1) What is the hardest part of reducing a nuclear bomb's size? 2) What is Lil' Kim looking thru?

Query 1 has been around since Trinity, but it’s now be-all and end-all (as it were) when mating the weapons with missiles; the “suitcase bomb” is the least of anyone’s worries for the time being, I presume.

See today’s NYTimes: As North Korea’s Nuclear Program Advances, U.S. Strategy Is Tested - The New York Times

Query 2 refers to the photograph leading the story.

2. Periscope binoculars


Fifteen years ago someone else asked around here, I now see…What are these binocular periscope type things? - Factual Questions - Straight Dope Message Board

As for #1, a nuclear bomb works when two subcritical chunks of fissionable material are smushed together very rapidly. They have to start out far enough apart physically to not react with each other prematurely, but not so far apart that it takes too long to smush them together, which results in a “fizzle.”

In the Hiroshima bomb (pretty much the simplest design) the two chunks started out 70.8" apart.

I am not a nuclear physicist. But my understanding is that a big bomb has some 'back-up" so that even if the reaction doesn’t occur precisely as planned, it will still be enough to cause a nuclear explosion. But as you shrink the bomb, you have to eliminate all these redundancies. You have to design a bomb that works exactly as it’s supposed to because any glitch will cause it not to work.

As an analogy, think about building a fire. If you’re not concerned about the resources you’re using, you’ll get a pile of wood, add a bunch of kindling, throw a little more kindling on just to be sure, maybe splash a little gasoline on the pile, light some matches and throw them in - you’ll get a fire going.

Now suppose you had to start a fire with the least amount of resources possible. You’re allowed one piece of wood. You can only use the absolute minimum amount of kindling. And you only get one match. Now you’re going to study different types of wood and kindling and matches to figure out which ones are the most reliable and work best together. You might carve your piece of wood into a special shape. You’ll carefully arrange your kindling in place one pinch at a time and adjust it until it’s perfect. You’ll pre-plan out how you’re going to strike your match and where you’re going to place it. And then you’ll hover over it waiting for the perfect moment when there’s no breeze.

Um, those “periscope binoculars”? They’re primarily optical range finders. One spreads the tubes horizontally, looks through the eyepieces at a target, and turns a knob to put the images in each eyepiece on top of each other. Then you look at a dial and tell the artillery battery how far away the target is. They did turn out to be popular with infantry officers in the trenches during WWI to get a better view of what the enemy was doing without exposing his head to rifle fire.

Assuming you are talking fission weapons: the modern fission weapon works by compressing a subcritical mass of plutonium to something like half its volume, or twice its density, at which point it becomes supercritical and initiates a very rapid chain reaction – much more rapid and complete than a critical mass at ordinary densities, e.g. what you mostly had in Little Boy and Fat Man.

As you would imagine, compressing a heavy metal like Pu to twice its normal density requires tremendous force. The force is readily available with modest amounts of modern high explosives. However, the problem is controlling the shock wave so that it uniformly compresses the Pu pit. Just as when you squeeze a bar of soap hard, the tiniest little irregularity or asymmetry will lead to an escape of the material along some axis and ineffective compression.

In practice you simply cannot machine explosives and control their detonation precisely enough to generate a perfectly symmetrical shock wave, so what is actually done is carefully design the explosives so that the natural asymmetries reinforce each other in predictable ways and generate an effective compression anyway.

The only way to do this is by doing very complex high-speed simulations and calculations on fancy computers. You have to understand exactly what is happening on a scale of 100ns or less, for at least several milliseconds. At one time, only the computers the US military had were capable of the feat, and only with complex exceedingly secret code that made use of all kinds of measured properties. Nowadays I imagine the computing power is widely available, and the material properties are probably relatively widely known. The code is probably still very challenging to reproduce, however, as it is a combination of clever theory and no doubt painfully worked out empirical understanding.

Now the smaller you make your weapon, the more precisely you need to control the explosion, which means the more accurately you need to predict what happens. That means not only very precise engineering of the device itself, but also exceedingly accurate computer simulation prediction of what will happen. If I had to guess, I would guess that getting an accurate computer simulation is the major stumbling block to very small weapons design. The next challenge I would guess is chemical engineering – making high explosive lens of the required uniform composition – and the third engineering – shaping and positioning the HE charges, and providing exquisitely precise ignition timing.

I am not a weapons scientist, so this is all guesswork.

What Carl said. And the explosives are extremely complex, consisting of shells of high and low velocity explosive material interspersed with metal shells, sized such that the various shock waves impinge on each other and reflect into the core. Only then can enough compression result from thin explosive shells.

There are several misconceptions here. A fission explosion does not require a perfectly symmetrical spherical shock wave, if that’s what you meant. The smallest devices use a linear implosion design:

This is definitely not the case. Absolute proof is the smallest nuclear weapons were designed and successfully tested long before sophisticated computerized numerical simulation was available.

The first versions of the tiny W54 were tested in 1957. There were no supercomputers then. This is before Cray and before even CDC existed. The first supercomputer is generally considered the CDC 6600 which was released in 1964: History of supercomputing - Wikipedia It’s performance was three megaflops, which is about 3% of a Cray-1 from 1976. The smallest nuclear warheads ever tested atmospherically were long before then. The W48 had a yield of 0.072 kilotons.

This revival is for part 2 of the OP, a continuing series:

What is Lil’ Kim looking at? (2nd photo)

North Korea fires three ballistic missiles in latest series of tests
18th July 2016

The picture below of him and a flunky pointing to a launch looks like a collage also, but it’s not worth reviving the Iran-Photoshop thread for that one.