The 21K spacecraft:
Conceived at the time when the first Soviet cosmonauts were still piloting one-seat Vostoks on pioneering missions around the Earth, the 21K spacecraft was designed for in-orbit propellant delivery to a proposed 200-ton Soviet expeditionary complex bound for the surface of the Moon. The 75-ton orbital tanker was a part of the earliest known architecture intended to put cosmonauts on the lunar surface.
The 21K vehicle at a glance:
The 21K tanker spacecraft would be a part of a four-spacecraft system proposed in the early planning of the Soviet lunar expedition. The work on the project was conducted at the OKB-1 design bureau led by Sergei Korolev and responsible for the Soviet lunar exploration effort.
In one of several flight scenarios formulated near the end of 1962 and the beginning of 1963, a pair of 21K tankers would precede the giant 19K lunar expeditionary complex to the low Earth's orbit.
All three vehicles (the space tug and the tankers) would be launched on the N1 rockets, which at the time were expected to have the capability of delivering 75 tons to the low Earth's orbit in a single launch.
Once all three spacecraft were in orbit, the two 21K tankers would dock one by one to the 19K spacecraft and pump a total of between 125 and 128 tons of propellant into the tanks of the 19K spacecraft.
Only then would the 7K transport vehicle with a crew lift off for a subsequent rendezvous and docking with the 19K complex in the Earth's orbit. The assembled stack would then embark on a direct flight to the lunar surface without entering lunar orbit.
The concept of launching the lunar expeditionary vehicle without propellant and then fueling it in orbit was essentially a scaled-up version of the Soyuz project, which was originally formulated for a circumlunar mission launched on R-7-based rockets.
At the time when these scenarios were under consideration, neither the automated rendezvous or in-orbit refueling had been attempted. The USSR achieved the first automated docking of two spacecraft during the mission of Kosmos-186 and 188 in October 1967, or at least four years after the 21K tanker had been conceptualized, but still almost two years before the first lunar landing of the Apollo-11 crew in July 1969.
In the meantime, the actual refueling of a spacecraft in orbit from a tanker would not be performed until the end of the 1970s with the introduction of the Progress series, or around 17 years after the work on the 21K project. The refueling procedures were never actually employed during the Moon Race and were only tested but not practically implemented for various Earth-orbiting missions in the 1980s, with the exception of the Soviet space stations.
Refueling operations are currently planned during the assembly of the cis-lunar gateway in the 2020s or six decades after the 21K tanker was proposed.
A hypothetical Soviet poster dedicated to the first flight of the 21K tanker.
This is apparently the only publicly available official depiction of the 21K tanker. It was part of a report discussed at the scientific council of the OKB-1 design bureau on April 22, 1963.
The 21K tanker approaches the 19 complex in the Earth's orbit. Click to enlarge. Copyright © 2019 Anatoly Zak
A hypothetical Soviet stamp dedicated to the first flight of the 21K tanker had it made it to orbit during the first launch of the N1 rocket as was envisioned around 1963. Copyright © 2019 Anatoly Zak