Advancements in Bomb Construction
The Soviets continued to search for better superbomb designs. They abandoned the layer-cake design in favor of radiation-pressure implosion in two stages. Although this concept matched the Teller-Ulam configuration, it was arrived at independent of the American design and resulted mostly from the work of Andrei Sakharov, Zeldovich and V.A. Davidenko. High-level mathematical modeling and computer work were necessary in order to make manageable the complex physical processes of exploding thermonuclear charges-not just in the U.S. and the Soviet Union but in all five countries that have built the H-bomb.
After the Soviets learned about the Americans' "Bravo" test in 1954 at Bikini, they realized just how far the U.S. had come in its development of thermonuclear weapons. Bravo yielded 15 megatons, confirming for the Soviets that a two-stage warhead was the way to go.
Advancements continued on the U.S. front as well. While all of the Castle series tests were conducted on the ground or from ocean barges, on May 31, 1956, the U.S. completed its first airdrop of a thermonuclear bomb, called the "Cherokee" shot. Further tests were conducted between May and July 1956, in order to further progress in constructing lighter and more efficient nuclear weapon prototypes designed to operate in various categories of warheads.