[Basic] Skill of the week: Explosives

[johndallman]

Originally Posted by Ulzgoroth

You could literally make a nuclear explosion by hand, by taking two large subcritical masses and fitting them together. (EDIT: Or would that be an underpowered (but catastrophic nonetheless) fizzle because the fissionables of the core would burn out over a slightly longer time than if it was slammed together mechanically?)

Maybe. The popular image of two hemispheres with their flat faces slammed together is not how it is done, because heating of the air between them will separate them very quickly.

Actual gun-assembly U235 devices put a cylindrical sleeve over a rod, or other configurations where heating of air doesn't strongly disassemble them. You might manage to assemble one of those by hand before it went off, if you didn't have a neutron source around: those are normally provided to initiate the reaction.

[malloyd]

Originally Posted by Ulzgoroth

You could literally make a nuclear explosion by hand, by taking two large subcritical masses and fitting them together. (EDIT: Or would that be an underpowered (but catastrophic nonetheless) fizzle because the fissionables of the core would burn out over a slightly longer time than if it was slammed together mechanically?).

It's not so much that the reaction is slower that causes the fizzle as the geometry never gets to the right point. As the subcritical bits get closer together they heat up from the increasing fission rates. The trick to getting a nuclear explosion is to get them close enough together in a shape with a low enough surface area (from which neutrons are escaping) relative to their volume. In principle heated air might push apart colliding hemispheres, but you might not get even that far doing it by hand, because the pieces get hard to hold on to as they heat up, and once they melt (or *vaporize*) you can't push on them at all and it doesn't take much flow before the assembled mass isn't a sphere, and hence has more surface area that you planned for.

The suddenly generated cloud of metal plasma glowing in neutrons and x-rays will certainly kill you, and likely anybody else in the same building, but still is several orders of magnitude less destructive than if you had set it off properly.

[whswhs]

Originally Posted by Ulzgoroth

You could literally make a nuclear explosion by hand, by taking two large subcritical masses and fitting them together. (EDIT: Or would that be an underpowered (but catastrophic nonetheless) fizzle because the fissionables of the core would burn out over a slightly longer time than if it was slammed together mechanically?)

Obviously not a survivable exercise.

They don't stay together long enough. I've read about a macho exercise called "twisting the dragon's tail" that physicists used to do during WWII involving lowering two subcritical masses together with a hand-held lever . . . at least in legend.

[Ulzgoroth]

Originally Posted by whswhs

They don't stay together long enough. I've read about a macho exercise called "twisting the dragon's tail" that physicists used to do during WWII involving lowering two subcritical masses together with a hand-held lever . . . at least in legend.

Oh, not just legend. Messing with near-criticalities was something that definitely happened, and killed a few physicists too.

I suspect the purpose of the lever there would be to let you carefully control how close you position the pieces, so as to let you approach fatal stupidity without actually quite committing it.

[johndallman]

Originally Posted by whswhs

Jones combined his Explosives (Pyrotechnics) and Mechanic (Clockwork) skill to prepare two casks full of powder timed to go off at a set interval after we dropped them over the stern. Thanks to a critical success on one of the rolls, they set off the other ship's magazine. . . .

Beautiful. There is one aspect in which Hollywood reality is correct: warships, tanks, and other combat vehicles armed with artillery usually do carry enough explosive to destroy them, if it can be set off.

[Ulzgoroth]

Originally Posted by johndallman

Beautiful. There is one aspect in which Hollywood reality is correct: warships, tanks, and other combat vehicles armed with artillery usually do carry enough explosive to destroy them, if it can be set off.

Although I would not think that a depth-charged wooden ship having its magazine explode is a likely outcome. As opposed to having it just break open and flood. Of course, as a critical result it doesn't need to be a likely outcome.

[fredtheobviouspseudonym]

Years ago I knew a guy who was a combat engineer in the National Guard. He told me two basic rules about explosives.

Rule 1: If you don't know what you are doing you will kill yourself.

Rule 2: Unless you've had professional training, and enough thereof, you don't know what you're doing.

So -- don't try this at home, sports fans.

[Not]

What about the Unabomer? He was trained as a math professor, and managed to produce workable explosives for decades. He'd still be at it if his brother hadn't turned him in.

[Ulzgoroth]

Originally Posted by Not

What about the Unabomer? He was trained as a math professor, and managed to produce workable explosives for decades. He'd still be at it if his brother hadn't turned him in.

Generalizations like that are pretty much always false, but it's often a very bad idea to assume that you personally are one of the exceptions.


EDIT: That said, I know there are plenty of people around here who have played around with explosives without professional training and done fine. They mostly stick to lower end of the difficulty-and-danger scale and hopefully have non-professional training from people who've done it before, though.

[vicky_molokh]

Originally Posted by Ulzgoroth

Generalizations like that are pretty much always false, but it's often a very bad idea to assume that you personally are one of the exceptions.


EDIT: That said, I know there are plenty of people around here who have played around with explosives without professional training and done fine. They mostly stick to lower end of the difficulty-and-danger scale and hopefully have non-professional training from people who've done it before, though.

Plus, judging by at least one that I talked to, there's a matter of self-training / study beginning with theory and moving on to practice.

I suspect the dangers of explosives are also routinely exaggerated in order to discourage people from trying. Kinda like the drug exaggerations of the . . . 80s, right?