Stanislaus Ulam (1909 – 1984) was a Polish mathematician. In 1943, after having worked as a university professor in the United States for three years, Ulam contacted Hungarian physicist John von Neumann to request a war job. Later that year, he received an offer from Hans Bethe to join a project near Santa Fe.
Ulam’s expertise was needed to ensure the proper design and construction of an implosion-type weapon. He was responsible for carrying out hydrodynamical calculations, which determine the behavior of solids at high temperature and pressure. This was crucial to the design of a proper explosive lens, which would be necessery to achieve perfect spherical implosion. Eventaully, Ulam and Neumann came up with the proper design.
Follwing the war, Ulam was invited to continue work on secret projects — notably the creation of thermonuclear weapons. He and Teller created the Teller-Ulam design, which to this day serves as the basis for all thermonuclear weapons.