Railgun

The theory and construction of a railgun are quite simple at first sight, but horribly complex beneath.

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 consits of two parallel metal rails (thus the name) which are slotted on the inside so that a metal projectile can slide between the rails. At one end, the rails are connected to an electrical power supply. When the 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. But since the rails and projectile are carrying an electrical current through this magnetic field, they experience a force; and it so happens that when you run through the maths, the force is trying to push the rails and projectile outwards. Since the rails are firmly mounted they cannot be pushed apart, but 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.

The complexity in railgun design comes from:

1. The need for strong conductive[?] materials to build the rails and projectiles from; 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 be very securely mounted.

1. Power supply design. The power supply must be able to deliver a large current for a tiny amount of time, so capacitors and compulsators are both being pursued; most other power supplies are designed to provide constant power levels for long periods, which is a very different design requirement.

1. Electromechanical design. What is the optimum distance between the rails, the optimum size of the rails, and the optimum shape for the projectile? The shape of the conductors influences the shape of the magnetic field. Ideally, the magnetic field should be as strong as possible in the region of the projectile in order to get the most acceleration. But anything that makes the conductive path longer increases its resistance, and so less current flows. Computer simulation and physical experimentation are being used to try to find optima.

Railguns as weapons

Railguns only fire bullets, not shells. Shells depend on their arrival at the target being more violent than their launching, so they can be detonated by impact without detonating in the barrel of the gun. However, being fired from an anti-tank railgun can be more violent than hitting a tank (the projectile accelerates to maximum velocity in the space of a metre or two, but when it hits the tank it punches straight through and decelerates for several metres), so the detonating mechanism for a shell would have to be fairly complex (and thus expensive, and prone to failure in dangerous ways). The railgun is still an effective weapon as the raw kinetic energy of a railgun projectile will not only punch through armour plating with impunity, it will spray vaporised metal carried on a sizeable shock wave which incinerates the interior of the target (and any occupants) - see KE-penetrator

One could construct a low-velocity railgun that fired shells, but there seems to be little interest in doing so to date; existing chemical propellant systems have that niche quite securely filled, although it is perhaps likely that future railgun artillery would also have a low-velocity shell firing mode for indirect fire and delivering chemical or biological payloads.

The United States military is funding railgun experiments. At the University of Texas' Centre for Electromechanics, military railguns capable of delivering tungsten armour piercing[?] bullets with kinetic energies of nine million joules have been developed. [1] (http://www.iat.utexas.edu/electrodyn) Nine million joules is enough energy to deliver a kilogram of projectile at three kilometres per second - 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 interested in railgun research, too. Current ship guns sit on top of a large room called a magazine, which is full of shells for the gun to fire. 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, all it needs to store to feed the gun are the tungsten bullets, which are much more compact than shells - and the electricity can be supplied from the ship's engines, perhaps buffered in capacitors. These things will explode much less violently than a room full of shells if hit.

Railguns in science fiction

Railguns have started to appear in sci-fi and become a mainstream idea. However, they have been portrayed somewhat inaccurately.

In the film Eraser, the lead character gets hold of a device like a chunky rifle that is said to be a man-portable railgun. It is shown firing bullets through great numbers of walls and so on, and it makes a blue trail in the air. The popular computer game Quake II features a very similar device.

However, man-portable railguns will not be revolutionary weapons; if power supply technology ever lets us make a railgun supply small enough to be carried then rail-handguns will probably only be able to fire projectiles at speeds not much higher than 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; we can quite happily build a handgun that fires 20mm cannon shells, but you couldn't fire it without having your hand broken.

One possible route to explore 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[?].

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.

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.

Further Reading

Theory

• http://www.physics.northwestern.edu/classes/2001Fall/Phyx135-2/19/railgun.htm

Practice

• http://www.powerlabs.org/railgun.htm
• http://www.railgun.org/