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Gunner

Fri Aug 14 2009, 07:04AM

Registered Member #1397 Joined: Mon Mar 17 2008, 12:47PM
Location: Finland
Posts: 43

I am not 100% sure what do you mean with "capillary" tube? :) PS. Is that hot glue thing somekind of recoil thing?

DYI

Sat Aug 15 2009, 02:26AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Posts: 62

Hot glue? No hot melt glue or anything similar is shown on any of the photos or schematics posted on this thread. The materials used in construction are the same as those indicated in the diagram on the first page.

As for the capillary tube, it's an important part of many plasma generators. It is a very small diameter plastic tube which creates and projects the plasma. A thin wire is "exploded" inside the tube, producing a high current arc which ablates and ionizes the walls of the tube, thus generating the plasma, which then expands and flows out the open end of the tube. You'll have to look online for a more detailed explanation.

rp181

Sat Aug 15 2009, 02:55AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Have you tried increasing the surface area of the discharge tube? Sanding with rough grit or grooving (or threading) may help, depending on the power level.

EDIT: You may want to get a LCR meter, to back up your claims.

DYI

Sun Aug 16 2009, 12:13AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Posts: 62

I haven't yet tried any methods of increasing the internal surface area of the capillary tube, and I doubt that my current measuring techniques would be sufficient to discern the difference. I'm not worried about backing up my claims at the moment. This is, after all, an R&D thread, not something from the Projects board. How would an LCR meter help anyway? Prove that the capacitor is really 16uF? How could I "prove" what the charge voltage is? Or how much the pendulum weighs, or if the round is really a painted steel ball bearing which carries more momentum? Trying to prove such things is pointless, in my opinion, and that's not even getting into the possibilities of video editing.

Today I measured the momentum transfer from the exhaust gases to the pendulum from the same distance as my previous tests, and it is surprisingly high - with no projectile impacting it, a 15g pendulum rose 0.10m above its starting position, meaning an imparted momentum of 0.021 kg*m/s. With this accounted for, the corrected ballistic efficiency is 20%, and the efficiency from capacitor to projectile (wadding excluded here) is ~17%, a more believable figure. Of course, with a tighter fitting barrel, wadding would be unnecessary. Once again, all figures stated are the most conservative they can be while still being realistic.

rp181

Sun Aug 16 2009, 02:57PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Posts: 1529

There will always be something, but may as well cover the basics. Any progress on the break wire chrono?

Increasing the surface area could result in a more compact gun, perhaps handheld.

DYI

Fri Aug 21 2009, 02:39AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Posts: 62

No progress on the break wire chrony yet, I've been working on other things. I also have a job, which does take up some of my time. All things considered, it's amazing how much I manage to get done on this.

And now for a picture. It doesn't really give me any new information, but I thought it was a cool looking crater...

It was hard to get a good angle, and the picture really doesn't do justice to the crater, which is 50mm deep and 36mm in diameter.

Gunner

Sat Aug 22 2009, 07:01AM

Registered Member #1397 Joined: Mon Mar 17 2008, 12:47PM
Location: Finland
Posts: 43

Jeesus what a crater :P What happens if you try to shoot with that power to the thin metal sheet?

DYI

Sat Aug 22 2009, 02:42PM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
Location:
Posts: 62

wrote ...
Jeesus what a crater :P What happens if you try to shoot with that power to the thin metal sheet?

I just thought I'd add that the crater shown wasn't done with an airsoft round, but a 0.42g ABS plastic slug. My tests have indicated that whether or not the slug is pointed is not nearly so important as its sectional density, which is what I expected - longer slugs go deeper, even if they have no point.

As to the question about the thin metal sheet: airsoft rounds (0.12 gram plastic spheres) launched from this design at full charge will penetrate 0.75mm mild steel. The heavier plastic slugs would definitely punch through, although it would be as a result of plastic deformation as the speed of sound in steel is much higher than that of plasticine.

DYI

Sat Dec 26 2009, 10:32PM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
Location:
Posts: 62

This project isn't dead yet, it's just been sleeping for a while.

The only advancement I've made recently is the implementation of a simple chamber liner to avoid excessive erosion and (hopefully) absorb enough water to improve gas production. The liner consists of a tightly wrapped rolled paper cylinder which is pressed into the chamber from the breech opening after the capillary tube is loaded, and is then allowed to absorb water for a few minutes. I have observed an increase in crater volume of roughly 5% (yet to be properly tested with the ballistic pendulum) and a very significant decrease in per-shot chamber damage resulting from the use of this liner.

There have been some durability issues with the chamber, and the capillary tubes are a real pain to machine. To counter these problems, and improve consistency so that I can do more detailed testing of variables such as barrel length, I have finished designing the second version of the electrothermal launcher and began to work on machining it.

Currently, my best estimate of the maximum chamber pressure exceeds the failure pressure of the brass chamber by a factor of two. The spike is not maintained long enough for actual failure of the pressure vessel to occur (pulse length <30us), but it does cause measurable plastic deformation, ~0.01mm/shot expansion of the inner diameter. This is not acceptable for a part intended to be used in extended testing, so I have redone the design to incorporate a 440C stainless steel chamber with a much higher hardness and failure pressure.

The redesign also incorporates a simplified, more robust capillary tube which will be much more convenient to produce in larger quantities.

A diagram of the new design can be seen here.

For more information on capillary tube discharges, this page is a good place to start. The EML symposium papers are a good source of information for those not wealthy enough to use ieeexplore or similar.

ramses

Sun Jan 03 2010, 12:10AM

Registered Member #1208 Joined: Thu Jan 03 2008, 05:30PM
Location: Chesterland, OH
Posts: 154

Amazingly, I have signed into this website after at least a year of spudfiles. I read/skimmed the paper about ablation, and I am still unclear about the effect ablated mass has on the velocity. On one hand, more mass implies higher pressure, and at the same time, more mass would tend to decrease the temperature (reducing pressure), while also reducing the speed of sound in the plasma (if plasma even has a speed of sound.)

I am unclear on whether you are optimizing the ablated mass for maximum mass or minimum mass. It would seem that you are maximizing the mass, otherwise you would eliminate the capillary tube altogether.

EDIT: wow. I can't read...

Anyway, how are you machining your capillary tubes, DYI? I assume you are turning them, but a smaller ID hole and longer length don't seem easier with that operation... Could you enlighten me?

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