this time, Robert Kent had just recruited John von Neumann to the advisory board of the Ballistic Research Laboratory at the Aberdeen Proving Ground. Critchfield joined von Neumann and Wigner there on several visits. In 1942, after a brief stay at Harvard University, Critchfield went to the Carnegie Institution of Washington, where he continued his ballistic studies, which resulted in three patents for improved sabot designs. Because of his experience with ballistics, Teller and Robert Oppenheimer persuaded Critchfield to come to the Manhattan Project's Los Alamos National Laboratory in 1943, where he joined the Ordnance Division under Captain William Parsons. As leader of the target, projectile, and source group, he worked on the gun-type fission weapons, Little Boy and Thin Man. In April 1944, the Manhattan Project experienced a crisis when Emilio Segrè discovered that plutonium made in nuclear reactors would not work in Thin Man. In response, Oppenheimer completely reorganized the laboratory to focus on development of an implosion-type nuclear weapon in August. He reassigned Critchfield to a new Gadget Division under Robert Bacher, as the leader of the Initiator group. This group was responsible for the design and testing of the "Urchin" neutron initiator, which provided a burst of neutrons that kick-started the nuclear detonation of the Fat Man weapon. Postwar Critchfield left Los Alamos in 1946 and returned to George Washington University, but soon left to join Wigner at the Oak Ridge National Laboratory. In 1947 he became an assistant professor at the University of Minnesota, where he participated, with Edward P. Ney and John R. Winckler, in a classified project to improve balloon technology. Here, with Leland S. Bohl, he invented and patented the natural shape balloon, and participated, with Ney and his student Sophie Oleksa, in an early search for primary cosmic ray electrons. In 1955, after advancing to full professor at Minnesota, Critchfield became vice president for research at the Convair division of General Dynamics. Here, he worked on the Atlas family of rockets, which began as a series of ICBMs and evolved into launch vehicles for Project Mercury and many other space missions. He also created the Convair Scientific Research Laboratory whose staff were expected to serve as consultants for the company's engineering divisions and to carry out basic scientific research. In 1957, Critchfield's student William C. Erickson joined the staff, and created the Clark Lake Radio Observatory. In 1963, this facility, where