The Physics of Explosion Design

It was John von Neumann who worked out the geometry of the implosion design with the plutonium bomb, wasn't it? It was a very... mostly him. How do you turn that around so that the blast wave, the shock wave, will go inward instead of outward? What they did was use different kinds of explosives, some of which burned fast, some of whom burned slow and shapes that must have basically been hemispheres. They needed one billionth of a second simultaneous in the 80s and all these 32 different explosive starts. And you can't get that with chemical fuses, but they don't burn that fast.