Railgun

From Academic Kids

For artillery running on railways, see railway gun.
A prototype railgun
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A prototype railgun

A railgun is a form of gun that converts electrical energy into projectile kinetic energy, rather than the more conventional chemical energy from an explosive propellant. It is not to be confused with a coilgun (Gauss gun).

Railguns utilize a magnetic force termed "Lorentz force" to propel an electrically conductive projectile that is initially part of the current path. The current flowing through the rails sets up a magnetic field between them and through the projectile perpendicularly to the current in the rail. This results in a mutual repulsion of the rails and the acceleration of the projectile along them.

Contents

Theory and construction

Schematic diagram of a railgun
Enlarge
Schematic diagram of a railgun

Although conceptually simple, the operation of a railgun involves several factors that have to this day made a practical design (one that can be fielded and replace conventional weapons) impossible.

A wire carrying an electrical current, when in a magnetic field, experiences a force perpendicular to the direction of the current and the direction of the magnetic field. This is the principle behind the operation of an electric motor, where fixed magnets create a magnetic field, and a coil of wire is carried upon a shaft that is free to rotate. When electricity is applied to the coil of wire a current flows, causing it to experience a force due to the magnetic field; the wires of the coil are arranged such that all the forces on the wires act to make the shaft rotate, and so the motor runs.

A railgun is even simpler than a motor. It consists of two parallel metal rails (hence the name) connected to an electrical power supply. When a conductive projectile is inserted between the rails (from the end connected to the power supply), it completes the circuit. Electrical current runs from the positive terminal of the power supply up the positive rail, across the projectile, and down the negative rail back to the power supply again.

This flow of current makes the railgun act like an electromagnet, creating a powerful magnetic field in the region of the rails up to the position of the projectile. In accordance with the right-hand rule, the created magnetic field circulates around each conductor. Since the current flows in opposite direction along each rail, the net magnetic field between the rails (B) is directed vertically. In combination with the current (I) flowing across the projectile, this produces a Lorentz force which accelerates the projectile along the rails. There are also forces acting on the rails attempting to push them apart, but since the rails are firmly mounted they cannot move. The projectile is able to slide up the rails away from the end with the power supply.

If you happen to do this with a very large power supply, providing a million amperes or so of current, then the force on the projectile will be tremendous, and by the time it leaves the ends of the rails it can be travelling at many kilometres per second.

Although these speeds are theoretically possible, the heat generated from the propelling the object is enough to actually vaporise the rails. This would result in very fast erosion of the rails themselves. So either they will have to be replaced often or a new material has to be made for them.

The complexity in railgun design comes from:

  1. The need for strong conductive materials with which to build the rails and projectiles; the rails need to survive the violence of an accelerating projectile, and heating due to the large currents involved and friction. The force exerted on the rails consists of a recoil force - equal and opposite to the force propelling the projectile, but along the length of the rails (which is their strongest axis) - and a sideways force caused by the rails being pushed by the magnetic field, just as the projectile is. The rails need to survive this without bending, and must be very securely mounted.
  2. Power supply design. The power supply must be able to deliver large currents, with both capacitors and compulsators being common.
  3. Electromechanical design. The rails need to withstand enormous repulsive forces during firing, and these forces will tend to push them apart and away from the projectile. As rail/projectile clearances increase arcing develops which causes rapid vaporization and extensive damage to the rail surfaces and the insulator surfaces. This limits most research railguns to one shot per service interval.

There are fundamental limits to the exit velocity due to the inductance of the system and in particular the rails. These limits are larger than currently attainable but do reduce the usefulness of the concept for space travel and military uses.

Railguns as weapons

Railguns are being pursued as weapons with projectiles that do not contain explosives, but are given extremely high velocities: 3500m/s or more (for comparison, the M16 rifle has a muzzle speed of 975m/s), which would make their kinetic energy equal or superior to that of an explosive-filled shell of greater mass. This would allow more ammunition to be carried and eliminate the hazards of carrying explosives in a tank or battleship. Also, by firing at higher velocities railguns have greater range, less bullet drop and less wind drift. It is postulated that the next generation of reactive armour will be impenetrable by conventional ammunition and shaped charges; the higher velocity KE-penetrator projectiles of railguns would be able to easily defeat such armour.

Although full scale guns have been built and fired, including a very successful 90 mm bore, 9MJ kinetic energy gun developed by DARPA, they all suffer from extreme rail damage and need to be serviced after every shot. Rail and insulator ablation issues still need to be addressed before railguns can start to replace conventional weapons. Probably the most successful system was built by the UK's Defence Research Agency (now QinetiQ PLC) at a gun range in Kirkcudbright, Scotland. This system has now been operational for over 10 years, has an associated flight range for internal, intermediate, external and terminal ballistics, and is the holder of several mass and velocity records.

The United States military is funding railgun experiments. At the University of Texas' Center for Electromechanics, military railguns capable of delivering tungsten armor piercing bullets with kinetic energies of nine million joules have been developed [1] (http://web.archive.org/web/20041010051811/http://iat.utexas.edu/electrodyn.html). Nine million joules is enough energy to deliver 1 kg of projectile at 3 km/s - which will tear a tank to pieces in a single shot.

Due to the very high muzzle velocity that can be attained with railguns, there is interest in using them to shoot down high-speed missiles.

Naval forces are also interested in railgun research. Current ship guns store their explosive shells in a large magazine underneath the gun. If a shell from a hostile ship should happen to penetrate into the armoury and explode, it is quite likely to cause all of the shells in the magazine to detonate, usually destroying the ship. However if the ship is instead equipped with railguns, the magazine would need only to store the non-explosive tungsten bullets. Additionally, the compact railgun projectiles would require less space to store than the shells used for current guns. Electricity for the railgun could be supplied from an on-board compulsator, which in turn could be powered by the ship's engines.

Man-portable railguns will not be revolutionary weapons; if power supply technology ever allows a railgun small enough to be carried then rail-handguns will probably only be able to fire projectiles at speeds similar to those currently achieved with chemical propellants. The simple reason is that the destructive power of a handgun or long gun is limited as much by recoil as anything else; it is quite possible to build a handgun that fires 20 mm cannon shells, but the recoil would make it impossible to aim or fire safely.

One possible solution is a portable railgun that fires very small bullets. The recoil of a weapon is caused by the momentum of the escaping projectile yet the damage done by the projectile is more related to its kinetic energy. The momentum of the projectile is its mass times its velocity, but the kinetic energy is one half of the mass times the velocity squared. So a very small, very fast, projectile could deliver a moderate recoil, but be carrying enough kinetic energy to vaporise upon impact and burn a large hole in armour and flesh alike. However, such a weapon would not fire through walls very well; the projectile would vaporise upon contact with the first wall (see needlegun). According to some sources, a railgun that fires .1 gram 6mm object to muzzle velocity of 16000 m/s has been built and according to the formula above, it has a recoil of near 2Nm. The only problem is that the power source is too big and bulky to be carried by humans.

Peaceful uses of railguns

There is interest in using railguns as mass drivers for space exploration and mining. They would be useful for launching bulk ores into space, particularly from low-gravity bodies such as moons and asteroids; electrically powered from solar panels, they would not require any consumables such as rocket fuels.

Rail guns have been proposed for use in delivering projectiles to space, especially from bodies without atmospheres (such as the Moon). Its main competitors are coil guns and ram accelerators.

Also, railguns may be used to initiate fusion reactions, by firing pellets of fusible material at each other. The impact would create immense temperatures and pressures, allowing nuclear fusion to occur. However current railguns are not yet sufficient to achieve the energies required.

Railguns in science fiction

Railguns have started to appear in sci-fi and become a mainstream idea. However, they have not always been portrayed accurately.

In Robert Heinlein's classic novel The Moon is a Harsh Mistress, rebelling Lunar colonists convert a kilometers-long mass driver system that delivers raw materials to Earth into a basic railgun that lobs metal-clad rocks.

The novel series Buck Rogers uses mass driver railguns on Luna.

In the 2003 remake of Battlestar Galactica, the title vessel has a main armament of 64 railgun batteries which are referred to as railguns, but which appear to be a strange and impractical hybrid of railguns and conventional firearms.

In the first season of the TV series Stargate Atlantis, a railgun prototype is used against the Wraiths' Darts. Intended to be used in the first generation of earth's space ship, Prometheus, Stargate Command (SGC) was ordered to send the prototypes to Atlantis of Pegasus galaxy before the Wraiths did attack Atlantis.

The blockbuster movie Eraser starring Arnold Schwarzenegger is based on a defence contractor employee who has stumbled on a secret government plot to sell a deadly new weapon to terrorists. This weapon is the railgun.

Certain computer games like System Shock, Quake 2, Quake 3, Deus Ex 2 and Shadow Warrior feature railguns as weapons.

In the video game Metal Gear Solid, the main weapon on the giant walking battle tank, Metal Gear REX, is a railgun (although the way it's described it seems more like a coilgun in operation) that fires nuclear warheads.

In the video game Metal Gear Solid 2, the boss character, Fortune, a member of Dead Cell uses a railgun as her weapon.

The computer games Red Faction and Red Faction 2 feature railguns that fire through walls with a heat-detection scope.

In the world of Warhammer 40,000, the Tau make use of rail technology with one of the largest and strongest guns in the game, and are experimenting with hand-held rail rifles.

In the role-playing game Rifts, the Occupational Character Class Glitter Boy is an individual piloting a large armoured suit equipped with a railgun (called "Boom Gun" in the game). Notably, the suit anchors itself into the ground when firing to maintain it's position.

In the Anime Macross, the SDF-1 has 4 heavy rail guns on its "shoulders" when it's in the transformed form. Also the Monster Mark III's have 4 heavy rail guns on its top mount that travel 4000 km/s in space which is so fast that making the warhead explosive would be redundant.

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