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Electrothermal Launcher Construction Tips

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Electrothermal Launcher Construction Tips

Unread postAuthor: DYI » Thu Nov 26, 2009 11:58 pm

Though not nearly as popular a topic as spudguns, ETGs have seen some popularity on the internet in recent times. Most suffer from similar sets of problems that cause them to be inefficient and short-lived.

For what it's worth, here are the basics of efficient ETG construction, as I know them:

Plasma generation. In small-bore designs (the only things us poor hobbyists can afford), a single capillary tube composed of most any plastic (polypropylene and polycarbonate seem to be favoured in professional applications), with an inner diameter less than 4.5mm and an aspect ratio (length/diameter) of around ten will work well. For fuse wire, thin strips of aluminium foil work well.

Chamber: Most chambers used in amateur designs are overly large and/or susceptible to corrosion, and are not properly maintained. The lowest possible volume to contain the necessary propellant and avoid excessive erosion should be chosen, typically not much larger than the projectile's volume. Remember, heat loss in an ETG should be avoided at all costs - heat is all you've got in this case. Brass and stainless steel both work well for generic chambers, various tungsten alloys would probably be better (chances are you'll stick to brass or SS after seeing the prices of custom-made tungsten parts).

Propellant: The goal is to produce a lightweight, fast moving propellant gas, and "absorb" the high temperature of the plasma jet. Polypropylene works well in theory, water is very good (and easy to use) in practice. All sorts of reasonably common substances are viable. High surface area for interaction between the plasma jet and the inert propellant is beneficial, and can improve chamber lifespan. Water-based foams are used in at least one high performance design, to great effect (138g projectile @ 1500m/s, 30% efficiency).

Power supply: discharge waveforms should be tailored to the best of the builder's abilities to the gun's characteristics - you don't want to blast the chamber into dust, and you also don't want a pulse lingering around slagging the chamber long after the projectile is gone.

Maintenance: consider the pressures and temperatures reached in the chamber before you begin complaining about all the black gunk that builds up in there. Proper cleaning, and replacement when necessary of parts will prevent poor performance and possible failures. Making sure that all seals are solid should be a priority. I've had my share of carbonized plastic and molten brass as a result of bad seals.

If the reader cares to take a look on 4hv, he can find the record of how I slowly figured all this out for myself. There are also a few pictures here.Comments and suggestions on how to improve these guidelines are, as always, welcome.
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Unread postAuthor: 245Tommy » Wed Feb 10, 2010 7:16 pm

Sorry to dig up an old thread but could the plastic tubes last more than one shot?
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Unread postAuthor: rp181 » Wed Feb 10, 2010 8:54 pm

They could, but the shots would get worse and worse as the tube is covered in residue.
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Unread postAuthor: Zeus » Thu Feb 11, 2010 4:53 pm

Does the melting temp of the capillary tube affect performance. I mean
say the tube was made of candle wax. Other than that, it's good to see
ETGs becoming more popular.
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Unread postAuthor: twizi » Thu Feb 11, 2010 7:48 pm

would a disposble camera capicter have enough volt to vaporize a piece of wool
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Unread postAuthor: 245Tommy » Thu Feb 11, 2010 9:07 pm

twizi wrote:would a disposble camera capicter have enough volt to vaporize a piece of wool
One cap could vaporize a tiny piece.
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Unread postAuthor: McCoytheLesser » Thu Feb 11, 2010 9:37 pm

To propel a projectile at a decent speed one would need a large capacitor bank (large as in small rail gun large). About 500 to 1000 Joules might be substantial enough to propel a small projectile (say, a nail) to a decent muzzle velocity. Now in order to propel a larger projectile (up to 10 grams) to higher speeds one would need a capacitor bank in the range of 20-40 kilo-Joules. Feasible but can get expensive. A really small scale test (BB size) might be possible with a flash capacitor bank. This gives me ideas :D
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Unread postAuthor: DYI » Fri Feb 12, 2010 9:06 pm

Considering the content, responding here doesn't count as "digging up an old thread".

rp181's answer is accurate, but not the main reason for the non-reusability of capillary tubes in small ETGs. The real problem, in the case of my design, is the immense mechanical stresses applied to the tube during the first shot. In my testing of reused tubes, roughly 50% failed at only 450J. Granted, the voltage I used was unnecessarily high (7.5kV) which stressed the tubes more than a lower voltage design would, but the tubes are heavily damaged after a shot. Although the tubes in my design are entirely contained by a stainless steel part, they still manage to rupture when reused. I've been working on a sturdier design that could prevent this failure mode, but it would be prohibitively expensive and difficult to build due to the required tolerances.

Does the melting temp of the capillary tube affect performance. I mean
say the tube was made of candle wax.


Take a look here. As you can see, there are a LOT of factors to be considered when looking into a capillary tube material. Most of us will be more limited by availability and mechanical strength of the materials. To attempt to answer your question, enthalpy of vaporisation will be of greater effect than melting temperature. There's obviously far more to it than that, far too much to get into here.

McCoytheLesser wrote:To propel a projectile at a decent speed one would need a large capacitor bank (large as in small rail gun large). About 500 to 1000 Joules might be substantial enough to propel a small projectile (say, a nail) to a decent muzzle velocity. Now in order to propel a larger projectile (up to 10 grams) to higher speeds one would need a capacitor bank in the range of 20-40 kilo-Joules. Feasible but can get expensive. A really small scale test (BB size) might be possible with a flash capacitor bank. This gives me ideas


I've been at that "really small scale test" stage for more than a year now. Even if my situation allowed me to up the power, I couldn't. It's taken this long just to get it to hold together at 500J input. Even at this low energy, brass chambers were plastically deforming beyond usefulness, and I was forced to switch to stainless steel. I don't claim to have access to the best in tools and materials, but it is not an easy process to make an efficient ETG which doesn't shred itself during each shot. Before I started this project, I had never seen Lexan shatter like cold PVC :wink:
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Unread postAuthor: rp181 » Fri Feb 12, 2010 9:38 pm

DYI wrote:I've been at that "really small scale test" stage for more than a year now. Even if my situation allowed me to up the power, I couldn't. It's taken this long just to get it to hold together at 500J input. Even at this low energy, brass chambers were plastically deforming beyond usefulness, and I was forced to switch to stainless steel. I don't claim to have access to the best in tools and materials, but it is not an easy process to make an efficient ETG which doesn't shred itself during each shot. Before I started this project, I had never seen Lexan shatter like cold PVC :wink:

:cry:
That made me want to go do a multi-kJ one even more....

Anyway, I have been planning on one after march. Chamber will likely be 2-3" SS and 3/8 ID with the plastic tube (.5" ID of SS). End caps will be 1" thick discs held on with 6 bolts each.
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Unread postAuthor: ramses » Sat Feb 13, 2010 1:56 pm

rp181 wrote:
DYI wrote:I've been at that "really small scale test" stage for more than a year now. Even if my situation allowed me to up the power, I couldn't. It's taken this long just to get it to hold together at 500J input. Even at this low energy, brass chambers were plastically deforming beyond usefulness, and I was forced to switch to stainless steel. I don't claim to have access to the best in tools and materials, but it is not an easy process to make an efficient ETG which doesn't shred itself during each shot. Before I started this project, I had never seen Lexan shatter like cold PVC :wink:

:cry:
That made me want to go do a multi-kJ one even more....

Anyway, I have been planning on one after march. Chamber will likely be 2-3" SS and 3/8 ID with the plastic tube (.5" ID of SS). End caps will be 1" thick discs held on with 6 bolts each.


what do you plan use as an insulator and where do you intend to get the SS?
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Unread postAuthor: Zeus » Wed Feb 17, 2010 5:26 pm

I have read through your ETG thread and I think I've got some of the more advanced concepts. If one was to fill the chamber with a more
easily ionised gas, like Ne how would that affect performance.
I assume that it would allow arcs without a fuse-like item when using
"low voltage" and because of the negative resistance curve, allow
the capacitor bank to fully discharge (assuming there is some Ne left
in the chamber).
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Unread postAuthor: rp181 » Wed Feb 17, 2010 5:55 pm

@ramses
Either a machine shop, or piercemetals. The insulator will probably be teflon.
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Unread postAuthor: ramses » Fri Feb 19, 2010 5:03 pm

Zeus wrote:If one was to fill the chamber with a more
easily ionised gas, like Ne how would that affect performance.


umm... neon is not easier to ionize unless the previous gas was hydrogen.

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Unread postAuthor: rp181 » Fri Feb 19, 2010 6:48 pm

ramses meant Helium.

Atoms are completely stable in octets, or 8 electrons. Without 8 electrons, the atom becomes more and more unstable, hence the oscillating pattern of the above picture.

I think you are confusing ionization (remove an electron) with excitation (electrons jumping levels). The jumping of the electrons between energy levels causes the electron to oscillate and produce light (neon tubes). Give it enough energy, the electron has enough energy to break free (ionization).
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Unread postAuthor: Zeus » Fri Feb 19, 2010 7:24 pm

Ok, I'm yet to have a solid grounding in chemistry. What I meant was excitation. If a gas with a lower arc-over voltage was introduced into the
chamber, would that change the performance. It could allow some degree
of triggering in the chamber, thus reducing losses in switching.
I hope all I'm saying isn't nonsense, if so please tell me to
pull my head in.
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