Swan was a U.S. test nuclear explosive, which was developed into the XW-45 warhead.
It was tested standalone on June 22, 1956 in shot Redwing Inca. It was tested again as the primary of a thermonuclear device on July 2, 1956 in shot Redwing Mohawk. Both tests were successful. It subsequently served as the primary in numerous thermonuclear devices during the 1950s.
Most subsequent U.S. primaries are Swan-derived, including the Robin, the Tsetse, the Python, and the much later (asymmetrical) ovoid (prolate) primaries such as the Komodo used in the W88.
Swan (nuclear primary) Wikipedia
The Swan device is the first design to incorporate a two-point ignition hollow-pit air-lens implosion assembly together with fusion boosting.
The Swan device had a yield of 15 kilotons, weighed 105 lb (47.6 kg), and had a (symmetrical) ovoid (non-prolate) shape with a diameter of 11.6 inches (29.5 cm) and a length of 22.8 inches (58 cm), a length to diameter ratio of 1.97.
The above schematic illustrates what were probably its essential features.
The Swan device has one-point safety designed into its structure. One-point safety is the ability to reliably avoid core fission when less than the full number of detonators are ignited, as the result of a crash, fire or impact. When only one of the two detonators of a Swan device is ignited, the core is imploded into two parts, neither of which achieves criticality.
The above schematics illustrate the essential features of this capability.
The super-precision diamond turning machines (lathes) which constructed the components of Swan and Swan-derived primaries could not accommodate workpieces which exceeded 15.5 dia. × 54 inches (39.5 dia. × 137.2 cm); Swan and Swan-derived components were no larger than 12 dia. × 24 inches (30.5 dia. × 61.0 cm).