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High power EM projectile launcher

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Kizmo

Sun Apr 12 2015, 09:05PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Todays experiments:

several shots at 800VDC 5.6kJ

First shot:
- 10.6ÂµH coil, 192g disk
- Measured velocity over 800mm travel 60m/s
- Kinetic energy 346J, efficiency 6.2%

Second shot:
- 10.6ÂµH coil, 192g disk, 85g additional non metallic load on the disk
- Measured velocity over 800mm travel 60m/s
- Kinetic energy 497J, efficinecy 8.9%

I also did couple shots with 40ÂµH coil but performance was much worse. I think next coil i will try will be around 8ÂµH and made from super heavy wire. Current coil is made from two parallel 2Â½mm^2 copper wires and it gets rather warm after couple shots.

Right now 60m/s seems to be difficult to beat, but funny thing is that i can add quit a lot of weight in the aluminum disk and it still flies 60m/s

hen918

Mon Apr 13 2015, 05:59PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

what do you use to dump the back EMF? some big-ass diodes?

Kizmo

Mon Apr 13 2015, 06:14PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

hen918 wrote ...

what do you use to dump the back EMF? some big-ass diodes?

Nothing. :)

(well, not true. I dont disconnect my charger and it has 50R series resistance on DC side so any negative voltage will be bled away by the rectifier bridge)

I did some more testing today and find out interesting thing: My chinese scale was reading wrong from the beginning. The 125g disk was actually 192g and so on

I have updated earlier posts with correct numbers

Also made 390g disk:

192g disk at 4kJ in, 346J to kinetic energy

Uspring

Tue Apr 14 2015, 08:53AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

Right now 60m/s seems to be difficult to beat, but funny thing is that i can add quit a lot of weight in the aluminum disk and it still flies 60m/s

Possibly the disk takes off long before the field has reached max. More mass implies slower takeoff and a perhaps a higher field before the disk leaves the range of the field. That would also explain the lower performance for your larger inductance coils, since there the field builds up even slower. But the lower performance might also be due to the larger resistance.

Hydron

Tue Apr 14 2015, 12:54PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

Chucking the coil in some liquid nitrogen could be interesting, dramatically lowers the resistance.

Kizmo

Tue Apr 14 2015, 04:24PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

I am planning to build about 10ÂµH coil that is wound with copper flat bar or perhaps flattened water pipe.

Sulaiman

Tue Apr 14 2015, 04:52PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

from the few small experiments that I have done
and noting performance of various EM launchers
it seems to me that increasing mass usually increases efficiency
with two problems;
great lumps of metal with lots of momentum as well as energy
unimpressive projectile speed.

I can speculate, but I don't actually KNOW why this is,
one of those laws of nature things ?

hen918

Tue Apr 14 2015, 05:02PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Kizmo wrote ...

I am planning to build about 10ÂµH coil that is wound with copper flat bar or perhaps flattened water pipe.

I can recommend flattened water-pipe. I used micro-bore pipe, wound in a spiral, for my can crusher / launcher. It worked well un-enclosed, but I noticed it was getting more bent and out of line every shot, so I 3D printed a surround for it and epoxied it in. Since then it's been on the back-burner and I haven't gotten around to removing the thick layer of plastic covering the coil.

Kizmo

Tue Apr 14 2015, 05:18PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Sulaiman wrote ...

great lumps of metal with lots of momentum as well as energy
unimpressive projectile speed,
more like artillery than small arms.

It is still very impressive bang.. kind of scary

I managed to hit c-clamp that was holding my projectile stopper together. Disk was stopped but the clamp got launched and it smashed ccfl tube from ceiling light fixture

Ash Small

Tue Apr 14 2015, 08:23PM

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

Sulaiman wrote ...

.it seems to me that increasing mass usually increases efficiency
.
I can speculate, but I don't actually KNOW why this is,
one of those laws of nature things ?

As Udo pointed out, if it's heavier, it will stay in the field being accelerated for longer.

I assume that while it is in the 'field' it is adding to the inductance, The more mass, the greater the inductance, and the longer the pulse lasts, also, a light disc will leave the field (and stop being accelerated) quicker, so won't be accelerated for as long as a heavier disc.

Maybe the 'law of nature' is to do with a heavier disc being accelerated for longer, so extra mass just means it's accelerated for longer, therefore mass has no bearing on velocity?

Obviously this would only hold true up to a certain point, the point I'm making is that as mass increases, efficiency increases, up to a point.

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