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ETG gun theoretical efficiencies

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rp181

Mon Mar 02 2009, 02:26AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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And we come back to plasma density. I think that is key here. Ever try conducting threw fire? I havn't, but i bet it doesn't work =)
I think there needs to be a balance between chamber volume and power.

hotcrazyfruit

Mon Mar 02 2009, 03:18AM

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DYI wrote ...

Does this mean that most of the energy in our ETGs is being lost in the connections and switching?

and thats why we use solder and SCR's.

DYI

Tue Mar 03 2009, 12:54AM

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hotcrazyfruit wrote ...
and thats why we use solder and SCR's.

I'm not quite sure where you're finding these SCRs that can survive 10-20kV and 50kA or more and switch to full conduction in less than 50us. Please elaborate.
For those of us who can't afford to spend thousands of dollars on a solid state switch that probably doesn't yet exist, spark gap switches are the way to go here - after the arc is established, there's essentially no resistance, and the rate of current rise is only limited by the connections. The only real problem is their low efficiency. The switch I'm currently working on uses two electrodes separated by about 1mm in a pressurized environment. When the pressure is dropped, the switch turns on. This is easily adjustable, cheap, and probably just as efficient as a solenoid actuated ball gap.

As far as solder, well that's a given. No matter how heavily the contacts are bolted together, the surface area in contact can always be increased with solder.

rp181 wrote ...
And we come back to plasma density. I think that is key here. Ever try conducting threw fire? I havn't, but i bet it doesn't work =)
I think there needs to be a balance between chamber volume and power.

I have tried conducting through fire, and it works, but not at low voltage because, as I think you're trying to say, it isn't a very dense plasma (at least not compared to a firing ETG or the core of the Sun). I think there should be as little chamber volume as possible, only enough to fit the material that's being vaporized. Basically, just the projectile almost resting against the breach plug with the propellant filing the gap. If there is a set amount of energy being dissipated through the propellant as heat, why give that heat extra surface area through which to escape? For a given amount of gas, high pressure / low volume is superior to low pressure / high volume, and that effect should be magnified here because all of the pressure is heat.

In that vein, I've constructed a (somewhat primitive) new test rig, which basically consists of the same barrel with a electrode epoxied into the breach. The tip of the electrode is almost flush with the breach and has 0.5mm clearance from the inner barrel wall. The aluminum foil or steel wool will be pushed down to the breach and the round loaded so that it touches the tip of the electrode, leaving around 20 cubic millimetres of "chamber" volume (versus 2cc for the previous version). I'm replacing all of the (heavily oxidized) old 4 gauge connections with shiny new ones as well. I'll update my thread with results of this experiment tomorrow.

rp181

Tue Mar 03 2009, 04:23AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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I was thinking back to my rogowski coil data, and the railgun data. The railgun showed lots of pulses, I am assuming from projectile bounce. Maybe a burst disc would help here, keeping the plasma contained until the bank is discharged.

badastronaut

Tue Mar 03 2009, 05:29AM

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Lot's of pulses? lemme see :)

Fnord

Tue Mar 03 2009, 02:34PM

Registered Member #2004 Joined: Sat Feb 28 2009, 11:43PM
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wrote ...
The only real problem is their low efficiency. The switch I'm currently working on uses two electrodes separated by about 1mm...

Put the switch inside the chamber.
Use it like a trigatron.
Wouldn't that solve a lot of problems?

rp181

Tue Mar 03 2009, 09:45PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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If you could get it to arc. The gap would b filled with the fuel, and just short the HV.

Fnord

Tue Mar 03 2009, 11:02PM

Registered Member #2004 Joined: Sat Feb 28 2009, 11:43PM
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If your fuel is powdered, yes. But thin wire or foil would be easy to use. The trig would take some precise engineering work, but I'm sure DYI could handle it.

rp181

Tue Mar 03 2009, 11:12PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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wire and foil give lower yields. A bigger piece makes it harder to heat until it can become plasma. When it is powder, It gives quick plasma production, and initiates the arc faster, as not to waste power. I only used foil because i don't have powder.
Here is the rogowski coil data. My conclusion was barely supported, so if you have any better idea's, shoot =)

Edit: Its on my dad's laptop, and he went on a business trip. Ile get it up saturday.

Backyard Skunkworks

Tue Mar 03 2009, 11:18PM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
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rp181 wrote ...

I only used foil because i don't have powder.

Just throw your foil in a blender filled with water (to prevent flash fires) and run it a while. Odds are you won't be able to say you don't have any aluminum powder after ten minutes of blending.

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