s, the U.S. Navy tested a railgun that accelerates a 3.2 kg (7 pound) projectile to approximately 2.4 kilometres per second (5,400 mph). They gave the project the Latin motto "Velocitas Eradico", which is Latin for "I, speed, eradicate", but may have been intended as "Speed kills" or similar.

In addition to military applications, railguns have been proposed to launch spacecraft into orbit; however, unless the launching track was particularly long, and the acceleration required spread over a much longer time, such launches would necessarily be restricted to unmanned spacecraft.