MEFP

[nbk2000]

Multiple Explosively Formed Fragments are one of those neat toys that the Big Boys have that have always seemed out of our reach.

Well, the idea occured to me that, instead of using complicated microsecond accurate timing circuits, EBW detonators, and all the other tech shit needed to do it the BB way, that it would be simplier to do it the O.G. way.

The nerds building the first atomic bombs had the problem of trying to synchronize the explosion of many different pieces of explosive, in such a manner as to create an imploding shockwave, to compress the Pu core.

Back in the days, they didn't have the electronics to do this, so they used "lenses" made of explosives of differing d/v to shape the shockwaves as needed.

So why not use that ourselves?

<img src="http://nbk2000.freeyellow.com/MEFP.jpg" alt=" - " />

By using an inert block (like wood/styro) as a carrier for pencil sized rod of cast explosives of varying velocity, you could create the multiple mach-stem interfaces needed to rupture an EFP platter into multiple fragments.

The centermost "pencil" would be of the lowest velocity, with the velocity of the pencils increasing as the distance from the center does.

The idea is, that as the shockwave from the detonator radiates outwards towards the outer edge, that the "pencils" pick up the shock and carry it towards the platter. The difference in velocity in the "pencils" needs to be such that the shockwave emerges from the bottom of the "pencil" carrier (and the outer circumference) at essentially the same time.

This is why cast explosives would be used, since they are fairly consistant in their d/v when there's no entrained air or other obstructions. Being able to consistantly replicate the effect would be the most critical skill to develop, since random/unpredictable effects are worthless.

We wouldn't be aiming for perfection like the BB's get with their smiley face MEFPs :) , but being able to duplicate an MEFP (if not the effectiveness) would be a major step in our advancement.

If the "pencil" carrier was removable, without disturbing the outer explosive casing, then carriers with different "pencil" configurations could be installed to form different patterns of fragmentation.

I'm posting this idea in the purely selfish hope that someone will be inspired to try it and post up pictures. <img border="0" title="" alt="[Wink]" src="wink.gif" />

If anyone would be interested in working with me on developing an annular waveform detonation technique (as part of my DVD), I'd like to hear from you. Established members only. Newbies, don't waste my time!

<small>[ April 04, 2003, 12:06 AM: Message edited by: nbk2000 ]</small>

[Microtek]

I use lenses whenever I need a shaped charge. Given the scale of my devices, the shockwave doesn't have enough of a "running start" to become planar before impacting the liner cone, so I use a circular low density material in the axial center of the device. The detonation flows around the edges and converges on the cone in a much more efficient way than if it was a planar wave.
The low density material doesn't have to be explosive in this application ( I think this is called an air lense ).

[nbk2000]

This isn't intended to form a planular wavefront. It's intended to form multiple initiation points that will intersect and constructivly reinforce each other to shatter the EFP platter into multiple fragments. If you used just an inert block, you'd get an EFP, but as a single massive slug, rather than many discrete fragments.

[Cricket]

It is hard for me to understand what exactly you mean. It looks like you want to concentrate or focus all the shockwaves to +/- one spot at +/- the same time on an Explosively Formed Plate? That looks interesting, but may be too difficult to correctly use outside of ideal conditions or high(er)tech stuff. To make the right density of cushion and have it all the right distance apart I mean. I also thought up something like this. You could have a cone of cast (or contained liquid) explosive with a slight point on the end that is to be detonated first. At the correct angle and with correct detonator construction and placement, I think it would detonate evenly once past the curve of the point. Once on the straight tube of explosive it should detonate as an even line going down it, instead of a meniscus or concave (or convex, depends on your point of view) shaped detonation wave. Then detonate the end of it (in the dome) and hopefully it will create a higher peak pressure and have more of a shattering effect. So the higher the detonation velocity, the more efficient I would think. If this is not at all what you mean then whoops. I have trouble understand a lot of stuff. Sorry.

[Mr Cool]

Figures giving VoD's of an explosive at different densities are easy to find. A graph could then be plotted of VoD/density, and used to find out the densities you need given the dimensions of your device.
You would know the density of your cast explosive of choice, and then if you knew the density of glass microspheres (weigh a volume of them and calculate the volume taken up by air around the spheres...) you could work out the HE/microsphere ratios required for the VoD's that you need. This way you would only need to use one explosive (finding several melt-castable, easily prepared HE's with the VoD's that you need would be practically impossible), although settling might be a problem while the explosive sets. Casting each charge a bit at a time would help this by limiting any settling that could occur.
Erythrite tetranitrate would be an excellent choice for the explosive, it is sensitive, VoD in the 8000 m/s range, and melts at 61*C. It is prepared in the same way as MHN, and erythrite can also be found as a sweetener, although it does not seem as common as other things. Xylitol, H(CHOH)₅H, is much easier to find, although I don't know the mp of the pentanitrate. And I haven't been able to find VoD/density figures for either which is a great problem. So, maybe PETN, RDX etc could be used as the main explosive, with cast ETN etc replacing the air. Then you could use the VoD/density figures of the main explosive to give a good approximation to those of your mixed explosive.
Annular: "in the shape of, pertaining to, or forming a ring." Maybe I'm not understanding correctly, but what exactly do you want to be ring-shaped?

Edit: <a href="http://www.boomspeed.com/mrcool/IdeaNotFullyThoughtThrough.jpg" target="_blank">Here's</a> an idea. It'll need more thought, but it's an idea...

<small>[ April 04, 2003, 11:12 AM: Message edited by: Mr Cool ]</small>

[nbk2000]

"...what exactly do you want to be ring-shaped?"

The shockwave.

By having two planular wavefronts collide with each other head-on, they form an annular shockwave. By having this take place in the center of a cylindrical fragmentation case, the majority of the casing will be directed out in an expanding ring of high velocity fragments running parallel to the ground, which is where the majority of the targets would be.

<img src="http://nbk2000.freeyellow.com/Annular_Frag_WM.gif" alt=" - " />

If, instead of a spherical expansion of the fragments, it proceeds as an annular expansion, you can get a 5x or better increase of hit count, and better than 99.5% hit probability, with the effectiveness increasing by orders of magnitude as the distance increases (compared to spherical). While a person would ideally be able to use pre-formed fragments (spheres), you could also use a steel pipe or other metal cylinder, as the casing. By using annular waveform detonation, you would be able to increase the lethality of a "simple" pipebomb by many times.

Also, because of the constructive collision between the shockwaves, this greatly accelerates the fragments to higher velocities than they'd otherwise obtain, resulting in a further increase in lethality. As you can see in the illustration, there's a very dense ring of high velocity orange and red fragments within a much more porous sphere of low velocity green fragments.

The important thing is to have the two wavefronts colliding in the center of the casing. If one is slower then the other, then the majority of the frags will be skewed off into the ground, or the sky, rather than parrallel to the ground where they'll do the most damage. Getting the two to meet where they should is the problem that needs working on. With det-cord, it'd be easy, but that's rather difficult shit to obtain, and to improvise. Reliability is paramont.

As for the MEFP, if you could get an essentially flat shockwave to impact the "pencils" at the same instant, then the "pencils" could be all the same explosive, their purpose being to convert a single wavefront into multiple colliding wavefronts.

<small>[ April 05, 2003, 12:57 AM: Message edited by: nbk2000 ]</small>

[Microtek]

That's where the explosive lenses come in: When a detonation has reached a steady propagation state travelling through a cylinder, drag effects cause the edge of the shockwave to lag behind the central part ( as was mentioned by Cricket ). An explosive lense can be used to flatten out the shockwave by having it encounter a cone of lower VOD HE. The arrangement is similar to an SC with the liner and hollow cone replaced by a solid charge of slower HE; when the central part of the shockwave propagates through the slow HE it is retarded relative to the edges which still propagate through the fast HE. These effects are reasonably robust so I think that fairly simple calculations would give a good starting point.

[Mr Cool]

</font><blockquote><font size="1" face="Verdana, Arial, Helvetica">quote:</font><hr /><font size="2" face="Verdana, Arial, Helvetica">I use a circular low density material in the axial center of the device. The detonation flows around the edges and converges on the cone in a much more efficient way than if it was a planar wave.
</font><hr /></blockquote><font size="2" face="Verdana, Arial, Helvetica">Is <a href="http://www.boomspeed.com/mrcool/SC-centralvoid.jpg" target="_blank">this</a> what you mean, Microtek? Please correct me if not. A picture is worth a thousand words, so I thought I'd knock one up to illustrate what I think is your point, to try to explain it better.