[Ultra-Tech] Mini-nuke TL

[Michael Thayne]

Maybe the thing we really need is stats for moderately optimistic, near-future 160mm and 400mm designs (calibers that got support in some Pyramid issue or other). The 160mm could even be launched with a 100mm cannon, rifle-grenade style (my understanding is that that's also how the Davy Crockett was supposed to work). Not sure what those stats would look like, though, or if you could get the range of dial-a-yield settings found in UT.

[Rupert]

Note that the Davy Crokett warhead had a yield of 10-20 tons of TNT. That's the 0.01 kiloton bottom limit of the mininukes in UT right there. As making such a small bomb is mainly about making a bigger bomb and then making it fizzle, making such a bomb and having it able to 'dial up' would be fairly easy, as these things go.

A variant of that warhead seems to have been dialable from 0.01 to 1KT. So the question really is "Can TL9 technology make a dialable mininuke smaller than about 11" diameter and about 50 pounds weight?" In the case of the diameter, almost certainly, given we can make bombs that fit into 8" artillery shells, and have been to since TL7. In the case of weight, it's far less clear because the minimum critical mass of U238 or Pu239 doesn't leave a lot of room for miniaturising the rest of the bomb unless, as John Dallman notes, TL9 means a much, much more powerful chemical explosive or some other solution (and the ones mentioned in UT would seem to be more appropriate to a fusion bomb).

Of course UT specifies a fusion bomb anyway, and they have no minimum yield if some means of triggering them that doesn't involve a fission primary can be arranged (and the attempts at fusion power show it can be, and fusion reactors are also TL9 in GURPS). The problem being getting that non-fission trigger system into a 100mm warhead. Again, making it a dial-a-yield bomb wouldn't be much of a problem if you can make the thing at all.

[Verjigorm]

Not really from Ultra-Tech, but the Nuclear Pulse Drive pretty much relies on miniaturizing nuclear bombs, to sizes comparable to the smaller warhead sizes.

[Ulzgoroth]

Originally Posted by Verjigorm

Not really from Ultra-Tech, but the Nuclear Pulse Drive pretty much relies on miniaturizing nuclear bombs, to sizes comparable to the smaller warhead sizes.

I don't think Project Orion was planning to use exceptionally small bombs.

[Fred Brackin]

I believe that antimatter initiated fission makes use of smaller masses of PU-239 or U-235 possible because when you impact an appropriate atomic nucleus with an anti-proton you get many more neutrons than you do with more natural fission processes.

I believe it also eliminates any need for conventional explosives. You don't have to keep the minimum mass in a lower density hollow sphere and then compact it into a solid sphere. It isn't a viable fission explosion waiting to happen until the anti-protons jump-start neutron production.

[Ulzgoroth]

I'm curious about the 'minimum critical mass' being referenced. With sufficiently effective neutron reflectors, wouldn't literally any mass be able to achieve criticality?

[AlexanderHowl]

You need a certain number of neutrons created every second that get a chain reaction. Since a certain number of neutron are always going to be absorbed by a reflector, pass through a reflector, or just bounce the wrong way, you always need a minimum mass for criticality. Antiprotons help, but less than 50% of them will hit protons in large nuclei, as the neutrons will get in the way and be annihilated. By the way, U-235 becomes Th-234 and then Pa-233 if it loses protons, both of which are useless for nuclear detonations.

[Ulzgoroth]

Originally Posted by AlexanderHowl

You need a certain number of neutrons created every second that get a chain reaction. Since a certain number of neutron are always going to be absorbed by a reflector, pass through a reflector, or just bounce the wrong way, you always need a minimum mass for criticality.

Well, yes, but that makes the minimum completely dependent on the effectiveness of the reflectors rather than something general.

[Fred Brackin]

Originally Posted by AlexanderHowl

Y Antiprotons help, but less than 50% of them will hit protons in large nuclei, as the neutrons will get in the way and be annihilated. By the way, U-235 becomes Th-234 and then Pa-233 if it loses protons, both of which are useless for nuclear detonations.

Shooting an anti-proton into an atomic nucleus does not result in simple subtraction. It injects enough energy into that nucleus to violently overcome even the strong nuclear force. The result is that many neutrons (it was my impression that we're talking more than dozen) are released instead of the average 2.5 we get from normal fission.

[Fred Brackin]

Originally Posted by Ulzgoroth

Well, yes, but that makes the minimum completely dependent on the effectiveness of the reflectors rather than something general.

No, not completely. Probably not even majorly. It's more likely that even the cleverest arrangement of neutron reflectors onlym changes the mass by a small number of percentage points.

For example, the usual number given for the mass of Pu-239 in the Nagasaki bomb is 6 kilos. I have seen speculation that the best modern designs might have lowered that to 5 kilos.