Lunar orbit rendezvous

Last updated
Diagram of LOR Lunar-orbit rendezvous.jpg
Diagram of LOR

Lunar orbit rendezvous (LOR) is a key concept for efficiently 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. [1]


Lunar orbit rendezvous was first known to be proposed in 1919 by Ukrainian Soviet engineer Yuri Kondratyuk, [2] as the most economical way of sending a human on a round-trip journey to the Moon. [3]

The most famous example involved Apollo command and service module and Apollo Lunar Module, 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.

Advantages and disadvantages


Representation of the lunar gravity well, illustrating how resources needed only for the trip home don't have to be carried down and back up the "well" LOR Gravity Well.png
Representation of the lunar gravity well, illustrating how resources needed only for the trip home don't have to be carried down and back up the "well"

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. [4]

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 as of 1962, because space rendezvous had not been achieved, even in Earth orbit. If the LEM 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. The fear proved to be unfounded, as rendezvous was successfully demonstrated in 1965 and 1966 on six Project Gemini missions [Note 1] 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. [Note 2]

Apollo Mission mode selection

Apollo 11 Lunar Module rendezvousing with Command Module in lunar orbit Apollo 11 lunar module.jpg
Apollo 11 Lunar Module rendezvousing with Command Module in lunar orbit

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 100,000 pounds (45,000 kg)) 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.

Tom Dolan [5] proposed the alternative of lunar orbit rendezvous, which had been studied and promoted by Jim Chamberlin and Owen Maynard at the Space Task Group in 1960 early Apollo feasibility studies. [6] This mode allowed a single Saturn V to launch the CSM to the Moon with a smaller Lunar Excursion Module (LEM). [Note 3] 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 4.6 metres (15 ft) above the surface, as opposed to being on their backs in a Command Module lander, at least 40 or 50 feet (12 or 15 m) 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.


John Houbolt explains Lunar orbit rendezvous John C. Houbolt - GPN-2000-001274.jpg
John Houbolt explains Lunar orbit rendezvous

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. [8]

In November 1961, Houbolt took the bold step of skipping proper channels and writing a nine-page private letter directly to associate administrator Robert C. Seamans. "Somewhat as a voice in the wilderness," Houbolt protested LOR's exclusion. "Do we want to go to the Moon or not?" the Langley engineer asked. "Why is Nova, with its ponderous size simply just accepted, and why is a much less grandiose scheme involving rendezvous ostracized or put on the defensive? I fully realize that contacting you in this manner is somewhat unorthodox," Houbolt admitted, "but the issues at stake are crucial enough to us all that an unusual course is warranted." [9] [10]

It took two weeks for Seamans to reply to Houbolt's letter. The associate administrator agreed that "it would be extremely harmful to our organization and to the country if our qualified staff were unduly limited by restrictive guidelines." He assured Houbolt that NASA would in the future be paying more attention to LOR than it had up to this time.

Comparison of lunar lander sizes, from an early Langley study Comparison of Lander Sizes - Direct vs LOR.gif
Comparison of lunar lander sizes, from an early Langley study

In the following months, NASA did just that, and to the surprise of many both inside and outside the agency, the dark horse candidate, LOR, quickly became the front runner. Several factors decided the issue in its favor. First, there was growing disenchantment with the idea of direct ascent due to the time and money it was going to take to develop a 50-foot (15 m) diameter Nova rocket, compared to the 33-foot (10 m) diameter Saturn V. Second, there was increasing technical apprehension over how the relatively large spacecraft demanded by Earth-orbit rendezvous would be able to maneuver to a soft landing on the Moon. As one NASA engineer who changed his mind explained:

The business of eyeballing that thing down to the Moon really didn't have a satisfactory answer. The best thing about LOR was that it allowed us to build a separate vehicle for landing.

The first major group to change its opinion in favor of LOR was Robert Gilruth's Space Task Group, which was still located at Langley but was soon to move to Houston as the Manned Spacecraft Center. The second to come over was the Von Braun team at the Marshall Space Flight Center in Huntsville, Alabama. These two powerful groups, along with the engineers who had originally developed the plan at Langley, persuaded key officials at NASA Headquarters, notably Administrator James Webb, who had been holding out for direct ascent, that LOR was the only way to land on the Moon by 1969. Webb approved LOR in July 1962. [11] The decision was officially announced at a press conference on July 11, 1962. [12] President Kennedy's science adviser, Jerome Wiesner, remained firmly opposed to LOR. [13] [8]

Other plans using LOR

LOR using Shuttle-Derived Heavy Lift Launch Vehicle Shuttleclunar.svg
LOR using Shuttle-Derived Heavy Lift Launch Vehicle

Episode 5 of the 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.


  1. Gemini 6A, Gemini 8, Gemini 9A, Gemini 10, Gemini 11, and Gemini 12
  2. Apollo 9 in Earth orbit; in lunar orbit on Apollo 10, Apollo 11, Apollo 12, Apollo 14, Apollo 15, Apollo 16, and Apollo 17.
  3. This was shortened to "Lunar Module" (LM) in June 1966. [7]

Related Research Articles

Apollo program 1961–1972 United States human spaceflight program, which landed the first humans on the lunar surface

The Apollo program, also known as Project Apollo, was the third United States human spaceflight program carried out by the National Aeronautics and Space Administration (NASA), which succeeded in landing the first humans on the Moon from 1969 to 1972. First conceived during Dwight D. Eisenhower's administration as a three-person spacecraft to follow the one-person Project Mercury which put the first Americans in space, Apollo was later dedicated to the national goal set by President John F. Kennedy of "landing a man on the Moon by the end of this decade and returning him safely to the Earth" in an address to Congress on May 25, 1961. It was the third US human spaceflight program to fly, preceded by the two-person Project Gemini conceived in 1961 to extend spaceflight capability in support of Apollo.

Apollo 9 3rd crewed mission of the Apollo space program

Apollo 9 was a March 1969 human spaceflight, the third in NASA's Apollo program. Flown in low Earth orbit, it was the second crewed Apollo mission that the United States launched via a Saturn V rocket, and was the first flight of the full Apollo spacecraft: the command and service module (CSM) with the Lunar Module (LM). The mission was flown to qualify the LM for lunar orbit operations in preparation for the first Moon landing by demonstrating its descent and ascent propulsion systems, showing that its crew could fly it independently, then rendezvous and dock with the CSM again, as would be required for the first crewed lunar landing. Other objectives of the flight included firing the LM descent engine to propel the spacecraft stack as a backup mode, and use of the Portable Life Support System backpack outside of the LM cabin.

Apollo 12 Second crewed mission to land on the Moon.

Apollo 12 was the sixth crewed flight in the United States Apollo program and the second to land on the Moon. It was launched on November 14, 1969, from the Kennedy Space Center, Florida, four months after Apollo 11. Commander Charles "Pete" Conrad and Apollo Lunar Module Pilot Alan L. Bean performed just over one day and seven hours of lunar surface activity while Command Module Pilot Richard F. Gordon remained in lunar orbit. The landing site for the mission was located in the southeastern portion of the Ocean of Storms.

Apollo 16 Fifth crewed mission to land on the Moon

Apollo 16 was the tenth crewed mission in the United States Apollo space program, the fifth and second-to-last to land on the Moon, and the second to land in the lunar highlands. The second of Apollo's "J missions," it was crewed by Commander John Young, Lunar Module Pilot Charles Duke and Command Module Pilot Ken Mattingly. Launched from the Kennedy Space Center in Florida at 12:54 PM EST on April 16, 1972, the mission lasted 11 days, 1 hour, and 51 minutes, and concluded at 2:45 p.m. EST on April 27.

Apollo Lunar Module A lander used in the Apollo program.

The Apollo Lunar Module, or simply lunar module, originally designated the Lunar Excursion Module (LEM), was the lander spacecraft that was flown from lunar orbit to the Moon's surface during the U.S. Apollo program. It was the first crewed spacecraft to operate exclusively in the airless vacuum of space, and remains the only crewed vehicle to land anywhere beyond Earth.

Constellation program Cancelled 2005–2010 United States human spaceflight program, aimed at crewed exploration of the Moon, Mars, and minor planets

The Constellation Program is a cancelled crewed spaceflight program developed by NASA, the space agency of the United States, from 2005 to 2009. The major goals of the program were "completion of the International Space Station" and a "return to the Moon no later than 2020" with a crewed flight to the planet Mars as the ultimate goal. The program's logo reflected the three stages of the program: the Earth (ISS), the Moon, and finally Mars—while the Mars goal also found expression in the name given to the program's booster rockets: Ares. The technological aims of the program included the regaining of significant astronaut experience beyond low Earth orbit and the development of technologies necessary to enable sustained human presence on other planetary bodies.

Apollo (spacecraft) American spacecraft

The Apollo spacecraft was composed of three parts designed to accomplish the American Apollo program's goal of landing astronauts on the Moon by the end of the 1960s and returning them safely to Earth. The expendable (single-use) spacecraft consisted of a combined command and service module (CSM) and an Apollo Lunar Module (LM). Two additional components complemented the spacecraft stack for space vehicle assembly: a spacecraft–LM adapter (SLA) designed to shield the LM from the aerodynamic stress of launch and to connect the CSM to the Saturn launch vehicle and a launch escape system (LES) to carry the crew in the command module safely away from the launch vehicle in the event of a launch emergency.

Apollo command and service module spacecraft

The Apollo command and service module (CSM) was one of two principal components of the United States Apollo spacecraft, used for the Apollo program, which landed astronauts on the Moon between 1969 and 1972. The CSM functioned as a mother ship, which carried a crew of three astronauts and the second Apollo spacecraft, the Apollo Lunar Module, to lunar orbit, and brought the astronauts back to Earth. It consisted of two parts: the conical command module, a cabin that housed the crew and carried equipment needed for atmospheric reentry and splashdown; and the cylindrical service module which provided propulsion, electrical power and storage for various consumables required during a mission. An umbilical connection transferred power and consumables between the two modules. Just before reentry of the command module on the return home, the umbilical connection was severed and the service module was cast off and allowed to burn up in the atmosphere.

Space capsule type of spacecraft

A space capsule is an often crewed blunt-body spacecraft that reenters the Earth's atmosphere without wings. Capsules are distinguished from satellites primarily by the ability to survive reentry and return a payload to the Earth's surface from orbit. Capsules make up the majority of crewed spacecraft designs, although one crewed spaceplane has launched to orbit.

The Apollo Applications Program (AAP) was created as early as 1966 by NASA headquarters to develop science-based human spaceflight missions using hardware developed for the Apollo program. AAP was the ultimate development of a number of official and unofficial Apollo follow-on projects studied at various NASA labs.. However, the AAP's ambitious initial plans became an early casualty when the Johnson Administration declined to support it adequately, partly in order to implement its Great Society set of domestic programs while remaining within a $100 billion budget. Thus, Fiscal Year 1967 ultimately allocated $80 million to the AAP, compared to NASA's preliminary estimates of $450 million necessary to fund a full-scale AAP program for that year, with over $1 billion being required for FY 1968. The AAP eventually led to Skylab, which absorbed much of what had been developed under Apollo Applications.

Earth orbit rendezvous

Earth orbit rendezvous (EOR) is a potential methodology for conducting round trip human flights to the Moon, involving the use of space rendezvous to assemble, and possibly fuel, components of a translunar vehicle in low Earth orbit. It was considered and ultimately rejected in favor of lunar orbit rendezvous (LOR) for NASA's Apollo program of the 1960s and 1970s, mainly because LOR does not require a spacecraft big enough to both make the return trip from earth orbit to splashdown in the ocean, and a soft landing on the lunar surface. Three decades later, it was planned to be used for Project Constellation, until that program's cancellation in October 2010.

Direct ascent is a method of landing a spacecraft on the Moon or another planet directly, without first assembling the vehicle in Earth orbit, or carrying a separate landing vehicle into orbit around the target body. It was proposed as the first method to achieve a crewed lunar landing in the United States Apollo program, but was rejected because it would have required developing a prohibitively large launch vehicle.

Moon landing Arrival of a spacecraft on the surface of the Moon

A Moon landing is the arrival of a spacecraft on the surface of the Moon. This includes both manned and robotic missions. The first human-made object to touch the Moon was the Soviet Union's Luna 2, on 13 September 1959.

Altair (spacecraft) Planned lander spacecraft component of NASAs cancelled Project Constellation

The Altair spacecraft, previously known as the Lunar Surface Access Module or LSAM, was the planned lander spacecraft component of NASA's cancelled Constellation program. Astronauts would have used the spacecraft for landings on the Moon, which was intended to begin around 2019. The Altair spacecraft was planned to be used both for lunar sortie and lunar outpost missions. On February 1, 2010, U.S. President Barack Obama announced a proposal to cancel the Constellation program, to be replaced with a re-scoped program, effective with the U.S. 2011 fiscal year budget.

Thomas Dolan was an American engineer who proposed the first fully developed concept of Lunar orbit rendezvous for the Apollo program while working at Vought Astronautics.

John Houbolt American aerospace engineer

John Cornelius Houbolt was an aerospace engineer credited with leading the team behind the lunar orbit rendezvous (LOR) mission mode, a concept that was used to successfully land humans on the Moon and return them to Earth. This flight path was first endorsed by Wernher von Braun in June 1961 and was chosen for Apollo program in early 1962. The critical decision to use LOR was viewed as vital to ensuring that Man reached the Moon by the end of the decade as proposed by President John F. Kennedy. In the process, LOR saved time and billions of dollars by efficiently using existing rocket technology.

Lunar escape systems series of proposed emergency spacecraft for the Apollo Program

Lunar escape systems (LESS) were a series of emergency vehicles designed for never-flown long-duration Apollo missions. Because these missions were even more hypothetical than the planned cancelled Apollo missions, the designs were never constructed. This concept was an outgrowth of the Lunar Flying Vehicle designed by Bell Aerospace.

Advanced Gemini is a number of proposals that would have extended the Gemini program by the addition of various missions, including manned low Earth orbit, circumlunar and lunar landing missions. Gemini was the second manned spaceflight program operated by NASA, and consisted of a two-seat spacecraft capable of maneuvering in orbit, docking with unmanned spacecraft such as Agena Target Vehicles, and allowing the crew to perform tethered extra-vehicular activities.

Apollo spacecraft feasibility study

The Apollo spacecraft feasibility study was conducted by NASA from July 1960 through May 1961 to investigate preliminary designs for a post-Project Mercury multi-crewed spacecraft to be used for possible space station, circum-lunar, lunar orbital, or crewed lunar landing missions. Six-month, $250,000 study contracts were awarded to General Dynamics/Convair, General Electric, and the Glenn L. Martin Company. Meanwhile, NASA conducted its own inhouse design study led by Maxime Faget, intended as a gauge of the competitors' entries. The three companies spent varying amounts of their own money in excess of the $250,000 to produce designs which included a re-entry module separate from the mission module cabin, and a propulsion and equipment module.


PD-icon.svg This article incorporates  public domain material from websites or documents ofthe National Aeronautics and Space Administration .


  1. "Lunar Orbit Rendezvous" – 1968 – NASA Mission Planning and Analysis Division on YouTube
  2. Harvey (2007), p. 6–7.
  3. Wilford (1969), p. 41-48.
  4. Reeves (2005).
  5. Brooks (1979).
  6. Gainor (2001), p. 62-66.
  7. Scheer, Julian W. (Assistant Administrator for Public Affairs, NASA). Memorandum from Project Designation Committee, June 9, 1966.
  8. 1 2 "The Rendezvous That Was Almost Missed: Lunar Orbit Rendezvous and the Apollo Program – NASA". December 1992. Fact Sheet NF175. Retrieved 2017-03-20.
  9. Tennant (2009).
  10. Hansen (1995).
  11. Witkin (1962).
  12. NASA (1962), p. 1.
  13. Nelson (2009), p. 209–210.
  14. Laxman (2012).