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some ETG help and questions..

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LAN_403

Killa-X

Tue Mar 03 2009, 02:05AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

I had a few questions about making a ETG. What type of capacitors do I need to make a working one? Right now I only have 400V 470uf (37.6 joules) capacitors..and 6 will get me near 255 joules. For messing around reasons, I've been shocking foil with 11 camera caps, and 1 large cap, Roughly around 100 joules at 300V. Tests have been done by placing foil over a tube, Then connecting a tube over that so its sandwich between 2 pip.es =|= Charged the capacitors, clipped +300V to the foil, and poked -300V with a screwdriver into the foil and BANG. Now, the screwdriver had a washer ring around it, so was a decent kinda seal. I put a lens cap from a telescope to cover the end of the pipe. Results, the 1 inch diameter cap poped off 2 ft with a bright flaming bit behind it.

What can you guys recommend to actually make a decent mini ETG?

How can I do a triger-spark gap inside the tube? I've saw one guy on 4HV who had a powerful ETG at 200 joules using nothing but a sparkgap in the tube. I thought caps couldn't spark gap?

Whats the best sized barrel to go for?

And finally, Can my capacitors (400V 470uf rubycon + - capacitors) work on this to get a decent ETG. Coilgun wise, guy on instructables, same caps, had EXCELLENT results with these capacitors.

Thanks

DYI

Tue Mar 03 2009, 03:00AM

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

I honestly don't think that we know enough about ETGs here to give you a very good idea of precisely what you need to make a good, efficient one.

Checking out the "ETG theoretical efficiency" thread should give you some good pointers. If the ideas spawned by that thread are correct, then you should have only enough chamber volume to hold the propellant, and no more, as little resistance in your connectors and switching as possible, and as much resistance in the arc as possible while still having a discharge fast enough to keep the plasma coherent, and be complete before the projectile leaves the barrel.

L0MY's ETG used a minature TC to start the arc through air in the chamber in a trigatron setup. How good this is for electrolytic capacitors is up for debate, but neither me nor rp181 have noted any reverse charging of the caps as a result (there definitely is ringing in the circuit though, as shown by rp181's Rogowski coil readout, which may reduce rated lifetime severely on polarized caps).

The bore should be as small as is feasible. Lower diameter equals less surface area for heat loss. Any bore larger than 1/4" at 250J probably isn't going to be of much use. The largest bore I've seen on a working ETG was 1/2", on Larda's 29kJ monster. Unfortunately, you'll really need to know what kind of efficiency and heat loss you're getting to predict the right barrel length, so "just winging it" seems a pretty valid approach to start with. I'm using a 20cm barrel right now, at 200J (although the earlier version used 1300J and the same barrel).

More voltage definitely helps, and faster, more heavily built caps can't go wrong either.

Good luck.

Killa-X

Tue Mar 03 2009, 03:34AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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So there's really no way to spark a capacitor in a tube without a mini tesla to arc accross first to help the capacitors discharge.

I'm using a wallwart transformer driven backwards and it easly shoots to 300V fast. It also was able to create 0.6mm arcs. Maybe I can have a switch on it. Left = Charge caps, Right = Spark gap. When it gaps, Bang?

DYI

Wed Mar 04 2009, 02:38AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Does it arc that far initially, or can you draw out the arc to that length? I've never been able to arc anything short of around 1kV. You could use a flyback for the HV source as well, and I've also heard an idea involving discharging a small cap through steel wool or something similar to create some plasma for the main power source to arc through (I'm pretty sure no one has actually tested that though...).

Your idea for triggering won't work - you need more voltage for the trigger electrode than there is between the other two. I used 2.5kV @ 1A from a MOT in my trigatron to great effect. It never misfired once.

A flyback or micro TC seems most likely for triggering in your case. These things get a lot easier if your bank stores a few kV, and probably more efficient as well.

Killa-X

Wed Mar 04 2009, 04:15AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
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Posts: 1039

Yes, it arcs 0.5mm and draws near 1cm. I do own an ignition coil that can go over 1 inch that I could try....

Today I open a highlighted and a marker. I foiled the top of the marker and shoved it down the highlighted. Then I glued the two together to seal it. I pluged the barrel with a marble, and then connected the foil in the highlighted to my caps. The marble simply fired out and hit a cd case 2.5 ft away since I expected nothing. Mind the way I do this, I push a ground wire through the highlighted tip. This means I got over 2mm of open space for air to fire out. So I lost pressure :(.

So yeah I don't know. Are you saying if I arc an ignition coil over 2 contacts, that allows the capacitor to discharge through the arc? How can you connect the two without sending over 30,000 volts trough the capacitors?

DYI

Thu Mar 05 2009, 01:22AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Regular trigatron switches seem to use corona discharge from the trigger electrode to ionize the gas between the electrodes. I used a high current HV source arcing between the trigger and one of the power electrodes to produce a conductive cloud that would envelope the whole setup, so it was something of a different design.

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