Railgun

From Wikipedia, the free encyclopedia

1 Overview
2 Theory and construction
- 2.1 Considerations in railgun design
  - 2.1.1 Strong materials needed
  - 2.1.2 Power supply design
3 Railguns as weapons
- 3.1 Tests
4 Railguns in science fiction
5 References
6 See also
7 External links

For artillery running on rails, see railway gun.

For deck railing mountable artillery, see swivel gun.

For the unlimited class target rifle, see benchrest shooting.

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

The term railgun is also used for conventional firearms used in the Unlimited class of benchrest shooting.

[edit] Overview

A Railgun is a type of Magnetic Accelerator Gun (MAG) that utilises an electromagnetic force to propel an electrically conductive projectile that is initially part of the current path. Sometimes they also use a movable armature connecting the rails. The current flowing through the rails sets up a magnetic field between them and through the projectile perpendicularly to the current in it. This results in the rails and the projectile pushing each other and in the acceleration of the projectile along the rails.

The world's first large-scale railgun was designed and constructed in the 1970s by John P. Barber, a Ph.D. Scholar from Canada and his advisor Richard A. Marshall from New Zealand, in the Research School of Physical Science at the Australian National University. The system used the very large (500 MJ of stored energy) Mark Oliphant homopolar generator as its energy source.

[edit] Theory and construction

Although conceptually simple, the operation of a railgun involves several problems that have to this day made a practical design (one that can be used in the field to 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.

In an electric motor, fixed magnets create a magnetic field, and a coil of wire is carried upon a shaft that is free to rotate. When an electrical current flows through the coil 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 make the shaft rotate, and so the motor runs.

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 (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.

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 slides up the rails away from the end with the power supply.

If a very large power supply providing a million amperes or so of current is used, 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. 20 kilometers per second has been achieved with small projectiles explosively injected into the railgun.

Although these speeds are theoretically possible, the heat generated from the propulsion of the object is enough to rapidly erode the rails. Such a railgun would require frequent replacement of the rails, or use a heat resistant material that would be conductive enough to produce the same effect.

[edit] Considerations in railgun design

The complexity in railgun design comes from:

[edit] Strong materials needed

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 and friction involved. 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.

[edit] Power supply design

The power supply must be able to deliver large currents, with both capacitors and compulsators being common.

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.

Some have speculated that there are fundamental limits to the exit velocity due to the inductance of the system, and particularly of the rails; but United States government has made significant progress in railgun design and has recently floated designs of a railgun that would be used on a naval vessel. The designs for the naval vessels, however, are limited by their required power usages for the magnets in the railguns. These limits are larger than currently attainable and do reduce the usefulness of the concept for space travel and military uses.

[edit] Railguns as weapons

Railguns are being pursued as weapons with projectiles that do not contain explosives, but are given extremely high velocities: 3500 m/s (11,500 ft/s) or more (for comparison, the M16 rifle has a muzzle speed of 975-1025 m/s, or 3,000 ft/s), which would make their kinetic energy equal or superior to the energy yield 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 naval weapons platform. Also, by firing at higher velocities railguns have greater range, less bullet drop and less wind drift, bypassing the inherent cost and 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."<ref>Lt. David Allen Adams (2003). "Naval Railguns Are Revolutionary".</ref>

[edit] Tests

Full-scale models have been built and fired, including a very successful 90 mm bore, 9 MJ (6.6 million foot-pounds) kinetic energy gun developed by DARPA, but 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 at Dundrennan Range in Kirkcudbright, Scotland. This system has now been operational for over 10 years as 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 at Austin Institute for Advanced Technology, military railguns capable of delivering tungsten armor piercing bullets with kinetic energies of nine million joules have been developed [1]. Nine million joules is enough energy to deliver 2 kg of projectile at 3 km/s - at that velocity a tungsten or other dense metal rod could penetrate a tank.

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.

[edit] Railguns in science fiction

Main article: Railguns in science fiction

Railguns are a popular device in science fiction. However, they are seldom portrayed accurately, often being confused with a coilgun. These fictional representations of the railgun sometimes appear as powerful handheld weapons in first person shooters, but more often as larger weapons mounted on mecha or starships.