Lunar orbit rendezvous
Encyclopedia
Lunar orbit rendezvous is a key concept for human landing on the Moon and returning to Earth.
In a LOR mission a main spacecraft
and a smaller lunar module travel together into lunar orbit
. The lunar module then independently descends to the lunar surface. After completion of the mission there, a part of it returns to lunar orbit and conducts a rendezvous
with the main spacecraft. The main spacecraft then returns to Earth.
First mention of LOR dates back to 1916. It was proposed by Yuri Kondratyuk
, a self-educated Ukrainian, who calculated that LOR was the most economical way of landing a human on the Moon.
LOR was used by the Apollo missions
for human spaceflight
to the Moon
.
The LOR mission "mode" was one of three modes considered for Apollo.
The first was direct ascent
. For this mission mode, a huge, monolithic rocket (called "Nova
") would have launched into space, landed on the Moon, and then returned to Earth.
The second was earth orbit rendezvous
, where two smaller rockets would have launched the capsule with the astronauts and a fuel tank. The astronauts would rendezvous with the fuel tank in Earth orbit, fill up the capsule and depart Earth orbit for the Moon.
The third mode was lunar orbit rendezvous, a plan first proposed by Tom Dolan
and later developed by a team led by John C. Houbolt
. In this mode, a rocket launches both the Command/Service Module
(CSM) with the astronauts and the Lunar Module
(LM). When the combined spacecraft reaches lunar orbit
, one of the three astronauts remains with the CSM; the other two undock the LM from the CSM and descend to the surface of the Moon. They then use the ascent stage of the LM to rejoin the CSM in lunar orbit, and use the CSM for the return to Earth.
A disadvantage is that the lunar lander needs a separate life-support system and two sets of engines were needed on the lunar lander and another attached to the command module.
Cutting systems back to the absolute minimum levels to achieve maximum fuel economy includes risks, also. Redundancy is often critical to survival. The LOR mode which was selected for Apollo offered some degree of redundancy for critical systems that gave the system more flexibility in handling unplanned events. For example, the separate systems enabled the survival of the Apollo 13
astronauts when the primary craft was disabled by an explosion. In that case, the LM's separate systems were pressed into service to save the lives of the astronauts, even though using the LM as a "lifeboat" was not part of its specifications (but was envisioned as a contingency prior to Apollo 13).
, today a National Historic Landmark
.
An Earth orbit rendezvous was considered far less risky for two reasons: (1) if the rendezvous in lunar orbit failed, the LM astronauts would be stranded and would not return to Earth, (2) if the Service Module
engine failed after the rendezvous in lunar orbit, the astronauts would never return. If the rendezvous in Earth orbit failed, no astronauts would be endangered. If the engine failed in Earth orbit, the orbit would naturally decay and reentry would occur.
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.
In November 1961, Houbolt took the bold step of skipping proper channels and writing a private letter, nine pages long, directly to Robert C. Seamans
, the associate administrator. "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."
It took two weeks for Seamans to reply to Houbolt's extraordinary 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.
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 the huge Nova rocket. Second, there was increasing technical apprehension over how the relatively large spacecraft demanded even 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 first major group to break camp in favor of LOR was Robert Gilruth's Space Task Group, which was still located at Langley but was soon to move to Houston
. The second to come over was the Von Braun
team at the Marshall Space Flight Center
in Huntsville, Alabama
. Then these two powerful groups of converts, along with the original true believers 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. With the key players inside NASA lined up behind the concept, Webb approved LOR in July 1962. He did it even though President Kennedy's science adviser, Jerome Wiesner
, remained firmly opposed to LOR.
Whether NASA's choice of LOR would have been made in the summer of 1962 or at any later time without the research information, the commitment, and the crusading zeal of Houbolt and his associates at NASA Langley is a matter for historical conjecture. However, the basic contribution made by the Langley researchers is beyond debate. They were the first in NASA to recognize the fundamental advantages of the LOR concept, and for a critical period of time in the early 1960s they were also the only ones inside of the agency to foster it and fight for it.
Thousands of factors contributed to the ultimate success of Apollo, but no single factor was more essential than the concept of lunar-orbit rendezvous. Without NASA's adoption of this stubbornly-held minority opinion, the United States may still have gotten to the Moon, but almost certainly it would not have been accomplished by the end of the decade, as President Kennedy had wanted.
In a LOR mission a main spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
and a smaller lunar module travel together into lunar orbit
Lunar orbit
In astronomy, lunar orbit refers to the orbit of an object around the Moon.As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by various manned or unmanned spacecraft around the Moon...
. The lunar module then independently descends to the lunar surface. After completion of the mission there, a part of it returns to lunar orbit and conducts a rendezvous
Space rendezvous
A space rendezvous is an orbital maneuver during which two spacecraft, one of which is often a space station, arrive at the same orbit and approach to a very close distance . Rendezvous requires a precise match of the orbital velocities of the two spacecraft, allowing them to remain at a constant...
with the main spacecraft. The main spacecraft then returns to Earth.
First mention of LOR dates back to 1916. It was proposed by Yuri Kondratyuk
Yuri Kondratyuk
Yuri Vasilievich Kondratyuk , was a follower, supporter and founder of cosmism, pioneer of astronautics and spaceflight. He was a theoretician and a visionary who, in the early twentieth century, foresaw ways of reaching the moon...
, a self-educated Ukrainian, who calculated that LOR was the most economical way of landing a human on the Moon.
LOR was used by the Apollo missions
Project Apollo
The Apollo program was the spaceflight effort carried out by the United States' National Aeronautics and Space Administration , that landed the first humans on Earth's Moon. Conceived during the Presidency of Dwight D. Eisenhower, Apollo began in earnest after President John F...
for human spaceflight
Human spaceflight
Human spaceflight is spaceflight with humans on the spacecraft. When a spacecraft is manned, it can be piloted directly, as opposed to machine or robotic space probes and remotely-controlled satellites....
to the Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
.
Apollo Mission modes
The first was direct ascent
Direct ascent
Direct ascent was a proposed method for a mission to the Moon. In the United States, direct ascent proposed using the enormous Nova rocket to launch a spacecraft directly to the Moon, where it would land tail-first and then launch off the Moon back to Earth...
. For this mission mode, a huge, monolithic rocket (called "Nova
Nova rocket
Nova was a series of proposed rocket designs, originally as NASA's first large launchers for missions similar to the production-level Saturn V, and later as larger follow-ons to the Saturn V intended for missions to Mars. The two series of designs were essentially separate, but shared their name...
") would have launched into space, landed on the Moon, and then returned to Earth.
The second was earth orbit rendezvous
Earth orbit rendezvous
Earth orbit rendezvous is a type of space rendezvous and a spaceflight methodology most notable for enabling round trip human missions to the moon...
, where two smaller rockets would have launched the capsule with the astronauts and a fuel tank. The astronauts would rendezvous with the fuel tank in Earth orbit, fill up the capsule and depart Earth orbit for the Moon.
The third mode was lunar orbit rendezvous, a plan first proposed by Tom Dolan
Tom Dolan (engineer)
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...
and later developed by a team led by John C. Houbolt
John Houbolt
John Cornelius Houbolt is a retired aerospace engineer. He is generally credited with having effectively promoted the lunar mission mode called Lunar Orbit Rendezvous or LOR. This flight path was first endorsed by Wernher von Braun in June 1961 and was chosen for Apollo program in early 1962...
. In this mode, a rocket launches both the Command/Service Module
Apollo Command/Service Module
The Command/Service Module was one of two spacecraft, along with the Lunar Module, used for the United States Apollo program which landed astronauts on the Moon. It was built for NASA by North American Aviation...
(CSM) with the astronauts and the Lunar Module
Apollo Lunar Module
The Apollo Lunar Module was the lander portion of the Apollo spacecraft built for the US Apollo program by Grumman to carry a crew of two from lunar orbit to the surface and back...
(LM). When the combined spacecraft reaches lunar orbit
Lunar orbit
In astronomy, lunar orbit refers to the orbit of an object around the Moon.As used in the space program, this refers not to the orbit of the Moon about the Earth, but to orbits by various manned or unmanned spacecraft around the Moon...
, one of the three astronauts remains with the CSM; the other two undock the LM from the CSM and descend to the surface of the Moon. They then use the ascent stage of the LM to rejoin the CSM in lunar orbit, and use the CSM for the return to Earth.
Advantages and disadvantages
One advantage of lunar orbit rendezvous is that the propellant necessary to return from lunar orbit back to the Earth need not be carried down to the Moon, and back up from the Moon again. This saves the propellant needed to move that propellant, which is considerable; it also reduces tankage weight and greatly reduces the sizes of the engines for the lunar lander.A disadvantage is that the lunar lander needs a separate life-support system and two sets of engines were needed on the lunar lander and another attached to the command module.
Cutting systems back to the absolute minimum levels to achieve maximum fuel economy includes risks, also. Redundancy is often critical to survival. The LOR mode which was selected for Apollo offered some degree of redundancy for critical systems that gave the system more flexibility in handling unplanned events. For example, the separate systems enabled the survival of the Apollo 13
Apollo 13
Apollo 13 was the seventh manned mission in the American Apollo space program and the third intended to land on the Moon. The craft was launched on April 11, 1970, at 13:13 CST. The landing was aborted after an oxygen tank exploded two days later, crippling the service module upon which the Command...
astronauts when the primary craft was disabled by an explosion. In that case, the LM's separate systems were pressed into service to save the lives of the astronauts, even though using the LM as a "lifeboat" was not part of its specifications (but was envisioned as a contingency prior to Apollo 13).
Risks
Lunar-orbit rendezvous required docking the lunar module with the command module in lunar orbit in order for the astronauts that landed on the Moon to return to Earth. Astronauts practiced the complex task of separating and uniting spacecraft to master docking techniques with Langley's Rendezvous and Docking SimulatorRendezvous Docking Simulator
The Rendezvous Docking Simulator, also known as the Real-Time Dynamic Simulator, is a simulator at Langley Research Center in building. It was constructed for the Gemini program in building 1244 and it became operatioal in June1963 at a cost of $320,000 and later reconfigured for the Apollo program...
, today a National Historic Landmark
National Historic Landmark
A National Historic Landmark is a building, site, structure, object, or district, that is officially recognized by the United States government for its historical significance...
.
An Earth orbit rendezvous was considered far less risky for two reasons: (1) if the rendezvous in lunar orbit failed, the LM astronauts would be stranded and would not return to Earth, (2) if the Service Module
Apollo Command/Service Module
The Command/Service Module was one of two spacecraft, along with the Lunar Module, used for the United States Apollo program which landed astronauts on the Moon. It was built for NASA by North American Aviation...
engine failed after the rendezvous in lunar orbit, the astronauts would never return. If the rendezvous in Earth orbit failed, no astronauts would be endangered. If the engine failed in Earth orbit, the orbit would naturally decay and reentry would occur.
Advocacy
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 rendezvousSpace rendezvous
A space rendezvous is an orbital maneuver during which two spacecraft, one of which is often a space station, arrive at the same orbit and approach to a very close distance . Rendezvous requires a precise match of the orbital velocities of the two spacecraft, allowing them to remain at a constant...
since 1959 and was convinced, like several others at Langley Research Center
Langley Research Center
Langley Research Center is the oldest of NASA's field centers, located in Hampton, Virginia, United States. It directly borders Poquoson, Virginia and Langley Air Force Base...
, 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
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
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.
In November 1961, Houbolt took the bold step of skipping proper channels and writing a private letter, nine pages long, directly to Robert C. Seamans
Robert C. Seamans
SSV Robert C. Seamans is a 134-foot steel sailing brigantine operated by the Sea Education Association for oceanographic research and sail training; she is named for a former Chairman and Trustee of SEA's board. She is equipped with hydrographic winches, bathymetric equipment, biological and...
, the associate administrator. "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."
It took two weeks for Seamans to reply to Houbolt's extraordinary 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.
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
Direct ascent
Direct ascent was a proposed method for a mission to the Moon. In the United States, direct ascent proposed using the enormous Nova rocket to launch a spacecraft directly to the Moon, where it would land tail-first and then launch off the Moon back to Earth...
due to the time and money it was going to take to develop the huge Nova rocket. Second, there was increasing technical apprehension over how the relatively large spacecraft demanded even 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 break camp in favor of LOR was Robert Gilruth's Space Task Group, which was still located at Langley but was soon to move to Houston
Lyndon B. Johnson Space Center
The Lyndon B. Johnson Space Center is the National Aeronautics and Space Administration's center for human spaceflight training, research and flight control. The center consists of a complex of 100 buildings constructed on 1,620 acres in Houston, Texas, USA...
. The second to come over was the Von Braun
Wernher von Braun
Wernher Magnus Maximilian, Freiherr von Braun was a German rocket scientist, aerospace engineer, space architect, and one of the leading figures in the development of rocket technology in Nazi Germany during World War II and in the United States after that.A former member of the Nazi party,...
team at the Marshall Space Flight Center
Marshall Space Flight Center
The George C. Marshall Space Flight Center is the U.S. government's civilian rocketry and spacecraft propulsion research center. The largest center of NASA, MSFC's first mission was developing the Saturn launch vehicles for the Apollo moon program...
in Huntsville, Alabama
Huntsville, Alabama
Huntsville is a city located primarily in Madison County in the central part of the far northern region of the U.S. state of Alabama. Huntsville is the county seat of Madison County. The city extends west into neighboring Limestone County. Huntsville's population was 180,105 as of the 2010 Census....
. Then these two powerful groups of converts, along with the original true believers at Langley, persuaded key officials at NASA Headquarters, notably Administrator James Webb
James E. Webb
James Edwin Webb was an American government official who served as the second administrator of NASA from February 14, 1961 to October 7, 1968....
, who had been holding out for direct ascent, that LOR was the only way to land on the Moon by 1969. With the key players inside NASA lined up behind the concept, Webb approved LOR in July 1962. He did it even though President Kennedy's science adviser, Jerome Wiesner
Jerome Wiesner
Jerome Bert Wiesner was an educator, a Science Advisor to U.S. Presidents Eisenhower and Kennedy and Johnson, an advocate for arms control, and a critic of anti-ballistic-missile defense systems...
, remained firmly opposed to LOR.
Whether NASA's choice of LOR would have been made in the summer of 1962 or at any later time without the research information, the commitment, and the crusading zeal of Houbolt and his associates at NASA Langley is a matter for historical conjecture. However, the basic contribution made by the Langley researchers is beyond debate. They were the first in NASA to recognize the fundamental advantages of the LOR concept, and for a critical period of time in the early 1960s they were also the only ones inside of the agency to foster it and fight for it.
Thousands of factors contributed to the ultimate success of Apollo, but no single factor was more essential than the concept of lunar-orbit rendezvous. Without NASA's adoption of this stubbornly-held minority opinion, the United States may still have gotten to the Moon, but almost certainly it would not have been accomplished by the end of the decade, as President Kennedy had wanted.
External links
- Article on nasa.gov
- LOR decision
- Correspondence between Houbolt and Seamans, including Houbolt's "Voice in the wilderness" memo