Von Neumann, in his head

From Robert Jungk's Brighter than a Thousand Suns: A Personal History of the Atomic Scientists.

The H-bomb project:

... Immediately after the White House directive the Theoretical Division at Los Alamos had started calculations for the new bomb.

... There was a meeting in Teller's office with Fermi, von Neumann, and Feynman ... Many ideas were thrown back and forth and every few minutes Fermi or Teller would devise a quick numerical check and then they would spring into action. Feynman on the desk calculator, Fermi with the little slide rule he always had with him, and von Neumann, in his head. The head was usually first, and it is remarkable how close the three answers always checked.

The MANIAC:

... When von Neumann released his last invention for use, it aroused the admiration of all who worked with it. Carson Mark, head of the Theoretical Division at Los Alamos, recollects that 'a problem which would have otherwise kept three people busy for three months; could be solved by the aid of this computer, worked by the same three people, in about ten hours. The physicist who had set the task, instead of having to wait for a quarter of a year before he could get on, received the data he required for his further work the same evening. A whole series of such three months' calculations, narrowed down to a single working day, were needed for the production of the hydrogen bomb.

It was a calculating machine, therefore, which was the real hero of the work on the construction of the bomb. It had a name of its own, like all the other electronic brains. Von Neumann had always been fond of puns and practical jokes. When he introduced his machine to the Atomic Energy Commission under the high-sounding name of 'Mathematical Analyser, Numerical Integrator and Computer', no one noticed anything odd about this designation except that it was rather too ceremonious for everyday use. It was not until the initial letters of the six words were run together that those who used the miraculous new machine realized that the abbreviation spelled 'maniac'.