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Electrothermal/chemical gun: Updated with first test results

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Unread postAuthor: Ragnarok » Sun Oct 19, 2008 4:30 pm

I think the most likely problem is that the supersonic crack would have been short lived at those ranges, and when there's a detonating fuel... well, that's going to start to drown those things out.

Most cameras don't pick up loud sharp sounds very well anyway.
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Unread postAuthor: rp181 » Sat Nov 01, 2008 1:11 pm

today i was reading a article, it says 125kJ discharged into water releases 1.5MJ of energy!
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Unread postAuthor: Ragnarok » Sat Nov 01, 2008 1:36 pm

Ever heard of the 1st law of Thermodynamics? 1.5MJ from 125kJ is, not to put too fine a point on it, impossible.
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Unread postAuthor: rp181 » Sat Nov 01, 2008 2:28 pm

it says the electrical energy releases the chemical energy in the water, so no free energy here.

I just tried my ETG again w/ water. My PVC shattered at the bottom, so it is reaching 600+ psi peaks, and this is with 80v left in the capacitor. I made a quick copper one, but it seems to be shorting through the water, whereas the PVC one only bled the capacitors slowly. With the copper one, mains pulsed was enough to shoot the water at me :D.

Heres some pics from the no water ETG.

ImageImage

ImageImage

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Unread postAuthor: Ragnarok » Sat Nov 01, 2008 2:52 pm

rp181 wrote:it says the electrical energy releases the chemical energy in the water, so no free energy here.

There is no usable chemical energy in water - it's already a combustion product, there is no more energy to be released!

My PVC shattered at the bottom.

Well, it sounds like you filled it completely - in which case, that would happen.
You're basically generating hydraulic shock of huge magnitude if you do that.
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Unread postAuthor: rp181 » Sat Nov 01, 2008 3:45 pm

doh, the article was on Electrothermalchemical =p, sorry.

The water was just above the electrodes, which where about a mm away from bottom.
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Unread postAuthor: TurboSuper » Sat Nov 01, 2008 5:09 pm

Ah yes, I have a water-based power plant actually, it's right next to my battery-powered battery charger. :D
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Unread postAuthor: rp181 » Sat Nov 01, 2008 6:55 pm

mine broke =(

=p
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Unread postAuthor: jimmy101 » Sun Nov 02, 2008 11:05 am

TurboSuper wrote:Ah yes, I have a water-based power plant actually, ...

Me too!
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Unread postAuthor: SpudFarm » Mon May 11, 2009 10:13 am

i think this is a good thread to start up again, people might have got new info by this time.

i made a rough 3D modell of the gun design yesterday, it is hard to get good pictures when you does not know how to make a halfcut though
the walls of the design is just made see through to give you a rough idea.

oh and the picture is taken with screenshot and made bigger in photoshop, that is the reason for the choppy edges at full resolution.

anybody uses Autodesk 3D's max that knows how to halfcut?

i may acsess DYI enough to get some replies from him in here by sending him screen shots of the page.

Thanks :)

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Unread postAuthor: Technician1002 » Mon May 11, 2009 10:58 am

jimmy101 wrote:
TurboSuper wrote:Ah yes, I have a water-based power plant actually, ...

Me too!
Image


Same here.
:)
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Unread postAuthor: rp181 » Mon May 11, 2009 8:51 pm

These designs are overly complicated, its quite simple to make a simple chamber. I have a CAD somewhere, ile look for it.

DYI was actually working on a ETG, an then later will be ETC. He made a post on a forum i referred him too, here it is:

This is going to be an ongoing log of my research involving electrothermal guns. For those who prefer to skip the big words and go right to the pretty pictures, they're at the bottom of the post.

I'll start off here with a little background: I've been interested in ETGs since before Larda posted his 29kJ monster on Spudfiles. My interests have always been directed toward propelling things at the highest speeds I can, and ETGs offer an interesting opportunity in that field: other than the mechanical properties of the chamber, projectile, and barrel, there is really very little limiting the speeds that can be achieved with them, unlike railguns, coilguns, or any chemical propellant systems - when you input more energy, you get more out. My first attempt at an ETG used a mechanical switch and a few camera flash capacitors connected in parallel and couldn't even push the wadding out the barrel. I later obtained a 450V 4900uF electrolytic capacitor and a 1.2kV 7.2kA SCR, and this allowed the old gun to shoot 0.12g airsoft rounds at around 180m/s after tweaking. At this point, my interests and money were directed elsewhere, and when the SCR fried, I put away the ETG.

In December, I regained my interest in the project, and came armed with slightly more knowledge than before. I ballasted a MOT with a 300W lightbulb as a power supply, found four old 400V 3900uF 'lytics, and put together a new airsoft ETG with a more solid design (1/4" machined yellow brass nipples and coupling, 0.257" ID seamless SS tube for a barrel, 1/4" SS rod for the electrode with copper busbar and 4AWG conductors). After the pain and suffering of getting the trigatron switch to trigger at such low voltage, I tested at 400V for a while before I got my 20kOhm 12W bleeder resistors. This produced interesting, but not altogether impressive results. With some work, the gun could just barely pierce 3/16" plywood with one of the 0.12g spheres. After getting the resistors, I rearranged the bank so that all four caps were in series, and tweaked the design extensively. I've tested salt water, tap water, Al, Cu, and steel as "propellants" (I only have 1.6kV right now, not enough to bridge the 3mm gap between the electrode and the chamber wall, so I need something to start up the arc) as well as various wadding methods. Aluminum seems most effective at the moment, as well as being much less messy to work with than salt water. On the better shots (performance degrades quickly via the deposition of oxide layers on the electrode and chamber and the ablation of the spark gap electrodes, and the whole thing needs to be cleaned regularly) the 6mm diameter projectiles will punch 15mm diameter entry holes into plasticine targets, which they can travel about 16mm through at the maximum.

Although this project is still in its infancy, I've gleaned quite a bit of potentially useful information already:
-Plasticine is a VERY GOOD target choice for SMALL guns (the 100kJ muzzle energy gas gun I built in the summer would have gone through a lot of plasticine...). The impact marks on the targets can tell a lot about what happened during the impact. Some pellets will shatter on impact and leave very wide but shallow holes, others will penetrate almost three times their own length and remain somewhat intact (although they do expand by about 1mm around their diameter and flatten into disks). As I lack a chrony, or any other way of accurately measuring speeds at the moment, I can tell which loading parameters extract the most efficiency by observation of the crater size. It's relative measurement, but that is better than no measurement at all.
- El Cheapo 0.12g "Airsoft" rounds are not particularly consistent in their manufacture and not effective as high speed projectiles. 0.25" Delrin rod or spheres would be more suited to the purpose, and I intend to acquire some as soon as is possible (Delrin has extremely high impact resistance and shape retention, and was the choice of insulator in the "sparkplug" for my HE gun project).
-Low voltage ETGs are very finnicky things - while a variation of 1% in the ratio of the fuel:oxidiser mix injected into a combustion gas gun makes no significant difference, small changes in ETG loading can affect muzzle energy by a factor of more than 2. Oxide buildup after a few tens of shots on the center electrode in this test device dropped its performance to the point that it was barely more powerful than a high-end commercial airsoft gun.
-Electrolytic caps should be avoided in favour of pulse capacitors for ETG use. There's no way that any shot I have taken yet has topped 5% overall efficiency (60J). That would equate to 1000m/s. Conveniently enough, I still have a target from the summer that was hit by an airsoft pellet that was measured to be traveling at roughly 1000m/s, so the comparison can be made directly, simply by firing at that target and comparing the effects.
-The "propellant", while important, is really only there for the purpose of starting up the plasma production. After the plasma is established, the remainder of the energy probably goes toward making it hotter. I obviously can't determine this for certain, but a very small (2mm x 15mm) strip of foil (the smallest that I can get to "stick" in the chamber and make solid contact with the electrode) is sufficient for full performance, and performance decreases noticeably with large amounts of conductor present in the chamber.
-For some reason, a slightly damp chamber and electrode is superior to a completely dry chamber or a very wet/completely full of water chamber, assuming that a constant amount of foil is used. Theories in regards to this phenomenon would be appreciated.
-Slight leaks in the chamber cut muzzle energy dramatically. The pressure isn't there for very long, but it's also very hot in the chamber, meaning that the plasma can escape through places that it isn't supposed to very easily.
-While there is probably ringing (I don't have the instrumentation to tell), there is no voltage reversal to speak of in this design. Caps always end up charged to between 200V and 300V after firing. So it would seem that I'm discharging at least 96% of the stored energy through the circuit. How much is being dissipated in the switch? I've no idea. There seem to be a lot of ~50% efficiency figures for trigatrons floating around, so perhaps I'm actually putting about 600J into the chamber of the gun. If these numbers are correct, then 1000m/s would be more like 10% efficient conversion of electricity to kinetic energy.


This brings me to where I am now. I hope to acquire a Chrony in a few weeks for measurement purposes. Pulse caps are definitely in the works, and I'd like to try some non-conductive propellants as well as play with the barrel lengths after I get some real speed data. One interesting idea I've come up with is to use a chamber pressurised with helium, with an insulated electrode in the middle. The pressure puts the breakdown voltage of the gas just above the length of the gap, and to fire the gun, the pressure is dropped via a crazy-fast burst disc valve into the barrel. This would have the beneficial effects of eliminating the switch (and by extension, the switching losses) altogether, as well as pre-accelerating the projectile to some extent before the effects of the pulse hit it, hopefully helping to keep it relatively intact. I am aware that this will require a long barrel, and a LOT more power than I currently have at my disposal to work properly.

And now for the pretty pictures (which should be pretty self-explanatory). No vids as of yet, because without a high speed camera or a massive muzzle blast, there's really not that much to see. All of the impacts shown were done with 0.12g airsoft rounds.

I dont feel like copying the pics...
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Unread postAuthor: inonickname » Tue May 12, 2009 4:04 am

Ragnarok wrote:
rp181 wrote:it says the electrical energy releases the chemical energy in the water, so no free energy here.

There is no usable chemical energy in water - it's already a combustion product, there is no more energy to be released!

My PVC shattered at the bottom.

Well, it sounds like you filled it completely - in which case, that would happen.
You're basically generating hydraulic shock of huge magnitude if you do that.


Depends on how you classify it. While you may not call it chemical energy some fuels burning such as magnesium can sever the bonds, using an oxygen for combustion and leaving a hydroxide ion. Probably doesn't count though.

Edit: ahh, gravedug topic. What was he using as a propellant?
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Unread postAuthor: rp181 » Tue May 12, 2009 7:05 am

In the above post, there was no chemical involved. That was just foil. As for the propellant he made, it is against the rules to discuss it.
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Unread postAuthor: Ragnarok » Tue May 12, 2009 9:15 am

inonickname wrote:While you may not call it chemical energy some fuels burning such as magnesium can sever the bonds. Probably doesn't count though.

No, it doesn't count. That's using the chemical energy from the fuel, which is using water as an oxidiser. There is no chemical energy available from the water.
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