Editing Stanisław Ulam (section)

===Hydrodynamical calculations of implosion===
A few weeks after Ulam reached [[Los Alamos, New Mexico|Los Alamos]] in February 1944, the project experienced a crisis. In April, [[Emilio G. Segrè|Emilio Segrè]] discovered that [[plutonium]] made in [[Nuclear reactors#Early reactors|reactors]] would not work in a [[gun-type fission weapon|gun-type]] plutonium weapon like the "[[Thin Man nuclear bomb|Thin Man]]", which was being developed in parallel with a uranium weapon, the "[[Little Boy]]" that was dropped on [[Atomic bombings of Hiroshima and Nagasaki#Hiroshima|Hiroshima]]. This problem threatened to waste an enormous investment in new reactors at the [[Hanford site]] and to make slow uranium [[isotope separation]] the only way to prepare [[fissile]] material suitable for use in bombs. To respond, Oppenheimer implemented, in August, a sweeping reorganization of the laboratory to focus on development of an [[Nuclear weapon design#Implosion-type weapon|implosion-type weapon]] and appointed [[George Kistiakowsky]] head of the implosion department. He was a professor at Harvard and an expert on precise use of explosives.<ref name="HODDESON 130-137"/>

The basic concept of [[Implosion (mechanical process)|implosion]] is to use chemical explosives to crush a chunk of fissile material into a [[critical mass]], where [[neutron]] multiplication leads to a [[nuclear chain reaction]], releasing a large amount of energy. Cylindrical implosive configurations had been studied by [[Seth Neddermeyer]], but von Neumann, who had experience with [[shaped charge]]s used in [[High-explosive anti-tank|armor-piercing ammunition]], was a [[John von Neumann#Manhattan Project|vocal advocate of spherical implosion]] driven by [[explosive lens]]es. He realized that the symmetry and speed with which implosion compressed the plutonium were critical issues,<ref name="HODDESON 130-137"/> and enlisted Ulam to help design lens configurations that would provide nearly spherical implosion.  Within an implosion, because of enormous pressures and high temperatures, solid materials behave much like fluids. This meant that [[Fluid dynamics|hydrodynamical]] calculations were needed to predict and minimize asymmetries that would spoil a nuclear detonation. Of these calculations, Ulam said:{{quote|The hydrodynamical problem was simply stated, but very difficult to calculate&nbsp;– not only in detail, but even in order of magnitude. In this discussion, I stressed pure pragmatism and the necessity to get a heuristic survey of the problem by simple-minded brute force, rather than by massive numerical work.<ref name='VITA'/>}}

Nevertheless, with the primitive facilities available at the time, Ulam and von Neumann did carry out numerical computations that led to a satisfactory design. This motivated their advocacy of a powerful computational capability at Los Alamos, which began during the war years,<ref name='COMPEARLY'/> continued through the cold war, and still exists.<ref name='COMPLATE'/> [[Otto Frisch]] remembered Ulam as "a brilliant Polish topologist with a charming French wife. At once he told me that he was a pure mathematician who had sunk so low that his latest paper actually contained numbers with decimal points!"<ref name='FRISCH'/>