You might think of the explosive part of a nuclear weapon as the "weapon" or "bomb," but in the technical literature it has its own kind of amusingly euphemistic name: the "physics package." This is the part of the bomb where the "physics" happens -- which is to say, where the atoms undergo fission and/or fusion and release energy measured in the tons of TNT equivalent.
Drawing a line between that part of the weapon and the rest of it is, of course, a little arbitrary. External fuzes and bomb fins are not usually considered part of the physics package (the fuzes are part of the "arming, fuzing, and firing" system, in today's parlance), but they're of course crucial to the operation of the weapon. We don't usually consider the warhead and the rocket propellant to be exactly the same thing, but they both have to work if the weapon is going to work. I suspect there are many situations where the line between the "physics package" and the rest of the weapon is a little blurry. But, in general, the distinction seems to be useful for the weapons designers, because it lets them compartmentalize out concerns or responsibilities with regards to use and upkeep.
The shape of nuclear weapons was from the beginning one of the most secret aspects about them. The casing shapes of the Little Boy and Fat Man bombs were not declassified until 1960. This was only partially because of concerns about actual weapons secrets -- by the 1950s, the fact that Little Boy was a gun-type weapon and Fat Man was an implosion weapon, and their rough sizes and weights, were well-known. They appear to have been kept secret for so long in part because the US didn't want to draw too much attention to the bombing of the cities, in part because we didn't want to annoy or alienate the Japanese.
But these shapes can be quite suggestive. The shapes and sizes put limits on what might be going on inside the weapon, and how it might be arranged. If one could have seen, in the 1940s, the casings of Fat Man and Little Boy, one could pretty easily conjecture about their function. Little Boy definitely has the appearance of a gun-type weapon (long and relatively thin), whereas Fat Man clearly has something else going on with it. If all you knew was that one bomb was much larger and physically rounder than the other, you could probably, if you were a clever weapons scientist, deduce that implosion was probably going on. Especially if you were able to see under the ballistic casing itself, with all of those conspicuously-placed wires.
In recent years we have become rather accustomed to seeing pictures of retired weapons systems and their physics packages. Most of them are quite boring, a variation on a few themes. You have the long-barrels that look like gun-type designs. You have the spheres or spheres-with-flat ends that look like improved implosion weapons. And you then have the bullet-shaped sphere-attached-to-a-cylinder that seems indicative of the Teller-Ulam design for thermonuclear weapons.
There are a few strange things in this category, that suggest other designs. (And, of course, we don't have to rely on just shapes here -- we have other documentation that tells us about how these might work.) There is a whole class of tactical fission weapons that seem shaped like narrow cylinders, but aren't gun-type weapons. These are assumed to be some form of "linear implosion," which somewhat bridges the gap between implosion and gun-type designs.
All of this came to mind recently for two reasons. One was the North Korean photos that went around a few weeks ago of Kim Jong-un and what appears to be some kind of component to a ballistic case for a miniaturized nuclear warhead. I don't think the photos tell us very much, even if we assume they are not completely faked (and with North Korea, you never know). If the weapon casing is legit, it looks like a fairly compact implosion weapon without a secondary stage (this doesn't mean it can't have some thermonuclear component, but it puts limits on how energetic it can probably be). Which is kind of interesting in and of itself, especially since it's not every day that you get to see even putative physics packages of new nuclear nations.
The other reason it came to mind is a chart I ran across on Pantex's website. Pantex was more or less a nuclear-weapons assembly factory during the Cold War, and is now a disassembly factory. The chart is a variation on one that has been used within the weapons labs for a few years now, my friend and fellow-nuclear-wonk Stephen Schwartz pointed out on Twitter, and shows the basic outlines of various nuclear weapons systems through the years. (Here is a more up-to-date one from the a 2015 NNSA presentation, but the image has more compression and is thus a bit harder to see.)
For gravity bombs, they tend to show the shape of the ballistic cases. For missile warheads, and more exotic weapons (like the "Special Atomic Demolition Munitions," basically nuclear land mines — is the "Special" designation really necessary?), they often show the physics package. And some of these physics packages are pretty weird-looking.
A few that jump out as especially odd:
In the Pantex version (but not the others), the W59 is particular in that it has an incorrectly-filled circle at the bottom of it. I wonder if this is an artifact of the vectorization process that went into making these graphics, and a little more indication of the positioning of things than was intended.
- The W52 has a strange appearance. It's not clear to me what's going on there.
- The silhouette of the W30 is a curious one ("worst Tetris piece ever" quipped someone on Twitter), though it is of an "Atomic Demolition Munition" and likely just shows some of the peripheral equipment to the warhead.
- The extreme distance between the spherical end (primary?) and the cylindrical end (secondary?) of the W-50 is pretty interesting.
- The W66 warhead is really strange — a sphere with two cylinders coming out of it. Could it be a "double-gun," a gun-type weapon that decreases the distance necessary to travel by launching two projectiles at once? Probably not, given that it was supposed to have been thermonuclear, but it was an unusual warhead (very low-yield thermonuclear) so who knows what the geometry is.
There are also a number of warheads whose physics packages have never been shown, so far as I know. The W76, W87, and W88, for example, are primarily shown as re-entry vehicles (the "dunce caps of the nuclear age" as I seem to recall reading somewhere). The W76 has two interesting representations floating around, one that gives no real feedback on the size/shape of the physics package but gives an indication of its top and bottom extremities relative to other hardware in the warhead, another that portrays a very thin physics package that I doubt is actually representational (because if they had a lot of extra space, I think they'd have used it).1
What I find interesting about these secret shapes is that on the one hand, it's somewhat easy to understand, I suppose, the reluctance to declassify them. What's the overriding public interest for knowing what shape a warhead is? It's a hard argument to make. It isn't going to change how to vote or how we fund weapons or anything else. And one can see the reasons for keeping them classified -- the shapes can be revealing, and these warheads likely use many little tricks that allow them to put that much bang into so compact a package.
On the other hand, there is something to the idea, I think, that it's hard to take something seriously if you can't see it. Does keeping even the shape of the bomb out of public domain impact participatory democracy in ever so small a way? Does it make people less likely to treat these weapons as real objects in the world, instead of as metaphors for the end of the world? Well, I don't know. It does make these warheads seem a bit more out of reach than the others. Is that a compelling reason to declassify their shapes? Probably not.
As someone on the "wrong side" of the security fence, I do feel compelled to search for these unknown shapes — a defiant compulsion to see what I am not supposed to see, perhaps, in an act of petty rebellion. I suspect they look pretty boring — how different in appearance from, say, the W80 can they be? — but the act of denial makes them inherently interesting.
- One amusing thing is that several sites seem to have posted pictures of the arming, fuzing, and firing systems of these warheads under the confusion that these were the warheads. They are clearly not -- they are not only too small in their proportions, but they match up exactly to declassified photos of the AF&F systems (they are fuzes/radars, not physics packages). [↩]