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December 1992

The Rendezvous That Was Almost Missed:
Lunar Orbit Rendezvous and the Apollo Program

In the opinion of many space historians, NASA Langley's most important contribution to the Apollo Program was its development of the lunar-orbit rendezvous (LOR) concept. The brainchild of a few true believers at Langley, LOR's basic premise was to fire an assembly of three spacecraft into Earth orbit on top of a single powerful rocket.
Lunar module during rendezvous


Pictured is the Apollo lunar module during rendezvous in lunar orbit with the command module. If rendezvous around the moon failed, the astronauts would have been too far away to have been saved. The large dark-colored area in the background is Smith's Sea. The Earth rises above the lunar horizon.

Apollo 11 crew portrait

Astronauts Edwin "Buzz" Aldrin, Neil A. Armstrong and Michael Collins after their selection to become the prime crew of the Apollo 11 landing mission.

More than twenty years have passed since July 20, 1969, when the lunar module "Eagle" with Apollo 11 astronauts Neil Armstrong and Buzz Aldrin aboard gingerly made its way down to the Sea of Tranquility, landing men on the moon for the first time.

Thousands of people and organizations in many different places played key roles in this "giant leap for mankind." As President Kennedy stated in the May 1961 speech to Congress in which he announced the nation's commitment to the lunar challenge, "It will not be one man going to the moon-it will be an entire nation. For all of us must work to put him there."

One place that was fortunate to participate more than a little in the achievement of the lunar objective was the NASA Langley Research Center in Hampton, Va., the nation's oldest civilian aeronautics laboratory and home of the Space Task Group that conceived and directed Project Mercury, America's first man-in-space program.

NASA Langley helped to establish many of the basic fundamentals and mission concepts central to the success of the Apollo program. In the laboratory's unique complex of wind tunnels, researchers studied the aerodynamic integrity of the Saturn-Apollo launch configuration and the problem of aerodynamic heating during the reentry of the Apollo command module into the Earth's atmosphere. Langley staff members and test facilities also played a major role in the training programs necessary to prepare NASA's astronauts for landing on the moon and moving around on its surface.

In the opinion of many space historians, however, Langley's most important contribution to Apollo was its development of the lunar-orbit rendezvous concept.

President John F. Kennedy's decision in 1961 to land a man on the moon "before the decade is out" meant that NASA had to move quickly to find the best method of accomplishing the journey. NASA gave serious consideration to three options: Initially, direct ascent; then, Earth-orbit rendezvous (EOR), and, finally, a darkhorse candidate, lunar-orbit rendezvous (LOR).

Direct ascent was basically the method that had been pictured in science fiction novels and Hollywood movies. A massive rocket the size of a battleship would be fired directly to the moon, land and then blast off for home directly from the lunar surface. The trip would be like that of a chartered bus, moving from point A to point B and back to A again in one brute of a vehicle.

Strong feelings existed within NASA in favor of direct ascent, largely because it meant the development of a proposed giant booster named the Nova. After the engineers made their calculations, however, NASA realized that any single big rocket that had to carry and lift all the fuel necessary for leaving the Earth's gravity, braking against the moon's gravity as well as leaving it, and braking back down into the Earth's gravity again, was clearly not a realistic option-especially if the mission was to be accomplished anywhere close to President Kennedy's timetable. The development of a rocket that mammoth would just take too long, and the expense would be enormous.

Saturn-Apollo wind tunnel testing

Extensive research into the aerodynamic forces affecting the Saturn-Apollo launch configuration was performed in Langley wind tunnel. Here, researchers study the effects of wind on the Saturn I and escape tower.

The demise of direct ascent led to a scrupulous evaluation of the second option: Earth-orbit rendezvous. The main idea of EOR was to launch two pieces into space independently using advanced Saturn rockets that were then in development; have the two pieces rendezvous and dock in Earth orbit; assemble, fuel, and detach a lunar mission vehicle from the modules that had joined up; and then proceed with that bolstered ship, exactly as in the direct flight mode, to the moon and back to Earth orbit. The advantage of EOR was that it required a pair of less powerful rockets that were already nearing the end of their development.

EOR enjoyed strong support inside of NASA, especially among those who recognized that selection of EOR as the mode for the Apollo mission would require the virtual construction of a space station, a platform in Earth orbit that could have many other uses, scientific and otherwise, beyond Apollo. For this reason, space station advocates like Dr. Wernher Von Braun and his associates at NASA's Marshall Space Flight Center in Huntsville, Alabama, favored EOR.

In the end NASA selected neither of the first two options: instead, it selected the third: lunar-orbit rendezvous.

The brainchild of a few true believers at the Langley Research Center who had been experimenting with the idea since 1959, the basic premise of LOR was to fire an assembly of three spacecraft into Earth orbit on top of a single powerful (three-stage) rocket.

John Houbolt explains LOR

Dr. John C. Houbolt explains the lunar orbit rendezvous concept that, in the opinion of many historians, was chief among the reasons why the U.S., in less than a decade, managed humankind's first extraterrestrial excursions.

This assembly included: One, a mother ship, or command module; two, a service module containing the fuel cells, attitude control system and main propulsion system; and three, a small lunar lander or excursion module. Once in Earth orbit, the last stage of the rocket would fire, boosting the Apollo spacecraft with its crew of three men in to its flight trajectory to the moon. Reaching lunar orbit, two of the crew members would don space suits and climb into the lunar excursion module (LEM), detach it from the mother ship, and take it down to the lunar surface. The third crew member would remain in the command module, maintaining a lonely vigil in lunar orbit. If all went well, the top half of the LEM would rocket back up, using the ascent engine provided, and re-dock with the command module. The lander would then be discarded into the vast darkness of space or crashed onto the moon (as was done in later Apollo missions for seismic experiments), and the three astronauts in their command ship would head for home.

Direct ascent, EOR, LOR diagrams

President John F. Kennedy's decision in 1961 to land a man on the moon 'before the decade is out' meant that NASA had to move quickly to find the best method of accomplishing the journey. NASA gave serious consideration to three options: initially, direct ascent; then Earth-orbit rendezvous (EOR); and, finally, a darkhorse candidate, lunar-orbit rendezvous (LOR).

Although the basics of the LOR concept had been expressed as early as 1923 by German rocket pioneer Herman Oberth, no one had recognized the fundamental significance of LOR until two separate groups of Langley researchers in 1959, not long after Sputnik and the creation of NASA, quietly began to think about the potential of LOR for the budding American space program.

One of these groups was the Lunar Mission Steering Group headed by Clinton E. Brown, head of the Theoretical Mechanics Division. The other was the Rendezvous Committee headed by Dr. John C. Houbolt, then the assistant chief of the Dynamics Load Division. Brainstorming by these two Langley groups, done at first independently, led to an intensive analysis of what were then thought to be two distinct subjects: one, the mechanics of a moon trip; and, two, the role of rendezvous in the operations of an Earth-orbiting space station. The idea of putting the two analyses together then led a few creative minds within the Langley study groups to consider the advantages of LOR for a manned lunar mission.

Apollo 11 launch

The basic premise of LOR was to fire an assembly of three spacecraft into Earth orbit on top of a single powerful rocket (the Saturn V). With the Apollo spacecraft, the Saturn V stood 363 feet tall. Pictured is the launch of Apollo 11, the first mission to land men on the moon, on July 16, 1969.

The first of these studies, a very brief paper by William H. Michael, Jr., examined the benefits of "parking" the Earth-return propulsion portion of a spacecraft in orbit around the moon during a landing mission. The main benefit, according to Michael's unpublished 1959 paper, was the weight advantage of a small lunar lander needing less fuel. The chief problems were the "complications involved in requiring a rendezvous with the components left in the parking orbit."

In December 1960, after different LOR mission concepts had been formulated, several Langley researchers, including Ralph W. Stone, Clinton E. Brown, John D. Bird, Max C. Kurbjun and Houbolt, made formal presentations on their concepts to the incoming associate administrator of NASA, Dr. Robert C. Seamans. Although Seamans seemed sufficiently impressed, the LOR concept was to remain something of an orphan within the NASA family at every place except Langley for some time to come.

Stage-by-stage lunar mission profile

A stage-by-stage lunar mission profile.

Twenty months later, on July 11, 1962, after much technical debate and in-fighting, Seamans and NASA Administrator James E. Webb announced during a press conference at NASA Headquarters in Washington, D.C., that lunar-orbit rendezvous had been selected as the primary mission mode for the initial manned moon landing. Considering the strong opposition to LOR during NASA's intensive evaluation of possible mission modes for Apollo, the choice seemed quite unlikely.

Illustration of Apollo boost toward Moon

Once in Earth orbit, the last stage of the Saturn rocket fires, boosting the Apollo spacecraft and its three-man crew into its flight trajectory to the moon.

Illustration of spacecraft docked in lunar orbit

Once in lunar orbit, two of the crew members donned spacesuits and climbed into the lunar excursion module, detached it from the mother ship and "flew" it down to the lunar surface.

When Langley engineers first suggested the concept of lunar-orbit rendezvous, NASA had rejected it out of hand for being too complicated and risky. If rendezvous had to be part of Project Apollo, critics of LOR felt that it should be done only in Earth orbit. if that rendezvous failed, the threatened astronauts could be brought back home simply by allowing the orbit of their spacecraft to deteriorate. But, if a rendezvous around the moon failed, the astronauts would be too far away to be saved. Nothing could be done.

In retrospect, we know that LOR enjoyed several advantages over the other two options. It required less fuel, only half the payload, and less brand new technology than the other methods; it did not require the monstrous Nova rocket; and it called for only one launch from Earth whereas EOR required two. Only the small, lightweight lunar module, not the entire spacecraft, would have to land on the moon. This was perhaps LOR's major advantage. Because the lander was to be discarded after use and would not need return to Earth, NASA could tailor the design of the LEM for maneuvering flight in the lunar environment and for a soft lunar landing. In fact, the beauty of LOR was that it meant that NASA could tailor all of the modules of the Apollo spacecraft independently.

Buzz Aldrin descends LM ladder

The second man on the moon, Buzz Aldrin, descends from the lunar module on July 20, 1969. He and astronaut Neil Armstrong spent two hours and 20 minutes walking on the moon. The small, lightweight Lunar Module was a major advantage of the LOR concept because it did not need to be returned to Earth.

But in 1962 all these advantages were theoretical. On the other hand, the fear that American astronauts might be left in an orbiting coffin was quite real. It was a specter that haunted the dreams of those responsible for the Apollo program and one that made objective evaluation of the lunar-orbit rendezvous concept by NASA unusually difficult.

Illustration of ascent from lunar surface

The lunar module ascent stage (upper portion) is shown using its ascent engine to rocket back into lunar orbit and rendezvous with the Command Module (still orbiting the moon). Success depended on Langley's ability to train the astronauts to master the techniques of landing the lunar module on the lunar surface and returning the ascent stage to orbit to dock with the mother ship.

In late 1961 and early 1962 NASA convened a number of internal task forces to help in the selection of the mission mode for Apollo. One of these committees (the Lundin Committee) evaluated the option of direct ascent and another (the Heaton Committee) investigated the feasibility of Earth-orbit rendezvous. But there was no committee to look into LOR. Only one of these study groups (the Lundin Committee) wanted to hear anything about lunar-orbit rendezvous, and in its final report LOR finished a distant third behind EOR and direct ascent.

But at least one tenacious Langley engineer, Dr. John Houbolt, would not let the advantages of LOR be ignored. As a member of 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, 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.

LOR simulations at Langley

Lunar-orbit rendezvous required docking the lunar module with the command module in lunar orbit. 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, pictured.

In November 1961, Houbolt took the bold step of skipping proper channels and writing a private letter, nine pages long, directly to 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.

Illustration of return to Earth

Upon return to Earth, the command and service modules separate, leaving the command module to plunge into the Earth's atmosphere at a velocity of 25,000 mph.

In the following months, NASA did just that, and to the surprise of many both inside and outside the agency, the darkhorse 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 in Huntsville. 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 advisor, Jerome Wiesner, remained firmly opposed to LOR.

Diagram of lunar de-orbit

Sequences of lunar de-orbit to Earth, which Michael Collins called the "get us out of here, we don't want to be a permanent moon satellite" maneuver.

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, we may still have gotten to the moon, but almost certainly it would not have been accomplised by the end of the decade, as President Kennedy had wanted.

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, 1917 - 1958