ETG Development / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

ETG Development

previous 1 2 3 4 next last

Move Thread

LAN_403

DYI

Thu Feb 12 2009, 02:39AM

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

Ah, dropping foil. I'm really hoping that you used something more remote than needlenose pliers for that...
73V left is almost exactly in line with what I'm getting: 96% discharge of the caps. I just tried wetting the barrel interior as well as the chamber, and while the crater in the target was excavated to roughly the same depth that it usually is, it was only 11mm wide. After that, I took one more shot with regular loading parameters, and results were in line with what was recorded yesterday. Maybe the discrepancy between our results lies in the voltage used? I still haven't heard any great explanation of why the damp chamber walls improve performance.

On the off-topic note you brought up, certain people could have figured out how to contact me if they really wanted to. I've heard that Spudfiles is degrading, but I don't have anything to contribute there right now.

rp181

Thu Feb 12 2009, 02:49AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I drilled a hole in the side of the barrel, and stick in a piece of 12AWG wire. I attached a string to it, and the foil rests on top. Pull the string, and BOOM! The disadvantage of this is that the barrel has to be at a steep angle, even more if the barrel is wet. I also setup a pneumatic piston to drop it in, but that was way more complicated then it needed to be. My only complaint about my current method is that some plasma escapes from it. Look at [ETG 2, ETG] on my site, you can see it in some of the stills.

My power was also limited by the fact my connection's where steel. I did not want to ruin the 1/0 AWG cable, so i put a steel bolt on, and hose clamped that to the (steel) barrel.

I think the voltage left in the bank could be two things:
1) ringback. I am going to install a diode and see what happens.
2) Plasma exit velocity. Our design looks very similar in size. Maybe having a flow choke will allow all the power to empty.
I will try to get a shot in tomorrow, but i have to limit myself to 2 shots a day. My mom yells at the noise (even if its JUST my injection system). I will record that. I have had problems with the video washing out even at lower powers. How would a polarized screen (for laptops) placed in front of it affect it?
OT: agreed =) oddly enough, a topic like that appeared here.

DYI

Thu Feb 12 2009, 03:37AM

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

How much damage do you really think that steel connections will do to energy transfer? I don't have accurate enough instrumentation to detect the differences in resistance between steel bolts, copper wires, and copper busbar (which are all alarmingly high, if my $20 multimeter is to be believed...).

About the ringback: why would the remaining charge always be in the correct polarity if ringback was the culprit? I'm more inclined toward believing #2, but I'm going to work on reducing the massive losses from my switch before I go worrying about 4% losses from something like plasma exit speed.

The washed out video could be difficult to fix, especially if it is caused by muzzle flash (and not the trigatron, which is what's causing it in my case). Taking the setup outside could reduce the problem, or using bright lighting. Theres not really much to see on the first frame anyway, unless you're using a Casio Exilim or something better for recording it.

That's too bad about the noise. It isn't much of an issue here - as long as the neighbours don't call asking about it, my parents are happy.

rp181

Thu Feb 12 2009, 04:15AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Its not just the fact that is was steel, but also it is a threaded rod connecting to round object. That means very low contact area. My neighbor's are not the problem (they didn't say anything when there backyard was filled with smoke), My mom just thinks im destroying the house.

I can't say much about the resistance, my multimeter is a 10$ one with the resistance function not working =)

Tomorrow i will try a bit of steel rod as a projectile aiming at 1/4" particle board after i machine some stuff for the railgun at school.

rp181

Thu Feb 12 2009, 10:35PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

So i just did another ETG shot, and recorded. This shot was alot worse, and ended up with 110v at the end, as the connection werent secure enough. This shot had alot more smoke than usualy, but not as much plasma.
Video:

oops, double post. merge unless no one really cares.

DYI

Sat Feb 14 2009, 03:00AM

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

Rp181, if you could get a DECENT video of a shot at a TARGET with a PROJECTILE, it would be much appreciated

. One needs a very big gun indeed before watching it fire is even remotely as entertaining as seeing what happened to the target.

There's not a whole lot more I can do until I get a new chrony, or miraculously find a high voltage energy discharge capacitor around here somewhere (that doesn't cost thousands of dollars).

Also, I've noticed that this thread has become more of a dialogue than a multi-party discussion. Does anyone else here have some experience in this field?

rp181

Sat Feb 14 2009, 03:29AM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Not to continue the dialog, but here it is:
I am currently sitting in the basement, with the rogowski coil connected to the laptop. I fired off a shot (very suckish), and the Coil peaked up AND down. There ther was a up peak, down peak, goes midway from top and mid, back up to the top, and down to zero. That means massive ringing, or a issue with the coil (worked fine for 1 capacitor). Does it affect the coil if the cable goe through the center, and the cathode is right outside of the coil?
I am doing another shot now.

j.azz

Sat Feb 14 2009, 06:19PM

Registered Member #888 Joined: Tue Jul 10 2007, 06:52PM
Location: Hannover, Germany
Posts: 40

DYI wrote ...

Also, I've noticed that this thread has become more of a dialogue than a multi-party discussion. Does anyone else here have some experience in this field?

Well, some years ago I built an ETG but it was too small to say that I really gained experience that could be useful for high power devices.
I discharged some paralled caps of an electric fence device (about 30J @ 1200V iirc) into a small 1Ohm resistor placed inside a plastic tube of a pen.
It could launch wet paper balls about 10 meters, so not very spectacular.
But I found out that at higher power with lower voltage (200J @ 360V, electrolytics), the discharge, though exploding the whole pen, seemed too long to transfer more power to the projectile.

When the hot phase in university has cooled down, I would be eager to do some more research with more power...

greetz, j.azz

DYI

Sun Feb 15 2009, 04:43AM

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

I tried a longer barrel today: 60" long, instead of the 10" one I'd been using before. While there would be sufficient pressure in the chamber to allow for a barrel this long at room temperature, it seems as though enough heat is lost through the barrel walls to ruin the muzzle energy. It would be interesting to see which barrel length optimizes muzzle energy for a given projectile here, but in my case that just isn't possible (such small bores necessitate the shipping of tubes across the border, and that just isn't fun at all). If someone could throw together a multi-kilojoule system, commonly available copper pipes could be used for barrels, and the barrel lengths adjusted easily. This much energy is not currently within my grasp.

Assuming that I'm right in the middle of my "plausible muzzle speed range", I'm guesstimating the chamber pressure at around 100bar, although it would depend to some extent on how hot it is in there.

rp181

Sun Feb 15 2009, 06:51PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

I did another shot, this time with a target. No video this time, battery died again =( This shot was the weakest yet, with 120v remaining after fired. The projectile was a small screw, and the target a sheet of 1/8" acrylic. It did not penetrate, but gave a spider-web crack about 2" in diameter. Next to the main target crater, there was another small one, I am attributing this to the foil being launched out. That means it is not vaporizing all of the foil. I presume that the weak power is due to several reasons:
1) electrode vaporizing
2)Electrode shooting out, which leaves another hole for the exhaust
3) hole in the side of the barrel used for triggering

I did however get a interesting rogowski coil shot.

This was with the anti- parallel diode.It is good news that it maxed out though, With integration. With a single capacitor and passive integration, the peak was below the first vertical div. I was put off by the long pulse time (almost 2ms). How is your pulse time?

I just now got my railgun enclosure to fit together, so that will occupy my time. My dad also saw the video of the ETG, and thought it would catch the house on fire =/

previous 1 2 3 4 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob