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9KJ coilgun construction

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big5824

Sun Feb 08 2009, 11:31PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

The guns still very early in development, but thanks to you guys im making good progress :) My final aim is high velocity, but without university standard knowledge yet im not getting my hopes too high.

The specs:
Caps:
5x 400v @ 22000uf, ~9KJ in total (25 pounds from ebay)
The current bank:

All the caps are in parallel as I plan on using one bank to power all the stages, by turning off the scrs with smaller caps once the projectile reaches the center of the coil, very similar to blackgrunge's gun, as this should eliminate suckback, and no energy will be wasted as whats left over will go into the next stage.

Barrel:
Im currently planning on using a glass tube, 9mm outer diameter with 1mm walls (from previous gun), as this will make optical triggering very simple. However the small bore limits me to a small projectile, which is going to make pulse times very short. With 5 stages the pulse times would be around 1.3ms for the first stage, dropping to 0.3ms on the last stage, with peak currents of around 20KA. This is a bit hard to manage so I may drop down to 10 stages to increase the pulse time. However if the glass shatters once i get a test coil made (with epoxy to strengthen), I will buy a carbon fibre tube of larger bore size and stick with the 5 stage plan.

Projectile:
Im currently just using a perfect fit 7mm steel rod (25mm length), but I think saturation is going to waste far too much energy, so like i said before i may change barrel and projectile, or try and get a rod of one of the special high saturation alloys made (i forget the names).

Charger:
Im just using the 20-450v boost converter from my previous gun, will be a rather long charge time, but using existing equipment should keep costs down a bit.

I now see that the pics are a bit low quality, il try and get some better ones tomorrow.

Like i said, the project is far from complete, but il try to keep this thread updated as I go :)

rp181

Mon Feb 09 2009, 02:51AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

You should go for a much bigger projectile, saturation will be horrible. I used a .5" bore with ~2kJ, and it worked great.

Glass will most likely break. Also, stay away from carbon fiber. It is conductive, and causes eddy currents. I suggest polycarbonate or garolite (check out mcmaster-carr. Scroll to the bottom, and click plastics under raw materials).

High saturation material's don't work very good at all. I tried hymu-80 (like the highest saturation), and it sucked. The cost of high saturation is low permeability. If you still wan't to try it, i can send you some, email me.

For optical triggering, I would just choose a clear plastic barrel, or incorporate electrical delay.

Very nice buy with the capacitor's. What company are they?

big5824

Mon Feb 09 2009, 04:36PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

Hmm I didnt know carbon fibre was conductive, thanks for the warning. As for the saturation, I agree its going to be a big hit on my efficiency, but finding the right mass to give the highest velocity is going to be hard....I guess I will have to try and figure out how to use FEMM again.

Oh and the capacitors are a custom model made for siemens by BHC, 12mohms ESR, and are 22cm by 9cm (weighs a ton)

Camel

Fri Feb 13 2009, 01:47PM

Registered Member #1694 Joined: Sat Sept 13 2008, 09:13AM
Location: Australia
Posts: 108

Carbon fibre isn't that conductive. Sure, the carbon fibres are, but they are embedded in epoxy. Eddy current losses would be insignificant.

Adrenaline

Fri Feb 13 2009, 05:17PM

Registered Member #235 Joined: Wed Feb 22 2006, 04:59PM
Location:
Posts: 80

Camel wrote ...

Carbon fibre isn't that conductive. Sure, the carbon fibres are, but they are embedded in epoxy. Eddy current losses would be insignificant.

Carbon fiber reinforced plastic panels are now being welded together using induction heating. So the eddy current losses can be significant, however in the duty cycle of a coil gun I would also consider them negligible.

big5824

Fri Feb 13 2009, 05:37PM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

Iv managed to model my coil in FEMM, and i think the peak field with the small projectile will be around 2-3 tesla. However iv been unable to find out at what level steel saturates, can anyone help me out? Iv also found a nice supplier for cheap carbon fibre tubing, as I think the combination of huge currents and the fact that the current wont rise fast enough will force me into using a larger mass projectile.

KLH

Fri Feb 13 2009, 07:29PM

Registered Member #1819 Joined: Thu Nov 20 2008, 04:05PM
Location:
Posts: 137

Camel wrote ...
Carbon fibre isn't that conductive. Sure, the carbon fibres are, but they are embedded in epoxy. Eddy current losses would be insignificant.

Good point... They're individual, very fine fibers. I'm also quite sure that the carbon fiber isn't very conductive.

Adrenaline wrote ...

Carbon fiber reinforced plastic panels are now being welded together using induction heating. So the eddy current losses can be significant, however in the duty cycle of a coil gun I would also consider them negligible.

I think he wants to use a carbon fiber tube, most of which are not welded together, so I think that these could easily be a drop-in-replacement for other barrel materials.

j.azz

Sat Feb 14 2009, 11:51PM

Registered Member #888 Joined: Tue Jul 10 2007, 06:52PM
Location: Hannover, Germany
Posts: 40

I have made ignitors for fireworks with small diameter carbon fiber tubes filled with black powder, they nicely glow at a few amps => they are conductive. But I'd still give a try.

big5824

Tue Feb 17 2009, 09:22AM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

I noticed this

auction would be good for my gun. The problem is, the datasheet he links say they can only handle a 1.5KA pulse, which seems very small considering their size, so I looked for other datasheets and found two more that both state 6KA. Has anybody bought this model from him before who can confirm this?

Electroholic

Tue Feb 17 2009, 10:04AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Again, that number is for 8.3ms half sine pulses, once every second i believe.
Use the action integral instead.

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