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Little coilgun, big cap..

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LAN_403

Tesladownunder

Thu Jul 20 2006, 12:42PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I made a quickie coil of 46 turns of heavy wire. I used a polypropylene barrel and a 33g iron projectile. I tried various energies from 500j to 3kj in sequence. Here is the last shot sequence. 2kj gave the most powerful shot and good deformation but not penetration of the can.
The 4 pics show the sequence of the 3kj shot.
1 the coil close up
2 the setup for a vertical shot with a can over the aperture.
3 my coil firing at 3kj with the coil exploding. The projectile did not come out of the coil with the last shot, presumably sucked back
4 the remains of the coil.

Peter

1153399350 10 FT0 Pulsecapscoilgunpredetail

Electroholic

Thu Jul 20 2006, 06:36PM

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

probably crushed on the 3kj shot. epoxy it, and play with the starting position.

Tesladownunder

Thu Jul 20 2006, 09:51PM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Electroholic wrote ...

probably crushed on the 3kj shot. epoxy it, and play with the starting position.

The forces on the coil are axial compression and radial expansion leading to coil disruption and inter turn shorting so if that is what you mean by crushed...
Epoxy will help but will tend to shatter and at these energies the leads will just blow off anyway.
Starting position was a few mm into the coil. It seemed OK at 2kJ and there was no coil deformation but moderate heating.
This was not really a serious attempt. To make a 5kJ+ capable coil the wire needs to be a lot larger. I was interested to see what happened with this larger projectile which is probably not suited to the much faster pulse of the 5.5kV of this cap bank. There was noticeable heating of the projectile on the 2kj shot suggesting significant energy going into hysteresis losses/eddy currents. Coil guns are only a few percent efficient anyway.
The 3rd photo above of the exploding coil (I have my Nikon D70S camera back and fixed

) shows the can streaking upward, hitting the top board and ricocheting backwards. This was not due to the projectile which didn't leave the barrel and was just due to the explosive force from the coil disruption.

Peter

Simon

Fri Jul 21 2006, 02:29AM

Registered Member #32 Joined: Sat Feb 04 2006, 08:58AM
Location: Australia
Posts: 549

Tesladownunder wrote ...

Coil guns are only a few percent efficient anyway.

Something tells me this doesn't even make that ranking. That's roughly equivalent to a 300J coilgun firing a 3g projectile (although bigger coilguns are harder to design).

Can you get any estimate of the efficiency?

I like it as an absurd project, though. A coilgun that blows itself up but does not penetrate the can with the projectile.

Quantum Singularity

Fri Jul 21 2006, 02:29AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Tesladownunder wrote ...

This was not really a serious attempt. To make a 5kJ+ capable coil the wire needs to be a lot larger.

What size wire were you using? I would guess you are correct though that youll need some heavier wire. Maybe a tougher barrel to and some good epoxy on a high power shot.

Tesladownunder wrote ...
There was noticeable heating of the projectile on the 2kj shot suggesting significant energy going into hysteresis losses/eddy currents. Coil guns are only a few percent efficient anyway....The 3rd photo above of the exploding coil (I have my Nikon D70S camera back and fixed

What material was the can... if it was aluminum I'd bet it was repulsed do to induction. I think the faster the pulse the more loss you will have due to induction. And I beleive it is much easier to make an induction launcher when your talking about using caps that large with high energy levels. I am not the expert with the ferromangetic launchers but it seems they are more efficient when the pulse is less like a pulse and more continous (like a square wave), the very high peak currents with short pulses that the real energy discharge caps provide seems better suited for IL's instead. Its cool though you got it to actually work with lower energies... did you get any pics of more successful launches?

Tesladownunder

Fri Jul 21 2006, 06:38AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

Simon wrote ...

...Can you get any estimate of the efficiency?
I like it as an absurd project, though. A coilgun that blows itself up but does not penetrate the can with the projectile.

Just the thing for a suicide coilgunner.
The 2kJ shot was enough to bend the can in half (it was supported by one end) but the wide round shape of the projectile didn't penetrate it. I was hoping to do an outside vertical shot to time it but was too enthusiastic about increasing the power.

Quantum Singularity wrote ...

What size wire were you using? ...
What material was the can... if it was aluminum I'd bet it was repulsed do to induction. ... Its cool though you got it to actually work with lower energies... did you get any pics of more successful launches?

Wire is 0.105 inch diameter.
You would have thought that after years of can crushing, ring launching and magneforming I should have realised that. Just a seniors moment.
No photos of successful launches however i may use a coilgun if I do any high speed photo work.

Peter

Marko

Fri Jul 21 2006, 10:37AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

If i'm not missing something you are a bit out of starting position there, with projectile just inside the small coil..? Maybe you could make good repulsion CG that way, just by moving projectile 2-3cm out of coil..

Tesladownunder

Fri Jul 21 2006, 10:55AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I could pursue this further but I have the pressure of so many projects. Single stage coil guns are a bit dead end any way and I don't feel ready for the complexity of a multistage one.

So at present this is my only functioning coilgun.

Hand powered from a telephone generator. This charges a 5400 uF 225 V capacitor to a total of 136 joules. I also had a power pack for times when the arms get sore as it takes a couple of minutes of vigorous winding to charge. Not really fast on the draw. Switching was with two metal contacts. It could just penetrate a cardboard box with an 8 g iron rod.

I will start a railgun thread to discuss a different direction I would rather pursue.

Peter

Steve Conner

Fri Jul 21 2006, 02:33PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The small capacitor size, high voltage, and low number of turns is going to be a lousy match to the heavy projectile. Like you found out, the discharge is on such a short time scale that it mostly rips the coil apart rather than putting momentum into the projectile.

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