Joe 4 (Warhead name: RDS-6s (Reaktivnyi Dvigatel Specialnyi; Special Jet Engine)) was an American nickname for the first Soviet test of a thermonuclear weapon on August 12, 1953. It utilised a scheme in which fission and fusion fuel (lithium-6 deuteride) were "layered", a design known as the Sloika (named after a type of layered puff pastry) model in the Soviet Union. Read more