Nuclear warhead efficiency

[NomAnor15]

So this is a pretty specific question, and it's entirely possible that no one on the board actually knows, but I figured I'd throw it out there, since I've had little success finding answers elsewhere.

When it comes to nuclear warheads, it is clearly more efficient (or rather yields more overall damage) to detonate, for example, ten 500 kiloton warheads than a single 5 megaton warhead. It is just as clear, however, that making a warhead with a yield of 100 tons is a complete waste of time. The question, then, is where do you achieve optimum efficiency? That is to say, at what yield (or range of yields) is it more effective to make two warheads than one?

[Purple]

That depends on entirely what you are trying to do.

For example, if you are attempting to decimate a city it is much more efficient to drop a ton of small bombs with overlapping blasts but if you want to penetrate a bunker dug into a mountain the tide turns.


That is because of the properties of how the bomb works. You see, the best way to destroy a target like a city is via air burst because that uses up most of the bombs energy into killing things. With a ground burst fully half (the bottom of the explosion sphere) of the energy is wasted in digging a crater. In an optimum altitude air burst most of the energy goes into blowing stuff up.

Now with a bigger bomb, the altitude from ground to bomb for an optimal air burst rises. So a huge part of the explosion is wasted into air.


That is at least how I understand it.

[Purple]

Missed the edit mark. You might want to check these out: http://homepage.mac.com/msb/163x/faqs/

Read the articles Nuclear Warfare 101, 102 and 103. They contain quite an interesting read not only on what you are asking but on the reasons on why and how to conduct nuclear warfare.

[Sea Skimmer]

Efficiency in terms of destroying the target, depends entirely on the target. Now Efficiency in terms of converting radioactive warhead material into atomic blast, that we can talk about a little in general terms. Basically efficiencies over 90% can be achieved by more then one possible combination of fission and fusion effects which dump energy into each other, and then create pressures and interact with radiation reflectors which help fully consume the fissile material before its blown apart ceasing the chain reaction. As far as I can tell once you get into the hundreds of kilotons of yield relatively high efficiency is basically a given (with a properly engineered weapon). Yields significantly under 100kt are ones that tend to waste fissile material. Making a nuke yield less then 15-20kt is very very hard, you basically have to make the nuke deliberately fail with reliability.

However you’ve got more then one kind of fissile material in a nuke, potentially three or four different ones working together. Some materials are a lot more expensive then others, and the rest of the hardware in a nuclear device isn’t exactly cheap, so building more bombs becomes expensive quickly. You need to upkeep and maintain these warheads after all, they aren’t like a conventional bomb that won’t mind sitting in a bunker for 20 years. Some fissile material like tritum gas has a fairly short half life, and while it might be cheap up front, it will need to be replaced every six years or so. So really what this means is no one can say where the optimal cost effective design point would be without a huge pile of classified information concerning detailed design of the devices. Observations of real arsenals would suggest warheads between about 50kt and 500kt are the most useful all around. Significantly higher yields are only required for deep underground bunkers, significantly low yields would only be for tactical use when the entire point is to yield as little as possible so you can use the nuke very close to your own ground forces.

[NomAnor15]

Thank you both for your answers. This has definitely shed some light on the subject.

Also, Purple, those articles were indeed interesting. Thanks.