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Rifle based ETG

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haxor5354

Mon Mar 28 2011, 09:07PM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

whats more efficient? ETG or coil gun

Arcstarter

Mon Mar 28 2011, 09:40PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

haxor5354 wrote ...

whats more efficient? ETG or coil gun

Well, for the amount of work involved, definitely the ETG. Perhaps one day there will be some breakthrough that makes reluctance coilguns more efficient, but i don't think a practical coilgun would be as efficient as practical ETG.

Chip Fixes

Mon Mar 28 2011, 10:13PM

Registered Member #3781 Joined: Sat Mar 26 2011, 02:25AM
Location:
Posts: 701

I am not sure this will work but what if you were to put the capacitors in a different location? I.E. a backpack or something? Thus you would be able to make the stock a lot more realistic and the gun would be lighter

Marko

Mon Mar 28 2011, 10:21PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi guys

I've seen a number of people attempting to build an electrothermal cannon in such a simple naive fashion - vaporize a bit of foil with a bunch of large capacitors in a steel tube, and the wonder why the projectile doesn't even come out and the "plasma chamber" explodes in bits. In reality it goes more like this:

Your capacitors dump their entire energy into a small bit of foil under a millisecond, turning it into a small ball of plasma that expands hyper-sonically, driving a shockwave into surrounding air which expands at speed of sound. The overpressure of this wave is very high, but very short lasting, and when it ultimately hits the projectile the amount of energy transferred to it will be pathetic - the shockwave will mostly just reflect back due to differences in acoustic impedance, and will actually just end up reverberating around the chamber, expending the energy into deformation of metal. Though I'm no expert in the field, I suspect the problem is a very severe case of "engine knocking" problem, which is caused by low octane fuel that tends to detonate.

I think most energy here will actually be wasted into chamber walls simply because their surface if far larger than the projectile's, and inertia of the projectile is so large that it acts almost like a wall in these conditions.

Now to solve these problems one should look into how a real, chemically powered cannon works. We have the combustion chamber filled almost entirely with fuel, that burns subsonically and evenly in entire chamber, with all the resulting gas receiving uniform amount of heat and expanding, again at less than speed of sound in them!

This leads to first and most obvious clue regarding an ETG - the discharge has to be slowed down! Some sort of a matching network in a must, and the simplest one to try would be a large inductor in series with the discharge, with a reverse freewheeling diode for protection of the caps. The idea of the inductor is to slow down the discharge so it stops producing a sonic boom. If you run your sparks in air, they should not produce the loud snap they normally do. I would probably just use an oversized inductor at first, so that only a dull pop is heard, and then proceed lowering it to the point of sweet performance.

But there is another point that needs to be addressed: the volume of the gas that it being heated. Considering the chemical cannon analogy, we want to heat as much gas we can at time rather than having one tiny, extremely hot plasma spot. In short, you want as much arc path in the volume as possible. Hence, my choice of a "spark plug" for the etg would be something like a coil of thin copper wire that is designed so it turns entirely into plasma upon discharge.

Of course, the windings would have to be as close as possible, but too close and they would arc over each other. The plasma coil would also tend to expand magnetically so it perhaps wouldn't be a best configuration, but the only way to test it would be to experiment. I'd first try to find the largest length of wire that can be exploded consistently with the given cap bank and then try to figure out how to mount it. Or, you might find out that using parallel strands of wire is easier.

Since I don't have time for projects anymore, I am left to enjoy manipulating others's projects for the sake of my evil plans... but in any case please take care not to blow yourself up, will you?

Marko

Killa-X

Mon Mar 28 2011, 11:48PM

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

"and the wonder why the projectile doesn't even come out"

Must disagree. How else did i shoot a film capacitor through the bottom of a soda can.

Even my steel tube works fine so far, Already shot marbles through soda cans at lower power levels, perfectly happy with the results, as to why im continuing and finishing this project slowly. It is a concept that works given you put time and effort into the design, while trying to keep low friction on your projectile in the barrel.

I also understand people use aluminum powder as it allows a much better reaction. IDK who made the video, but i have seen on youtube aluminum powder ETGs shoot an object through concrete cinder blocks.

Marko

Tue Mar 29 2011, 12:01AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi killa,

I couldn't figure from your posts that it worked. But I thought you wanted to maximize it's efficiency in any case? A matching network would also reduce the pulse burden of the scr. With 1000J input output should be in range of a small handgun, IIRC.

Marko

haxor5354

Tue Mar 29 2011, 12:36AM

Registered Member #2063 Joined: Sat Apr 04 2009, 03:16PM
Location: Toronto
Posts: 352

ok so shooting a real connon with C4 would not work neither will a electro thermal cannon. because the sudden explosion of C4 would just blow up the cannon before launching a cannon ball? same goes to a electro thermal cannon?

Marko

Tue Mar 29 2011, 11:23AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

haxor5354 wrote ...

ok so shooting a real connon with C4 would not work neither will a electro thermal cannon. because the sudden explosion of C4 would just blow up the cannon before launching a cannon ball? same goes to a electro thermal cannon?

Well, I think you could say so. I'm not sure why nobody ever took it into consideration when building ETG's. I suspect just using a 10 or so cm length of wire in the plasma chamber instead of foil bit (in line with the middle of the barrel) would help a lot, and slowing down the discharge likely much more. This is of course all just my theory, I never really got to play with these things in person.

Marko

Steve Conner

Tue Mar 29 2011, 11:55AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Marko wrote ...

Since I don't have time for projects anymore, I am left to enjoy manipulating others's projects for the sake of my evil plans

Very nicely put

I think since ETGs are usually built with electrolytic capacitors, the peak power will be relatively low and the discharge time quite long, so it's probably more like gunpowder than C4. If you say that it discharges in 1ms, well, the speed of sound is about 1ft per millisecond, and the combustion chamber is smaller than 1ft.

Aluminium foil also burns quickly when blasted into pieces and ignited by the discharge, which adds more fuel. I remember spending a happy afternoon watching Mike Harrison's Destructotron vaporise various things, and aluminium foil was far and away the most violent. It went with a blinding flash and a really loud bang, and there was practically nothing left. I guess this is why aluminium powder works better still.

Marko

Tue Mar 29 2011, 12:45PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

It went with a blinding flash and a really loud bang, and there was practically nothing left. I guess this is why aluminium powder works better still.

How does one put a sustainable arc through aluminum powder though?

Marko

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