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28,8Kj ElectroThermal Gun! (Soon handheld)

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: MaxuS the 2nd » Sun Apr 20, 2008 1:00 pm

Truly is one of the best.
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Unread postAuthor: dongfang » Sun Apr 20, 2008 2:03 pm

Hi Larda

Wow I am deeply impressed. May I see it if I come to Sweden this summer?

I made an electromagnetic chrono once: The projectile is magnetic (doesn't have to be much; a few cubic mm of a magnet from a hard disk drive is enough), and it is fired through 2 coils (mine are 300 turns each I think). The coils are connected to a pre-amp followed by comparators, high pass filters, and finally a flip-flop that gates a microsecond timer. The idea is to get the difference in time between the transition-thru-zero of induced voltage in the 2 coils.

I measured sonic speeds over only 75 cm with such a set-up, and with 4 significant digits of accuracy.

I know that your gun probably screws up the whole electromagnetic spectrum from DC to daylight when fired, but as far as I can see the magnetic field should be parallel to the wires of the chrono coils, and not affect them!

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Unread postAuthor: Larda » Mon Apr 21, 2008 8:48 pm

Hi dongfang

Your idea with a electromagnetic chrono sounds really interesting, maybe i try something similar.
But i think the coils will need some shielding to make it work without interference from the gun.
Would be fun to analyse the EM output from the gun with a spectrum analyzer. :lol:

How big did you make your coils? It would be nice if i could make them big enough so i could place them a few meters from the gun and cables to reduce the interference without the risk of the projectile hitting them.
The drawback is that they will be harder to shield properly and i also would need a much bigger magnet on the projectile.

I cant give any promises right now, but i don't see any problem to give you a demo of the gun if you visit. :)
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Unread postAuthor: dongfang » Tue Apr 22, 2008 4:22 am

Hi Larda,

Sure you could shield the coils again electric fields, but my first guess it that it won't be necessary. And, hmm they run on the magnetic field. If the magnetic field from cannon interferes too much, there is not much to do but to move the chrono further away.

My coils were just about 10 cm. dia. I just hung one of them around the (plastic) muzzle of the gun ;) and I fixed the other one in front of it. Yeah, it's probably better to have a spare or 2, just in case of a hit ;)

I think that with your velocities, you will have plenty of signal with even very small magnets.

PS: The simplified version just has the coils in series, and hooked to a triggered oscilloscope or even just a PC sound card with a 'scope program running. Works great.

Regards
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Unread postAuthor: Ragnarok » Tue Apr 22, 2008 5:31 am

Unfortunately, I'm not sure that this magnetic chrono is a good idea for launchers with metal barrels.
When a magnet moves through a conductive tube, it creates eddy currents which will act as a retarding force on the projectile.

If you drop a neodymium magnet through a copper tube, instead of accelerating at 9.81 m/s<sup>2</sup> as you might expect, the magnetic eddies slow it to around 10 centimetres per second.

That means that at ~10 centimetres per second, the eddy forces are approximately 1 G (as they counteract the 1G of gravity :P).
I'm not an expert, but I believe this scales linearly - at 100 metres per second, the forces on the magnet are going to be ~1000 G. Although Larda has a very powerful launcher to overcome this, and he won't be using a large magnet (I assume, anyway), this is still going to have an effect on projectile velocity.
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Unread postAuthor: Larda » Tue Apr 22, 2008 6:35 am

Then i just have to switch to a PVC chamber and barrel....No just kidding. :)

Ragnarok have a valid point, maybe you could have a current running through the coil and use the fluctuations in magnet field it to detect the projectile?
The easiest and most reliable way would probably be just letting the projectile break two wire or optic fibers, the drawback is that you have to replace the wires between each shoot.
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Unread postAuthor: dongfang » Wed Apr 23, 2008 4:24 am

Hi,

Yeah OK induction will happen in the barrel and current will flow.

But still I'd say, for the magnets that I used, that it is totally insignificant. I know I should support that by some kind of computation.....

I did the shoot-through-the-wires thing, too. That was so cumbersome that I designed the magnetic set-up instead. It was not much fun sewing the wires into place, getting a reliable connection to the flimsy wire ends, avoiding to tear the thin wires and finally having the projectile sometimes whooting over the wires, sometimes (for pointed projectiles) just pushing the wires aside.

I measured the muzzle velocity of my sister's (!!) BB gun with the wire set-up a long time back. The timer was the ingenious 6526 "CIA" chip of the Commodore 64 computer. The gun was just sub-sonic when fired the normal way - and just supersonic when putting a drop of oil in the hollow space in the back of the pellet. Dieseling.

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Unread postAuthor: SpudFarm » Sat May 03, 2008 12:45 pm

i am REALLY sorry to bump this mods but:

when i saw this at random cannon it hit me that mabe aluminium is a solid propellant isn't it? :S

just a question..
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Unread postAuthor: Larda » Sat May 03, 2008 1:56 pm

No, it doesn't qualify as a solid propellant.

Solid propellant: Any substance containing all the chemical elements required to enable sustained combustion without the presence of additional oxidizers. This includes but is not limited to gunpowder, thermite, and many commercial and/or homemade pyrotechnic compounds.

1. For this purpose, nitroglycerine, though liquid, would qualify.
2. For this purpose, ordinary flour, though solid, would not qualify.
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The ETG is going handheld. :twisted:

Unread postAuthor: Larda » Sat May 03, 2008 9:39 pm

Here is a mock-up of how its going to look with a rifle stock from a BB gun. I am also going to insulate the barrel and add a lexan shield to prevent any contact with the high voltage parts.
I am not planing to do any full power shots when i am holding it in my hands and will take the necessary safety precautions so it will be relatively safe.
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Mock-up of the handheld ETG.
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Unread postAuthor: psycix » Sun May 04, 2008 6:39 am

I hope for you you dont vaporize, ehm... I mean shock yourself.
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Unread postAuthor: rp181 » Sun May 04, 2008 2:17 pm

You need a LONG range scope and a rifled barrel =)
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Unread postAuthor: drex » Sun May 04, 2008 3:19 pm

look'in good. im looking forward to seeing a video of the handheld one
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Unread postAuthor: Gunner » Wed May 07, 2008 11:46 am

Larda, could you say what kind of capacitor source are you planning to you with your handheld rifle?Thanks!
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Unread postAuthor: SpudFarm » Wed May 07, 2008 11:48 am

i think he is going to use the full 28.8KJ of energy

(ouch my shoulder hurts just of the thought)
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