Discover

A 'coilgun' (not to be confused with a railgun) is a type of cannon which uses one or more electromagnetic coils to accelerate a magnetic projectile to high velocity.

Contents

Coilguns versus Gauss guns

Overview

Construction

Main Coilgun Types

Potential uses

Limitations

Coilguns in science fiction

Calculations

External links

References

Coilguns versus Gauss guns

The name 'Gauss gun' is a reference to Carl Friedrich Gauss, who formulated mathematical descriptions of the electromagnetic effect used by magnetic accelerators. Coilguns work according to the same magnetic principle as the Gauss gun, but their methods vary: both guns use an attractive magnetic force to accelerate the projectile, but Gauss guns use electromagnets while coilguns use solenoids.^[1]

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 (or solenoid) with a ferromagnetic projectile placed at one of its ends. A large current is pulsed through the coil and a strong magnetic field forms, pulling the projectile to the center of the coil. When the projectile nears this point, the coil is switched off and a next coil 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 coils around the track. Power is supplied to the coils 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 the capacitors from electric backpressure after the initial discharge.
A superconducting coilgun called a 'quench gun' could be created by successively quenching a line of adjacent coaxial superconducting coils forming a gun barrel, generating a wave of magnetic field gradient travelling at any desired speed. A travelling 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.

Main Coilgun Types

There are two main types or setups of a coilgun, single stage and multistage. A single stage coilgun uses just one coil to propel a ferromagnetic projectile. A multistage coilgun uses multiple coils 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 coil to increase the speed of the projectile.

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, and the only noise produced is by the projectile moving. 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 betweens drive coils are needed to compensate for this rate. As the projectile increases in speed it reaches drive coils 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 coil 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 also on top of the list of major limitations, when dumping large amounts of electrical energy into a coil, the majority of its energy is lost as it is converted to heat because of resistance. A way of overcoming this would be to use some sort of superconducting material.
:'Energy in the Magnetic field of the coil'
The energy in the magnetic field does not dissipate, it returns to the capacitor when the EMF is removed and the 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.

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]
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 the planet central to the storyline in order to attempt to destroy a target within a deep ocean on that planet. In Peter F. Hamilton's Night's Dawn Trilogy, gauss rifles are small portable coilguns of about 30 kg mass which are mostly used by augmented Marines.
The coil gun first appeared in literature in the 1897 science fiction novel ''A Trip to Venus'' by John Munro (published by Jarrold & Sons, London) as the "'electric gun'". The novel described it in detail as a way to launch vehicles into outer space from the Earth's surface. The book is out of copyright and freely available as a text file from Project Gutenberg here.
In the book 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. Amazingly, the book also describes a passenger capsule equipped with on-board rockets, compressed gas jets, and even retrofired bullets as a means to increase velocity and change direction, as well as the use of planetary atmosphere aerobraking and parachutes for landing on a planet.
John Munro was also author of the ''The Wire and the Wave'' and ''The Story of Electricity''.

Calculations

Energy stored in capacitor:
$E\; =\; \{V^2\; C\; over\; 2\}$
Projectile Kinetic Energy:
$E\_K\; =\; \{mv^2\; over\; 2\}$
Peak Current:
$V\; .\; sqrt\; \{C\; over\; L\}$
Discharge Time:
$pi\; sqrt\; \{LC\}\; over\; 2$
Coil Inductance:


: ''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)

External links

★ Another Coilgun Site

★ Homemade Camera Flash Coilgun

★ Coilgun theory and design

★ Coilgun Systems

★ Homemade Coilgun

★ EM-15 Coilgun project Thinkbotics Labs

★ World's Coilgun Arsenal

References

1. B. N. Turman and R. J. Kaye, EM Mortar Technology Development For Indirect Fire, Sandia National Laboratories, Albuquerque, NM, 87185-1182
2. Mercenary: Traveller Volume 4, , Frank, Chadwick, Game Designers' Workshop, ,