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LAN_403

Microwatt

Thu Nov 08 2012, 04:18PM

Registered Member #3282 Joined: Wed Oct 06 2010, 05:01PM
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Posts: 224

I want this thread to be a depository of coil gun, solenoid physics, pages that are dedicated to coil guns, amateur as well as military. please post all of the technical goodies below. hopefully in the future you could refer to this thread. If you know of any peer reviewed articles as well please list them.

BigBad

Sat Nov 24 2012, 09:12PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

OK here's what I know:

1) axial coils suck balls; Eric Laithwaite says so, and I believe him, the flux path is shit

2) if you use an iron projectile, applying a field that is stronger than its saturation means you've wasted (probably a lot of) energy

3) if your gun isn't saturating your projectile, you might as well use a presaturated projectile (i.e. a hard magnet) instead.

4) if you use permanent neo magnets for your projectile, you can pull up to 1000g for the length of your barrel; you can get about 2000g with iron- if you saturate it.

BigBad

Mon Jan 07 2013, 06:02PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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In a multiturn coilgun, for maximum speed, all the coils should optimally be the same physical size.

Later coils need to have less turns than earlier coils, but the turns need to be thicker, carry more current, to give the same current density and size and they must be pulsed more quickly to keep up with the projectile and give the same acceleration along the barrel.

Later coils really need to be litz of some kind.

Yandersen

Tue Jan 08 2013, 03:52AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
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Posts: 340

For *maximum speed* coils must decrease in size being wound with the same wire as first stage - this way efficiency of each stage will stay the same while increase in kinetic energy will be higher and higher. Keeping size the same is the way to make kinetic addition the same while heat losses are getting smaller and smaller due to shortening of the pulse time.

BigBad

Tue Jan 08 2013, 02:28PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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You're not restricted to using the same wire, and/or you can lay wires connected in parallel.

There's a difference between constant power acceleration, and constant acceleration; in most cases you want constant acceleration if possible, because it gives a higher top speed.

If you want constant power, then you wind later coils smaller, if you want constant acceleration, then they're the same size. (This is obvious if you look at the situation from the point of view of the projectile, it doesn't know if the current is flowing in one fat turn or many thin, both have the same current density, but the inductance and resistance possible switching time is a lot lower with a lower number of turns, but the same total amount of wire)

Note that in both cases efficiency goes up the same with speed. Power done is F x v, whereas power losses are just current density^2 x p.

BigBad

Tue Jan 08 2013, 03:05PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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OK, you can get constant acceleration by making the later coils physically smaller, and running them at higher current density, but you lose more energy by doing that, and you need higher voltages.

Yandersen

Tue Jan 08 2013, 09:44PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Acceleration will be the same for given coil geometry - if all have the same dimensions, they pull with the same force. Decreasing the number of layers increases current density (so as heat losses) but makes mag fileld closer to the projectile increasing the pull force this way.
I like to apply A = F*s formula here. It clearly shows why the same force and geometry result in a constant energy increase rate for each coil of a multistage coilgun (if timing is ideal, of cource).

BigBad

Wed Jan 09 2013, 06:20PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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I'm pretty sure that independent of the number of turns, there's nothing useful, performance wise, to be gained by winding a coil smaller, with higher resistance; I think for a given amount of wire, each coil should have the same mass of wire.

I did wonder about the time constant: L/R, which implies that you want high R for any L; but that struck me as fishy, but then I realised that the current approaches V/R after one time constant, so higher resistance means you end up with less current, and the time constant is a red herring, higher R means you don't get the same current, but you get there more quickly.

Yandersen

Thu Jan 10 2013, 01:01AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Sorry, BigBad, I forgot that you guys used to monkey technologies such as looping current with damper diode and limiting current by coil' resistance. Of course, my LC-oscillator approach, operation of which can be easily described by school formulas, operate on different rules.

BigBad

Thu Jan 10 2013, 01:37AM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
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Posts: 600

Apparently you're not following me.

I'm saying it will certainly work if you use progressively smaller coils, but it will be suboptimum.

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