Lunar orbit rendezvous
Lunar orbit rendezvous (LOR) is a key concept for landing humans on the Moon and returning them to Earth and was first utilized for the Project Apollo missions in the 1960s and 1970s. In a LOR mission, a main spacecraft (such as the Apollo CSM) and a smaller lunar lander (such as the Apollo LM) travel together into 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, 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 Ukrainian and Soviet engineer, who calculated that LOR was the most economical way of landing a human on the Moon. When NASA was looking to land astronauts on the Moon by the late 1960s, the idea was controversial, and after NASA's administrator, James Webb announced that Apollo would utilize this method, he was publicly criticized by President Kennedy's National Science Policy Advisor, Jerome Wiesner. As history has shown, the method worked, and allowed NASA to use only one Saturn V per lunar landing mission, something other landing options did not offer.
A robotic lunar orbit rendezvous is planned for use by the Chinese Chang'e 5 lunar sample return mission scheduled in 2017. The American Golden Spike Company plans on landing humans on the Moon in 2020 using LOR.
Apollo Mission modes
The LOR mission "mode" was one of three modes considered for Apollo.
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, though also promoted by Jim Chamberlin and Owen Maynard at the Space Task Group in 1960. 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.
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 required for the lunar lander.
Disadvantages include the lunar lander needing a separate life-support system, and requiring two sets of engines and docking mechanisms: one set on the lunar lander and another on the translunar ferry craft.
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 oxygen tank 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).
Lunar-orbit rendezvous required docking the lunar module with the command module in lunar orbit in order for the astronauts who had landed on the Moon to transfer (with their collected lunar material) to the command module for the return journey to Earth. Astronauts practiced the complex task of separating and uniting spacecraft to master docking techniques with Langley's Rendezvous and Docking Simulator, today a National Historic Landmark.
An Earth orbit rendezvous was considered far less risky because it avoided the two riskiest elements of LOR: 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, no astronauts would return. With EOR, if the rendezvous in Earth orbit failed, the astronauts would be able to return to Earth : either with a powered re-entry or, if the engine had failed, when the orbit would naturally decay.
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.
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 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. 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. The decision was officially announced at a press conference on July 11, 1962. President Kennedy's science adviser, Jerome Wiesner, remained firmly opposed to LOR.
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