Railgun
A railgun or rail gun, sometimes referred to as a rail cannon, is a linear motor device, typically designed as a ranged weapon, that uses electromagnetic force to launch high-velocity projectiles. The projectile normally does not contain explosives, instead relying on the projectile's high kinetic energy to inflict damage. The railgun uses a pair of parallel rail-shaped conductors, along which a sliding projectile called an 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. It is based on principles similar to those of the homopolar motor.
As of 2020, railguns have been researched as weapons utilizing electromagnetic forces to impart a very high kinetic energy to a projectile rather than using conventional propellants. While explosive-powered military guns cannot readily achieve a muzzle velocity of more than ≈, railguns can readily exceed. For a similar projectile, the range of railguns may exceed that of conventional guns. The destructive force of a projectile depends upon its kinetic energy at the point of impact. Because of the potentially higher velocity of a railgun, its force may be much greater than conventionally launched projectiles of the same mass. The absence of explosive propellants or warheads to store and handle, as well as the low cost of projectiles compared to conventional weaponry, are also advantageous.
Basics
The railgun in its simplest form differs from a traditional electric motor in that no use is made of additional field windings. This basic configuration is formed by a single loop of current and thus requires high currents to produce sufficient accelerations. A relatively common variant of this configuration is the augmented railgun in which the driving current is channeled through additional pairs of parallel conductors, arranged to increase the magnetic field experienced by the moving armature. These arrangements reduce the current required for a given acceleration. In electric motor terminology, augmented railguns are usually series-wound configurations. Some railguns also use strong neodymium magnets with the field perpendicular to the current flow to increase the force on the projectile.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. The high current required to power a railgun can be provided by various power supply technologies, such as capacitors, pulse generators and disc generators.
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 with better aerodynamically streamlined projectiles 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 incorporating hit-to-kill guidance, as replacements for explosive shells. Therefore, typical military railgun designs aim for muzzle velocities in the range of with muzzle energies of 5–50 megajoules
. For comparison, 50MJ is equivalent to the kinetic energy of a school bus weighing 5 metric tons, traveling at. 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 tesla, most contemporary railgun designs are effectively air-cored, i.e., they do not use ferromagnetic materials such as iron to enhance the magnetic flux. However, if the barrel is made of a magnetically permeable material, the magnetic field strength increases because of the increase in permeability. The field 'felt' by the armature is proportional to , so the increased field increases the force on the projectile.
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. A light gas gun, the Combustion Light Gas Gun in a 155 mm prototype form was projected to achieve 2500 m/s with a 70 caliber barrel. 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 safe, compact, reliable, combat survivable, 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. Arguably such technology has been matured with the introduction of the Electromagnetic Aircraft Launch System . 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.
History
The concept of the railgun was first introduced by French inventor André Louis Octave Fauchon-Villeplée, who created a small working model in 1917 with the help of the Société anonyme des accumulateurs Tudor. During World War I, the French Director of Inventions at the Ministry of Armaments, Jules-Louis Brenton, commissioned Fauchon-Villeplee to develop a 30-mm to 50-mm electric cannon on 25 July 1918, after delegates from the Commission des Inventions witnessed test trials of the working model in 1917. However, the project was abandoned once World War I ended later that year on 11 November 1918. Fauchon-Villeplee filed for a US patent on 1 April 1919, which was issued in July 1922 as patent no. 1,421,435 "Electric Apparatus for Propelling Projectiles". In his device, two parallel busbars are connected by the wings of a projectile, and the whole apparatus surrounded by a magnetic field. By passing current through busbars and projectile, a force is induced which propels the projectile along the bus-bars and into flight.In 1923, Russian scientist A. L. Korol'kov detailed his criticisms of Fauchon-Villeplee's design, arguing against some of the claims that Fauchon-Villeplee made about the advantages of his invention. Korol'kov eventually concluded that while the construction of a long-range electric gun was within the realm of possibility, the practical application of Fauchon-Villeplee's railgun was hindered by its enormous electric energy consumption and its need for a special electric generator of considerable capacity to power it.
In 1944, during World War II, Joachim Hänsler of Germany's Ordnance Office proposed the first theoretically viable railgun. By late 1944, the theory behind his electric anti-aircraft gun had been worked out sufficiently to allow the Luftwaffe's Flak Command to issue a specification, which demanded a muzzle velocity of and a projectile containing of explosive. The guns were to be mounted in batteries of six firing twelve rounds per minute, and it was to fit existing 12.8 cm FlaK 40 mounts. It was never built. When details were discovered after the war it aroused much interest and a more detailed study was done, culminating with a 1947 report which concluded that it was theoretically feasible, but that each gun would need enough power to illuminate half of Chicago.
During 1950, Sir Mark Oliphant, an Australian physicist and first director of the Research School of Physical Sciences at the new Australian National University, initiated the design and construction of the world's largest homopolar generator. This machine was operational from 1962 and was later used to power a large-scale railgun that was used as a scientific experiment.
In 1980, the Ballistic Research Laboratory began a long-term program of theoretical and experimental research on railguns. The work was conducted predominantly at the Aberdeen Proving Ground, and much of the early research drew inspiration from the railgun experiments performed by the Australian National University. Topics of research included plasma dynamics, electromagnetic fields, telemetry, and current and heat transport. While military research into railgun technology in the United States ensued continuously in the following decades, the direction and focus that it took shifted dramatically with major changes in funding levels and the needs of different government agencies. In 1984, the formation of the Strategic Defense Initiative Organization caused research goals to shift toward establishing a constellation of satellites to intercept intercontinental ballistic missiles. As a result, the U.S. military focused on developing small guided projectiles that could withstand the high-G launch from ultra-high velocity plasma armature railguns. But after the publication of an important Defense Science Board study in 1985, the U.S. Army, Marine Corps, and DARPA were assigned to develop anti-armor, electromagnetic launch technologies for mobile ground combat vehicles. In 1990, the U.S. Army collaborated with the University of Texas at Austin to establish the Institute for Advanced Technology, which focused on research involving solid and hybrid armatures, rail-armature interactions, and electromagnetic launcher materials. The facility became the Army's first Federally Funded Research and Development Center and housed a few of the Army's electromagnetic launchers, such as the Medium Caliber Launcher.
Since 1993 the British and American governments have collaborated on a railgun project at the Dundrennan Weapons Testing Centre that culminated in the 2010 test where BAE Systems fired a 3.2 kg projectile at 18.4-megajoules . In 1994, India's DRDO's Armament Research and Development Establishment developed a railgun with a 240 kJ, low inductance capacitor bank operating at 5 kV power able to launch projectiles of 3–3.5 g weight to a velocity of more than. In 1995, the Center for Electromagnetics at the University of Texas at Austin designed and developed a rapid-fire railgun launcher called the Cannon-Caliber Electromagnetic Gun. The launcher prototype was later tested at the U.S. Army Research Laboratory, where it demonstrated a breech efficiency over 50 percent.
In 2010, the United States Navy tested a BAE Systems-designed compact-sized railgun for ship emplacement that accelerated a 3.2 kg projectile to hypersonic velocities of approximately, or about, with 18.4MJ of kinetic energy. It was the first time that such levels of performance were reached. A 32-megajoule earlier railgun of the same design resides at the Dundrennan Weapons Testing Centre in the United Kingdom.
Low power, small scale railguns have also made popular college and amateur projects. Several amateurs actively carry out research on railguns.