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Electrothermal/chemical gun: Updated with first test results

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Unread postAuthor: DYI » Sat Sep 20, 2008 8:27 pm

I was hoping it wasn't that kind of propellant really.


You were hoping what wasn't what kind of propellant? An ETC, by definition, uses a solid propellant.

In the initial stages (i.e., this winter) it will be just an electrical ignition causing a normal propellant burn. It will hopefully become an ETC gun next summer, when I can get the cap. bank and start playing around with that.
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Unread postAuthor: TurboSuper » Sat Sep 20, 2008 9:14 pm

I'm anything but an expert on this, so I can't help but ask: Won't the electrode degrade over time given the heat and pressure generated from the combined plasma and (insert other fuel here)?
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Unread postAuthor: Hotwired » Sat Sep 20, 2008 9:15 pm

Chemical doesn't have to be solid, liquid is unlikely but a high pressure gaseous mix ignited by a plasma jet still counts.

Actually that sounds like one of my sketchbook designs.

Well actually it's nothing like it but a large electrical power dump into a pressurised chamber of gaseous fuel/air was certainly part of it.
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Unread postAuthor: DYI » Sat Sep 20, 2008 9:17 pm

Won't the electrode degrade over time given the heat and pressure generated from the combined plasma and (insert other fuel here)?


Yes. I wouldn't be surprised if the electrode degraded very quickly.
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Unread postAuthor: Lentamentalisk » Sun Sep 21, 2008 12:23 am

Hotwired wrote: a large electrical power dump into a pressurized chamber of gaseous fuel/air

point of intrest:
that is one way of creating DDT without a long pipe actually. DDT either requires a long runup distance, or a massive ignition source.

anyways, back on subject. Go.
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Unread postAuthor: DYI » Thu Sep 25, 2008 6:46 pm

I finished turning down the bar yesterday (which took off all the rust and got rid of any small bends that there might have been in it). This is critical, as the next stage (drilling a hole through its entire length) requires it to be completely flat and straight.

On Monday, or whenever we find the correct adapter for the lathe, I can start boring the hole. Unless anyone here has a better idea, the teacher's going to need to order an extra-long 1" drill bit (or a smaller diameter bit as well as a boring bar that can fit a smaller hole), and even then, it'll need to be drilled from both sides to make it all the way through. Perhaps someone with experience in machining can tell me how long it's going to take to drill a 1" hole from one end to the other of a 24" long bar (it'll obviously be started with a smaller bit and gradually bored bigger).

This project is coming together quite nicely so far. I'll avoid discussing fuels on here, as it's probably against the spirit of the rules, whether it's against their letter or not. Anyone who wants to discuss fuels with me can do so via chat or PM.
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Unread postAuthor: rp181 » Thu Sep 25, 2008 9:04 pm

it will take quite awhile and alot of cutting fluid. It took about 5 sec just to drill 3/4in through 1/8in alu (no lead hole)
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Unread postAuthor: DYI » Sat Oct 18, 2008 7:37 pm

Finally got to do a test with a "proper" fuel on the little gun I built in tech from that 2.375" bar. The results were quite impressive - they indicate an acceleration rate of roughly
4 800 000 m/s<sup>2</sup>. And no, that is not a typo.

After fuel was accounted for, the barrel was 3.8cm long, and the measured projectile speed, about 5 feet in front of the gun, was 1404 ft/s, or 428m/s. Imprint on the plate showed that the 3/8" ball bearing was still in one piece, and at least relatively spherical, when it hit the target.

And like all my outlandish claims, this comes with a video.
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Unread postAuthor: TurboSuper » Sat Oct 18, 2008 8:10 pm

Impressive. Seems a bit underwhelming size-wise, I guess that's because I'm used to the relatively long barrel of multi-staged coil guns.

Either way, nice work :D
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Unread postAuthor: DYI » Sat Oct 18, 2008 8:50 pm

Just to correct an error caused by my overly simplistic calculations, the acceleration is actually 2.4 million m/s<sup>2</sup>, NOT 4.8 million.

Due to some oddly good luck, however, the error in my acceleration calculation cancelled out the error in my projected muzzle speed equation, and the 2km/s prediction still stands :lol:

Also, it would seem as though the average barrel pressure was about 17000 psi, which seems low. Obviously, some experimenting needs to be done here :twisted:
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Unread postAuthor: jimmy101 » Sun Oct 19, 2008 11:39 am

Seems awful quite for a supersonic round. Very little bang at all. Should't it have sounded more like a rifle?
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Unread postAuthor: DYI » Sun Oct 19, 2008 11:48 am

Seems awful quite for a supersonic round. Very little bang at all. Should't it have sounded more like a rifle?


Small rounds in the just-barely-supersonic range aren't nearly as loud as most people here like to think they are (having fired many relatively small supersonic rounds myself, often in my back yard). I did, however, hear the characteristic crack. It wasn't pronounced enough to overcome the wind or the poor sound quality of my camera.

I'll fire a shot with it in an area more conducive to sound transmission sometime, to give a good idea of what it really sounds like.
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Unread postAuthor: jimmy101 » Sun Oct 19, 2008 1:51 pm

Perhaps, but even a 0.22 gives a very distinct "crack", though I suppose a typical 0.22 long rifle round is up around Mach 1.5 or better instead of MacH 1.1 or so.
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Unread postAuthor: DYI » Sun Oct 19, 2008 2:59 pm

The only way to tell for sure is a chrony, and it said 1404fps. I'm aware that Chronys sometimes give erroneous readings, and I can't tell for certain until I do several more shots, but I'm pretty sure that the number was accurate.
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Unread postAuthor: THUNDERLORD » Sun Oct 19, 2008 3:43 pm

Perhaps, but even a 0.22 gives a very distinct "crack"...


True. But at that close range (video), the cracking super sonic sound is usually not heard for some reason. (just speaking from experience).

Unfortunately I don't have time to check out this whole post right now. :(
Looks pretty awesome though DYI! 8)
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