SpudFiles

After I got the 450V, 4700uF capacitor for use in my ignition system and witnessed its power, I though I could put together a little electrothermal gun to start out on the road to my goal of a high powered plasma launcher, to be built some time in the future.

This launcher hasn't come close to living up to my expectations for it, with a shot-to-shot speed variation of almost 400%. Even the most powerful shot never came close to achieving 2% efficiency, which would have been about 300m/s (as I can only charge the cap to 250J or so at the moment). I'm guessing that the solution to this problem is, as Larda so eloquently put it on MSN: "MORE POWER IGOR! MOHAHAHA!!!! ". The lack of voltage is probably killing efficiency, and the cap is obviously not fully discharged after each shot.
GGDT says that 200psi room temperature air could have achieved the same power, but that's not the point: the point is, there is no compressed air, no conventional fuel, and no valve needed, only electricity and some aluminum foil.

So, to sum it up, in lieu of b*tching and griping about the lack of power, just donate me a few of those 200kJ 20kV pulse caps from General Atomics

I thought the video was a bit too pathetic to include, but here it is, for posterity's sake:

[youtube][/youtube]



The whole setup



The launcher - it's not anything really complex, just two 1/4" couplings with a close nipple in between that the wire is welded to (it used to just be soldered, but it's welded now )



The electronics - once again, nothing very complex. Camera flash circuit and a D cell inside the box for charging (currently takes about 10 minutes to charge), along with the capacitor. On top there's a key switch to turn on power to the circuit, a charging switch, and two others that aren't currently in use (although one of them is going to get wired to the SCR). Outside the box, there's a ridiculously oversized stud type SCR that I got on eBay, rated for a surge of 8.5kA at 800V that does the switching, powered by two 6V lantern batteries wired in series.



And this is just to illustrate what goes on in the chamber, and my frustration with such low muzzle speeds.

That is pretty sweet. So all you are using is aluminum foil?

Hope the chamber doesn't look like that every shot

Maybe it depends on how the foil touches the terminals as to how it vaporises and produces the spike.

Woo, unexpected mention in the credits!

I think your limited voltage is probably holding you back. If you can get it up to the full 450V, you should see real improvement.

Awesome!
And what if you used something like aluminum or magnesium powder instead of the foil?

Wouldn't a weak burst disk help this gun like it does propane combustions?

I'm not going to try for the full 450V charge, because the next version will be using 800V and 1.9kJ, and I don't want to have to make a charging rig for both voltages. This was just to see if I could do it. I'm hoping to reach at least 1000fps with the next version. After that... Well then it starts getting expensive.

The big spark in the last picture was a dud shot (foil wasn't contacting the electrode properly or something), so I set an aluminum rod between the two wires and activated the switch. Currently, that's how I discharge a failed shot.

As for the propellant - I'm getting a pound of 4000 mesh aluminum powder (it was shipped yesterday), which I can work with to improve reliability for this launcher. That way, I can measure out precise amounts and see what works best. I suppose that my maximum of 0.8% efficiency isn't that bad considering how little voltage I've been using. Larda's ETG was getting over 5% efficiency at 1200V, and it would be interesting to see what could be achieved with higher voltages (say 20kV or so ).

Wouldn't a weak burst disk help this gun like it does propane combustions?

In this case, I believe the pressure spike is so fast that a burst disk would actually decrease performance (as well as heat loss, which is the real killer here - aluminum doesn't really like being a gas, and it won't stay that way long if it doesn't have to).

i like it. simply the best.

well you should try to get mere power on that capacitor.

i can't say much more when i am not a geek in this since it does not interest me.