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LAN_403

DYI

Mon Feb 16 2009, 01:03AM

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

I'm guessing that the launcher is too leaky, and uses insufficient voltage. Although I haven't tried it, I'd be pretty confident in my ETG's ability to put a pointed steel projectile through 1/8" acrylic, or at least to come close (although, in fairness, I've done a LOT more tweaking than you've had the chance to).

I'm probably not reading this this right, but it looks on that readout like the reversal is larger than the initial pulse... What is that initial downward spike?

rp181

Mon Feb 16 2009, 01:09AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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just flip it on the X axis. The direction of the pulse spike depends on which way the current is traveling through the coil.

DYI

Sat Feb 21 2009, 03:01AM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Flipping it on the x-axis doesn't fix any problem, the second spike is still bigger than the first.

In other news, I've obtained a small pulse capacitor (7.5kV, 16uF). While I don't have the equipment to charge it to full power or accurately monitor its charge level yet (I still need to build a voltage divider, get a new meter, and scavenge at least two more MOTs), the initial results at only 100J (or thereabouts, I can't tell right now

) are quite promising. While it is not as powerful as the 1.3kJ electrolytic version yet, I think that's to be expected.

I'll try to update this thread after I get the equipment I need to test the new version properly.

rp181

Sat Feb 21 2009, 04:25AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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My theory on the rogowski coil is that the limits are different. So the upper limit is not equidistant from zero to the lower limit. The railgun will be fired tomorrow, so i will see how the coil readout comes.

DYI

Fri Mar 06 2009, 11:40PM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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As was mentioned on the "ETG theoretical efficiency" thread a few days ago, I rebuilt my ETG to have almost no chamber volume (20 cubic millimetres). Results of that reconfiguration, in terms of performance, were quite promising. At 300J, this design produced similar effects to what was done by the one shown at the top of the page at 1300J, with no other changes.

Unfortunately, the new design builds enough chamber pressure that my simple epoxied electrode designs are not holding together very well. Both attempts cracked severely and immediately. I won't be doing any more testing now until I can access a lathe and make a proper test chamber from brass and Teflon. This may happen as early as two weeks from now, depending on circumstances. At that point, I should also have my voltage divider and charging circuit built as well.

Actual muzzle speed measurements may be attempted within a month, but I won't be releasing them directly because of my fear that they could fall into the wrong hands.

DYI

Thu Jul 09 2009, 10:48PM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Well, I've finally got around to doing some proper research and spent some time building a sturdy, well designed chamber. I'm still using very low energy, in the 100 - 300J range, as I will be for quite a while yet, but I have made some tangible progress - efficiency has climbed to around 10% (although I still haven't managed to measure anything directly).

The reason that the wetted chamber walls increased efficiency was the contribution that steam made to the propelling gas (the hot aluminum plasma ablates the water clinging to the chamber walls), and the reason for decreased efficiency with a chamber full of water was heat loss to the un-vaporized water. Aluminum plasma alone loses heat too quickly to the chamber walls and barrel, and its molecular weight is too high to make it an effective propellant gas. Instead, the plasma should be used to heat something that will produce a gas with a low molecular weight and high speed of sound

Switching to a simple capillary tube plasma generator from the old "electrode in center of chamber" design, and reducing the chamber volume to something in the neighborhood of 200 cubic millimetres has made the efficiency increase possible. However, the fact that I need something on the order of 5mg of water in the chamber has made experimentation with other working fluids difficult. If anyone is interested, I'll post the chamber design for the 10% efficient version.

Gunner

Thu Jul 16 2009, 04:46PM

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

I would like to see you design :)

DYI

Sun Jul 19 2009, 08:29PM

Registered Member #1917 Joined: Fri Jan 09 2009, 02:38AM
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Here's the basic design:

ETA11

The barrel is a slip fit connection. When I had problems with it detaching, I made a similar looking part out of 30mm diameter aluminum rod, and drilled and tapped holes which extend to the center hole. Bolts thread into these holes and are tightened, firmly holding the barrel in place.

And now a few impact marks with the new design. All were done using 0.12g airsoft rounds:

PlasticineCraterETA11

This next one has an unused round in the crater for comparison:

Gunner

Tue Jul 21 2009, 12:38PM

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

Do you have any ideas of output velocity?

rp181

Tue Jul 21 2009, 03:36PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
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Thats what you mean by capilary plasma injection. I have seen that before, but it was reffered to with different names. I made a design for a high power version a while back. This one was not intended to fire a projectile, but study the plasma with a high speed camera. It has a replacable plastic linear, that requires minimal machining.
wires screw inbetween the two red pieces (copper), come out, and tighten beneath the aluminum casing and cap.

EDIT: forgot to mention, for size, the red pieces are .5" diameter, and the widest diameter is 1.5".

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