Published on Feb 15, 2021
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. 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 hypersonic velocities of approximately 2.4 kilometres per second (5,400 mph), about Mach 7.
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.
A rail gun is basically a large electric circuit, made up of three parts: a power source, a pair of parallel rails and a moving armature.
1. The power supply is simply a source of electric current. Typically, the current used in medium- to large-caliber rail guns is in the millions of amps.
2. The rails are lengths of conductive metal, such as copper. They can range from four to 30 feet (9 meters) long.
3. The armature bridges the gap between the rails. It can be a solid piece of conductive metal or a conductive sabot -- a carrier that houses a dart or other projectile.
Some rail guns use a plasma armature. In this set-up a thin metal foil is placed on the back of a non-conducting projectile. When power flows through this foil it vaporizes and becomes a plasma, which carries the current.
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.
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 (B) is directed at right angles to the plane formed by the central axes of the rails and the armature. In combination with the current (I) 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. A very large power supply, providing on the order of one million amperes of current, will create a tremendous force on the projectile, accelerating it to a speed of many kilometres per second (km/s). 20 km/s has been achieved with small projectiles explosively injected into the railgun. Although these speeds are possible, the heat generated from the propulsion of the object is enough to erode the rails rapidly. Under high-use conditions, current railguns would require frequent replacement of the rails, or to use a heat-resistant material that would be conductive enough to produce the same effect.
Notice that the Lorentz force is parallel to the rails, acting away from the power supply. The magnitude of the force is determined by the equation F = (i)(L)(B), where F is the net force, i is the current, L is the length of the rails and B is the magnetic field. The force can be boosted by increasing either the length of the rails or the amount of current.
Because long rails pose design challenges, most rail guns use strong currents -- on the order of a million amps -- to generate tremendous force. The projectile, under the influence of the Lorentz force, accelerates to the end of the rails opposite the power supply and exits through an aperture. The circuit is broken, which ends the flow of current.
A) Launch of Spacecraft For space launches from Earth, relatively short acceleration distances (less than a few km) would require very strong acceleration forces, higher than humans can tolerate. Other designs include a longer helical (spiral) track, or a large ring design whereby a space vehicle would circle the ring numerous times, gradually gaining speed, before being released into a launch corridor leading skyward.
In 2003, Ian McNab outlined a plan to turn this idea into a realized technology. The accelerations involved are significantly stronger than human beings can handle. This system would only be used to launch sturdy materials, such as food, water, and fuel. Note that escape velocity under ideal circumstances (equator, mountain, heading east) is 10.735 km/s. The system would cost $528/kg, compared with $20,000/kg on the space shuttle. The railgun system McNab suggested would launch 500 tons per year, spread over approximately 2000 launches per year.
Because the launch track would be 1.6 km, power will be supplied by a distributed network of 100 rotating machines (compulsator) spread along the track. Each machine would have a 3.3 ton carbon fibre rotor spinning at high speeds. A machine can recharge in a matter of hours using 10 MW. This machine could be supplied by a dedicated generator. The total launch package would weigh almost 1.4 tons. Payload per launch in these conditions is over 400 kg.There would be a peak operating magnetic field of 5T – Half of this coming from the rails, and the other half from augmenting magnets. This halves the required current through the rails, which reduces the power fourfold.
Railguns are being researched as weapons with projectiles that do not contain explosives or propellants, but are given extremely high velocities: 3,500 m/s (11,500 ft/s) (approximately Mach 10 at sea level) or more (for comparison, the M16 rifle has a muzzle speed of 930 m/s (3,050 ft/s), and the 16"/50 caliber Mark 7 gun that armed World War II American battleships has a muzzle speed of 760 m/s (2,490 ft/s)), which would make their kinetic energy equal or far superior to the energy yield of an explosive-filled shell of greater mass. This would decrease ammunition size and weight, allowing more ammunition to be carried and eliminating the hazards of carrying explosives or propellants in a tank or naval weapons platform. Also, by firing at greater velocities, railguns have greater range, less bullet drop, less time to target, and less wind drift, bypassing the physical limitations of conventional firearms: "the limits of gas expansion prohibit launching an unassisted projectile to velocities greater than about 1.5 km/s and ranges of more than 50 miles [80 km] from a practical conventional gun system."
Trigger for inertial confinement fusion
Railguns may also be miniaturized for inertial confinement nuclear fusion.
Fusion is triggered by very high temperature and pressure at the core.
Current technology calls for multiple lasers, usually over 100, to concurrently strike a fuel pellet, creating a symmetrical compressive pressure.
Railguns may be able to trigger fusion by firing energetic plasma from multiple directions. The process developed involves four key steps.
Plasma is pumped into a chamber.
When the pressure is great enough, a diaphragm will rupture, sending gas down the rail.
Shortly afterwards, a sufficient voltage is applied to the rails, creating a conduction path of ionized gas.
This plasma accelerated down the rail, eventually being ejected at a large velocity.
The rails and dimensions are on the order of centimeters.
Full-scale models have been built and fired, including a 90 mm (3.5 in) bore, 9 MJ kinetic energy gun developed by the US DARPA. Rail and insulator wear problems still need to be solved before railguns can start to replace conventional weapons. Probably the oldest consistently successful system was built by the UK's Defence Research Agency at Dundrennan Range in Kirkcudbright, Scotland. The Yugoslavian Military Technology Institute developed, within a project named EDO-0, a railgun with 7 kJ kinetic energy, in 1985. In 1987 a successor was created, project EDO-1, that used projectile with a mass of 0.7 kg (1.5 lb) and achieved speeds of 3,000 m/s (9,800 ft/s), and with a mass of 1.1 kg (2.4 lb) reached speeds of 2,400 m/s (7,900 ft/s). It used a track length of 0.7 m (2.3 ft).
According to those working on it, with other modifications it was able to achieve a speed of 4,500 m/s (14,800 ft/s). The aim was to achieve projectile speed of 7,000 m/s (23,000 ft/s). At the time, it was considered a military secret. The United States military is funding railgun experiments. At the University of Texas at Austin Centre for Electro-mechanics, military railguns capable of delivering tungsten armor piercing bullets with kinetic energies of nine megajoules have been developed.
9 MJ is enough energy to deliver 2 kg (4.4 lb) of projectile at 3 km/s (1.9 mi/s) – at that velocity a rod of tungsten or another dense metal could easily penetrate a tank, and potentially pass through it. The United States Naval Surface Warfare Center Dahlgren Division demonstrated an 8 MJ railgun firing 3.2 kg (7.1 lb) projectiles in October 2006 as a prototype of a 64 MJ weapon to be deployed aboard Navy warships. The main problem the U.S. Navy has had with implementing a railgun cannon system is that the guns wear out due to the immense heat produced by firing. Such weapons are expected to be powerful enough to do a little more damage than a BGM-109 Tomahawk missile at a fraction of the projectile cost.Since then, BAE Systems has delivered a 32 MJ prototype to the U.S. Navy.
1. Adams, E. "Electromagnetic railgun," Popular Science. September 7, 2005, http://www.popsci.com/popsci/technology/generaltechnology/
2. The Defence Science and Technology Ministry (UK Ministry of Defense), http://www.dstl.gov.uk/pr/science_spot/off_the_rails.htm
3. Encyclopedia Britannica 2005, s.v. "military technology." CD-ROM, 2005.
4. Encyclopedia Britannica 2005, s.v. "magnetism." CD-ROM, 2005.
5. Folger, T. "The guns of Brooklyn," Discover. August 1992.
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