received D + T cross section data obtained from intelligence gathering without mentioning the source. The large advantage of lithium deuteride became evident and the deuterium design was abandoned. Both the 'first' and 'second' idea were used in the RDS-6s. The result was similar to the US 'Alarm Clock', but there is no indication that the Soviets were aware of the concept of the 'Alarm Clock'. After the United States tested Mike in November 1952, Lavrentiy Beria sent a memo to spare no effort on the development of the RDS-6s. In the final development report from June 1953 the yield was estimated at 300 +/- 100 kilotons. The RDS-6s was tested on August 12, 1953 (Joe 4). The measured yield was 400 kilotons, 10% from fission of the Uranium-235 core, 15-20% from fusion and 70 - 75% from fission of the Uranium-238 layers. After the successful test Sakharov proposed a more powerful version of the RDS-6s, code named RDS-6sD. Attempts to increase the yield of the RDS-6s however proved unfeasible. In December 1953 all research on the RDS-6t was also stopped after it was proven that thermonuclear ignition was not possible in the RDS-6t. Both the RDS-6s and the RDS-6t were dead ends and research focused again on a two-stage thermonuclear weapon. A variant of the RDS-6s was developed later, code named RDS-27. The difference between the RDS-6s and the RDS-27 was that the RDS-27 did not use tritium. This improved the operational usefulness of the RDS-27, but reduced the yield from 400 kilotons to 250 kilotons. The RDS-27 was intended as a warhead for the R-7 ICBM. The RDS-27 was tested November 6, 1955 (Joe 18). Despite the inability of the RDS-6s to be scaled into the megaton range, the detonation was still used by Soviet diplomats as leverage. The Soviets claimed that they too had a hydrogen bomb, but unlike the United States' first thermonuclear weapon, theirs was deployable by air. The United States didn't develop a deployable version of the hydrogen bomb until 1954. The first Soviet test of a "true" hydrogen bomb was on November 22, 1955 under the directive of Nikolai Bulganin (influenced by Nikita Khrushchev), code-named RDS-37. All were at Semipalatinsk Test Site, Kazakh SSR. Like RDS-6, it was a "dry" weapon, using lithium-6 deuteride instead of liquid deuterium. See also 2013 Chelyabinsk meteor airburst, whose estimated explosive force slightly exceeded the RDS-6s test's