Схемы сближения на лунной орбите: от Кондратюка до Apollo

Spaceflight maneuver

Lunar orbit rendezvous (LOR) is a process for landing humans on the Moon and returning them to Earth. It was utilized for the Apollo program 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 proposed in 1919 by Ukrainian 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 Project Apollo's command and service module (CSM) and lunar module (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 plans proposed for Shuttle Derived Heavy Lift Launch Vehicle, Golden Spike and the 2029/2030 Chinese crewed effort, are also considered to be 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.

Lunar orbit rendezvous was considered risky in 1962, because space rendezvous had not been achieved, even in Earth orbit. If the LM could not reach the CSM, two astronauts would be stranded with no way to get back to Earth or survive re entry into the atmosphere. Rendezvous was successfully demonstrated in 1965 and 1966 on six Project Gemini missions with the aid of radar and on board computers. It was also successfully done each of the eight times it was tried on Apollo missions.

When the Apollo Moon landing program was started in 1961, it was assumed that the three man command and service module combination (CSM) would be used for takeoff from the lunar surface, and return to Earth. It would therefore have to be landed on the Moon by a larger rocket stage with landing gear legs, resulting in a very large spacecraft (in excess of ) to be sent to the Moon. If this were done by direct ascent (on a single launch vehicle), the rocket required would have to be extremely large, in the Nova class. The alternative to this would have been Earth orbit rendezvous, in which two or more rockets in the Saturn class would launch parts of the complete spacecraft, which would rendezvous in Earth orbit before departing for the Moon. This would possibly include a separately launched Earth departure stage, or require on orbit refueling of the empty departure stage. Wernher von Braun and Heinz Hermann Koelle of the Army Ballistic Missile Agency presented lunar orbit rendezvous, as an option for reaching the Moon efficiently, to the heads of NASA, including Abe Silverstein, in December 1958. During 1959 Conrad Lau of the Chance Vought Astronautics Division supervised a complete mission plan using lunar orbit rendezvous which was then sent to Silverstein at NASA in January 1960. Tom Dolan, who worked for Lau, was sent to explain the company's proposal to NASA engineers and management in February 1960. This alternative was then studied and promoted by Jim Chamberlin and Owen Maynard at the Space Task Group in the 1960 early Apollo feasibility studies. This mode allowed a single Saturn V to launch the CSM to the Moon with a smaller Lunar Excursion Module (LEM). When the combined spacecraft reaches lunar orbit, one of the three astronauts remains with the CSM, while the other two enter the LEM, undock and descend to the surface of the Moon. They then use the ascent stage of the LEM to rejoin the CSM in lunar orbit, then discard the LEM and use the CSM for the return to Earth. This method was brought to the attention of NASA Associate Administrator Robert Seamans by Langley Research Center engineer John C. Houbolt, who led a team to develop it. Besides requiring less payload, the ability to use a lunar lander designed just for that purpose was another advantage of the LOR approach. The LEM's design gave the astronauts a clear view of their landing site through observation windows approximately 15 feet (5 m) above the surface, as opposed to being on their backs in a Command Module lander, at least above the surface, able to see it only through a television screen. Developing the LEM as a second crewed vehicle provided the further advantage of redundant critical systems (electrical power, life support, and propulsion), which enabled it to be used as a "lifeboat" to keep the astronauts alive and get them home safely in the event of a critical CSM system failure. This was envisioned as a contingency, but not made a part of the LEM specifications. As it turned out, this capability proved invaluable in 1970, saving the lives of the Apollo 13 astronauts when an oxygen tank explosion disabled the Service Module.

Dr. John Houbolt would not let the advantages of LOR be ignored. As a member of Lunar Mission Steering Group, Houbolt had been studying various technical aspects of space rendezvous since 1959 and was convinced, like several others at Langley Research Center, that LOR was not only the most feasible way to make it to the Moon before the decade was out, it was the only way. He had reported his findings to NASA on various occasions but felt strongly that the internal task forces (to which he made presentations) were following arbitrarily established "ground rules." According to Houbolt, these ground rules were constraining NASA's thinking about the lunar mission—and causing LOR to be ruled out before it was fairly considered.

The proposed Soviet lunar landing plan, using the N1 rocket, LK Lander and Soyuz 7K LOK, would have used a similar LOR mission profile. The Constellation program would have used a combination of EOR and LOR for Moon landing. The Artemis program plans to use LOR to land humans on the lunar south pole region.

Episode 5 of the 1998 television miniseries From the Earth to the Moon, "Spider", dramatizes John Houbolt's first attempt to convince NASA to adopt LOR for the Apollo Program in 1961, and traces the development of the LM up to its first crewed test flight, Apollo 9, in 1969. The episode is named after the Apollo 9 Lunar Module.