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1kJ Electrothermal gun (ETG)

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Shrad

Mon Jan 14 2013, 11:56AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

what about using a pyro mix in the combustion chamber?

what accelerates the projectile here is gas expansion right?

what about a mixture of steel or alu powder and CaCO3 which would vaporize great amounts of CO2 when firing?

Ash Small

Mon Jan 14 2013, 02:10PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sounds like some good suggestions above, however, increasing energy 'could' blow the whole thing apart (inevitable if energy is increased beyond a certain point) as this is manufactured from brass/copper, so beware of flying shrapnel and use protective screens when trying to increase power. I suppose metal fatigue or similar could also be a problem here, so take adequate safety precautions.

Nice work, very nice work

MrFlatox

Mon Jan 14 2013, 02:17PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

@ Yandersen :

I see your point. I did not thought that the long plasma jet at the barrel exhaust would imply that my barrel is too short, but now that you have mentioned it, it seems pretty obvious. Unfortunately, the initial design has a barrel twice as long as it is now, but I cut it thinking that the bad performance I was getting at the moment were the consequence of the too-long barrel. I could increase the lenght by simply soldering another bit of tube at the end.

I also see your point in the pulse lengh, if it's too short, the power dissipated in the capacitor, switch and wires (RIÂ²) is larger than when making a longer pulse. But I also think that a shorter pulse would create hotter plasma than a longer one, thus increasing the pressure. Afterall, it's just a combination of pulse and barrel lengh. But judging from the plasma jet I get, one of the two criterion are wrong (solution : longer barrel or shorter pulse, or a combination of the two) You are saying that a longer pulse, and a longer barrel would be better and I think you are right. But if the plasma stay longer in the chamber, it would have time to transfer heat through chamber and barrel wall, decreasing temperature and pressure.

If I want to have a longer pulse, you're saying that I must put an inductor in series with the main circuit, right ? Could I just simply "wind a few turns of my 16mmÂ² main wire and a ferrite torroid ? will it works ? Something is worried me however, this will create a LC circuit, which will probably oscillate, and as my capacitors are electrolythics polarised ones, they will not like it very much. Should I put some protective diodes across them ? Is it a redhibitory restriction ? And by the way, in your opinion, is there already a LC circuit in my ETG capable of generating high back EMF voltage ? I don't have a clue about the last question...

Talking about that gave me another idea : why not using a heavier projectile ? it will theoretically decrease the velocity, increase the time when the projectile travels throught tha barrel, and thus it will take more energy from plasma pressure ? So efficiency would be greater ? I read on the internet that ETG are bad with heavy projectiles due to the fact that the pulse time is so short that light projectile are more susceptible to come out of the barrel before the pulse is over. I tried bullet shape steel rod, but the results were absolutely terrible because the bullet jammed in the barrel every time.

The main inconvenient of ETG is that there are so few valuable information on internet.

DYI has the most advanced one (10-20% efficiency) but his design is nowhere near cheap and easy to operate.

Larda has the most powerful 29 kJ.

and... that's it.

@ Shrad

I prefer staying away from chemically powered thing, because this is field I feared and where my knowledge is small compared to electronics (I am not a "beast", far from that)

@ Ash Small

When the ETG is fired, I am not even in the same room (thanks to the rope triggered switch, so a concrete wall is between me and the ETG. Thanks for safety advices

You can never do too much for safety.

Yandersen

Mon Jan 14 2013, 04:26PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

"soldering another bit of tube at the end" - are you sure there is no way to replace the whole barrel? Soldering is the worst solution possible. Or your projectile has smaller diameter than a barrel? It must fit without a gap, actually.
How about halving your cap's bank instead of doubling the barrel?

Once again about efficiency. Heating up the unexpandable plasma means heating up wires instead. The hotter the plasma, the more heat is dissipated in wires while plasma stays at the same temperature emitting just a light. When plasma expands (only when projectile moves giving the volume for the gas) it cools down increasing it's resistance and heat dissipation in a gap. Using a heavier projectile will lower efficiency dramatically as plasma will not be able to expand during the short pulse time, so it will get close to superconductive state almost instantly and dissipation will continue on wires. The projectile must be as small as possible. But you may try to stabilize the current by use of inductor and try heavier projectile - this should work.

Forget about ferrite thoroids. You have incredible currents running through, so only rods are acceptable cores. Better to use steel. Go to Home Depot to get an 1" hex screw half feet in length and a few meters of thick wire to wind around it.
When inductor is inside the circuit you will be able to judge currents and pick up an appropriate damper diode. But I doubt it is needed as if everything works right way (all the voltage is dropping on the air gap) then minimal energy is stored in wires.

Oh, I just got an interesting idea for a second function of an inductor - it may work as a trigger so you can avoid using your mechanical switch at all as well as a foil! Say you put hundred turns on your screw. Then wire few more turns over it as secondary coil. Make an air gap inside the chamber as small as possible - just a little bit more than your cap's voltage can blow through. Take a little non-polar cap and charge it from the same source. To trigger the shot, apply this small cap to the terminals of a secondary coil. This will lead to a high voltage spike on a primary coil which will penetrate an air gap between a terminals in a chamber by a spark, and once air will become conductive, main cap bank will discharge through it. Cheap solution, isn't it? And no losses in switches. :)

MrFlatox

Mon Jan 14 2013, 06:10PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

My projectile does not fit perfectly in the barrel they have a diameter of 5.95mm (airsoft plastic bbs) while the barrel is approx. 6mm or more (standard copper pipe 8mm OD 6mm ID). I actually tried to fire a projectile that fit better in the barrel, but it was too heavy, and it jammed in the barrel. Having a projectile free in the barrel is a problem ? If it's tigh enough, it offer no resistance (friction) while still blocking the barrel (few gas loss). The whole barrel is soldered at the base of the chamber.

Would you please develop the details about the inductor ? How many turns are we talking about ? The pulse absorb a huge amount of amps, does having a large inductor will make it worse about the resistivity of the circuit ?

I like your idea of triggered spark gap ETG ! I am not sure if this would work but I could definitely give a try. Same question, how many turns are talking about on primary and secondary ? This would definitely ease the operation of the gun since putting the little Al foil is really a pain in the back. With a triggered spark gap, it would only need some cleaning after each shot. Is there any danger of damaging the main capacitor bank with the HV spike produced across their terminals ? I know that the energy involved is low but I am just questioning. I'm not in favor of halving my capacitor bank, my main goal is power. If halving my capacitor bank give me a 1% increase in efficiency, thus decrease the kinetic energy of 49%, I will not do that, and will coninue to run the set up as it is right now.

Another thought : what about re-arranging my capacitor array to make : 900V 2500ÂµF instead of 450V 0.01F ? What will I get ? Certainly a shorter and heavier pulse, is it right ? how will this affect efficiency ?

Yandersen

Mon Jan 14 2013, 10:17PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

5.95 to 6.00 is good enough.
How many turns? I don't know. Say, 100-200 should be good. Actually, if after all function of inductor will turn into just a triggering transformer, then you can use your thoroid - doesn't matter that it will get saturated and current will be uncontrolled during the main discharge.
Try triggering with just one of the main caps in an independent assembly. Regulate the gap between electrodes to get desired result - no spontaneous sparking at operating voltage but triggered discharge when supplementary cap touching the secondary of a transformer.
If you have 100 turns on primary inductor and just, say, 5 turns on secondary over it (insulate all wires heavily!), then applying 450V on secodary will result in 9kV on primary. 9kV will add to 450V of cap's bank and result in 9450V applied to an air gap during the triggering pulse - this should be enough to turn 9mm of air into plasmed spark, so when triggering pulse will finish current will continue to run through that ionized gas even at 450V of main cap's charge.

I was playing with 780V and spark was easily penetrating through the water drop covering electrodes distanced by 1mm away from each other. Though, without inductor they melt and get damaged due to overcurrenting. With inductor, I still had to clean them over as water electrolized them. Transformer triggering I didn't tried - go ahead, be first. Should work, actually.

If you say there is no info about ETG, then WTF you started from 1KJ instead of touch-try-investigate it's operation at safe energy levels?! Playing with lethal energies is a final build step.

Do not rearrange your caps. It will only lower the efficiency due to the increase of ESR. The only reason to go overvoltage is to be able to make an air gap wider and trigger the pulse by simple switch. You say caps are left 80V charged? It means that plasma stopped to be conductive when voltage dropped to that level - current falled to the level when heat dissipation got higher than plasma's heating - it's resistance started to rise and heating got even lower and so on. Closing the channel happened almost instantly. Anyway, by that time, I'm sure, Al was all evaporated and chamber was filled with plasma. So all the distance between electrodes was conductive even at just 80V. That's why I'm expecting transformer triggering to work well - when gas is ionized, much lower voltage is needed to keep the current running through the big distance filled with conductive plasma.

MrFlatox

Mon Jan 14 2013, 10:33PM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

I did not said that there was no info on ETG, but there are a few, far less than for a coilgun for example. With a dirty design like mine, I think it would be difficult for it to work below the kJ level of energy.

100 to 200 turns... Do you realise the size of that transformer, and lengh of cable required ? I think that the resistance of all those turns would be too high to enable the ETG to work properly. Plus, due to the very high current pulse flowing through the inductor/transformer I guess that a high voltage peak will appear on the secondary. Is there any other means to trigger my spark gap without placing a montruous transformer accross the main circuit ? What about placing in parrallel of my main circuit, another circuit that would generate a Hv pulse that would simply cross my spark gap, without damaging my capacitor bank, nor a voltmeter placed across the main capacitor bank ?

Yandersen

Mon Jan 14 2013, 11:22PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Big size? Say, 2mm wire, 100 turns - less than a feet screw you need to wire around if single layered - is that bigger than your current mechanical switch? Less than 4 meters of such wire will be used for a screw 1 inch in diameter; given the actual wire is 1mm in diameter (0.5mm insulation) it will add 87mOhm of resistance. The benefits? No losses in mechanical contacts of switch, possibility of running without the foil (just an air gap).

The HV spike on secondary will not be higher than applied 450V. Main caps will not be affected at all.
There is no other way except of using HV diode (really-really pretty much HV diode...) or smg... I'm not sure what else and how can work here without messing up with the cap's output.

MrFlatox

Tue Jan 15 2013, 08:31AM

Registered Member #9349 Joined: Mon Jan 07 2013, 08:50AM
Location: France
Posts: 102

I know you seem to like inductors but, seriously, 1mm wire ? 87mohms is far too high, I think it will ruin all the efficiency it is getting right now. I truly don't understand. You want to lenghten the pulse but in what proportions ? +100% ? +1000% ?the "spark" will be pretty weak.

I am okay to try a triggered spark gap first, we shall see next for the inductor as i am absolutely not sure of the results. How could I add a feature on my main circuit to generate HV pulse to cross the gap between my two eletrodes without harming the rest of the circuit ?

Yandersen

Tue Jan 15 2013, 12:03PM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

The HV will be as like another cap connected in series with the main cap. HV will be applied only to the gap. Temporarily. And it will not contain any significant energy - just enough to ionize the gas between electrodes to make it conductive at 450V.
Don't want don't try. Don't try - stay the same without risking and taking chances. Not my business, I'm just suggesting.

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