Lunar Orbit Rendezvous

Lunar orbit rendezvous (LOR) is a key concept for efficiently landing humans on the Moon and returning them to Earth. It was first utilized for the Project Apollo missions in the 1960s and 1970s. In a LOR mission, a main spacecraft and a smaller lunar lander travel to lunar orbit. The lunar lander then independently descends to the surface of the Moon, while the main spacecraft remains in lunar orbit. After completion of the mission there, the lander returns to lunar orbit to rendezvous and re-dock with the main spacecraft, then is discarded after transfer of crew and payload. Only the main spacecraft returns to Earth.

Lunar orbit rendezvous was first known to be proposed in 1919 by Soviet engineer Yuri Kondratyuk, as the most economical way of sending a human on a round-trip journey to the Moon.

The most famous example involved Apollo CSM and Apollo LM, where they were both sent to a Translunar flight in a single rocket stack. However, variants where the landers and main spacecraft travel separately, such as the lunar landing plan proposed for Shuttle-Derived Heavy Lift Launch Vehicle and Golden Spike, are also considered as Lunar Orbit rendezvous.

The main advantage of LOR is the spacecraft payload saving, due to the fact that the propellant necessary to return from lunar orbit back to Earth need not be carried as dead weight down to the Moon and back into lunar orbit. This has a multiplicative effect, because each pound of "dead weight" propellant used later has to be propelled by more propellant sooner, and also because increased propellant requires increased tankage weight. The resultant weight increase would also require more thrust for lunar landing, which means larger and heavier engines.

Another advantage is that the lunar lander can be designed for just that purpose, rather than requiring the main spacecraft to also be made suitable for a lunar landing. Finally, the second set of life support systems that the lunar lander requires can serve as a backup for the systems in the main spacecraft.

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