The Soviet crewed lunar programs were a series of programs pursued by the Soviet Union to land humans on the Moon, in competition with the United States Apollo program. The Soviet government publicly denied participating in such a competition, but secretly pursued two programs in the 1960s: crewed lunar flyby missions using Soyuz 7K-L1 (Zond) spacecraft launched with the Proton-K rocket, and a crewed lunar landing using Soyuz 7K-LOK and LK spacecraft launched with the N1 rocket. Following the dual American successes of the first crewed lunar orbit on 24–25 December 1968 (Apollo 8) and the first Moon landing on July 20, 1969 (Apollo 11), and a series of catastrophic N1 failures, both Soviet programs were eventually brought to an end. The Proton-based Zond program was canceled in 1970, and the N1-L3 program was de facto terminated in 1974 and officially canceled in 1976. Details of both Soviet programs were kept secret until 1990 when the government allowed them to be published under the policy of glasnost. Early concepts As early as 1961, the Soviet leadership had made public pronouncements about landing a man on the Moon and establishing a lunar base; however, serious plans were not made until several years later. Sergei Korolev, the senior Soviet rocket engineer, was more interested in launching a heavy orbital station and in crewed flights to Mars and Venus. With this in mind, Korolev began the development of the super-heavy N-1 rocket with a 75-ton payload. Soyuz-A-B-C and N1 In its preliminary Moon plans, Korolev's design bureau initially promoted the Soyuz A-B-C circumlunar complex (A-B-V in Russian) concept under which a two-crew spacecraft would rendezvous with other components in Earth orbit to assemble a lunar flyby excursion vehicle. The components would then be delivered by the proven middle-sized R-7 rocket. While developing the N1, since 1963, Korolev began to plan a Moon landing mission using two launches and docking. Later Korolev managed to increase the payload of the N1 to 92-93 tons (by switching to liquid hydrogen in the upper stage(s) and increasing the number of engines in its first stage from 24 to 30), providing enough power to accomplish the mission with a single launch. UR-500K / LK-1 and UR-700 / LK-3 Another main space design bureau, headed by Vladimir Chelomei, proposed a competing cislunar orbiting mission using a heavy UR-500K rocket (later renamed the Proton rocket) and a two-crew LK-1 spacecraft.