Coilgun
A coilgun is a type of
Coilguns generally consist of one or more coils arranged along a barrel, so the path of the accelerating projectile lies along the central axis of the coils. The coils are switched on and off in a precisely timed sequence, causing the projectile to be accelerated quickly along the barrel via magnetic forces.
Coilguns are distinct from railguns, as the direction of acceleration in a railgun is at right angles to the central axis of the current loop formed by the conducting rails. In addition, railguns usually require the use of sliding contacts to pass a large current through the projectile or sabot, but coilguns do not necessarily require sliding contacts.[2] While some simple coilgun concepts can use ferromagnetic projectiles or even permanent magnet projectiles, most designs for high velocities actually incorporate a coupled coil as part of the projectile.
Coilguns are also distinct from Gauss guns, although many works of science fiction have erroneously confused the two. A coil gun uses electromagnetic acceleration whereas Gauss guns predate the idea of coil guns and instead consists of ferromagnets using a configuration similar to a Newton's Cradle to impart acceleration.[3]
History
The oldest electromagnetic gun came in the form of the coilgun, the first of which was invented by Norwegian scientist Kristian Birkeland at the University of Kristiania (today Oslo). The invention was officially patented in 1904, although its development reportedly started as early as 1845. According to his accounts, Birkeland accelerated a 500-gram projectile to approximately 50 metres per second (160 ft/s).[4][5][6]
In 1933, Texan inventor Virgil Rigsby developed a stationary coil gun that was designed to be used similarly to a machine gun. It was powered by a large electrical motor and generator.[7] It appeared in many contemporary science publications, but never piqued the interest of any armed forces.[8]
Construction
There are two main types or setups of a coilgun: single-stage and multistage. A single-stage coilgun uses one electromagnetic coil to propel a projectile. A multistage coilgun uses several electromagnetic coils in succession to progressively increase the speed of the projectile.
Ferromagnetic projectiles
For ferromagnetic projectiles, a single-stage coilgun can be formed by a coil of wire, an
In a multistage design, further electromagnets are then used to repeat this process, progressively accelerating the projectile. In common coilgun designs, the "barrel" of the gun is made up of a track that the projectile rides on, with the driver into the magnetic coils around the track. Power is supplied to the electromagnet from some sort of fast discharge storage device, typically a
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.[9][10]
Non-ferromagnetic projectiles
Some designs have non-ferromagnetic projectiles, of materials such as
Though the cost of power switching and other factors can limit projectile energy, a notable benefit of some coilgun designs over simpler railguns is avoiding an intrinsic velocity limit from hypervelocity physical contact and erosion. By having the projectile pulled towards or levitated within the center of the coils as it is accelerated, no physical friction with the walls of the bore occurs. If the bore is a total vacuum (such as a tube with a plasma window), there is no friction at all, which helps prolong the period of reusability.[14][15]
Switching
One main obstacle in coilgun design is switching the power through the coils. There are several common solutions—the simplest (and probably least effective) is the
A quick-and-dirty method for switching, especially for those using a flash camera for the main components, is to use the flash tube itself as a switch. By wiring it in series with the coil, it can silently and non-destructively (assuming that the energy in the capacitor is kept below the tube's safe operating limits) allow a large amount of current to pass through to the coil. Like any flash tube, ionizing the gas in the tube with a high voltage triggers it. However, a large amount of the energy will be dissipated as heat and light, and, because of the tube being a spark gap, the tube will stop conducting once the voltage across it drops sufficiently, leaving some charge remaining on the capacitor.
Resistance
The
At low speeds the heating of the coils dominates the efficiency of the coilgun, giving exceptionally low efficiency. However, as speeds climb, mechanical power grows proportional to the square of the speed, but, correctly switched, the resistive losses are largely unaffected, and thus these resistive losses become much smaller in percentage terms.
Magnetic circuit
Ideally, 100% of the magnetic flux generated by the coil would be delivered to and act on the projectile; in reality this is impossible due to energy losses always present in a real system, which cannot be eliminated.
With a simple air-cored solenoid, the majority of the magnetic flux is not coupled into the projectile because of the magnetic circuit's high
Reverse charging can be prevented by a diode connected in reverse-parallel across the capacitor terminals; as a result, the current keeps flowing until the diode and the coil's resistance dissipate the field energy as heat. While this is a simple and frequently utilized solution, it requires an additional expensive high-power diode and a well-designed coil with enough thermal mass and heat dissipation capability in order to prevent component failure.
Some designs attempt to recover the energy stored in the magnetic field by using a pair of diodes. These diodes, instead of being forced to dissipate the remaining energy, recharge the capacitors with the right polarity for the next discharge cycle. This will also avoid the need to fully recharge the capacitors, thus significantly reducing charge times. However, the practicality of this solution is limited by the resulting high recharge current through the equivalent series resistance (ESR) of the capacitors; the ESR will dissipate some of the recharge current, generating heat within the capacitors and potentially shortening their lifetime.
To reduce component size, weight, durability requirements, and most importantly, cost, the magnetic circuit must be optimized to deliver more energy to the projectile for a given energy input. This has been addressed to some extent by the use of back iron and end iron, which are pieces of magnetic material that enclose the coil and create paths of lower reluctance in order to improve the amount of magnetic flux coupled into the projectile. Results can vary widely, depending on the materials used; hobbyist designs may use, for example, materials ranging anywhere from magnetic steel (more effective, lower reluctance) to video tape (little improvement in reluctance). Moreover, the additional pieces of magnetic material in the magnetic circuit can potentially exacerbate the possibility of flux saturation and other magnetic losses.
Ferromagnetic projectile saturation
Another significant limitation of the coilgun is the occurrence of
Projectile magnetization and reaction time
Apart from saturation, the B(H) dependency often contains a
Induction coilguns
Most of the work to develop coilguns as hyper-velocity launchers has used "air-cored" systems to get around the limitations associated with ferromagnetic projectiles. In these systems, the projectile is accelerated by a moving coil "armature". If the armature is configured as one or more "shorted turns" then induced currents will result as a consequence of the time variation of the current in the static launcher coil (or coils).
In principle, coilguns can also be constructed in which the moving coils are fed with current via sliding contacts. However, the practical construction of such arrangements requires the provision of reliable high speed sliding contacts. Although feeding current to a multi-turn coil armature might not require currents as large as those required in a railgun, the elimination of the need for high speed sliding contacts is an obvious potential advantage of the induction coilgun relative to the railgun.
Air cored systems also introduce the penalty that much higher currents may be needed than in an "iron cored" system. Ultimately though, subject to the provision of appropriately rated power supplies, air cored systems can operate with much greater magnetic field strengths than "iron cored" systems, so that, ultimately, much higher accelerations and forces should be possible.
Formula for exit velocity of coilgun projectile
An approximate result for the exit velocity of a projectile having been accelerated by a single-stage coilgun can be obtained by the equation[17]
m being the mass of the projectile, defined as kg
V being the volume of the projectile, defined as m3
μ0 being the vacuum permeability, defined in SI units as 4π × 10−7 V·s/(A·m)
χm being the magnetic susceptibility of the projectile, a dimensionless proportionality constant indicating the degree of magnetization in a material in response to applied magnetic fields. This often must be determined experimentally, and tables containing susceptibility values for certain materials may be found in the CRC Handbook of Chemistry and Physics as well as the Wikipedia article for magnetic susceptibility.
n being the number of coil turns per unit length of the coil, which can be found by dividing the total turns of the coil by the total length of the coil in meters.
and I being the current passing through the coil in
While this approximation is useful for quickly defining the upper limit of velocity in a coilgun system, more accurate and non-linear second order differential equations do exist.[17] The issues with this formula being that it assumes the projectile lies completely within a uniform magnetic field, that the current dies out instantly once the projectile reaches the center of the coil (eliminating the possibility of coil suck-back), that all potential energy is transferred into kinetic energy (whereas most would go into frictional forces), and that the wires of the coil are infinitely thin and do not stack on one another, all cumulatively increasing the expected exit velocity.[17]
Uses
Small coilguns are recreationally made by hobbyists, typically up to several joules to tens of joules projectile energy (the latter comparable to a typical air gun and an order of magnitude less than a firearm) while ranging from under one percent to several percent efficiency.[18]
In 2018, a Los Angeles-based company
In 2022 Northshore Sports Club, an American gun club in Lake Forest, Illinois began distributing the CS/LW21, also referred to as the "E-Shotgun", a compact, 15 joule magazine fed coil gun, manufactured by the China North Industries Group Corp.[21] They project distribution to reach 5000 units per year in the US,[22][23] and the manufacturer has also unveiled plans to supply the Chinese police and military with units for "non-lethal riot control".[24]
Much higher efficiency and energy can be obtained with designs of greater expense and sophistication. In 1978, Bondaletov in the USSR achieved record acceleration with a single stage by sending a 2-gram ring to 5000 m/s in 1 cm of length,
Though they face the challenge of competitiveness versus conventional guns (and sometimes railgun alternatives), coilguns are being researched for weaponry.[27]
The DARPA Electromagnetic Mortar program is one example, if practical challenges like sufficiently low weight can be achieved. The coilgun would be relatively silent with no smoke giving away its position, though a supersonic projectile would still create a sonic boom. Adjustable, smooth acceleration of the projectile along the barrel length would allow higher velocity, with a predicted range increase of 30% for a 120mm EM mortar over the conventional version of similar length. With no separate propellant charges to load, the researchers envision the firing rate to approximately double.[27][28]
In 2006, a 120mm prototype was under construction for evaluation, though a tenuous time for deployment was then estimated to be 5 to 10+ years by Sandia National Laboratories.[27][28] In 2011, development was proposed for an 81mm coilgun mortar to operate with a hybrid-electric version of the future Joint Light Tactical Vehicle.[29][30]
Coilgun potential has been perceived as extending beyond military applications.
Few entities could overcome the challenges and corresponding capital investment to fund gigantic coilguns with projectile mass and velocity on the scale of
- An ambitious lunar-base proposal considered in a 1975 L5 in support of massive space colonization utilizing a large 9900-ton power plant).[33]
- A 1992 NASA study calculated that a 330-ton lunar quench gun (superconducting coilgun) could launch 4400 projectiles annually, each 1.5 tons and mostly liquid oxygen payload, using a relatively small amount of power, 350 kW average.[34]
- After a NASA Ames study of aerothermal requirements for heat shields with terrestrial surface launch, Sandia National Laboratories investigated electromagnetic launchers for spacecraft and researched other EML applications using both railguns and coilguns. In 1990 a kilometer-long coilgun was proposed for launch of small satellites.[35][36]
- Later investigations at Sandia included a 2005 proposal of the StarTram concept for an extremely long coilgun, one version conceived as launching passengers to orbit with survivable acceleration.[37]
- A mass driver is essentially a coilgun that magnetically accelerates a package consisting of a magnetizable container with a payload. Once accelerated, the container and payload separate, the container is slowed and recycled to receive another payload.
See also
- Electromagnetic propulsion
- Carl Friedrich Gauss
- Helical railgun
- Light-gas gun
- Mass driver
- Edwin Fitch Northrup
- Plasma railgun
- Railgun
- Ram accelerator
- Solenoid
- Tubular linear motor
References
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- ^ "Compact Coilgun". lukeallen.org. Archived from the original on 2011-05-18. Retrieved 2011-05-08.
- ^ "Coil Gun Kit Instructions From Disposable Camera". Angelfire. Archived from the original on 2011-06-24. Retrieved 2011-05-08.
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- ^ "You Can Now Buy a Practical Gauss Gun". Hackaday. 2018-07-12. Archived from the original on 2018-08-07. Retrieved 2018-08-07.
- ^ "Handheld railgun as powerful as an air rifle to go on sale in the US". NewScientist. 2021-08-12. Archived from the original on 2021-10-07. Retrieved 2021-10-07.
- ^ "China Rolls Out Electromagnetic Weapon to Quell Violent Protests". Bloomberg. 17 April 2023.
- ^ "Coil Accelerator". North Shore Sports Club. Archived from the original on 2022-07-18. Retrieved 2022-07-06.
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