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Imagine you've somehow tranquilized or tied up Superman with a kryptonite rope or something, and the military wants to do a stress test so they can start building a series of anti-Superman bombs. So they drop increasingly large nuclear devices on him, upping the yield until they can noticeably injure Superman and thus determine the upper limit of his invulnerability.

But there's a bigger problem here than mad science or pissing off Superman. It's the limits of our weapons technology. Which leads me to ask.

What's the practical limit for building bombs with today's technology? And I don't mean practical as in efficient or cost-effective. As long as the bomb works and works reliably, that'll do. All it's designed to do is injure a single superhumanly durable target, so it doesn't matter if using it in war on a soft civilian target would be excessive or impractical. Also, saying "antimatter" is forbidden. It has to be doable right now.

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    $\begingroup$ Gentlemen. You can't fight in here. This is the War Room! $\endgroup$ – Scott Downey Nov 6 '16 at 21:02
  • $\begingroup$ Well, if nothing else, it's going to weight somewhere around 500 ton/per gigaton (just going off the Tsar bomb as a guide) and would probably need a couple percent of all known uranium and create a fireball tens of kilometers across. $\endgroup$ – Samwise Nov 6 '16 at 22:08
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    $\begingroup$ "But there's a bigger problem here than mad science or pissing off Superman." What!!!??? Apart from the insanity of wanting to kill Superman. Also, if your gigaton bomb fails to harm Superman, it will most likely vaporize the kryptonite rope. This should happen well below the stress test level of causing Superman any actual harm. May I suggest there should be more responsible experimental methods for testing the limits of Superman's invulnerability. The Test Ban Treaty is still in place, $\endgroup$ – a4android Nov 6 '16 at 22:24
  • $\begingroup$ Further issue: Repeatedy bombing the same location with massive megatonnage would probably crack the world in half before Superman died, if we're theorising that multi-gigatonnage would be required. $\endgroup$ – Ieuan Stanley Nov 7 '16 at 11:57
  • $\begingroup$ The real question is why you'd need a multi-gigaton bomb to take care of Superman. Since Superman is made of normal matter, even a little Hiroshima-sized device will be sufficient to vaporize him. $\endgroup$ – jamesqf Nov 8 '16 at 3:53
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Yes. Note: Your Challenges will Include Transportation and Assembly.

The only limits to what we have built and plan today are delivery logistics. ICBM's can only handle so much weight, and even large transport planes have limits. However, if you were able to build and detonate a nuclear weapon in the same place transportation becomes a nonissue.

The specifics of nuclear bomb construction are not in the public domain so it is hard to comment on more specifically, but I would strongly expect to run into a construction issue. At a point you need to put a LOT of uranium in a limited area, which is going to probably make robot workers a requirement. That seems within the realm of current technology though, so it probably could be overcome if we really wanted to.

Here's an interesting statement on the topic...

Because the maximum theoretical yield-to-weight ratio is about 6 megatons of TNT per metric ton, and the maximum achieved ratio was 5.2 megatons of TNT per metric ton, there is a practical limit on the total yield for an air-delivered weapon. Most later generation weapons have eliminated the very heavy casing once thought needed for the nuclear reactions to occur efficiently, and this has greatly increased the achievable yield-to-weight ratio. For example, the Mk-36 bomb as built had a yield-to-weight ratio of 1.25 megatons of TNT per metric ton. If the 12,000 pound casing of the Mk-36 were reduced by 2/3s, the yield-to-weight ratio would have been 2.3 megatons of TNT per metric ton, which is about the same as the later generation, much lighter 9 megaton Mk/B-53 bomb.

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  • $\begingroup$ Why do you need a lot of uranium/plutonium? I'd think that you could use a number of small fission bombs to ignite a very large fusion device, in much the same way as chemical explosives are used to trigger the fission device. (I am not a nuclear bomb expert, though :-)) $\endgroup$ – jamesqf Nov 7 '16 at 5:16
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    $\begingroup$ @jamesqf in a hydrogen bomb, the explosion is not due to the greater energy of fusion as is commonly beleived. Rather, it provides a neutron flux so that more plutonium reacts before being blown apart. $\endgroup$ – JDługosz Nov 7 '16 at 9:08
  • $\begingroup$ @JDługosz: Why do you think that? References would be nice. There would seem to be a lot of easier ways of providing more neutrons. $\endgroup$ – jamesqf Nov 7 '16 at 17:40
  • $\begingroup$ @jamesqf - I would start at Wikipedia, specifically here. It's definitely technical, but as you read through it you can see that the fusion material is used to boost the fission reaction. Well, at least in theory... actual specifications and justifications are obviously not available to the public :) $\endgroup$ – GrinningX Nov 7 '16 at 17:55
  • $\begingroup$ Stage 1: Fission explosion generates x rays to heat and ablate the second stage. Stage 2: Ablation crushes the Lithium deutride and triggers fusion reaction. Stage 3: The fast neutrons from the fusion reaction trigger fission in the depleted uranium "tamper" which holds the entire assembly together. If the tamper is lead then you only have a two stage device. $\endgroup$ – Thucydides Nov 8 '16 at 2:13
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The ever useful Atomic Rockets website has a section on theoretical yields for nuclear weapons, and some very interesting figures come out of it:

Or, to take another tack, and returning to the initial impetus for me looking at this topic, we know that the famous “Tsar Bomba” of the Soviet Union weighed 27,000 kilograms and had a maximum yield of 100 Mt, giving it a yield-to-weight ratio of “only” 3.43 kilotons/kilograms. That’s pretty high, but not for a weapon that used so much fusion energy. It was clear to the Atomic Energy Commission that the Soviets had just scaled up a traditional H-bomb design and had not developed any new tricks. By contrast, the US was confident in 1961 that they could make a 100 Mt weapon that weighed around 13,600 kg (30,000 lb) — an impressive 7.35 kiloton/kilogram ratio, something well above the 6 kt/kg achieved maximum. By 1962, after the Dominic series, they thought they might be able to pull off 50 Mt in only a 4,500 kg (10,000 lb) package — a kind of ridiculous 11 kt/kg ratio. (In this estimate, they noted that the weapon might have an impractically large diameter as a result, perhaps because the secondary was spherical as opposed to cylindrical.) So we can see, without really knowing much about the US had in mind, that it was planning something very, very different from what the Soviets set off.

So it seems there is an avenue to approach to create gigaton and and larger weapons. As an aside, on the NEOFuel site, Anthony Zuppero writes of an experience he had as a young man being asked to design a hypothetical way to eliminate all of Russia's missile fields at once. His recollection of this project was doing calculations to loft a 5000 megaton bomb to Western Russia in 2 minutes, so he designed an ORION nuclear pulse drive rocket to do the job. There are remarkably few details of size or mass of the device or the total assembly, although the USAF had proposed a 4000 ton ORION Battleship during the Kennedy administration, so there is no theoretical limit to how large the thing could be. The bomb itself, if it worked near the theoretical limit of 11kt/kg would be @ 460000 kg (hopefully didn't drop a decimal somewhere), so it would be a freaking huge rocket overall.

William Black has this theoretical reconstruction:

enter image description here

So short answer is it has been possible since the 1960's to create gigaton yield warheads, but there is no practical reason to do so, nor is there a practical means of delivering them. To put it into a final perspective, the former USSR is reputed to have had a regiment of SS-18 Satan ICBMs armed with a 20 megaton warhead each, who's wartime mission was to convert Cheyenne Mountain and other hardened command centres into lakes.

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Yes, but the other answers are missing the point.

The basic design of an H-bomb isn't a secret anymore.

Stage 1, you crush a Pu-239 ball with conventional explosives, it goes boom.

Stage 2, the energy from that detonation (of course you won't capture it all) is used to crush a chunk of lithium deutride axially. There is a piece of heavy material between the initial atomic explosion and the lithium deutride to slow down the direct crushing, thus ensuring the axial crushing happens first.

Stage 2b, the heavy material can't provide much time, the axially-crushed lithium deutride now gets crushed in the third dimension. There's a Pu-239 rod in it that's crushed by this step, it goes supercritical and ignites the fusion burn in the lithium deutride.

Now, there's a limit to the energy available for crushing and the lithium deutride takes a lot of crushing in order to sustain a fusion reaction. Thus there is a ratio of the input energy to the size of the fusion stage of the bomb.

If this were the end of the story there would be an upper limit on the size of an h-bomb. (The numbers are of course not disclosed!) However, there are two ways around this.

First, note that the fusion stage did not capture all the energy from the initial a-bomb. You can mount multiple fusion stages (how many????) around the initial a-bomb, each explodes independently but at the same time.

Second, while normal thought is that you use an a-bomb to set off an h-bomb there is nothing about the fusion stage that cares that the initial blast was an a-bomb. Anything with enough energy will do--and the detonating h-bomb certainly qualifies. You can pile another, bigger h-bomb after the first. (Note, though, that the second h-bomb must be in the shadow of the first so the energy from the a-bomb doesn't tear it apart.

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  • $\begingroup$ While a pure fusion device would be very advantageous, for many reasons, so far there has been no appreciable progress to date: infogalactic.com/info/Pure_fusion_weapon $\endgroup$ – Thucydides Apr 6 '18 at 12:24
  • $\begingroup$ @Thucydides I'm not talking about a pure fusion device, but rather conventional -> fission -> fusion -> bigger fusion. So long as each stage is in the blast shadow of the previous one you can chain any number of them. $\endgroup$ – Loren Pechtel Apr 7 '18 at 0:56
  • $\begingroup$ "Second, while normal thought is that you use an a-bomb to set off an h-bomb there is nothing about the fusion stage that cares that the initial blast was an a-bomb. Anything with enough energy will do". Unfortunately this isn't possible with current technology or materials. Laser driven ICF fusion comes close, but requires enough equipment to fill a stadium sized building. $\endgroup$ – Thucydides Apr 9 '18 at 3:52
  • $\begingroup$ @Thucydides All that's saying is that currently LCF doesn't have enough energy. I'm talking about something that certainly does have enough energy--an h-bomb. Line up bomb after bomb if you want, each provides the energy to set off the next one. $\endgroup$ – Loren Pechtel Apr 10 '18 at 0:33

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