Early ideas of the fusion bomb came from espionage and internal Soviet studies. Though the espionage did help Soviet studies, the early American H-bomb concepts had substantial flaws, so it may have confused, rather than assisted, the Soviet effort to achieve nuclear capability.[36] The designers of early thermonuclear bombs envisioned using an atomic bomb as a trigger to provide the needed heat and compression to initiate the thermonuclear reaction in a layer of liquid deuterium between the fissile material and the surrounding chemical high explosive.[37] The group would realize that a lack of sufficient heat and compression of the deuterium would result in an insignificant fusion of the deuterium fuel.[37]

Andrei Sakharov's study group at FIAN in 1948 came up with a second concept in which adding a shell of natural, unenriched uranium around the deuterium would increase the deuterium concentration at the uranium-deuterium boundary and the overall yield of the device, because the natural uranium would capture neutrons and itself fission as part of the thermonuclear reaction. This idea of a layered fission-fusion-fission bomb led Sakharov to call it the sloika, or layered cake.[37] It was also known as the RDS-6S, or Second Idea Bomb.[38] This second bomb idea was not a fully evolved thermonuclear bomb in the contemporary sense, but a crucial step between pure fission bombs and the thermonuclear "supers".[39] Due to the three-year lag in making the key breakthrough of radiation compression from the United States the Soviet Union's development efforts followed a different course of action. In the United States they decided to skip the single-stage fusion bomb and make a two-stage fusion bomb as their main effort.[37][40] Unlike the Soviet Union, the analog RDS-7 advanced fission bomb was not further developed, and instead, the single-stage 400-kiloton RDS-6S was the Soviet's bomb of choice.[37]

The RDS-6S Layer Cake design was detonated on 12 August 1953, in a test given the code name by the Allies of "Joe 4".[41] The test produced a yield of 400 kilotons, about ten times more powerful than any previous Soviet test. Around this time the United States detonated its first super using radiation compression on 1 November 1952, code-named Mike. Though the Mike was about twenty times greater than the RDS-6S, it was not a design that was practical to use, unlike the RDS-6S.[37]

Following the successful launching of the RDS-6S, Sakharov proposed an upgraded version called RDS-6SD.[37] This bomb was proved to be faulty, and it was neither built nor tested. The Soviet team had been working on the RDS-6T concept, but it also proved to be a dead end.

In 1954, Sakharov worked on a third concept, a two-stage thermonuclear bomb.[37] The third idea used the radiation wave of a fission bomb, not simply heat and compression, to ignite the fusion reaction, and paralleled the discovery made by Ulam and Teller. Unlike the RDS-6S boosted bomb, which placed the fusion fuel inside the primary A-bomb trigger, the thermonuclear super placed the fusion fuel in a secondary structure a small distance from the A-bomb trigger, where it was compressed and ignited by the A-bomb's x-ray radiation.[37] The KB-11 Scientific-Technical Council approved plans to proceed with the design on 24 December 1954. Technical specifications for the new bomb were completed on 3 February 1955, and it was designated the RDS-37.[37]

The RDS-37 was successfully tested on 22 November 1955 with a yield of 1.6 megaton. The yield was almost a hundred times greater than the first Soviet atomic bomb six years before, showing that the Soviet Union could compete with the United States.[37][42] and would even exceed them in time.