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1.4kJ electrothermal gun

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Wizzup

Fri Mar 01 2013, 09:14PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

MrFlatox got me interested in electrothermal guns so I decided to make one:

In the video it shoots a 0.25g 6mm plastic airsoft round through 12mm thick plywood, the power is really good in my opinion, I didnt expect anything near that power. I dont have a chrony so I dont know the exact speed but I would say it is for sure over 500m/s because of the damage it makes, that would be 30J+ kinetic energy.

I will post some pictures tomorrow of my setup but for now ill just tell you the specs:

Capacitor bank: 3x 4700uF 450V inverter grade electrolytic caps, 22mOhm ESR each, so in parallel about 7mOhm.
Chamber: 31mm diameter nylon rod, electrodes are two 6mm diameter threaded rods.
Switch: homemade spring loaded mechanical switch

I use some water in the chamber with a piece of aluminum foil, without the water the shot has only about half of its power than what it has with water.

Okay I took a new video this morning and some pictures:

The aluminum plate is 2mm thick. After the shot I found a small 6mm diameter aluminum "bowl" that the projectile had ripped out of the plate. Also the hole in the plate is 7mm in diameter, that is 1mm larger than the projectile originally. I have to make some improvements on the barrel attachment as the force is so big that it flies off.

Overview of the whole etg:

Projectile:

just a regular plastic airsoft round, not aluminum
Chamber overview:

Chamber inside:

UPDATE: New video using the new chamber:

It fires a 6mm diameter 9mm long steel projectile

into some wooden blocks that are 4cm thick, about 6cm total penetration. There was a bad connection on another one of the electrodes where the cable is attached, you can see the sparks in the video, so I lost some power there. Sorry for the camera shaking so much, I had some adrenaline in my blood that time :D I will fix that connection and then I will probably try a longer projectile and see if I can get even more power out of this.

Maxwell

Fri Mar 01 2013, 10:00PM

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

I have a strong feeling we're going to be seeing a lot of ETG's on this forum in the coming months.

Very nice shot there...

We should really attempt measure some quantitative efficiency or energy conversion output.

Ash Small

Fri Mar 01 2013, 10:08PM

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

Nice one

I was thinking myself that water in the chamber should improve performance considerably.

I've also been thinking, as E=1/2CV^2, does it make more sense to connect capacitors in series, rather than in parallel? (more voltage, but less charge, but energy is a factor of V^2)

Yandersen

Fri Mar 01 2013, 10:30PM

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

Wizzup, I can't believe that plastic bullet can damage a plywood - I would expect it to smash over it in a little cloud of plastic dust. Maybe it was an aluminum ball?

I keep trying the pure air discharge increasing energy up to 70J already. I lost hope in air-operating ETG - plastic 6mm bullet can't go through the beer can - nonsense. So don't waste time - aluminum foil rules. Still I wouldn't recommend water as electrodes will not survive for long then.

Thomas W

Sat Mar 02 2013, 12:01AM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Yandersen wrote ...

Wizzup, I can't believe that plastic bullet can damage a plywood - I would expect it to smash over it in a little cloud of plastic dust. Maybe it was an aluminum ball?

I keep trying the pure air discharge increasing energy up to 70J already. I lost hope in air-operating ETG - plastic 6mm bullet can't go through the beer can - nonsense. So don't waste time - aluminum foil rules. Still I wouldn't recommend water as electrodes will not survive for long then.

theres a video of someone shooting a candle though a wood door
so i would think that a plastic ball could get though with enough force behind it

Maxwell

Sat Mar 02 2013, 04:49AM

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

Ash Small wrote ...

I've also been thinking, as E=1/2CV^2, does it make more sense to connect capacitors in series, rather than in parallel? (more voltage, but less charge, but energy is a factor of V^2)

You typically get more bang for your buck as you increase your voltage rating.... but the capacitance quickly drops off. In CG's, the higher voltage, lower capacitance, faster pulses. In these ETG's, if a longer pulse is beneficial, greater capacitance is needed.

Wizzup

Sat Mar 02 2013, 08:28AM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

First post updated with new video and pictures.

I think that I would not get much more power out of this if I had my capacitors in series, because I tried how it will fire at 300V charge and the sound was pretty much nothing, just a small puff. I think that my barrel length is pretty good for my current energy level, because with 300V the pulse is not long enough to push the projectile all the way out of the barrel before it ends so I guess that I would get worse performance with even shorter pulse.

Yandersen: The projectile is plastic, not aluminum. After the shot I find lots of small white plastic pieces that are parts of the projectile.

MrFlatox

Sat Mar 02 2013, 02:28PM

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

Very nice ETG ! I am glad to see that my presentation inspired some ideas !

It would indeed be very nice to see how it performs with figures, I would like to know how does the efficiency looks like on this one, but it should be pretty good judging from the damages you get. I am supposing something in the range of 4-5% range might be a good guess.

Keep up the good work !

Wizzup

Sat Mar 02 2013, 03:53PM

Registered Member #3302 Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42

I made a test shot with a 6mm diameter 10mm long steel projectile, the kinetic energy with it is a lot more than with a plastic airsoft round, and I used just 250V in the shot. It went through the plywood and sunk 0.5cm into my wooden backstop, that is about the same power as I have with an airsoft round at 400V. Only negative thing is that it broke my chamber, I found a crack going on the side of the chamber from top to bottom so I have to make a new one from some other material. I guess I dont have many other choices than steel because the old nylon chamber had over 1cm wall thickness and it is pretty strong material.

RateReducer

Sat Mar 02 2013, 05:02PM

Registered Member #4603 Joined: Wed Apr 25 2012, 07:33PM
Location: Austria
Posts: 159

This thing is unbelievable Wizzup!
2mm Aluminum with soft-air projectiles - just awesome! :)
How are you going to construct your new chamber?

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