One-Point Implosion: “Palm Fan”

2022.05.18   prev     next

THE Gadget/Fatman device employed thirty-two simultaneous detonators to generate a spherical implosion. A decade later, the 1956 Swan device reduced that to just two detonators.

Question: Is it possible to do it with only one detonator?

Let’s look at the Swan device (recreated here from Wikipedia’s current diagram):

The device has two detonators, one at each end. When they’re fired, the outer shell of high explosive detonates from both ends, in a manner that creates two hemispherical shock waves propagating through the inner air pocket. These waves merge together in the middle and set off the spherical layer of high explosive from all sides at once, which results in implosion.

Half-Swan

So how to make it work with just one detonator? Maybe create a half-Swan, like this, sitting on top of a metal plate, on top of a concrete slab on the ground:

Issue 1: Do the metal plate and concrete slab have to withstand the power of a nuclear bomb? No, because once we get a nuclear explosion going, the desired effect is already achieved. More power to it, if it obliterates the plate and slab! The plate and slab only have to be able to withstand the chemical high explosive, and only for long enough to achieve compression of the plutonium.

Issue 2: Do the device materials need to be securely attached to the metal plate? No, they don’t. They can just sit on top of it. As long as the materials are in the correct position when the detonator fires, everything else will follow as planned.

Issue 3: With only half as much plutonium, will the bomb work at all? No, it won’t! So increase the size of everything 26% (by scale), and the volumes double. Then we’re back to the same amount of plutonium as before.

Issue 4: Will the plutonium fission adequately if it is squeezed to a small hemisphere, not a small sphere? I’m gonna say yes. I don’t think a runaway fission reaction is that particular to the exact shape; any small, tight lump oughta do the trick.

Issue 5: How do you deploy a weapon that has to sit on a slab on the ground? Yup, that’s a problem.

What we need is a way to have the detonation wave wrap around the bottom of the fission package, generating a spherical implosion without the need for any backing inertial slab. In Swan, the fast explosive can reach the middle before the slower air wave reaches the sphere. Could it reach the other side of the sphere before the slow wave reaches the near side?

Palm Fan

I tried a few shapes and wound up with this:

This is much better than a half-Swan. It does have a disadvantage compared to full Swan: It’s a little larger. But since Swan is only two feet by one foot, a little larger than that isn’t bad.

The Advantage

The big advantage over Swan is that there’s only one detonator. All the problems with making sure your detonators go off extremely simultaneously (and very reliably so) simply disappear. A single detonator is always perfectly synchronized with itself — just fire that thing.

Update 2024.01.04 — Check out this from about a year ago, an allegedly leaked W88 warhead design. Note that the secondary is spherical; it’s just the cutaway view that makes it look heart-shaped. But the primary looks to be a really advanced, single-detonator design that has no spherical core — instead it just throws the plutonium together in a way that makes it all meet in the same place at the same time. Nice!

Update 2024.10.15 — Note that the text accompanying the above-linked W88 image implies that there are two detonators (“capacitance discharge units which fire both detonators”); however, the depicted shape of the primary strongly suggests to me that there is only one detonator, at the forward end of the primary (hidden by the firing assembly in the depiction).

See also:
The Gift of Nukes
&
Little Plutonium Boy
&
Implosion: Were Those Two-Speed Lenses Really Necessary?
&
One-Point Implosion: “Palm Fan”
&
Pu
&
Swan Device 1956 — Probable Shape

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