SpudFiles

After vaguely mentioning EFPs elsewhere, I came up with this bit of mental flatulence certainly there is nowhere near the power provided by high explosives available from a hybrid mix, but it would be interesting to see the effects

I see your thinking, but it wouldn't form the penetrator without very high mixes.

I think at reasonable mixes, <40X, it would just pop out with one hell of a bang. At higher mixes, it'd be louder.

My bet is, just a dome shaped blown out piece of copper or nothing at all, it's just a thicker burst disk.

Maybe, if the chamber was long enough to get DDT the results may be more interesting.

Seems like it could be awesome if someone could get it to work but I think that a barrel would be necessary for it to even be worth anyone's time. The supersonic shockwaves created by the explosives in real efp's are already well.. supersonic and the copper plate is accelerated to high velocity almost instantly as opposed to the slow acceleration a hybrid mix would provide. maybe if I find some copper sheets... Petitlu we need you....

al-xg wrote:My bet is, just a dome shaped blown out piece of copper or nothing at all, it's just a thicker burst disk.

At the reasonable mixes Zeus mentioned, I would expect the same effect. Tomorrow's a public holiday, lots of other projects on the table but I might give this a go on a small scale. A 2 euro cent coin came to mind, but then i remembered its actually copper coated steel.

it would be very interesting to test

I think to get even close to this working with a hybrid, you'd need a very thin sheet and some form of conical barrel to slowly form the penetrator.

I'm thinking, put a plastic burst disc behind a very ductile metal disc (maybe with a small amount of buffer material between them), then a conical barrel to try and force the copper to form a dart. I can see it just crumpling though, and the disc not remaining lined up correctly. It'll no doubt be a fun experiment but the forces involved on this scale are far below what a HE can produce, so my money's on a fun but dismal failure. Haha.

Insomniac wrote:a fun but dismal failure.

That sounds like a lot of my projects interesting thought about the conical barrel... EFP meets squeeze bore

My initial impression is that, while the necessary pressure and detonation speed could probably be achieved to some extent by, say, detonating a supercritical fluid mix from a starting pressure of several thousands of atmospheres, the starting pressure would require a sheet too thick for the forming to actually happen.

Explosive gas mixes are simply not useful for explosive forming. If you want to do explosive forming, use solid or liquid high explosives

I was waiting for you to chime in

Explosive gas mixes are simply not useful for explosive forming. If you want to do explosive forming, use solid or liquid high explosives

You're perfectly right of course, but this is more of a "how much fun can we have with a gas mix" sort of idea as opposed to a "lets make a successful EFP" concept.

Taking this concept off on a bit of a tangent, there is potential for some vaguely interesting experimentation here if you have access to a vacuum pump;

You essentially have a relatively low pressure flat plate accelerator with no valve mass to worry about. The vacuum is required to prevent tumbling and reduce the otherwise tremendous air resistance. Having fired U.S. quarters face-on at clay blocks at ~500m/s with an EM disc shooter a few years back, it's possible to get some nice cratering effects this way. Not really your style though - penetration is, understandably, very low

Hehe interesting.

Another tangent would be to examine the squeeze bore effect.

Certainly a diablo style airgun pellet can be swaged down in a similar manner to the sort of projectiles used by taper bore guns:

I know I'm a little late, but this is an interesting idea. It might work if you used thin copper foil and managed to achieve a DDT, like al-xg suggested. The main problem I see is that copper has a fairly high melting point, when compared to lead. Perhaps you could start out with some thin lead instead?

On another note:
I don't know if you know this already, but the last few inches of some airgun barrels are choked, something similar to the choke tube on a shotgun.

The main problem I see is that copper has a fairly high melting point...

Sarcasm doesn`t translate too well over the internet. Just to keep the record straight in case you were serious, shaped charge jets are typically well below the melting point of the metal, even in the case of copper.

Having tried an actual EFP once, maybe I can lend a small amount of practical advice here (Mods feel free to edit this, not really sure how much I can say on the topic).

As DYI has already explained, You're probably not going to get anything interesting with normal gas mixes. I guess the best way to approximate the effects would be to use gasEQ to get figure the energy density of your desired mix, the compare it to that of a conventional explosive. Your best bet might possibly be natural gas/oxygen at exceptionally high mixes, initiated by an exploding wire of some sort.
To increase the pressure your concave 'disc' can take, drill a small hole in it and run a 'breakwire' through it, attached to the back of the chamber.

As to the real-deal, I used several ounces of a KClO₄/ NaC₆H₅CO₂ mix initiated by a small low-order primer. Containment was a 2" length of 2" sch80 steel with a 3/16" plate welded on one end.
Penetrator was a copper disc which weighed a bit over an ounce.

This test punched a two-inch hole through .063 steel backed with 3/4" plywood, but did not appear to be traveling at sufficient velocities to cause the 'liquid deformation' effects seen in successful EFPs. The slug that had been punched out had not separated completely, but was still attached by a small 'hinge' of metal. It had folded back completely and made contact with the rear of the target plate. I never found the projectile; it's possible it could have broken up.
The launcher was taped to a cinderblock to absorb the recoil. Upon initiation, the cinderblock was reduced to many small, golf-ball sized fragments and scattered about for twenty feet of so.
The launcher survived, but the back plate had bulged noticeably.