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Explorations in Data Destruction 5: The Munroe Effect

Zoz continues his series of HDD destruction experiments as he gives annular and radial shaped charges a shot and tries the compression welding technique.

Insufficient amount of stearic acid

The stearic acid turns out to be a really important component of this explosive. And if you don’t get that amount right, it doesn’t work. So this is a test shot using pyro aluminum powder (watch video above). It’s stearic acid coated, but not that much stearic acid. Here’s a high-speed shot. And what you’ll see there is that the blasting cap just throws it around. That is a non-detonation, right? So that is a total fail.

The effect with enough stearic acid content

Principle of the Munroe effect

Principle of the Munroe effect

When you get the stearic acid content approximately right, this is what it looks like (watch video above). So, I’m sure many of the people here already know this, but this is 101, so let’s talk a little bit about the Munroe effect (see right-hand image). That’s the official name of the effect when we say “Shaped Charge”. What it means is you have a groove, often conically shaped but it can’t be linear, for example like a cutting charge, in your high explosive. So when you put that with the groove facing the material you want to cut and you set it off, the cavity concentrates the shock wave and forms a kind of a jet. And you can actually line the cavity with copper or tantalum and form a liquid metal jet that will cut through whatever you are trying to cut through. Very-very useful technique. A lot of the anti-tank warheads and that kind of stuff uses it.

Leveraging the Munroe effect

Leveraging the Munroe effect

So here are a few design tips for doing it (see left-hand image). What I was doing is I was layout out a cup to hold the FELIX in OpenSCAD. A few rules of thumb: apex angle should be 40º-90º – the narrower the angle the greater the penetration until your jet collapses and doesn’t work; you want to stand it off by about 2-3.5 cone diameters; and your explosion charge height should be a little bit more than the height of the cone.

The annular shaped charge idea

The annular shaped charge idea

First of all, what I thought was, well, what about doing a linear shaped charge in the shape of a ring (see right-hand image), and putting that on top of the drive so that you cut through the platters? I designed this all so that it would fit within 1U.
FELIX inside

FELIX inside

So there it is, viewed from the top, 3D printed; and viewed from the bottom.

We can fit 60 grams of FELIX in this little container, and using a plastic cup there for the standoff. Don’t concern yourself too much at this stage with the containment, because I’ll work on that later. This (watch video below) is shot at normal speed and then just slowed down. Here’s another shot, same technique, same amount. You can see a bit of that drive exit stage right.

Annular shaped charge vs HDD demo

Results

Results

Here are the results (see right-hand image). Not as impressive as I’d hoped, unfortunately. The first thing you’ll notice is there’s a lot of unconsumed aluminum, so that stoichiometric mix was not correct. That was over-aluminized. Turns out, you know, you don’t need too much to sensitize the nitromethane. It stripped all of the platters off the spindle, which is pretty cool. And it has crushed the platters amongst themselves. So, you know, definitely it’s done some damage. Definitely it would be difficult to exploit information from this drive. And there is one place where the shaped charge has done its job and it has cut through, but that corresponds to where the cap was placed on that shot. So, basically, we’re doing the right with this charge, but we’re having a problem capping it, because the charge is not propagating around the ring the way that we want it to.

Radial shaped charge

Radial shaped charge

So I thought about another idea. What about if we make our shaped charges radial (see left-hand image), coming out like that? Here’s another OpenSCAD model. And also, to try and stop everything from flying around – we found a lot of aluminum around the place with those shots – I made a lid for it as well, with a little hole to feed some detonation cord through.

Charge all set for the test

Charge all set for the test

So there’s a 3D printed charge (see right-hand image). 100 grams of FELIX this time, a bigger physical thing. You can see the det cord that we’re using to set it off all around the place, 18 inches of 80 grain det cord (watch video below). Where did it go? This particular camera, by the way – that’s the close camera – it’s inside an ammo box with a 1-inch acrylic window on it. You can see it gets a good shake from the shockwave. Another one is from a GoPro 120fps, and you can see bits of that drive go in all directions, nothing very big.

Radial shaped charge works wonders

Damage made

Damage made

So we had to search a bit to find the pieces. This one (see left-hand image) is interesting because all of the surface-mount components
This used to be platters

This used to be platters

have been ripped off the board. Here’s part of the platters (see right-hand image). Some more of the platters (see left-hand image below). And we’ve actually got some explosive welding happening here.
Components welded together

Components welded together

That’s actually the top plate and the platters have been welded together. So, very nice.

How about compression welding?

How about compression welding?

That made us think, well, let’s try and do some compression welding. Let’s actually just try and exploit that alone. This is just a straight-up det cord shot, a 100 grain det cord in that top one (see right-hand image). And then the double-sided version – doing some on each side to have the shockwave move from the outside in and compress everything together. I’m going to show you the single shot later on, because we did another interesting experiment with that at the same time.

Compression welding shot

Setting it up

Setting it up

This is the double shot – that’s how we set it up (see right-hand image). So actually, the drive is not in frame anymore, but it didn’t move very far (watch video above). It was actually quite well-balanced. Here is a slowed down shot. You can see it just hop actually only a couple of feet, but it’s a sloped piece of land there,
Experiment aftermath

Experiment aftermath

so it just drops down to where you can’t see it in that shot.

There’s the drive that has still got the plates on it (see left-hand image).

Platters compressed together

Platters compressed together

When we took that top plate off, you can see it did not strip the platters off the spindle like the FELIX shots did, but it did compress them together very nicely and explosively welded them (see right-hand image).
Read head welded to platter

Read head welded to platter

You can’t see because they’re welded together, but you can see that the read head is welded to the top platter (see left-hand image).

Top and bottom of the drive

Top and bottom of the drive

This is the double shot (see right-hand image). It didn’t do nearly as much damage. I mean, okay, the single shot is 2.5 times the double shot in terms of total explosive weight. So this is 40% of the charge of the single shot. It did deform the platters quite nicely and made this really cool groove in them (see left-hand image).
The groove in platters

The groove in platters

But we can see here that they were not welded together in any way, shape or form (see right-hand image).
Not quite enough damage

Not quite enough damage

This is a Seagate drive, by the way. It just goes to show anything you do to a Seagate drive doesn’t work. But we do know that the charge that we need to use to compress the platters and weld them is between those two levels somewhere. So that told us something.
 

Read previous: Explorations in Data Destruction 4: Kinetic Methods

Read next: Explorations in Data Destruction 6: Oil Well Perforators

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