RDS-37

RDS-37 (Russian: РДС-37, CIA: JOE 19) was the Soviet Union's first two-stage thermonuclear weapon test, dropped from a Tupolev Tu-16 bomber over Semipalatinsk Test Site on 22 November 1955. The yield was 1.6 megatons, scaled down from a 3 megaton design. It was the world's first air-dropped multi-stage thermonuclear test; prior US thermonuclear tests Ivy Mike (1952) and in Operation Castle (1954) were ground-based experiments. The Soviet nuclear weapons project had previously tested thermonuclear reactions RDS-6s in 1953. This was a 400-kiloton single-stage layer cake design (Russian: 'sloika') achieved using layers of lithium deuteride-tritide fusion fuel and natural uranium surrounding a highly enriched uranium core. Scientists expanded sloika designs, testing the tritium-free RDS-27 at 250 kilotons, and outlining a 1.8 megaton 'RDS-6sd', but the single-stage design proved very difficult to scale. Physicists Andrei Sakharov and Yakov Zeldovich then independently invented the radiation implosion design, using the X-rays from a fission-based nuclear primary to compress a fusion-fuelled secondary, specifically a spherical sloika. Dubbed "Sakharov's Third Idea", this was equivalent to the US Teller-Ulam design.