Coilgun

A coilgun or Gauss gun is a type of projectile accelerator that uses one or more electromagnetic coils to accelerate a magnetic projectile to high velocity. Coilguns accelerate the projectile using contactless means. Not to be confused with Gaussian Gun

Coilguns consist of one or more coils arranged along the barrel that are switched in sequence so as to ensure that the projectile is accelerated quickly along the barrel via magnetic forces.

It is distinctly separate from a railgun, which pass a large current through the projectile or sabot via sliding contacts.

Because coilguns have no sliding contact, no wear or erosion occurs to the barrel, and the working life of a coilgun is potentially infinite.

Etymology

The name Gauss gun is a reference to Carl Friedrich Gauss, who formulated mathematical descriptions of the electromagnetic effect used by magnetic accelerators. The term Coilgun refers to the coils of wire forming electromagnets that are used to accelerate the projectile. Some confusion is caused by the use of separate terms. However, there is no difference between a coilgun and a Gauss gun.

Overview

Many hobbyists use low-cost rudimentary designs to experiment with coilguns, for example using photoflash capacitors from a disposable camera, or a capacitor from a standard cathode-ray tube television as the energy source, and a low inductance coil to propel the projectile forward.

Construction

A coilgun, as the name implies, consists of a coil of wire - an electromagnet - with a ferromagnetic projectile placed at one of its ends. Effectively a coilgun is a solenoid: an electromagnetic coil with the function of drawing a ferromagnetic object through its center. A large current is pulsed through the coil of wire and a strong magnetic field forms, pulling the projectile to the center of the coil. When the projectile nears this point the electromagnet is switched off and the next electromagnet can be switched on, progressively accelerating the projectile down successive stages. In common coilgun designs the "barrel" of the gun is made up of a track that the projectile rides on, with the driver electromagnetic coils around the track. Power is supplied to the electromagnet from some sort of fast discharge storage device, typically a battery or high-capacity high voltage capacitors designed for fast energy discharge. A rectifier is used to protect polarity sensitive capacitors (such as electrolytics) from damage due to inverse polarity of the current after the discharge.

A superconductor coilgun called a quench gun could be created by successively quenching a line of adjacent coaxial superconducting electromagnetic coils forming a gun barrel, generating a wave of magnetic field gradient traveling at any desired speed. A traveling superconducting coil might be made to ride this wave like a surfboard. The device would be a mass driver or linear synchronous motor with the propulsion energy stored directly in the drive coils.^{[citation needed]}

Main coilgun types

There are two main types or setups of a coilgun, single stage and multistage. A single stage coilgun uses just one electromagnet to propel a ferromagnetic projectile. A multistage coilgun uses multiple electromagnets in succession to progressively increase the speed of the projectile. The animated figure (top right) is an example of a multistage coilgun, as it uses more than one electromagnetic coil to increase the speed of the projectile.

Switching

One main obstacle in coilgun design is switching the power through the coils. There are several main options- the most simple (and probably the least effective) is the spark gap, this releases the stored energy through the coil when the voltage reaches a certain threshold. A better option is to use solid-state switches - these include IGBTs (which can be switched off mid-pulse) and SCRs (which release all stored energy before turning off).^[1]

Potential uses

Like railguns and ram accelerators, coilguns have been proposed for use in delivering payloads to space.

A coilgun has no moving parts other than the projectile, the only noise produced is by the projectile moving and the theoretic maximum speed of the projectile is extremely high. These attributes make it attractive as a weapon.

Limitations

Despite heavy research and development by the amateur and professional community, great obstacles have yet to be overcome.

Projectile saturation

One of the greatest limitations to the coil gun is the rate at which the ferromagnetic projectile becomes fully saturated by the magnetic field and the rate at which it loses its magnetic saturation. Once a ferromagnetic object becomes completely saturated the amount of force in which it can be attracted stops increasing. The rate at which the projectile loses its saturation is critical; as this rate is constant, greater distances between drive electromagnets are needed to compensate for this rate. As the projectile increases in speed it reaches drive electromagnets at progressively faster rates. Without compensation for desaturation time, there will be less and less effect to the velocity of the projectile, resulting in exponentially lower efficiency per drive electromagnet stage as the projectile travels down the line. Once the amount of force exerted to the projectile is less than or equal to the amount of resistance exerted on the projectile due to air friction and friction in the barrel the projectile will no longer gain velocity.

Resistance

Coil resistance is a major limitation because when dumping large amounts of electrical energy into a conductor the majority of the energy is converted to heat due to resistance and therefore effectively lost as it is not driving the projectile. This could be overcome through the use of a superconducting material, however presently these are impractical.

Energy in the magnetic field of the coil

The energy in the magnetic field does not dissipate, much of it returns to the capacitor when the EMF is removed and the electric current is decreasing. Unfortunately it does this in the reverse direction (via a 'ringing' mechanism), which can seriously damage polarized capacitors (such as electrolytics).

In the circuit it appears as if the magnetic field keeps the current in the coil flowing after the capacitor has discharged, so that it keeps discharging and builds up a negative voltage. This is similar to an LC Oscillator.

The capacitor charging to a negative voltage can be prevented by placing a diode across the capacitor terminals.

Calculations

Energy stored in capacitor:

$E={CV^{2} \over 2}$

Projectile Kinetic Energy:

$E_{K}={mv^{2} \over 2}$

Peak Current (assuming $R\ll {\sqrt {L \over C}}$ where R is coil resistance):

$V.{\sqrt {C \over L}}$

Discharge Time:

$\pi {\sqrt {LC}} \over 2$

Coil Inductance:

$L={\frac {0.8r^{2}N^{2}}{6r+9l+10d}}$

L = inductance in µH

r = mean radius of coil in inches

l = physical length of coil winding in inches

N = number of turns

d = depth of coil in inches (i.e., outer radius minus inner radius)

Images

Typical single stage coilgun

Typical single stage coilgun
Multistage coilgun

Multistage coilgun
Multistage coilgun

Multistage coilgun
Two coils from a multistage coilgun

Two coils from a multistage coilgun

Coilguns in science fiction

Coilguns are a popular device in science fiction, especially science fiction role playing and video games, where they go under such names as Gauss cannon, Gauss rifle, or Magnetic Accelerator Cannon. The examples are numerous. The earliest role-playing game appearance was in Traveller book 4, Mercenary, in the form of the vehicle-mounted VRF (very rapid fire) Gauss Gun and the more advanced small arms Gauss Rifle. ^[2]

The coil gun first appeared in literature as the "electric gun" in the 1897 science fiction novel A Trip to Venus by John Munro (published by Jarrold & Sons, London). The novel described in detail a way to launch vehicles into outer space from the Earth's surface. In the novel, Munro describes in great detail multiple coils fired in sequence by solenoids timed to achieve acceleration without generating g forces that would harm passengers. The gun could be angled on a hillside if desired.

A recent literary example is Neal Asher's novel, The Voyage of the Sable Keech, in which an alien spaceship deploys a coilgun in orbit around a planet to attempt to destroy a target within a deep ocean on that planet. Another example is Peter F. Hamilton's Night's Dawn Trilogy, in which gauss rifles are small portable coilguns of about 30 kg mass which are mostly used by augmented Marines and specially adapted mercenary troops.

Video games

In the Fallout series game Fallout 2, the Rheinmetall M72 Gauss Rifle and the Walther PPK12 Gauss Pistol represent cutting edge technology lost in the great war. There is also a Gauss Minigun in Fallout Tactics.

In the original Half-Life game one of the main weapons was a gauss gun, renamed the Tau cannon. The recoil could also be used to gain height.

In Bungie's Halo series, most of Earth's capital starships are equipped with MAC (Magnetic Accelerator Cannon) platforms which fired tungsten shells with a depleted uranium core at their targets. A later version of the Warthog ATV was mounted with a gauss gun in place of the previous chaingun.

In the popular Battletech universe the "Gauss rifle" was a mainstay weapon throughout most of the fiction, associated tabletop game, and the long-running video game franchise.

In the popular Real Time Strategy Game "Starcraft" Gauss rifles are the main wepons for the Marine units.

The battleships in the Stargate television series are equipped with rail guns for the majority of their use in the series.

In the popular table-top wargame, Warhammer 40,000, the Necrons, use a weapon system which is referred to as "gauss weapons",^[3] but which bears no resemblance to actual Gauss technology. The Necron weapons are depicted as having transparent green barrels, which rapidly remove layers of atoms from the target and pull them towards the weapon.^[4]

In the video game Crysis, one weapon made available to the player in the second half of the game is a Gauss rifle.

In Syndicate, the "Gauss gun" is essentially a rocket launcher with a huge explosion radius.

References

^ "Room 203 Technology". Coil Gun. {{cite web}}: Unknown parameter |accessmonthday= ignored (help); Unknown parameter |accessyear= ignored (|access-date= suggested) (help)
^ Chadwick, Frank (1978). Mercenary: Traveller Volume 4. Normal Illinois: Game Designers' Workshop. pp. 36–37, 39, 52.
^ "Who are the Necrons?". Games Workshop.
^ Codex: Necrons. Games Workshop. ISBN 1841541907.

External links

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