This was Britain's first deployed "true" H-bomb. Violet Club incorporated fusion fuel but represented an awkward, expensive, inefficient, dead-end design. Yellow Sun Mk 1 employed the radiation implosion technology demonstrated during Grapple in 1957. This was a megaton range weapon that entered service in 1958. Since the first such design had been successfully tested only in November 1957, it may be assumed that these weapons were akin to the U.S. "emergency capability" thermonuclear weapons deployed in 1954. That is, they were thermonuclear systems that would work, and could be delivered, but cut a lot of corners in engineering and military requirements areas like safety, reliability, cost, stockpile life, flexibility, efficiency, etc. The high yield tests of April and September of 1958 may have been in part refinements of this design.
The Yellow Sun Mk 1 warhead was about 4 feet wide and 9 feet long, the whole weapon was 21 feet long. Probably only a few were deployed. The decision to adopt the advanced American Mk-28 thermonuclear weapon design, made in September 1958, brought Yellow Sun Mk 1 manufacture and development to a halt
