Coilgun

A coilgun (not to be confused with a railgun) is a type of magnetic accelerator gun (MAG) or magnetic accelerator cannon (MAC). It uses one or more electromagnetic coils to accelerate a magnetic shell to very high velocities. 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.

Overview

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

Construction

Simplified coilgun diagram with three coils, a barrel and ferromagnetic projectile. Each of the coils is connected to a fast discharge energy storage source (capacitor bank) and activated in sequence by a timing device thus producing a strong magnetic field causing the rapid acceleration of the projectile.

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Electrical Schematic of a Single Stage Coilgun switched by SCR(Silicon Controlled Rectifier).

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Multistage coilgun

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Two coils from a multistage coilgun

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Multistage coilgun

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 of high-capacity high voltage capacitors designed for fast energy discharge. A rectifier "absorbs" the energy when the electrical current is finished and stops the capacitor from burning out from the electrical surge.

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 above 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 may 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 to increase. 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 resistance exerted on the projectile due to air friction and friction in the barrel the projectile will no longer gain in 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 polarised 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 sci-fi role playing and video games, where they go under such names as Gauss cannon, Gauss rifle, or Magnetic Accelerator Cannon. The examples are numerous. One example is presented in Neal Asher's recent 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. It is also seen in the Mechwarrior and Battletech games in the form of the naval and vehicle mounted Gauss rifles. Another example is Valve's first Half-Life, the gauss gun being a popular weapon for gaining massive speed and altitude in deathmatch. Yet another example is in the Halo series of video games by Bungie,in which the Magnetic Accelerator Cannon is used by the humans as a weapon in ship-to-ship combat or to destroy ships(Super MAC gun)(Example-Cairo Defense Platform/Cairo Station)

The coil gun was actually first called the "electric gun" and first described in detail as a way to launch vehicles into outer space from the earth's surface in the 1897 science fiction novel "A Trip to Venus" by John Munro and published in 1897 by Jarrold & Sons, London. The book is out of copyright and freely available as a text file from the project gutenberg.org website at http://www.gutenberg.org/etext/13716 .

In the book Munro describes in great detail multiple coils fired in sequence by solenoids at a proper timing to achieve acceleration without too high of g forces affecting the passengers. The gun could be angled on a hillside if desired. Amazingly, this book also describes in detail electric gun launch for a passenger capsule equipped with onboard 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:

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

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)

External links

• Camera Flash Coilgun
• Another Coilgun Site
• Coilgun Systems
• EM-15 Coilgun project Thinkbotics Labs
• World's Coilgun Arsenal