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Variable Yield Thermonuclear Bomb Dialing

  1. Aug 12, 2017 #1
    Ok so this week I was studying aerodynamic differences between a "Fat Man" bomb which was dropped on Nagasaki and the Aim 120c Air to Air Missile. What intrigued me with nuclear weapons was the "Variable Yield", SO I did some research to see how it works . I couldn't mind much on how to adjust a nuclear weapon - Not surprising.

    Im just curious whether its adjusted by adding or removing fissionable material or is there some shielding mechanism to prevent all of the material undergoing fission. My second idea seems more safe and logical to me, I was thinking of a mechanism similar to that of a Artillery fuse in mortar shells which are adjusted with a special tool. Then again fuse has nothing to do with the size of the explosion in a shell.
  2. jcsd
  3. Aug 12, 2017 #2
    Reviewing the B61 wiki article (https://en.wikipedia.org/wiki/B61_nuclear_bomb#Design) it notes a few configuration options that will change the total mass of bomb - and the yield selection is not included. Also, there is a description of the operators panel - and the "mechanical" operations listed is a handle too disable the bomb. So I suspect that the "dial" is simply a digital selection - entered through a data entry terminal.

    I also notice that this is a "thermonuclear" bomb - that is, a hydrogen bomb. My first thought as to how such a bomb would be made variable would be to control the amount of hydrogen that was fused. The technical name for "Dial a Yield" is the "Full Fuzing Option".

    I don't know how it is actually done, but controlling the amount of hydrogen fused could be done fully digitally by controlling the precise timing of the initial chemical explosive detonations.
  4. Aug 14, 2017 #3


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    The yield on current nukes can reportedly be dialed over a fairly wide range, possibly a factor of ten.
    These are thermonuclear devices, as .Scott points out, with a fission fusion fission package. The initial fission blast serves to compress and fuse the tritium fuel,
    the neutrons from that fission the uranium casing material. Presumably the quantity of tritium provided is what determines the ultimate yield achieved.
    Hopes for a 'clean' fusion bomb initiated without a fission trigger have not come to fruition, or at least have never been reported.
    The large contribution to the yield from the second fission element ensures that these devices leave plenty of undesirable residues. The Soviet Tzar Bomba, the largest device ever detonated, was unusual in that it did not use a fissile casing, so a record fraction of the yield was from fusion. Adding a uranium casing would have at least doubled the megatonnage achieved.

    PS the AIM 120C is a conventional warhead air to air weapon. There was a nuclear predecessor design, the AIR-2 Genie, in service from the 50s to the 80s.
  5. Aug 15, 2017 #4
    << Post edited by a Mentor >>

    Here is the meaning of dial-a-yield:

    In essentially all modern nuclear weapons, the yield of the fission primary is “boosted” by D-T (deuterium-tritium) fusion reactions, where the D-T mixture is contained in a hollow cell buried somewhere inside the plutonium sphere. The amount of D-T fusion neutrons generated can change the amount of Pu fission over a gigantic range, from a sub-kiloton fizzle to many times what Pu alone could yield.

    Most importantly for the present discussion, there is external access to the D-T cell. At some arbitrary time before a weapon is fired, a variable amount of tritium gas (a “dialed” amount!) is bled into the cell. When the weapon is initiated, the Pu starts fissioning, the D-T mixture heats up to fusion temperature and ALL the tritium is burned. The D-T neutrons contribute markedly to the fissioning of the Pu, and the total yield of the fission primary influences the magnitude of the reaction in the secondary of the weapon as well.
    Last edited by a moderator: Aug 15, 2017
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