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4mm ETG

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Unread postAuthor: DYI » Sat Jan 16, 2010 9:45 pm

Good work 245Tommy, not many people have this much success on their first try at an ETG.

It would be nice to have some more specific details on its operation though; what the propellant gas is, and how it is generated (my best guess based on your design is that you create an arc through the propellant to ablate it) are what I'm most interested in.

Also, it may be worthwhile for you to try a simple ballistic pendulum test to try to measure muzzle speed/energy.
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Unread postAuthor: kjjohn » Sat Jan 16, 2010 9:46 pm

I have always read that they take only a few seconds (<5) to discharge.
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Unread postAuthor: ramses » Sat Jan 16, 2010 10:19 pm

Anything more than a few thousandths is way too slow. The idea is to expend all the energy before the projectile moves very far. Idealy, you want the pulse to be as short as possible, to raise the temperature and thus pressure as fast as possible. 1000 watts for 1 second isn't very impressive, but 10^6watts for 1/1000 of a second can seriously damage things. And make a report like a rifle shot.

DYI will know more.

It is interesting that I just dug out my capacitor bank. I was playing with it drawing arcs through a saltwater resistor. Very bright. :D

I gotta get a lathe.
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Unread postAuthor: 245Tommy » Sat Jan 16, 2010 11:14 pm

DYI wrote:Good work 245Tommy, not many people have this much success on their first try at an ETG.

It would be nice to have some more specific details on its operation though; what the propellant gas is, and how it is generated (my best guess based on your design is that you create an arc through the propellant to ablate it) are what I'm most interested in.

Also, it may be worthwhile for you to try a simple ballistic pendulum test to try to measure muzzle speed/energy.
Actually, my first etg used a cut up co2 cartridge as a barrel and chamber, and it worked alright. The propellant is a 5x22mm shred of aluminum foil dipped in water bridging the screw electrode to the barrel.
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Unread postAuthor: Ragnarok » Sun Jan 17, 2010 9:15 am

kjjohn wrote:Ever think of using ultracapacitors?

Supercapacitors/ultracapacitors... whatever they may be called, are pretty useless for this kind of work.

They don't have the required voltage (and having enough of them in series to allow it would need a LOT of capacitors - and would drop that capacitance heavily), but more importantly, they have a relatively high effective series resistance (ESR).

For the discharge rates needed for ETGs/Railguns/coilguns, we ideally need an ESR of fractions of a milliohm per volt. An average flash capacitor can manage less than half a milliohm per volt, allowing theoretical currents of thousands of amps.
The best supercapacitors are closer to 30 milliohms per volt. They just can't supply the same currents - maybe 30 amps maximum, and that just isn't enough.

You see, I may not know much electronics, but I do know some.

~~~~~

Anyway, to return to the original topic, impressive results from such a small calibre.
You're providing me with unfortunate drive to return to my coilgun project - I just don't have the time or money, gorram it!

In the interests of safety, can I suggest that you please insulate or otherwise protect those bare connections from stray fingers? The voltages and energies you're dealing with could easily cause cardiac arrest if someone (that someone being you, most likely) got a shock off them.

I'm notoriously resistant to electric shocks, as I have abnormally high skin resistance (around two orders of magnitude above the norm, believe it or not), but even I would be wary of what you're doing.
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Unread postAuthor: rp181 » Sun Jan 17, 2010 11:08 am

@ragnarok: me too. At first I thought I had gotten used to it because I have gotten shocked so many times since I was 6 (I guess this is also part of it), but It turns out Im a freak too :)
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Unread postAuthor: 245Tommy » Sun Jan 24, 2010 2:42 am

I'm working on a cartridge version, same caliber.
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Unread postAuthor: jackssmirkingrevenge » Sun Jan 24, 2010 3:19 am

Looks promising :) will the cartridge eject or is it a locked breech?
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Unread postAuthor: 245Tommy » Sun Jan 24, 2010 3:58 am

I didn't even make the barrel yet but I'm thinking of using a simple blowback with a sideways swinging hammer to eject the cartridge.
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Unread postAuthor: qwerty » Sun Jan 24, 2010 4:38 am

I think Jacks getting a bit jealous, Common tommy say some more stuff about how amazing it's gonna be :wink:
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Unread postAuthor: jackssmirkingrevenge » Sun Jan 24, 2010 4:49 am

qwerty wrote:I think Jacks getting a bit jealous, Common tommy say some more stuff about how amazing it's gonna be :wink:


eff off :D

a sideways swinging hammer


Hmm... interesting...
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Unread postAuthor: 245Tommy » Mon Jan 25, 2010 2:02 pm

The barrel and chamber is done, it's made of three sleeved brass tubes soldered and jb welded together.
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Unread postAuthor: jhalek90 » Tue Jan 26, 2010 8:00 pm

Holy mother of jesus.

Lookin good. i wanna see the cartridge shoot now :-)
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Unread postAuthor: 245Tommy » Tue Jan 26, 2010 10:01 pm

I only got a few shots before the insulator started arcing over, here's the testing setup.
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Unread postAuthor: jackssmirkingrevenge » Wed Jan 27, 2010 12:07 am

I like your "bolt spring" :) any chance of a video?
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