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1kJ Electrothermal gun (ETG)

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MrFlatox

Wed Feb 27 2013, 11:43PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

clever design ! As an airsoft player, I am thinking of the applications your design could have for airsoft skirmish, but it seems underpowered, and too bulky, not to mention the reloading. Otherwise it is very well arranged and it's a nice physics toy !

In your opinion, would your design be upgradable on a higher scale (efficiency or ouput energy) ? Is the other end of the barrel blocked ? what king of efficiency would be achieved compared to a regular ETG ?

Maxwell

Thu Feb 28 2013, 12:10AM

Registered Member #8497 Joined: Tue Dec 04 2012, 06:24PM
Location:
Posts: 74

@Yandersen - Very cool.
Do you have any idea yet what sort of efficiency you're obtaining?

Yandersen

Thu Feb 28 2013, 12:12AM

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

MrFlatox, it IS a regular ETG! Efficiency should rise with the energy input, I think. I just modified the discharge chamber to blur the spark to the whole electrodes length to prolong their life.
Reloading? Just take an airsoft toy gun and replace plunger with my discharge chamber - it has one end closed, yes.

Efficiency is a total crap. More energy needed for longer pulse - maybe I'll add caps. But I don't think my thin-walled plastic barrel will survive. And plastic ball 6mm in diameter and 0.126g in weight - how can it fly normally with high Ekin?!

MrFlatox

Thu Feb 28 2013, 09:04AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

Well, it can fly pretty well. With addition of a "hop-up unit" (a device that introduce Magnus effect on the BB) it can fly horizontally for about 35 to 40 meters with 1J of kinetic energy (.2g bb), or for about 65 meters for better setup with 2J KE (.36g bb). BBs weight is different too : generally .2g for KE =< 1J, heavier bbs for KE > 1J.

This is how it works in my country, I know things are different elsewhere.

So this is a regular ETG... I thought that the discharge path was "moving" from the back of the barrel to the front with the help of your B-field, as you described earlier the "sonic wave". So actually, when the discharge occurs, it happens in all the area between your two electrodes, but ions are pushed to the front of the barrel ? What actually push the BB ? The increasing air pressure in the barrel or the ions travelling forward that transfer their momentum ?

Yandersen

Thu Feb 28 2013, 12:05PM

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

I come to the conclusion that the B field can only bend the arc, not to rip it apart into individual ions flying out, and as result of this bend, discharge mess consumes the whole area between electrodes, expanding like a wave of ionization up to the ends of electrodes. So current flows everywhere between electrodes anyway, and the result is a rapid expansion of this whole area. The thing is that as soon as short pulse is over, air is cooling down due to a contact with the walls, and pressure lowers in the same rapid manner. Explosion followed by implosion. If bullet is not leaving the barrel before implosion, it will be sucked back.
I tried 3 caps instead of two and did not mention any difference except more plasma exhaust. Adding an additional inductor had negative effect. I think the length of wire in a magnetization coils should be minimized. The discharge chamber should be shorter and thicker, maybe coil around it is obsolete.
...

MrFlatox

Thu Feb 28 2013, 12:52PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

In your actual design , all coils and discharge gap are wired in series ?

Yandersen

Thu Feb 28 2013, 08:31PM

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

Yes, but this is not necessary. You may control all elements individually. I just did it this way to limit current by coil's inductance and resistance.
I'm thinking about DC motor magnets - I'll try to replace magnetization coils with those. Bad I do not have neodymium magnets of such bean shape...

Magnetization by DC motor magnets worked as expected (even better), but actual discharge didn't got caught on camera even once. Still, you can see the setup:

Well, as my eyes seen the spark, I can tell it indeed moved out of the part of the pipe between magnets as I have seen shining path only at the ends of electrodes. Howhether, all electrodes, even inside the pipe, were covered by shining air - that means that even if the actual spark was outside the magnetic trap, still some chaotic ions happened to be everywhere.

As I recall previous considerations about air discharge for ETG application, we need to maximize the length of the discharge arc. But that means that triggering voltage must be very high. So here is my solution: as shown in the video above, we align electrodes in a triangle way and put this part inside the magnetic trap. Arc will happen in a shortest distance, but it will be pushed out immediately and actual discharge will happen between distanced electrodes' parts which is just outside the magnetic trap, where field is not strong enough to work against electric field between electrodes.

Yandersen

Fri Mar 01 2013, 11:02PM

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

Well, the last air-operating ETG setup I tried, so here is the video for history:

My conclusion: air-operating ETG is a dead end due to a miserable efficiency it shows.

MrFlatox, how are you?

Ash Small

Fri Mar 01 2013, 11:40PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Yandersen wrote ...

My conclusion: air-operating ETG is a dead end due to a miserable efficiency it shows.

I don't agree, as efficiency isn't limited to saturation of the projectile.

DerAlbi

Sat Mar 02 2013, 01:15PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

But its limited because its a heat engine.

The saturation argument is no argument at all. The theoretical limit of efficiency of a coilgun is much closer to 100% as this heat stuff. Here you always end up with energy stored in warm air even if everything electrical is superconductive. In a superconducting coilgun however you only end up with... hmmh.... a fast projectile^^

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