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LAN_403

Killa-X

Sat Mar 05 2011, 01:39AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

I decided to come back around to electrothermal guns. Today i made a quick scrap one firing 700J of power. I just put 2 wires in a PVC pipe. bridge them with foil. Put a PVC barrel on the coupler, nail-bolt it shut, and fired.

I managed to make this horrible scrap gun and shoot a marble. I just wanted to know a few things.

A) Whats best to use, AL strips or AL 600mesh powder? If i use powder, does it need to be packed?

B) Is it better to use foil, or try to blow the capacitor over an arc? Meaning arc a HV transformer between 2 bolts, and use a 3rd bolt to discharge my 700J into it, thus creating plasma. What method is the best to go at?

I will keep progress going here. Just something I wanted to finish as a project, and experiment with. Thanks!

MArked One

Fri Mar 11 2011, 07:47AM

Registered Member #3688 Joined: Mon Feb 14 2011, 07:39PM
Location: Europe
Posts: 38

Greetings.

I am on the way to create an ETG myself, and I found Sam BarrosÂ´s Powerlabs website to be very helpful. Foil has worked better than steel wool, but i have not had the chance to try powder. My barrel is reinforced steel pipe, with several plumbing attachments, and an internal nyoln insulator for the center electrode, securely epoxied in place. If you wish, IÂ´ll supply pictures when I get home. I do not think, that you will be successful in creating the nesccecary plasma by blowing it over an Arc. AFAIK, you need something to plasmify, rather than air. (I could be wery wrong, though). You could try mixing the powder with water, creating a very powerful propellant.

Kind Regards,
Ivan Kozlov

pauleddy

Fri Mar 11 2011, 11:09PM

Registered Member #2909 Joined: Wed Jun 09 2010, 12:31AM
Location: fort belvoir, Va USA ( south of DC)
Posts: 145

have you tried the little steel strips that are in those anti-theft srips from stuff you buy, there every where if you have a game you probly have one. they make quite a bang when you discharge a few Kj though i have yet to try an ETG but it blows apart bambo reeds and cups full of water
heres a pic of one of them

Killa-X

Sun Mar 13 2011, 08:26PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Have not tried those actually.

My gun now has a metal barrel, but there is a little weld seem. I was able to manually blow the projectile out with very little air pressure, slowly, so i know it should shoot. I did 1KJ to it, and it didnt even fire out.

I think because i just used what i had, 20awg wires for electrodes, it failed. The electrodes evaporated after 4 shots, all 1CM of the copper. Also, a open-barrel shot wasnt even as loud as my PVC system. Not at all.

I'm hoping this fail was due to wires, and not the barrel. Plan to go to what I have on my PVC one, 12awg. Will post updates on how that goes when i get the wire tomorrow!

MArked One

Mon Mar 14 2011, 10:15AM

Registered Member #3688 Joined: Mon Feb 14 2011, 07:39PM
Location: Europe
Posts: 38

Greetings.

Just to update, my barrel failed miserably. The epoxy seam around the insulated electrode detached, causing the gun to become more of a sparkler. Verdict: do NOT use epoxy on your barrel! weld or solder it instead, and use pressure fittings or threads for the insulated electrode.

Kind Regards,
Ivan Kozlov

Killa-X

Mon Mar 14 2011, 11:14AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Ouch...Usually i take a threaded endcap and drill 2 holes at the bottom for 2 electrode (12awg insulated wires) to come through and bridge foil between. Worked good on my old PVC one.

DYI

Sat Apr 09 2011, 05:23PM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
Location:
Posts: 62

The first issue apparent to me is that you're trying to use a metal as a propellant gas. Water is a more suitable choice. Even liquid hydrocarbons would be better than metals. You'll be seeing huge losses using aluminum, as it has essentially all the undesirable characteristics you could look for in a propellant gas - high thermal conductivity, high molecular mass, high boiling point (it may even begin to condense in the barrel in many cases).

Also, the direct wire-explosion heating method you're using has a history of horrible inefficiency. In professionally built designs it's considered only in single stage electric LGGs, and tends to manage around one percent ballistic efficiency in those cases. The capillary tube plasma generator approach is far more suited for relatively low speed hobbyist designs, having achieved up to 20% efficiency in my design. I've done a short writeup on the basics of designing effective ETGs which can be viewed here. I'd also like to note that suitable aspect ratios for the capillary tubes are heavily dependent on voltage used. In the typical low voltage designs, much lower length/diameter ratios are required for full discharge to occur. I don't recommend trying to use less than 5kV or so in a small design, as they tend to be rather inefficient and finicky to work with.

If you're very crafty, you can build a rudimentary ETG with only a drill press and basic tools. It's not going to happen by epoxy and plumbing parts alone.

EDIT: Fixed link.

Ash Small

Sat Apr 09 2011, 05:44PM

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

DYI wrote ...

. I've done a short writeup on the basics of designing effective ETGs which can be viewed here.

Your link doesn't work, this one does:

,p,268939.html

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