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railgun bullet energy vs. damage (bullet resistant plastic)

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railgun bullet energy vs. damage (bullet resistant plastic)

Unread postAuthor: rp181 » Tue Feb 23, 2010 10:51 pm

First read this post on my website: http://rp181.fortscribe.com/?p=384

Any ideas? That damage seems way too severe for a recorded 850fps on a 8.11g projectile (around 200 joules kinetic energy).
This is supposed to be UL level 3 bullet resistant plastic (from thickness, I think its level 2). A level two piece is supposed to protect from a .357 Magnum Jacketed Lead Soft Point. This cartridge is supposed to be nearly 800 joules. Level 3 is supposed to take a .44 Magnum Lead Semi-Wadcutter Gas Checked shot.
Is it because of the small piece (3" x 3"), or is my chronograph data wrong?
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Unread postAuthor: roboman » Tue Feb 23, 2010 11:09 pm

What kind of chrono are you using? You mentioned something about "needing an optical chronograph"...
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Unread postAuthor: rp181 » Tue Feb 23, 2010 11:20 pm

Railgun's produce a large muzzle flash, and that combined with the EM pulse makes it very difficult if not impossible to use a optical chronograph.
For this, I am using a breakwire chronograph. The black broken wires in the third picture of from the chronograph.
The optical chronograph is to calibrate the breakwire chronograph.
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Unread postAuthor: Technician1002 » Tue Feb 23, 2010 11:21 pm

I looked at your wire break setup. If the first wire break is too close to the muzzle, the magnetic field can induce a false signal into the wire break causing a false first break reading. The shockwave from the discharge may also cause an early break of the first wire. As a control, a 3 wire instead of 2 wire gate can be used. Anomalies in the time between the first and second to the second to third break indicates a false signal in the first break.

The light flash produced and ejected fireball of sparks cause major problems with an optical chrony at close range. Stick with wire break or use a high speed camera with intense external lighting.
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Unread postAuthor: rp181 » Tue Feb 23, 2010 11:32 pm

That sounds like a good idea, Ile give it a try next week.
The shockwave theory is interesting, that would explain some pieces of wire that remained unbroken, but a random piece of insulation in the middle was stripped off. I also found that one end of the broken wire was driven with enough force to stick into a piece of wood ~3/8" deep (30AWG wirewrapping wire).
The first wire is placed around 2.5" from the end of the rails, and from looking at the rails, it seems the discharge ended around 3" from the end.

EDIT: The breakwire works by charging the capacitor (started by break of first wire) and the second wire stops the charging. So that means interference could only cause a small increase of voltage in the capacitor (.33v measured, that indicates 850 FPS). The capacitor is 100uf (~97 measured) so that should damp a lot of it.
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Unread postAuthor: jackssmirkingrevenge » Wed Feb 24, 2010 1:25 am

Interesting, it does seem to be a disproportionately high amount of damage for such a low recorded velocity.

On closer examination however, given the fact that you were using such a small block, I think it's feasibly that such damage could have been caused by a projectile at 850 fps. Aside from the crack the projectile doesn't seem to have penetrated much, as one would have expected from a higher velocity impact, and also aluminium is somewhat harder than lead so one would expect less projectile deformation and more target damage compared to firearm bullets.
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Unread postAuthor: jeepkahn » Wed Feb 24, 2010 9:45 am

Bullet resistent media (glass/lexan/etc) are designed to dissipate the energy through deformation(lexan) and energy consumption through cracking... On such a small piece there isn't enough surface area to effectively consume the energy before the glass was destroyed... I believe most ratings are based on a 12"x12" piece of materiel...
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Unread postAuthor: rp181 » Wed Feb 24, 2010 5:41 pm

Yes, they are rated for 12" pieces.

After looking through some more, I am fairly confident that it is accurate due to the hardness reason JSR said and the size.
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Unread postAuthor: 245Tommy » Wed Feb 24, 2010 5:48 pm

What switch are you using?
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Unread postAuthor: jeepkahn » Wed Feb 24, 2010 5:54 pm

I know I said otherwise in my earlier post, but I think your chrony is wrong.... the reason I say this is because the coppercane shooting ".40shellcasing/wirenut combo" projectiles at 1085+ fps are stopped by 3/8" lexan motorcycle windscreen materiel when fired from 30yds on a warm day(on colder aka"sub 40f" days they punch clean through), not sure the exact weight but in the 7-10g range...
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Unread postAuthor: rp181 » Wed Feb 24, 2010 6:02 pm

okay, I still try the three wire method mentioned.

@tommy: It is a injector, working on a trigatron.
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Unread postAuthor: DYI » Wed Feb 24, 2010 6:13 pm

This is closer to 300J than to 300J, and for a solid aluminum slug I don't believe it is unreasonably severe target damage.

Try shooting into a block of clay, and I can give you a much better opinion. Hard targets are not well suited to such low energies.

You could always check to see if you're in the right ballpark with a simple ballistic pendulum :wink:
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