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Electrothermal Launcher Construction Tips

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Unread postAuthor: DYI » Fri Feb 19, 2010 9:46 pm

Doing switching in-chamber may save some energy, but actually achieving such a thing in a proper ETG would be difficult. Considering the pressure generated and the necessity of low volume, any sort of valve attached to the chamber would be a bad idea.

Reducing the resistance in the arc is actually a bad idea - low resistance in the chamber is what ruins the performance of many amateur ETGs. It is, unfortunately, not a good time of night for me to be getting into plasma physics, so suffice it to say that one wants a plasma with as much resistance as feasible, but no more. Too little resistance, and not enough energy is dropped in the chamber as kinetic energy of the plasma. Too much, and the plasma will either not form at all, or be insufficiently hot and energetic to ablate the propellant properly. Increasing resistance, for a plasma, means decreasing temperature, or increasing a few other variables which I've not been able to positively identify yet (I'm certain of a density dependence, and would guess it to be in direct relationship to resistance). Any direction on this topic would be appreciated.

Assuming that I'm correct, the relatively low pressure plasma formed at the switch contacts has very little resistance compared to the high pressure plasma formed in the chamber, and won't be the source of very severe losses. Just how much? Your guess is probably as good as mine. The texts I've found on the matter are too far out of my depth for me to generate any real solid numbers. Eliminating switching losses would improve performance for a given capacitor bank. I can't tell you by how much.

[/rambling]

Without a "fuse", one would be relying on dielectric breakdown in the gas in the capillary tube occurring before dielectric breakdown somewhere less desirable and repairable. By using a fuse wire, there will always be a very low resistance path through where we want the current to go, simplifying design somewhat.

These have been some interesting questions. To anyone looking for a better understanding, Introduction to Plasma Physics (by R. J. Goldston and Paul Harding Rutherford) would be an interesting read, as would a few other such texts. Don't let the title fool you though, it's nearly indecipherable to anyone short of at least a second year physics course :roll:

I'm trying to develop a better understanding of some of these issues in between schoolwork and other tedium. I'll keep you posted if I come across anything new and interesting.
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Unread postAuthor: 245Tommy » Fri Feb 19, 2010 10:07 pm

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Unread postAuthor: DYI » Fri Feb 19, 2010 10:19 pm

I've seen that launcher before, but never any details on its operation.

I suspect that it would be difficult to scale up, and less efficient than a capillary tube based design without some creative modification. I'd like to see exactly how the triggering is done.
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Unread postAuthor: rp181 » Sat Feb 20, 2010 1:56 am

From the pictures, My guess is he put a coil in series with the capacitor bank, and the plasma is generated just from the dielectric breakdown. The coil serves to accelerate the plasma.
My guess is that was the intended, but the coil does not do anything, perhaps increase pressure by a minuscule amount.
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Unread postAuthor: 245Tommy » Sun Feb 21, 2010 2:53 pm

The 10mm long plastic tubes (labeled styrene) in my gun aren't getting eroded at all and the efficiency has to be way under 10%, any ideas why?
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Unread postAuthor: rp181 » Sun Feb 21, 2010 3:11 pm

the most common problem is way too much chamber volume. Try to decrease the volume somehow (add more plastic perhaps). Try wetting the barrel a little (very little, run some water through it, and shake it all out).
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Unread postAuthor: 245Tommy » Sun Feb 21, 2010 3:24 pm

There's only enough chamber to contain a strip of foil and a little water, it's shaped very similar to DYI's gun. http://s813.photobucket.com/albums/zz59 ... =ETA11.jpg
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Unread postAuthor: rp181 » Sun Feb 21, 2010 3:28 pm

try decreasing the amount of foil, and water.water should not be able to drip.
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Unread postAuthor: DYI » Sun Feb 21, 2010 4:58 pm

There are all sorts of reasons that efficiency might be low. The first one I can think of is that you either have too little working fluid in the chamber, or too much. I use a small paper liner with two or three layers of standard thickness paper, saturated with water.

After that, the barrel could be too long, or possibly too short (I've found the former to be a more serious issue in my designs), chamber volume could be too high, the projectile may not fit tightly enough, there may be plasma escaping at the breech, pulse length could be too long for your energy and projectile mass...

As to the erosion issue, how many shots have you fired, and at what energy? In your case, ablated mass would be around 1mg per shot, although it will depend on the internal diameter of the tube. The mass loss wouldn't be noticed immediately without fairly sensitive equipment. Also, I'm reasonably certain that the tubes are mislabeled, as the melting point of styrene is -30 degrees Celsius. :lol:
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Last edited by DYI on Mon Mar 01, 2010 4:48 pm, edited 1 time in total.
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Unread postAuthor: rp181 » Sun Feb 21, 2010 5:21 pm

What, you have never used a liquid straw?
Of course its not styrene, its probably polystyrene, and styrene is the base.

Make your tube rough to increase the surface area. I have found a sort of film forms after low power shots, try sanding it.
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Unread postAuthor: 245Tommy » Sun Feb 21, 2010 9:40 pm

I put a drop of water on the base of the bullet and a bit above the capillary tube but there's only 3mm between the end of the capillary tube and base of the bullet. The barrel is 10.5" long including the breech. I think the cartridge expands during firing to seal it in the breech because there's not a lot of black soot on the outside and it's noticeably stretched after a few shots. The bullet has a skirt made of tape (it doesn't break up in barrel) that should expand like a mini ball, it weighs about 6 grains. I shot it five times at 1000v 360 joules, a rod fit in the (1/8") capillary tube a tiny bit looser than when it was new and I assumed that was because it was stretched.
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Unread postAuthor: Zeus » Mon Feb 22, 2010 6:02 pm

I will start constructing a test bed for capillary tube testing with a 4700uF
450V capacitor to supply the power. I will post if I come across something
special.
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Unread postAuthor: 245Tommy » Mon Feb 22, 2010 6:26 pm

BTW I tried using a 2x22mm piece of foil (instead of 5x22mm) and the muzzle blast wasn't as bright but the power was exactly the same.
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Unread postAuthor: inonickname » Tue Feb 23, 2010 5:00 am

245Tommy wrote:BTW I tried using a 2x22mm piece of foil (instead of 5x22mm) and the muzzle blast wasn't as bright but the power was exactly the same.


Less energy coming out of the muzzle (that isn't from the projectile). That's a good thing.
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