Railgun

For artillery running on rails, see railway 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.

Overview

A Railgun is a type of MAG (Magnetic Accelerator Gun) that utilizes 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 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 (500 MJ 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. Twenty 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.

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.

While some have speculated that there are fundamental limits to the exit velocity due to the inductance of the system, and in particular the rails, the 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.

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."^[1]

Although 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, 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.

Naval forces are also interested in railgun research from the standpoint of survivability in combat. Current ship guns store their explosive shells in armouries located beneath the gun(s), i.e. the magazine. If a shell fired from an enemy's gun or warhead from an attacking missile penetrates into the the magazine and explodes, it is likely to cause all of the shells located there to detonate. The catastrophic explosion would usually destroy the ship - the accepted cause for the loss of the British Royal Navy's HMS Hood (51) when she fought the Bismarck at the Battle of the Denmark Strait during WWII, and the sinking of the United States Navy's USS Arizona (BB-39) during the attack at Pearl Harbor, Hawaii.

However if a ship is instead equipped with railguns, the magazine would only need 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 (400 km) on naval vessels as early as 2011.^[2]

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 a modern tank's 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. It is theoretically possible to make a recoiless design by firing an equal mass at an equal velocity out the back of the railgun, making it comparable to a recoilless rifle, but this would make the weapon much heavier and double the weight of the ammo. Such a design, if built, would likely serve the same role as current recoilless rifles, that is, as a man-portable anti-tank weapon.

The ability for the soldier to adjust the muzzle velocity gives the potential for a much more versatile weapon. Such a weapon could have two or more power settings: a high velocity setting for single-shot long-range precision shooting and a low velocity setting for shorter-range bursts of automatic fire. It also makes it more versatile. For example, rifle grenades would be much easier to implement, since the gun could switch to an appropriate velocity without changing the ammunition. Thus, the soldier wouldn't need to bring along a grenade launcher to be able to launch grenades on short notice.

The lower-velocity setting, of perhaps 975 m/s using the same bullet currently used in the 5.56 x 45 mm NATO would be approximate to the current performance of the cartridge. Such a setting allows automatic fire in a controllable manner, which is suitable for close quarter combat, or rapidly aimed semi-automatic fire against targets at intermediate ranges.

If we take the recoil of the larger 7.62x51mm NATO cartridge as the maximum allowable recoil, we could take the 62-grain bullet from the 5.56 mm x 45 mm NATO cartridge and accelerate it up to over twice the velocity of the larger cartridge, in the region of 2000 m/s. In this scenario, the muzzle energy of the 5.56 mm round increases by 4.5 times, and is 2.3 times as powerful as the larger 7.62 mm round.

Such a bullet would be very flat-shooting, which minimizes the effects of wind and distance-estimating error, and would transfer very large quantities of energy to the target. This would make it a very effective sniper weapon and would represent the long-range precision shooting aspect of a variable-power weapon.

Much like the assault rifle combined the close-combat automatic-firing ability of the submachine gun with the accuracy and power of a full-power battle rifle, a variable-velocity railgun could combine the assault rifle with long-range sniping and/or armor-penetrating ability.

One problem with variable-velocity railguns is the rate of twist of the rifling. A rate of twist that works well at lower velocities might cause enough spin in higher velocities to literally tear the bullet apart from centripetal force as soon as the bullet leaves the barrel, and a rate that works well at high velocities might not sufficently stabilize a lower-velocity projectile. Most likely is that the difference between the high and low velocities will not be as great as described above, but even an increase of 1,000 ft/s at the muzzle results in nearly twice the kinetic energy at 500 yards and 60% more energy at 1,000 yards.

If the power source was small enough, it would also enable the soldier to carry several times the ammunition than a conventional rifle cartridge, which is about 30% to 50% bullet (or payload) and 50% to 70% powder, primer, and brass case, by weight.

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 railgun may be more of a problem than the recoil of a weapon with the same projectile momentum, but a lower velocity and heavier projectile. Since a high-velocity railgun would accelerate the projectile over a shorter time, the force exerted on the projectile (and hence, the recoil force exerted on the weapon) would be greater. For example, 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. A shock absorber built into the gun could compensate for this, but would add to the weight.

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 projectile such as a 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

File:Glitterboy.JPG

A Glitter Boy from Rifts brandishes its 'railgun'

Railguns are a popular device in science fiction. However, they are rarely portrayed accurately, often being confused with a coilgun. These fictional representations of the railgun sometimes appear as powerful weapons like in the first person shooters of the Quake series, but more often as larger weapons installed on boats or spaceships.

References

^ Lt. David Allen Adams (2003). "Naval Railguns Are Revolutionary" (PDF). {{cite journal}}: Cite journal requires |journal= (help)

External links

Theory

Amateur

University

Auburn University Rail Gun

Press and Media

USN sets five-year target to develop electromagnetic gun Jane's Defence Weekly, 20 July 2006

[1] Lt. David Allen Adams (2003). "Naval Railguns Are Revolutionary" (PDF). {{cite journal}}: Cite journal requires |journal= (help)

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