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1.4kJ electrothermal gun

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Yandersen

Mon Mar 04 2013, 08:46PM

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

Please delete this doubled post.

Ash Small

Mon Mar 04 2013, 10:14PM

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

Wizzup wrote ...

Seems like I have to design my chamber all over again... The nylon parts inside the chamber that work as insulators cant take the pressure created when firing heavier projectiles. I will spend more time designing this next chamber and I will probably start constructing it next weekend.

The idea I've come up with to tackle this problem is to use modified automotive spark plugs. You'd probably want un-suppressed ones (eg Ford model 'T', or similar). Grind off the 'earth' electrode, and the part that protrudes past the end of the threads, leaving the centre electrode intact. You can buy taps for tapping spark plug threads.

This is the method I'll use when I build one.

You could weld them in, but threading will probably be stronger and welding could damage the porcelain insulation.

MrFlatox

Tue Mar 05 2013, 01:20PM

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

I did try automotive sparks plug in the first attempt, even before the actual setup. But as you mentioned, I did even thought taht spark plugs could have internal resistance so my design gave... nothing. I found out that using a cylindrical electrode at the end of the chamber sealed with epoxy between groove I made on the electrode and threads of the chamber to enable mor contact surface and more strenght. I do not know if this is understandable... This has not failed yet, and does not show any sign of wear. However I do not know how this would behave under higher imput energies.

Wizzup

Sat Mar 16 2013, 09:46PM

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

I am currently waiting for some parts to arrive so that I can finish my chamber...

In the meantime I checked how many pulse caps there are for sale in ebay and found this one:

its values look very good except for the pulse current rating, just 2kA? I thought that pulse rated caps with this voltage rating could do like atleast 10kA, or is this just a bad capacitor model for pulse discharge use? Or can the capacitor do more than that but can blow up on higher values or something? My knowledge on these type of capacitors is very limited, what type of capacitors are generally the best suited for this type of use? With the capacitor type I mean like foil, paper and so on what different types there are.

Also if you know any other places than ebay where to look for this type of stuff it would be great if you can post it here. I tried googling "energy storage capacitor" and similar keywords and couldnt really find any sites that sold those other than those 20kJ+ monsters.

I will first make this thing fully working with my current capacitors and then look if I will find a pulse cap somewhere to get shorter pulse time.

Yandersen

Sun Mar 17 2013, 03:42PM

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

As far as I know russian traditions, they put halved numbers as official parameters for max ratings and x10 values for parasitic parameters comparing to real values. Make sure this cap is not an old timer from sixties - you don't want cracked dielectric.
Still, I would just recommend any polypropylene stacked caps - with negligible ESL the only thing that may limit current in those caps is a resistance of the terminal wires, so look for screw-mount types. Usually those caps are called surge or surge-suppressing caps or pulse caps. They are box-shaped, not cylinder. The one you linked is the perfect example, but I wouldn't play with 4kV - look for higher capacitance and <1kV rating - you still can survive a 1kV shock if any accident happen, but 4kV will probably kill you. :)

Wizzup

Fri Apr 19 2013, 12:20PM

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

Long time since i last posted here... most of the time i have waited for parts to arrive. I made a new chamber from aluminum and it should be strong enough that i cant break it with the current energy levels.

First a video of the new design:

I have two new barrels made from steel, 4mm and 6mm ID, the one used in this video is the 4mm one. It shot a 15mm long 4mm sharpened steel rod through about 11cm of wood and then it hit a pile of glass panels that were behind that wooden box, shattered few of them to small pieces and stopped to the bow target that is the grey thing behind the box. I didnt think it would go through all that wood so it was a surprise for me also. The "raining" sound right after the shot is pieces of glass dropping on the floor and table. I stopped filming on the plywood because i thought it didnt go through it but found out the real damages later.

Also the new barrels are attached to the aluminum chamber with threads so there is no longer need for any difficult attachment things like i had with the softair gun barrel.

EDIT: I made another shot with the 4mm barrel, this time with a 30mm long projectile and 10mm thick aluminum plate as a target:

EDIT2: I tested the 6mm barrel, made a 40mm long 6mm diameter steel rod almost through 10mm aluminum:

I will also get a chrony in a few weeks so then i will be able to say how much kinetic energy the projectile has and what is the effiency :) With it I can also adjust the projectile lengths and so on to get the most energy out.

Also no one has anything to comment?

Maxwell

Mon Apr 22 2013, 11:42PM

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

@Wizzup,

I'm very impressed with your above tests. Can you provide a few more pictures of your setup?

Caps? Trigger? Voltage?

Wizzup

Tue Apr 23 2013, 01:13PM

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

I can get some more pictures later today but you can find all the specs in the video describtion in youtube.

Caps: 3x 4700uF 450V inverter grade electrolytics in parallel.
Switch is a homemade spring loaded switch with flattened coper pipe as contacts. Voltage is in the range of 400-420V in the shots.

EDIT: Also I found a guy who sells these capacitors:

The terminals and the capacitors size look sturdy enough that it should be able to take the "short out" on an ETG with no problems and an ETG is not fired that often either so it would probably survive well in my usage. I wouldnt shoot any heavy steel projectiles etc with those because the pressures would be massive but i should be able to get a airsoft round going well over 1000m/s with one or two of those. They also cost very little as they are used, i dont know yet where they have been used but i sent the seller a message about them. I would expect a capacitor like that to discharge fully in under 50Âµs so it would for sure be fast enough to discharge fully before an airsoft round leaves the barrel. And yes, i know these capacitors are WAY more dangerous than my current electrolytic ones so i will be super careful if i will buy some of these.

radhoo

Wed Jun 12 2013, 07:27PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

amazing stuff. did you have the chance to make any additional progress on this?

I'd also like to see more pics and vids with your project

Wizzup

Wed Jun 12 2013, 09:10PM

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

I am currently waiting for some mosfets to arrive for a zvs driver for the charger so that the charging would be a lot faster and then I will be able to charge them a bit higher voltage than about 410V that is the maximum with the current driver to get about 250J more energy. The mosfets should arrive this week so I will shoot a new video of it + a shot with a bit higher energy after it is working.

I also got some 6mm 440 SS ball bearings to shoot with this thing, I am waiting to get to shoot them at some steel plates and that 10mm thick aluminum plate.

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