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DYI

Wed Jul 22 2009, 04:11AM

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

With further research, I have gained further insight. The writers of one of the papers from the 13th EML symposium had a setup almost identical to mine, and it incorporated a few points I will likely test with the "ETAv2" (Electro-Thermal Accelerator - because it's really not a gun in all but the loosest sense of the word).
1. My capillary tubes are way too short - I've got 10mm, and this paper tests two tube lengths - 23mm, and 38mm. They used 600J maximum, so I think it would be safe to assume that increasing the length of the tubes I use by a factor of two would be unlikely to damage performance, and may in fact improve it.
2. Lower diameter is, as I assumed, better in capillary tubes. Their study indicated that ablated mass increased with decreasing capillary diameter for a given energy level. In this vein, I may try decreasing diameter to 2.5mm, or even 2mm (a trivial change in the case of my design, requiring no major modifications, and it only makes machining the things easier, as does an increase in length).
3. The cathode/nozzle. There's was 4mm in diameter (for testing 3mm and 4.5mm ID tubes), and made of a tantalum/tungsten alloy. I haven't checked the shop yet, but there's a good chance that I don't have any tantalum/tungsten bar stock left over from my previous projects (no dense metals at all, unless you count those M829A3 rounds in the storage bin...). I don't think this will be a problem at the moment, but I may have to upgrade to a tungsten alloy cathode later on, when I eventually get more energy to play around with. My wallet shudders at the thought...
4. The anode. They used a carbon disk to protect the brass anode from ablation. Apparently the carbon disk suffers almost no ablation at all, which also reduced the number of variables in their experiments. This is another factor which hasn't bothered me much yet, but which may become nasty at higher energies. However, I would likely use a copper or brass disk instead, simply to avoid having to machine complex parts every other shot - it's certainly cheaper than a tungsten anode. Carbon introduces very significant resistance, and it sounds a bit too explodey for my liking.

I feel that I can also make an improvement to the working fluid and its ablation. If anyone has ideas for turning the water or another suitable working fluid into a low density foam or similar, they would be greatly appreciated. The current setup simply wets the chamber walls and saturates the paper towel wadding with water. This works (the chamber and wadding remains are reasonably dry after a shot), but I feel that it could definitely be improved. Areas of further study will include optimizing barrel length, efficiency changes with different projectile masses, and what maximum speed I can obtain with this supply before I am forced to upgrade. An even smaller bore to allow ridiculous muzzle speeds with insignificant chunks of plastic (~50mg) is also being considered. This would, of course, happen after I come up with a way to actually measure the muzzle speed :p . 20% efficiency is my goal, and it is looking more reachable all the time.

Reid

Thu Jul 23 2009, 10:14PM

Registered Member #2064 Joined: Sat Apr 04 2009, 09:26PM
Location: NJ, USA
Posts: 16

here's an idea for a DIY chrony, it'll need some modifications depending on the ammount on noise produced by the ETG firing and the projectile impact, bit it will give you a good idea of (downrange) speed.

disclaimer....
yes, I am the author of this 'ible, but it can definitely be made to work.

good luck and good work, I'll definitely be building an ETG next!

DYI

Mon Jul 27 2009, 11:18PM

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

Here's the problem with a sound-based system: airsoft rounds moving at this kind of speed will slow down at a very high rate. Measured over a large enough distance to separate the two noises, the result will be a reading lower than the actual muzzle speed. And I don't aim on getting into ballistics modeling to obtain results from the apparatus, so this option seems unlikely to be used.

My current idea is a ballistic pendulum. While it may not be accurate enough for fine-tuning, I will be able to get a reasonable idea of the speeds involved, and characterize the crater formation properties with increasing impact speed. If money and equipment allows, I will eventually build a very accurate wire break based system to do the fine-tuning. This will likely be necessary to reach my 20% efficiency goal (the current estimate of system efficiency lies around 10%, but it is only a guess at the moment).

rp181

Tue Jul 28 2009, 02:18AM

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

Wire break chrono still presented problems when trying to chrono the railgun. It seems the plasma is either conducting a little, or it is electromagnetic interfereance from the pulse. It may work for you, with the lower energy.

DYI

Wed Jul 29 2009, 01:54AM

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

wrote ...
Wire break chrono still presented problems when trying to chrono the railgun. It seems the plasma is either conducting a little, or it is electromagnetic interfereance from the pulse. It may work for you, with the lower energy.

EM interference from the pulse should be distinguishable from the wires actually breaking, unless the pulse is still going when the wires are broken. As the pulse in this design is likely complete long before the projectile leaves the barrel (including the ring-down time, the current flow lasts less than 400us, and projectile exit should occur somewhere in the neighborhood of 1000us after the beginning of the pulse), it should be easy to make the distinction. I doubt that there will be enough plasma in the muzzle flash to mess with the readings significantly. Any plasma there is will be very low density, and not nearly conductive enough for a few volts to flow through.

The main chamber parts have been replaced, and the design slightly modified, following a plasma blowby incident which damaged them sufficiently to prevent their continued use. If anyone wants the exact specifics, I can post them here.

DYI

Tue Aug 11 2009, 02:31AM

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

I recently constructed a simple ballistic pendulum and ran a single test. The result is likely anomalous, as it indicates efficiency in the neighbourhood of 20% (achievable, but difficult, especially without optimization). The camera was positioned wrong because of my expectation of ~10% efficiency, so the exact height reached by the 44g pendulum is unknown. Absolute minimum is 0.42m above initial position. You do the math

I'll be cutting out more capillary tubes, and testing a new material (UHMW polyethylene) to build them with. I will conduct several pendulum tests with an improved setup which will eliminate the possible anomalous results, and update with real efficiency figures in a week or so. On the off chance that the first test was accurate, well, I'd have to say that I've got a hell of a contraption here...

Gunner

Wed Aug 12 2009, 06:09PM

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

What was your cap bank and how much did the projectile weigh? I am very intrested of your project :)

DYI

Thu Aug 13 2009, 01:52AM

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

wrote ...
What was your cap bank and how much did the projectile weigh? I am very intrested of your project

The capacitor is currently 7.5kV, 16uF, for 450J at full charge. The first shot with the pendulum was done at 300J. The projectile is still a 0.12g airsoft round, as it is the only commonly available thing that fits the barrel. Eventually I'll have to get a barrel with a more useful diameter like 6.4mm (instead of 6.1, which is the current size) so that I can simply cut off chunks of plastic bar or use steel/nylon spheres. I don't ever intend on using projectiles much in excess of 1g, as the aim is to produce a launcher which reaches higher speeds than can be achieved through "normal" means, for impact testing.

On another interesting note, I rebuilt the pendulum to weigh 96g instead of 44g, and did a test shot at full power (around 450J, the multimeter/voltage divider was giving screwy readings again, but the current draw through the divider equals the current through the charger at just under 8kV, a neat little safety feature which prevents me from accidentally making the cap go boom). At more than double the pendulum mass, with a different energy level, and at a distance where the minute muzzle blast would have no measurable effect on the momentum transferred to the pendulum, the previous efficiency level was maintained (the pendulum rose to 0.22m) - ~20% efficiency is seeming more and more likely. Even with the absolute most conservative assumptions, 21% of bank energy is converted to kinetic energy of the projectile (the wadding, which weighs 25% as much as the projectile carries some too, so the actual electric to kinetic conversion efficiency is higher).

I should be able to do three more pendulum tests, and obtain an average muzzle speed from the full power results. At the moment, based on results of these and previous tests, I'd feel safe in claiming a minimum of 10% efficient conversion from stored electrical energy to kinetic energy of the projectile. Does anyone know of other amateur ETGs reaching similar efficiency levels?

Gunner

Thu Aug 13 2009, 06:39AM

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

Huh...that`s crazy...if your effiency is 20% with 450Joules that makes 90 Joules to the bullet and when 0.12 gram bb has 90Joules, it has to go +1200m/s :)

E: And if you could it would be great to see few extra pics of your cannon :)

DYI

Fri Aug 14 2009, 12:02AM

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

I conducted another pendulum test today, this time with a 99.5g pendulum. The mass of the round+wadding was measured at 140mg, +/-5mg. Height reached was 0.18m, consistent with 125J total kinetic energy, and 107J kinetic energy in the projectile. Unfortunately, the voltage reading system is broken, so I can say only that a minimum efficiency of 21% was reached. It is worth noting that the pendulum completed a half rotation before it reached maximum height, so the reading obtained will actually be slightly low. Once again, any assumptions made here were conservative - the pendulum probably reached more like 0.19m, and it was likely slightly heavier than 99.5g because of the mass of the strings.

I have two more capillary tubes left. One will be used in a final pendulum test, the other will be used in a "dry fire" (no projectile) to see how much momentum the propellant gases contribute to the pendulum. I'll have to use a far lighter pendulum, probably completely empty or close to it. This will be compared to the total momentum imparted by the previous shots, and I should be able to get a good idea of how fast the rounds are moving.

Also, I have the pictures you requested. These show the full launcher from the outside. I'm not getting into posting a whole archive of pictures until I make it nice and pretty and post it on the Projects board. I might also upload a picture of an airsoft round in about 50 pieces sometime later.

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