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1kJ Electrothermal gun (ETG)

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MrFlatox

Fri Jan 11 2013, 01:06PM

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

Hi !

Let me present another project (I also made a marx generator w/ homemade caps

from this account because I can't get the good login for the older one...

So here I am again with a (dirty) electrothermal gun made from electrolytics capacitors, homemade mechanical switch, brass chamber, and custom charging circuit. Let me go over the main features of it :

First, here is the main "box" containing the charging circuit composed of a voltage doubler from mains (230VAC), the capacitor bank made from 10 electrolytics capacitors (450V 1000ÂµF) wired in parallel to achieve a bank with 450V 0.01F (which correspond to 1kJ at full charge). The box also contains a the charging circuit controller with a flip/flop base to start or stop the charging sequence. There is also a selectable voltage limit which would stop the charging procedure when desired voltage is reached, but this doesn't work for mysterious reasons... the box also include a safe discharge function wich safely discharge the capacitor bank into light bulbs thanks to an SCR.

You can see above the main switch in opened and closed position, wich is pretty much self-explainatory. The switch is triggered by a small rope, this enables me to be at a great distance when the thing is fired.

Then we move on to the chamber wich is made of brass fittings (I know this is bad, but it's also cheap and easy to use). As you can see, the chamber is composed of three parts.

The "back part" wich houses the main electrode made of flattened copper pipe, of course, the electrode is electricaly insulated from the body by strong epoxy glue.

Then the front part composed of a barrel (6mm ID). The intermediate part is just used to secure both part by tightening them.

The main fuel of this ETG is aluminium foil, the foil is placed between the center copper electrode and the body, traped in the body thread as you see :

Results :

here is a video showing its operation :

The ETG fires 6mm plastic BBs of .23g

Achieved velocity measured by a ballistic pendulum is 510m/s or 1670fps (supersonic) wich give a muzzle energ of approximately 30J, and an efficiency of 3.5%. Considering the quality of the chamber and it relatively large internal volum, I find those results pretty convincing.

Here some pictures of muzzle flash taken from video. I personnaly can't help watching those as I find this to be very... appealing

That's it ! please feel free to make commentaries both good or bad, as long as they are "constructive".

English is not my native language, so it's not impossible that errors or words that does not exist might be hidden in this text

klugesmith

Fri Jan 11 2013, 05:08PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Thank you for showing us your creation.
Spectacular results, and I admire its simplicity.
Will you try different metals for the exploding bridge,
or additional propellants (even water) in the chamber?
Let's see a red, blue, or green muzzle flash, or a plume of steam or smoke.

Your written English is fine -- better than that of the average American user of the Internet.

I hope your ballistic pendulum inspires other readers to try proper instrumentation, instead of just blowing things up.
Here are a couple of things that might make it give falsely high results, and would be easy to mitigate.
1. BB bouncing back toward the gun, instead of being captured.
2. Impingement on the pendulum of other ejecta (such as a puff of gas) from the gun.

Officially (to respect users on mobile devices), you are not supposed to embed large pictures in your posts. One simple alternative is to upload them using a button in the editor; then they appear in post as pretty large thumbnails that readers can click on.

-Rich
p.s. Feel free to share your real name. Or else I might address you as Pierre, or Henri, or GeneviÃ¨ve !

MrFlatox

Fri Jan 11 2013, 06:22PM

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

Thanks for your advice, call me as you wish

As for the large pictures, is it ok if I let them there and do as you said for future picture posting or should I change them ?

As for projectile velocity measurements, the calculated efficiency is in adequation with other ETG of the same energy, so I would not be surprised that the achieved velocity would be true or at least in that range.

1. The BB does not bounce in the pendulum (I took care of that problem) because the pendulum is made of wood, and the BB get trapped into it.

2. This, however, might be true and is one of my concerns. I think that the hot gases/plasma jet participate in the movement of the pendulum, but I don't know in what proportion, and I don't know how could I find it. So i just guess it plays a small role, but to be frank (don't call me frank btw

) I do n ot have a clue.

As for trying other metals as propellant, why not, but I do not know what materials could I use.

I tried to add humidity in the chamber (by breathing in there) but I did not measured the velocity difference that could have appeared.

ZakWolf

Fri Jan 11 2013, 10:59PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

Very cool, good job :)

Yandersen

Sat Jan 12 2013, 03:57PM

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

Are you willing to do some experiments? If so, I have some ideas for you.
First, mechanical switch is not as good as SCR. Afraid of currents? Insert an inductor in the discharge circuit. Capacitance and inductance will give you the current pike value. Inductor must be BIG, yes, but I'm sure your "switch" right now is bigger than inductor and SCR all together.
Another profit of using an inductor is that it prolonges the discharge time so you can make barrel longer and achieve higher efficiency due to lower acceleration - burning is better than blowing.
Second thing is HV - over a kilovolt you do not need anything to burn at all - just make 1mm air gap between electrodes and air will expand the same good as aluminium foil you use right now. Or you can fill chamber with water to higher the efficiency. For air discharge I would strongly recommend inductor because uncontrolled current during the strike will also melt electrodes and blow your ears. I've tried those with 80J and 2mH inductor and 1mm air gap for 800V and get effect of match lightened - just a stable flame for around half a second instead of lightning strike.
I would also recommend to use non-polar caps as those are actually able to give the current you need - electrolithics don't.
So what do you think?

klugesmith

Sat Jan 12 2013, 06:20PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Re. your ballistic pendulum, and shielding it from the muzzle flash.

If the gun shoots straight, you might put one or two thin barriers
well in front of it, with holes to let the projectile pass through.
A wooden board could serve as your "optical bench".

Yanderson's idea of shaping the current pulse, e.g. with a simple series inductor, sounds promising.
For that, as he says, non-polarized high voltage caps are good.
Unlike a coilgun, the ETG can't be matched to source voltage by tuning the wire diameter.

But IMHO your mechanical switch is just fine, and perfectly respectable, as long as you don't need to synchronize the trigger to some other event.
If you are working at 1 kV or higher, then the switch contacts need not touch. It becomes a mechanically triggered spark gap, used with consistent and reliable performance in can crushers and coin shrinkers at 10's of kA.

MrFlatox

Sat Jan 12 2013, 07:03PM

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

@ Yandersen

I think I don't have enough money

I'm ready to make improvements as long as they are cheap and do not imply to totally change my design so that I would have to rebuilt an entire ETG. Please do not take that as an offence, but I know that your advices are meant to enhance an ETG, but my goal is to build cool "science projet" made from scrap or cheap parts. However I'm okay to do minor modifications on the actual design. Space is not a problem, money is.

@ klugesmith

This might work indeed, but the "thing" would have to be precisely aligned and suposed that the ETG fires straight wich I doubt. This could look like a flash hider for firearms ( deflect gases to the side instead of the front).

I also thought about blank firing the ETG in front of the pendulum and see what effet it has on it , and what momentum it will transfer. Then I brutely substract projectile momentum by "blowing" momentum to obtain the corrected velocity.

BigBad

Sat Jan 12 2013, 07:31PM

Registered Member #2529 Joined: Thu Dec 10 2009, 02:43AM
Location:
Posts: 600

Really cool.

Is the aluminium burning and giving extra kick though? Possibly that's why it's going so very fast.

Either way it does seem to have good performance! Cute!

MrFlatox

Sat Jan 12 2013, 07:55PM

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

As I understand it, the BB is propelled thanks to a combinaison of two things :

- the vaporization of the Al foil create hot gases and high pressure spike. It also ionises a path between the electrodes.

- the current continues to flow through ionised gases in the chamber, thus heating up more the small volume of gases contained in the chamber, creating more pressure.

I guess what we can see in the muzzle flashes is actually plasma, correct me if I'm wrong.

As Ã side note, after the shot, only 80v are left in the capacitor, independantly of how small is the Al foil, confirming that electrical discharge through ionised air continues after the Al foil is gone. It also indicate that most of the stored energy is gone.

Yandersen

Mon Jan 14 2013, 10:24AM

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

On the pictures above I see a plasma's trace as long as the barrel in length. How about a longer barrel or halving the energy?
Checking effect of inductor in series is not expensive - just wire any thick wire over the big iron screw. Howhether, as barrel is too short by now, effect will hardly be positive as prolonging the pulse will just lower the current during the short acceleration period.

Just a side thoughts about the efficiency... IMHO, ETG has more potential than coilguns as here heat is used for acceleration which contributes to 90+% of losses in coilguns. Howhether, as I know, plasma decreases resistance with temperature. It means, that during the shot plasma heats up increasing it's conductivity, which results in increase of heat dissipation on wires and caps - heat there does not accelerate projectile and should be avoided.
In other words, short pulse is not effective - if plasma heats up faster than expands, then heat dissipation shifts to wires instead of the air gap. Inductor may stabilize energy supply rate resulting in linear gas expansion, I think.

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