Railgun
From Free net encyclopedia
- For artillery running on rails, see railway gun.
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.
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Overview
Railguns utilize an electromagnetic force called the Lorentz 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 a mutual repulsion of the rails and the acceleration of the projectile along them.
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 Sciences at the Australian National University. The system used the very large (500MJ of stored energy) Mark Oliphant homopolar generator as its energy source.
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 employed in the field in order 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. 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 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 using a heat resistant material that would be conductive enough to produce the same effect.
The complexity in railgun design comes from:
- 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.
- Power supply design. The power supply must be able to deliver large currents, with both capacitors and compulsators being common.
- 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 and 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: 3500 m/s or more (for comparison, the M16 rifle has a muzzle speed of 975-1025 m/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 guns - "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>Template:Citepaper</ref>
Although full scale guns have been built and fired, including a very successful 90 mm bore, 9 MJ 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 at a gun 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 Center for Electromechanics, 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.
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 happens 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 (this is accepted as having happened to Hood when she fought Bismarck during WWII). 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. However the main advantage for naval forces is range; the US Navy plans to deploy railguns with ranges over 250 miles on naval vessels as early as 2011.Template:Fn
Research into tank-portable railguns is in very early stages; high-velocity kinetic energy projectiles are becoming more important as the primary means of penetrating reactive armor.
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.
While it would be possible to reduce recoil by constructing a railgun which fires very lightweight projectiles at high velocity, such projectiles are extremely inefficient at wounding compared to larger, heavier projectiles at moderate velocity. While energy correlates with the amount of penetration in armor, this is due to the nature of the impact involved; metal on metal. Despite the prevalence of the "energy transfer" hypothesis, energy will only correlate with damage done in soft tissue if bullets enter with a large impact mass to speed ratio.
Additionally, the recoil of a hand-held weapon is not solely dependent on the momentum of the projectile. The force (mass * acceleration) of the recoil is often a larger factor than the momentum. For instance, commercially-loaded .357 magnum (9 x 33 mm R) ammunition which uses 125 grain (8.1 gram) bullets at 1450 ft/s (442 m/s) is almost universally regarded as having more unpleasant recoil than ammunition with 158 grain (10.2 gram) bullets at 1235 ft/s (376 m/s). Despite the higher momentum of the latter load, the higher velocity of the former means that the bullet (and therefore the gun) accelerates faster, making the recoil more forceful. A railgun which fires a very light projectile at very high speeds would inevitably have an extremely high recoil force, though the impulse could be kept relatively low.
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 coilguns 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 have also been used to research impacts on orbiting satellites. Railguns can simulate the high velocity of a low mass projectiles such as lost nut or bolt in a zero gravity environment. This allows engineers to verify computer models, test, and develop proper enclosures to protect satellite electronics.
Railguns in science fiction
Railguns are a popular device in science fiction. However, they are rarely portrayed accurately, often being confused with coilguns or directed-energy weapons. These fictional representations of the railgun sometimes appear as powerful hand-held weapons like in the first person shooters of the Quake series, but more often as larger weapons installed on robots, tanks or spaceships. Robert A. Heinlein's classic novel The Moon Is a Harsh Mistress features not railguns, but huge fixed coilguns (instead called "catapults") installed on the moon for launching material back to Earth.
References
See also
- Superconducting slingshots
- Impulse accelerators
- Homopolar generators
- Pulse transformers
- Coilguns
- Scram cannon
- Kinetic energy penetrator
- Light gas gun
- Ram accelerator
External links
Theory
- Railgun Theory by Matthew E. Massey
- Jengel and Fatro's Rail Gun Page
- Overview of Electromagnetic Guns
- Theoretical limits on exit velocity
Amateur
- PowerLabs Rail Gun research
- Railgun Construction
- Railgun theory, design, construction, and testing
- Railgun Blog by Jason Rollette
- Miniature Experimental Railgun