The Mark 21 nuclear bomb was a United States nuclear gravity bomb first produced in 1955. It was based on the TX-21 "Shrimp" prototype that had been detonated during the Castle Bravo test in March 1954. While most of the Operation Castle tests were intended to evaluate weapons intended for immediate stockpile, or which were already available for use as part of the Emergency Capability Program, Castle Bravo was intended to test a design which would drastically reduce the size and costs of the first generation of air-droppable atomic weapons (the Mk 14, Mk 17 & Mk 24). At 12 feet six inches long, 56 inches in diameter, and weighing 15,000 pounds (6,800 kg), the Mk-21 was half the length and one-third the weight of the Mk-17/21 weapons it replaced. Its minimum yield was specified at four mega