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Lunar orbit rendezvous
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== Apollo mission mode selection == [[File:Apollo 11 lunar module.jpg|thumb|[[Apollo 11]] [[Lunar Module Eagle|Lunar Module ''Eagle'']] rendezvousing with [[Command module Columbia|Command module ''Columbia'']] in lunar orbit]] When the Apollo Moon landing program was started in 1961, it was assumed that the three-man [[Apollo command and service module|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 {{convert|100000|lb|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 rocket|Nova]] class. The alternative to this would have been [[Earth orbit rendezvous]], in which two or more rockets in the [[Saturn (rocket family)|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.{{sfnp|Godwin|2019}} During 1959 [[Conrad Lau (engineer)|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 (engineer)|Tom Dolan]], who worked for Lau, was sent to explain the company's proposal to NASA engineers and management in February 1960.{{sfnp|Brooks|1979}}{{sfnp|Godwin|2019}} 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.{{sfnp|Gainor|2001|pp=62โ66}} This mode allowed a single [[Saturn V]] to launch the CSM to the Moon with a smaller Lunar Excursion Module (LEM).{{refn|group=Note|This was shortened to "Lunar Module" (LM) in June 1966.<ref>Scheer, Julian W. (Assistant Administrator for Public Affairs, NASA). Memorandum from Project Designation Committee, June 9, 1966.</ref>}} 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 Houbolt|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 {{convert|15|ft|m|disp=flip}} above the surface, as opposed to being on their backs in a Command Module lander, at least {{convert|40|or|50|ft|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. === Advocacy === [[File:John C. Houbolt - GPN-2000-001274.jpg|thumb|[[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 missionโand causing LOR to be ruled out before it was fairly considered.<ref name=":0" /> 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."{{sfnp|Tennant|2009}}{{sfnp|Hansen|1995}} 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. [[File:Comparison of Lander Sizes - Direct vs LOR.gif|thumb|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, 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 {{convert|50|ft|m|adj=on}} diameter [[Nova (rocket)|Nova rocket]], compared to the {{convert|33|ft|m|adj=on}} 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: <blockquote>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.</blockquote> The first major group to change its opinion in favor of LOR was [[Robert R. Gilruth|Robert Gilruth]]'s Space Task Group, which was still located at Langley but was soon to move to Houston as the [[Lyndon B. Johnson Space Center|Manned Spacecraft Center]]. The second to come over was [[Wernher von Braun]]'s 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 E. Webb|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.{{sfnp|Witkin|1962}} The decision was officially announced at a press conference on July 11, 1962.{{sfnp|NASA|1962|p=1}} President Kennedy's science adviser, [[Jerome Wiesner]], remained firmly opposed to LOR.{{sfnp|Nelson|2009|pp=209โ210}}<ref name=":0">{{Cite web|url=https://www.nasa.gov/centers/langley/news/factsheets/Rendezvous.html|title=The Rendezvous That Was Almost Missed: Lunar Orbit Rendezvous and the Apollo Program โ NASA|date=December 1992|website=www.nasa.gov|language=en|id=Fact Sheet NF175|access-date=2017-03-20|archive-date=2020-12-23|archive-url=https://web.archive.org/web/20201223201720/https://www.nasa.gov/centers/langley/news/factsheets/Rendezvous.html|url-status=dead}}<!-- This NASA Facts was prepared by the NASA Langley Research Center Office of Public Affairs, with the assistance of Dr. James R. Hansen, author of ''Engineer in Charge: A History of the Langley Aeronautical Laboratory, 1917โ1958'' --></ref>
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