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Induction Coil Arrow Gun

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Aussienitro

Tue Jan 26 2010, 06:29AM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
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Posts: 27

The idea is to take the thompson coil disk launcher and impart that force onto a ballistically efficient projectile. The picture below shows the basic idea, the arrow sheathing a ferrite core is pushed by an aluminuim disk "sabot" which is in turn pushed by a moving coil.

Has this sheathed "arrow" idea been tried before? What about the moving coil?

I do worry that the force exerted on the moving piston will be too great, and if its not too great its probably not worth having however it cant hurt. In theory the coils velocity should be added to the arrows.

Anyway the picture is self explanatory so ideas or opinions?

Barry

Wed Jan 27 2010, 05:43PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

The Discovery Channel built a 7KJ coilgun using a principle like this. A flat aluminum plate was launched from a flat spiral coil, pushing a lightweight projectile shaped like a rocket. The plate was captured a few inches later inside the launcher by impacting a donut-shaped cushion. The projectile flies out the barrel under its own momentum.

Link: Discovery Channel Coilgun

It worked pretty well, and it looks good in the video. The project was one episode in the series "Discovery Project Earth" for saving the planet from global warming via far-fetched ideas.

However, they have no measurements of velocity or efficiency. All we know for sure is that it was definitely powerful enough to cave in the flat aluminum disk so it became a bowl shape. The coilgun itself has been decommissioned and the parts long since sold on eBay.

Cheers, Barry
One can never know for sure what a deserted area looks like. - George Carlin

Aussienitro

Thu Jan 28 2010, 12:47AM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
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Thanks barry thats one mean machine. Seems crazy to take so much time and money designing and making that then giving it a few shots then scrapping it without any useful results. Such is the nature of TV i guess.

One thing I just realised with my drawing is that the coil is going to want to grab onto the ferrite core isn't it? thus limiting the effectiveness of the moving coil.

Barry

Thu Jan 28 2010, 01:16AM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Yes, a TV production schedule is all about the visual effect and not about doing science. As long as it looks like science that's all that matters. I find this understandable when the daily videotaping schedule is so expensive. The ROI is all from a working demonstration, and they get no return from performing a controlled experiment or even collecting any data. I brought my oscilloscope and lab notebook on site in Arizona for the shoot, but the director was 100% focused on a successful production in the two short days available. So I got no data or waveforms.

In the sketch from the opening post, I believe you'll always get better results from a stationary coil and backing plate. The mounted parts should be as unmovable as possible to give the best thing to push against. Otherwise there's wasted energy in moving things that don't contribute to the exit velocity.

Also I believe the backing plate just behind the coil should be non-conductive. Otherwise it will experience eddy currents and generally dissipate energy that we'd rather have converted into kinetic energy (or recovered and saved back into capacitors).

I think I see your idea behind a moving coil: It extends the time that its magnetic field interacts with the projectile. The practical matters of implementation would be immense, and I doubt the extra work would pay off.

For example, the Discovery Channel guys discovered that the aluminum disk doesn't move away from the coil in a reliably flat or parallel fashion. It tended to lift one edge more than others as if it were flipping a coil. This caused no end of problems and variability for the optical speed detector mounted at just one spot and inch above the resting position.

I really like the proposed inner ferrous guide rod. This also keeps the projectile going straight, and at the same time improves performance by carrying the magnetic field much farther. Of course, one must design the ring-down time to continue long enough to match the time while the projectile is still on the guide rod. On the other hand, a ferrous part brings the issue of magnetic saturation back into play.

Cheers, Barry
A man walks into a bar with a slab of asphalt under his arm, and says: "A beer please, and one for the road."

Aussienitro

Thu Jan 28 2010, 07:15AM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
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But isnt the magnetic field out the back of the coil always wasted anyway? If it was iron backed it would be used pulling and compressing it thus wasted, aluminium it would repulse and yes in the process adding velocity to the arrow through longer contact (think of throwing a ball from a moving vehicle, the best thing to "push against" is something moving in the direction you want to go). Since we are working with such a short pulse, any lengthening of the time the pulse is in contact may have very significant results. It would be easy to test on a low powered gun but taming it in a powerful one would be somethng else.

I do fully agree that making this in a practical sense would be very hard, much harder then what the simple picture suggests. And theres the problem of it grabbing the rod, so it may be an issue of either having a moving coil or a ferrite guide rod whichever is best.

I did suspect having a thin disk may twist and "lock" onto the guide rod. Thanks for coonfirming that, I mentioned in the end of this thread

that it would be best to have the disk sit over the Al tube, so even if pressure is not even, it will still glide straight.

Maybe cast the backing plate out of bismuth to squeese the last few microjoules out of the gun :P Wouldn't it be repulsed by both eddy currents and diamagnetism? Anyways thats no a serious suggestion but a theoretical one. I'm serious with the rest of it, the Aluminium tube and ferrite rods are in the mail so we'll see what happens.

Aussienitro

Thu Jan 28 2010, 07:42AM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
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Oh and another question, would the Al tube try to "levitate" around the ferrite rod when fired? No friction would be good.

Barry

Sat Jan 30 2010, 05:03PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

Aussienitro wrote ...

Oh and another question, would the Al tube try to "levitate" around the ferrite rod when fired? No friction would be good.

There is a small tendency for the Al tube to center itself on the ferrite rod. The effect is negligible in a coilguns and launchers where the time scale is just a few milliseconds.

Low friction is good; it can easily be accomplished with a loose sliding fit between smooth parts. No special effort is needed as the force of frictional drag is a few orders of magnitude less than the force of acceleration.

I look at it this way: if frictional force were say 1% of the accelerating force (which we hope is in the range 100G to 1,000G), then no matter how much work you invested in reducing friction then you could only get a mere 1% improvement.

Cheers, Barry
A dwarf, who was a mystic, escaped from jail. The call went out that there was a small medium at large.

Aussienitro

Tue Feb 09 2010, 09:36AM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
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Posts: 27

A short on the discovery channel coil gun

It was just advertised on Australian TV, but I didn't take note of when it aired, this week sometime I think.

Bit of a setback on mine, the ferrite came however it wasn't straight enough to use with the Al tubing I had. Now considering using a spiral flute reamer or slow turn helix drill bit to have "reverse rifling" stabalisation.

Aussienitro

Sun Feb 21 2010, 10:46AM

Registered Member #2399 Joined: Mon Sept 28 2009, 09:29AM
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Posts: 27

The ferrite core was a bust due to the rods not being straight enough.

Now the next alternative I'm looking at is carbon fibre tube filled with iron powder, but am unsure what effect eddy currents will have on the carbon fibre. I'm assuming it will act like the solid iron to powder relationship where as long as the carbon is not a homogenous solid, which it isnt, the limited eddy current formation will be a non issue.

Dr. Slack

Fri Feb 26 2010, 07:03PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Using the sabot in contact with the projectile is inefficient in terms of energy. You have to make them both travel with the same velocity, which means they both need to be supplied with energy, ultimately from your power supply. And then you go and wastefully stop the sabot.

If you can mount the projectile ahead of the sabot/driver, seperated by a gap, with a resilient material between them, then the situation changes. The driver will be pushed up to speed by the coil. If it the same weight as the projectile, then it will transfer all its energy to the projectile while being brought to a halt by it. The quality of the resilient material affects things here, it will absorb energy if too lossy, and not do its proper job if too soft or too hard. Driving a golf ball with a solid driver of the same weight is a good pairing, as the ball itself provides all the resiliency to transfer the energy efficiently.

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