PowerLabs Coil Gun Page:
I believe most of us science types have played with
solenoids (coils of wire forming an air cored electromagnet) when we were kids.
I remember being particularly interested in them when I was about 7 years old,
and sure enough it did not take long for me to realize that if a solenoid could
pull metal into its core, it could also shoot that metal metal out if the field
was turned off at just the right time...
It would be many years until I came back to that concept, but when I was working in Holland 1998 with a high energy capacitor bank the idea re-occurred to me and I decided to put it to work. After several tries and some very interesting results I have decided to share some my acquired knowledge on the web. Partly because many people have asked me to, but also because the few information that is out there is either incomplete, or inaccurate, and the gauss guns on display are (in my humble opinion) too weak to be of any real interest. After several years investing money on energy storage and switching systems, and researching the underlying principles of these devices, POWERLABS was able to develop linear electromagnetic mass accelerators which are now capable of matching muzzle energies of small caliber firearms. This page is dedicated to showcase the latest advances in those accelerators and their development. But first a warning: The devices described on this page deal with energy storage systems that are easily capable of delivering LETHAL electric shocks, and can fire projectiles capable of causing serious injury of even death. Do not attempt to replicate any of this research unless you are fully qualified to do so! POWERLABS and its creator assume no responsibility for any damages occurred from experiments derived from this page.
Also, a quick note: some of the information displayed here is fruit of very extensive (both time and money consuming) research I have done independently. I would appreciate it if you were so kind as to give me credit for any ideas implemented from the knowledge acquired on this page.
Gauss Guns, also known as Coil Guns, derive their name from the Mathematician Gauss, who's name is a measure of magnetic field strength (and since the propellant force in a gauss gun is a magnetic field, it makes sense to call them by the unit of that field). The scientific definition of a gauss gun is that of an "Asynchronous linear induction motor". In their most basic level, gauss guns can be described as pulsed solenoids with a moveable core. A solenoid of course being an air cored electromagnet. When power is applied to the coil, it will produce a bipolar magnetic field originating at it's center and fading in strength as it gets further from the middle. Anything placed in one of it's extremities will be pulled towards the center. This makes solenoids useful for many applications, such as remotely actuated valves, relays, switches, or control surfaces in airplanes and robotics, in which this pulling motion is used to do any type of mechanical work. However, what differs a gauss gun from just a regular solenoid is that in the gauss gun the core is allowed to move freely into and than THROUGH the solenoid. For the coil must pulsed with a short, but intense electric current, which will form the magnetic field that pulls the core towards the center, but is depleaded by the time the core has reached the middle point of the field; Hence, with nothing to hold it back, the core (which has now become a projectile) exits through the other end... That's it in simple terms... However, there are problems...
First of all the practical limitations: The complete design of a gauss
gun with all of its parameters kept dynamic (I.E. Capacitor, Coil and Projectile
parameters chosen at random and matched to achieve maximum efficiency) is a MATHEMATICAL
IMPOSSIBILITY! Just like attempting to calculate maximum
efficiency for propeller shapes (New Scientist, Nov 99) or calculating dynamic gravitational interaction
between several distinct bodies, all of which are allowed to move (typical
calculus practice problem), designing a
coil gun so that all of its parameters are freely variable an yet match up to provide maximum
efficiency in accelerating a projectile is impossible BY DEFINITION.
But this of course does not make it impossible to near perfection to as high a degree of closeness as can be afforded. The reason why the mathematical approach breaks down is due to the fact that not only are there a very large number of variables, but also these variable are infinitely variable within themselves (E.G. There can be any number of turns, any coil length, any capacitor energy storage, any projectile mass that fits the numeric system). So, unless certain variables are fixed, the equation simply becomes unsolvable. In the two other examples mentioned above (the gravitational attraction for multiple dynamically moving masses and the propeller shape vs efficiency) it is possible to reach a solution by finite integrals: The simulation is re-run several number of times with an efficiency result being recorded. As the simulation is run more and more times several high efficiency results are obtained and the largest one is stored for comparison. It is seen that as the number of integrations nears infinity the difference between the highest efficiency values reached with different parameters nears zero. The simulation is stopped when a high enough efficiency value has been reached. I believe it may be possible to design such a program to project any coilgun given a set of values for, say, energy storage and desired projectile mass. However, finite element analysis computer programs are highly specialized and can not be adapted from other tasks. Furthermore, their design is well beyond all of my (admittedly feeble) programming abilities.
Hence, I have developed another approach. First, let us look at the variables:
The final kinetic energy (1/2MV^2) of the projectile will be a direct
product of the energy delivered to it times the gun's overall efficiency loss.
Since high energy power supplies are very expensive, dangerous, and difficult to
obtain, it follows that one would want to maximize efficiency in order to obtain
the best possible results with the least energy possible. Maximizing efficiency
also preserves the components of the gun since most energy losses are dissipated
in the system as damaging heat or destructive back currents.
In order to maximize the efficiency of the solenoid in pulling the projectile through we must first maximize its force. The force is a direct product of magnetic field strength (B), which is in turn given by the equation B = �0NI/L. We can thus see that the magnetic field of a solenoid is directly proportional to the current and the number of turns. Without going into details on the equation (we will do that on the Solenoid part of this file), it is important to know that both a high number of turns and a high current are necessary. However, a high number of turns implies a high impedance (sqrt(L+R)). Therefore it follows that a Gauss Gun's Energy Storage System (or Power Supply) will have to be capable of supplying not only a very high current pulse, but also at a high voltage. This, unfortunately, rules out all but the most specialized kinds of batteries and chemical energy storage devices, that produce very modest voltage outputs and are incapable of supplying very large currents (the currents spoken off here run in the thousand of amperes range). Generators, alternators, and other such energy storage devices are also unfeasible. The only energy storage devices capable of producing a pulse of sufficient magnitude to power a coil gun are:
Super conducting rings (experimental, but very high energy densities and peak current capabilities).
Compulsators (overly complex to build, but currently the most promising (high power density, high current capability, multiple shot ability).
Homopolar Generators (capable of high current pulses, but unfeasible at small scales due to lower voltages).
Pulse Transformers (inductors); sometimes employing explosive magnetic field compression. The actual design of a basic inductor is simple, but the need of winding extremely thick wires around heavy metal cores makes them very difficult to construct).
Capacitors. Capacitors are the most widely used power source for energy discharge experiments: From Z-pinch driven nuclear fusion to lasers, rail guns, coil guns, EMP generators and hypersonic metal forming, capacitors provide the ultra fast, extremely high power impulses required to achieve the currents to drive the enormous magnetic field these devices require. Although high performance high voltage pulse capacitors are extremely expensive, costing several thousand dollars per kilojoules, the (relatively) lower current requirements of coil guns make it possible for non pulse rated capacitors to be used on them with good results. Amongst the worst capacitors in terms of pulse performance are electrochemical capacitors (these are also very low voltage, thus being unpractical on coil guns) and electrolytic capacitors. However, electrolytic capacitors also happen to be the cheapest capacitors around, so it is not surprising that just about amateur every coil gun uses. Electrolytic capacitors come in many types: Computer grade, inverter grade, and pulse rated. Pulse Rated electrolytic capacitors include those used in camera flashes, and those I use on my multi stage coilgun. They differ from regular electrolytics by making use of thicker aluminium plates and stronger internal connections. The larger ones also have much bigger terminals, which are a direct indication of their intended high peak current. These capacitors are the perfect compromise between price, energy density and performance on a coilgun: They cost less than pulse capacitors and are lighter for the same energy storage, and their performance is only slightly below what a real pulse cap can achieve. Photo Flash capacitors also work well, though the smaller units require so many connections that a lot of resistance builds up on larger banks. Inverter grade capacitors are reasonable since they are made to operate at higher RMS currents and duty cycles than regular electrolytics; the computer grade capacitors. Even the worst electrolytic capacitors can be made to work in a coilgun, provided, as was said in the beginning of this tutorial, that they are of high enough voltage. 450V peak capacitors and 550V peak capacitors are the best ones because they do not require excessive series connection; because they have a very high ESR (Equivalent Series Resistance), electrolytic capacitors should not be placed in series for pulse applications because the ESR of the bank can very quickly become larger than the impedance of the load, and if that happens most of the energy of the discharge will be dissipated internally, damaging the capacitors. With pulse rated electrolytics it is possible to place some in series and thus obtain performance gains. My current systems operate at 900V, though even higher voltages would be desirable. The most efficient coilgun ever developed was fired at 3,5kV per stage.
The second greatest problem encountered in coilgun design is devising a
means to switch the power of the discharge, which very frequently runs in the
multi-megawatt range. Because the current leaving the energy storage system are
extremely high (thousands to tens of thousands of amperes), any small amount of
resistance will generate massive I^2R losses. If a mechanical switch is used to
deliver the pulse, the very moment the switch surfaces come into contact
microscopic irregularities in the switch material will start to conduct the
pulse at a higher resistance, dissipating so much energy as to be vaporized
right off. When the switch finally closes in its entirety, these vaporized
surfaces and the molten metal beneath them weld together, and the switch is thus
ruined. On higher power coil guns, the use of high voltage makes it possible for
the impulse to be switched by the means of a spark gap; triggered gaps
(trigatrons) are most often the choice as they allow precise control over when
the discharge happens. However, any spark occurring at these powers will mean
very large (30 - 40%) losses and will be very destructive to the electrodes. Air
gaps are unadvisable as the noise levels encountered are excessive. The most
efficient way to switch the discharge is through a solid state switch. These can
be Thiristors (SCRs), IGBTS, or even some types of MOSFETS and Transistors. SCRs
are currently the cheapest and most powerful solid state switches in the market
due to mass production arising from the newly implemented DC power grids around
the world. This unfortunately does not mean they are cheap, as an SCR capable of
reliably switching a coilgun impulse is not a common market device and tends to
cost over 100 dollars. However, the gains over spark gaps (1 - 3% losses
compared to 30 - 40% for spark dissipation) and the absolute lack of noise,
electrode erosion, and maintenance makes them highly attractive and professional
on coil guns. The SCR must have an RMS voltage rating equal to or superior to
the maximum capacitor charge voltage, and a peak current rating equal to or
larger than the peak current encountered on the circuit during a maximum power
shot. Furthermore, because they are essentially diodes (SCR = Silicon Controlled
Rectifier), the SCRs used have to be protected by a diode wired in reverse
across the junction, so as to shunt all cEMF coming back from the inductor
A coil gun projectile must be
ferromagnetic (attracted by magnetic field) but also non magnetizeable. The
reason for that is because if the projectile was to become magnetized during
firing, energy would be lost in the magnetization process which would not
contribute to the final kinetic energy. It must have a high magnetic
permeability, since the higher the magnetic permeability, the stronger the
magnetic field it will concentrate. The use of silicon steel alloys
which can not be permanently magnetized (also good because they are dense and
very hard, thus making effective projectiles) or ferrites, as are used in high
frequency transformers. Ferrite, although being easier to find, makes inferior
projectiles for being of lower density, lower magnetic permetivity, and also
being very brittle and hard to machine. The second point to be considered in the
projectile is its mass: Very light projectiles will accelerate faster and to
higher velocities and will therefore require faster pulses. Since faster pulses
come with higher amperages, it is very easy to exceed the maximum ratings on the
switching system and destroy it. That is the main reason why supersonic coil guns
have so far been impossible for amateurs. Finally, the projectile should be as
long as possible and not employ a sharp point, as it would add extra weight with
very little extra impulse (magnetic coupling). The ideal projectile is
cylindrical and has a length that amounts to several times its diameter. The
worst possible projectile is spherical, as it concentrates the maximum possible
mass on the lowest possible volume, and thus achieves very little acceleration.
Here is where all the components of the gun come together and must
match perfectly: The solenoid must provide the greatest amount of magnetic field
coupling possible with the projectile, as this determines the pulling force and
thus the acceleration. Ideally, it would have the same length as the projectile
and have zero wall thickness. Since the windings must be protected from abrasion
by the moving projectile, some sort of guiding tube is required; this should be
as thin as possible while still having enough structural integrity to withstand
the compressive forces encountered as the two poles of the coil attract each
other. My first experiments with plastic and glass coils indicate that only
metal tubes can be thin enough and yet withstand the forces. Unfortunately,
metals inside changing magnetic fields will produce very high eddy losses, which
means that the coil form must have a gap across its entire length while inside
the coil in order to work. This makes the design rather non-trivial, but is a
necessity. The actual length of the solenoid will determine the coupling ratio
with the projectile. A 1:1/2 coupling ration would mean that 25% of the energy
could be delivered into the projectile (since it starts outside with near 0
coupling and ends up taking up half of the coil, with 50% coupling, the final
value is an average of both couplings). That value minus resistive losses gives
us the efficiency. The most efficient gun ever developed used a 1:0.8 coupling
ratio. Unfortunately, as with the high speed projectile example, the higher the
coupling ratio, the shorter the pulse required to accelerate the projectile
within such a short distance. This very quickly runs into a power switching
barrier (pulse length becomes so short that the current required goes above the switch's
ratings). What is typically done is a realistic coupling ratio is chosen and
than the number of turns is varied so as to vary the inductance to obtain the
pulse length required to accelerate the projectile right to the middle of the
coil while spending all of the energy stored at the capacitors.
This represents the best possible way to obtain maximum efficiency
without excessive trial-and-error.
UNDER CONSTRUCTION. Pictures and further information soon to come!
PowerLabs 3000-Joule coilgun: Kinetic
energies matching a .22 rifle with a single magnetic impulse! Videos up for
PowerLabs 7000-Joule Multi Stage GaussGun: 5 stages. The first stage is now complete, and is able to fire the projectile at over 270km/h! Videos available for download.
PowerLabs Advanced Coil Gun Research: My latest and most efficient Coil Gun so far.
Questions? Comments? Mail me!
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Last updated 11/02/10
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