complexity of the system. Eventually, a combination of radar and contact fuzing was chosen. Two fuze designs were under development; the first used existing components while the latter (called TX-28 Prime) required more development and used pyrotechnic-activated components. A contact backup was not initially included for the airburst design due to the concern that anti-aircraft fire might activate the contact fuze. Additionally, there was a desire to prevent fallout from a contact burst in the tactical mission. Los Alamos informed the TX-Theta Committee in the same month that the nuclear system could enter production by January 1958, a date that matched Sandia's availability date for the non-nuclear components. By May 1955 the TX-28 design firmed-up. The warhead itself would be in diameter and long, with each end capped in a hemisphere. The TX-28 Prime fuze design was also rapidly advancing. The design used pyrotechnically actuated switches to control preheat current, timer selection, thermal battery arming, ground- and air-burst selection, contact fuze arming, timer motor starting, trajectory-arm-switch pressure-port sealing, gas boosting and even thermal battery monitoring. These switches were small, lightweight and resistant to shock. A baroswitch was included to improve operational safety of the weapon. It was a two-chamber design in which one chamber would close at weapon release and then the baroswitch would measure the pressure difference between the sealed and open chamber as the weapon fell. At some point in the design the fuzing system was changed to allow for contact preclusion to be selected on the ground. With contact preclusion selected the contact fuze would be disabled in the airburst option. The initial Mod 0 used internal initiation, but in October 1955 Sandia described progress on external initiation of the weapon which eventually became the Mod 1 design. The external initiator system consisted of a power supply, precision timer and a neutron source called an S-unit. The S-unit was a tube filled with tritium gas while one end was titanium-coated and loaded with deuterium. During function the tritium ions were accelerated into the deuterium target which fused, releasing neutrons. The Mk-28EX Mod 0 (external) and Mk-28IN Mod 0 (internal) were design-released in June 1957 and early production was achieved in August 1958. The weapon had a diameter of . The external configuration weapon had a length of and weighed approximately with the nose section containing the fuze. In the internal configuration, the warhead section was turned around