Lunar orbit rendezvous (LOR) 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.[1][2]
LOR was used by the Apollo missions for human spaceflight to the Moon.
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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[3] 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.
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 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).
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 Simulator, 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.
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."[4][5]
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. 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.
This article incorporates public domain material from the United States Government document "Enchanted Rendezvous: John C. Houbolt and the Genesis of the Lunar-Orbit Rendevous Concept". By James R. Hansen. Dec 1995. Monographs in Aerospace History Series #4. NASA-TM-111236.