Railgun

For railroad artillery, see railway gun.

For fictional railguns, see Electromagnetic projectile devices (fiction).

Schematic diagram of a railgun

A railgun is a purely electrical gun that accelerates a conductive projectile along a pair of metal rails using the same principles as the homopolar motor.

Railguns use two sliding or rolling contacts that permit a large electric current to pass through the projectile. This current interacts with the strong magnetic fields generated by the rails and this accelerates the projectile. The U.S. Navy has tested a railgun that accelerates a 7 pound bullet to seven times the speed of sound..

Railguns should not be confused with coilguns (Gauss guns), which are contactless and which use a magnetic field generated by external coils arranged along the barrel to accelerate a magnetic projectile.

Theory and construction

A wire carrying an electric 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. An electric 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 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 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 is in opposite direction along each rail, the net magnetic field between the rails (B) is directed vertically. In combination with the current (I) 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.

Considerations in railgun design

Materials

The rails and projectiles must be built from strong conductive materials; the rails need to survive the violence of an accelerating projectile, and heating due to the large currents and friction involved. The recoil force exerted on the rails is equal and opposite to the force propelling the projectile. The seat of the recoil force is still debated. The traditional equations predict that the recoil force acts on the breech of the railgun. Another school of thought invokes Ampère's force law and asserts that it acts along the length of the rails (which is their strongest axis). The rails also repel themselves via 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.

Design Considerations

The power supply must be able to deliver large currents, sustained and controlled over a useful amount of time. The most important gauge of power supply effectiveness is the energy it can deliver, measured in joules. The largest energy ever used in a railgun was 10 million joules. The most common forms of power supplies used in railguns are capacitors and compulsators.

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 limited some early research railguns to one shot per service interval.

The inductance and resistance of the rails and power supply limit the efficiency of a railgun design. Currently different rail shapes and railgun configurations are being tested, most notably by the United States Navy.

Heat dissipation

Massive amounts of heat are created by the electricity flowing through the rails, as well as the friction of the projectile leaving the device. This leads to three main problems: melting of equipment, safety of personnel, and detection by enemy forces. As briefly discussed above, the stresses involved in firing this sort of device require an extremely heat-resistant material. Otherwise the rails, barrel, and all equipment attached would melt or be irreparably damaged.

In practice the rails are, with most designs of railgun, subject to erosion due to each launch; and projectiles can be subject to some degree of ablation also, and this can limit railgun life, in some cases severely.

Mathematical formula

In relation to railgun physics, the magnitude of the force vector can be determined from a form of the Biot-Savart Law and a result of the Lorentz force. It can be expressed mathematically in terms of the permeability constant ($\mu _{0}$), the radius of the rails (which are assumed to be circular in cross section)($r$), the distance between the centerpoints of the rails($d$) and the current in amps through the system ($I$) as follows:
Failed to parse (SVG (MathML can be enabled via browser plugin): Invalid response ("Math extension cannot connect to Restbase.") from server "http://localhost:6011/en.wikipedia.org/v1/":): {\displaystyle F = \frac{\mu_0 I^2}{ 2\pi} \ln{ \frac{d-r}{r}}}

The formula is based on the assumption that the distance($l$) between the point where the force ($F$) is measured and the beginning of the rails is greater than the distance of the rails ($d$) by a factor of about 3 or 4 ($l>3d$). Of course, some other simplifying assumptions have also been made; to describe the force more accurately, the geometry of the rails and the projectile must be taken into consideration.

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, approximately Mach 10 at sea level) or more (for comparison, the M16 rifle has a muzzle speed of 930 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."

If it is even possible to apply the technology as a rapid-fire automatic weapon, a railgun would have further advantages in increased rate of fire. The feed mechanisms of a conventional firearm must move to accommodate the propellant charge as well as the ammunition round, while a railgun would only need to accommodate the projectile. Furthermore, a railgun would not have to extract a spent cartridge case from the breech, meaning that a fresh round could be cycled almost immediately after the previous round has been shot.

Tests

Naval Surface Warfare Center test firing in January 2008

Full-scale models have been built and fired, including a very successful 90 mm bore, 9 megajoules (6.6 million foot-pounds) kinetic energy gun developed by DARPA. Rail and insulator wear issues still need to be addressed 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. This system has now been operational for over 10 years at an associated flight range for internal, intermediate, external and terminal ballistics, and achieved 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 megajoules have been developed. Nine MJ is enough energy to deliver 2 kg of projectile at 3 km/s - at that velocity a rod of tungsten or of 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 rail gun firing 3.2 kilogram (slightly more than 7 pounds) projectiles in October 2006 as a prototype of a 64 MJ weapon to be deployed aboard Navy warships. The main problem the 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 Navy.

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.

In February 2008 the US Navy tested a magnetic railgun; it fired a shell at 5,600 miles (9,000 km) per hour using 10 megajoules of energy. Its expected performance is over 13,000 miles (21,000 km) per hour muzzle velocity, accurate enough to hit a 5 meter target from 200 nautical miles away while shooting at 10 shots per minute. It is expected to be ready in 2020 to 2025. Image and comments

Railguns in popular culture

Railguns have made several appearances in popular culture, namely as weapons in science-fiction media.

• 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.
• In the Arnold Schwarzenegger film Eraser, Schwarzeneggar's character uses a pair of man-portable railguns capable of firing bullets through walls.
• In the video games Red Faction, Red Faction II, and the Quake series, railguns are common weapons.
• Railguns appear several times in the Metal Gear video game series. In Metal Gear Solid, a railgun is mounted on and used as a primary weapon of Metal Gear REX. In Metal Gear Solid 2: Sons of Liberty, a man-portable railgun is used by the character 'Fortune.' In Metal Gear Solid 4: Guns of the Patriots, the character 'Crying Wolf' has a railgun mounted on her exo-suit, which later becomes a man-portable weapon useable by the player.
• An experimental railgun called "Reason" was used in the novel Snow Crash, by Neal Stephenson.
• In the novel series Buck Rogers, the protagonist uses railguns on the planet Luna.
• The Tau in Warhammer 40,000 use railguns extensively.

See also

• Coilgun
• Mass driver
• Light gas gun
• Ram accelerator
• Scram cannon
• Homopolar generator
• Kinetic energy penetrator
• Project Babylon

External links

Theory

• Railgun Theory by Matthew E. Massey
• howstuffworks.com - explanation with animations
• iop.org - Theoretical limits on exit velocity
• Jengel and Fatro's Rail Gun Page

Amateur

• PowerLabs Rail Gun research
• Railgun theory, design, construction, and testing
• Railgun Blog by Jason Rollette
• Miniature Experimental Railgun
• Tim Ventura's 90's-era Railgun Designs
• American Antigravity's Mass-Driver Demo + Video
• +EML Laboratory－ Japanese Experiment＋Video

University

• Auburn University Rail Gun

Press and media

• USN sets five-year target to develop electromagnetic gun Jane's Defence Weekly, 20 July 2006
• US Navy Invents Railgun Wired News blog article, Thurs January 18, 2007
• Electromagnetic Railgun Popular Science Article
• Video of Navy railgun test firing, Navy Electromagnetic Launch Facility, Test Shot #1, 2 October 2006. Source: Fredericksburg.com, accessed 30 January 2007
• World’s Most Powerful Rail Gun Delivered to Navy, 14 November 2007