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why doesn't the military like Coilguns?

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GreenPenguin

Sun Jun 19 2011, 08:00PM

Registered Member #3957 Joined: Sun Jun 19 2011, 02:17AM
Location: Minnesota, US
Posts: 8

I am beginning a railgun or coilgun project, and I can't really decide what to make. I notice from my research that it is quite possible to make a coilgun with small power supplies such as camera capacitors, this appeals to my small budget, but what are the inherent inefficiencies of a coilgun, that makes the military want to build railguns instead? Railguns it seem take more power for a small gun, and seem to have design flaws limited by our available materials, like the fact that we can't make a railgun that doesn't destroy it's own rails.
I'm hoping some people on this forum who have experience building such devices can explain this. Thanks.

Forty

Sun Jun 19 2011, 08:08PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

it is very, very difficult to design coilguns with a muzzle velocity that the military would find useful. with a railgun you can just dump ridiculous amounts of energy into it to achieve high velocities.

GreenPenguin

Sun Jun 19 2011, 08:12PM

Registered Member #3957 Joined: Sun Jun 19 2011, 02:17AM
Location: Minnesota, US
Posts: 8

so the railguns are inefficient in the fact that they take loads of power, but coilguns are just difficult? What is the power limit on coilguns come from?

Forty

Sun Jun 19 2011, 08:32PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

with high velocity reluctance coilguns, you need fast switching of a lot of energy. as the projectile accelerates faster and faster, the subsequent coils have to be pulsed and turned off within the time frame that the projectile is passing through the coil. this requires powerful semiconductor devices as well as fast and accurate triggering. also, magnetic saturation of the projectile causes less of the energy to be imparted to it as coil energy increases. inductive coilguns do not suffer from the magnetic saturation problem, but their coils must be turned on and off even faster, and also require fast triggering for subsequent coils.
a railgun can be as simple as two mounted rails, with a conducting, non ferromagnetic projectile that fits between them. the acceleration of the projectile is created by the lorentz force, a result of the interaction or the field lines created by the current as it flows up one rail, across the projectile, and down the other rail. since this resultant force depends almost exclusively (<- i'll probably get corrected for saying that) on the current involved, so more energy in equals more energy out, with the only additional complication being the structural reinforcement of the rails.
amateurs more often build coilguns because they are cheaper, safer, and more interesting to design and build

edit: sorry for the wordy, possibly run-on sentences.

GreenPenguin

Sun Jun 19 2011, 09:45PM

Registered Member #3957 Joined: Sun Jun 19 2011, 02:17AM
Location: Minnesota, US
Posts: 8

thanks forty. I am aware of the operation of the two types magnetic acceleration. so the costs of a railgun are higher because of the required power supplies i suppose. I am thinking about building/modding an alternator to deliver pulses of current like a compulsator http://en.wikipedia.org/wiki/Compensated_pulsed_alternator I originally thought it might be a more economical solution when i considered the price of 30-40 caps.

Forty

Sun Jun 19 2011, 09:56PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

they're also more expensive because of the need to replace the rail material. as i once read somewhere "railgun rails last for about 100 shots. give or take 100 shots." If you think you can build a compulsator then go for it. But i know of no amateur built compulsators. it is also difficult to find information about their construction, although i imagine that if one can fully understand all of the electromagnetics of ac motors and alternators, that it would become trivial to construct a compulsator.

GreenPenguin

Sun Jun 19 2011, 10:43PM

Registered Member #3957 Joined: Sun Jun 19 2011, 02:17AM
Location: Minnesota, US
Posts: 8

Just as Trivial as any other project concocted by people on this forum.

I think i shall start a new thread on building a compulsator....

mike0t4ever

Tue Jul 05 2011, 07:58PM

Registered Member #321 Joined: Wed Mar 15 2006, 03:33AM
Location:
Posts: 14

the limiting issues with reluctance coilguns are: a) saturation, b) eddycurrents

that being said if you have access to the ieee transactions most of the coilgun research uses induction based coilguns.

Neuuubeh

Tue Sept 27 2011, 06:38AM

Registered Member #3411 Joined: Sat Nov 13 2010, 08:25PM
Location:
Posts: 33

Or it may be that the larger part of the military coilgun research isnt made public yet? :)

Of course they are more complicated than railguns. However there are already some pretty advanced research in the area - a EM mortar that launches a round with 450m/s with an efficiency of around 25% (i believe, search for it, there is a pdf somewhere on the net), some missile launcher (ejecting the rockets from the tubes so to speak) and probably a few more. Record accelerations of 5km/s for a 2gr armature have also been achieved, quite a long time ago too :)

The rail erosion on railguns is still not quite solved i believe...

Sulaiman

Tue Sept 27 2011, 12:26PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I suspect that the primary reason is the cheapness, energy density and volume density of explosive charges is far better than any electrical energy storage device that I've heard of.
Plus, the exploding gass technology is well understood and reliable.

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