Following President John F. Kennedy’s 25 May 1961 “moon speech” before a special joint session of Congress, NASA re-directed Project Apollo. Originally Apollo was to have been the next U.S. piloted spacecraft program after Mercury. In a plan drafted in 1959-1960, the Apollo spacecraft was described as an Earth-orbital vehicle designed for independent flight or ferry flights to an Earth-orbiting space station. It was hoped that eventually – by about 1970 – the Apollo program might lead to a circumlunar or lunar-orbital flight. Following JFK’s speech, however, the program’s goal became to land a man on the moon by 1970 and return him safely to the Earth. Almost immediately, the obvious question was asked: how would Apollo accomplish this epic feat?
As many as six lunar mission modes received consideration in 1961-1962, though two – Earth-Orbit Rendezvous (EOR) and Direct Ascent – emerged as early favorites. Each included several variants. In the EOR mode, one or more propulsion stages and a piloted moonship (and sometimes tankers for filling the propulsion stages) were brought together in Earth orbit. The propulsion stage or stages fired to place the moonship and its crew on course for the moon. In Direct Ascent, a single large rocket boosted the piloted moonship from Earth’s surface directly to the moon. After the piloted spacecraft was placed on course for the moon, the EOR and Direct Ascent missions were essentially identical.
The process by which the Apollo lunar mission mode decision was taken was complex and involved many players at NASA Headquarters and the NASA field centers, some of whom backed different modes at different times. Throughout the entire untidy 14-month-long process, however, engineer John Houbolt of the NASA Langley Research Center (LaRC) in Hampton, Virginia, staunchly advocated Lunar-Orbit Rendezvous (LOR). Houbolt did not, however, originate the LOR mode; it dates back at least to 1948, when H. E. Ross described it in London before a meeting of the British Interplanetary Society.
Houbolt gave briefings on LOR to several of the groups involved in the Apollo mode decision, including the Lundin Committee, the Ad Hoc Task Group for Study of Manned Lunar Landing by Rendezvous Techniques (the Heaton Committee), and the LaRC-based (but LaRC-independent) Space Task Group. The Large Launch Vehicle Planning Group (the Golovin Committee) requested a detailed written report on LOR after Houbolt briefed it in August 1961.
Houbolt and his colleagues at LaRC explained in their 31 October 1961 report to the Golovin Committee that LOR would differ from EOR and Direct Ascent in the nature of its spacecraft. As noted above, in EOR and Direct Ascent a single Apollo spacecraft would accomplish all phases of the lunar landing mission. It would bear its occupants from the Earth to the moon, land them on the moon, and then transport them back to Earth.
LOR, on the other hand, would see lunar mission phases divided between two distinct piloted vehicles. In the LOR scenario LaRC described, these spacecraft were the Apollo (which acted as a mothership) and the single-stage “Bug” lunar lander.
The three-man Apollo spacecraft, which in the LOR mode would come no nearer to the moon than lunar orbit, would include the mission’s Earth-atmosphere reentry vehicle and a pair of propulsion modules for performing major maneuvers. The Bug would detach from the Apollo in lunar orbit, descend toward the moon’s surface, land one or two astronauts gently on the moon, and then return to the Apollo mothership in lunar orbit. The crew would cast off the spent Bug, then the Apollo would return to Earth.
LaRC examined three Bug designs, which it dubbed “Shoestring,” “Economy,” and “Plush.” The first, with a dry (no propellants or other expendables) mass of only 1270 pounds, would land one man on the moon for only a brief period and return no more than 50 pounds of lunar samples to the waiting Apollo for transport to Earth. Of the three designs, the Shoestring Bug would come closest to fulfilling the strict letter of President Kennedy’s directive – that “a man” land on the moon.
The second design, the Economy Bug, would support two men on the moon for 24 hours. The lander would have a dry mass of 2234 pounds and could lift 100 pounds of rock samples to the orbiting Apollo.
The Plush Bug, the scientists’ favorite, would support two men on the moon for one week, providing them with adequate time to perform field geology at the lunar landing site. It would have a dry mass of 3957 pounds and could lift 150 pounds of samples to the orbiting Apollo.
According to Houbolt’s team, an LOR landing would be safer than either a Direct Ascent or EOR landing because the Bug would be designed only for that function – in other modes the landing function would be compromised by the need to take into account other functions, such as Earth atmosphere reentry. Houbolt also proposed a spacecraft configuration that would further enhance astronaut safety by including two Shoestring Bugs. If the first Shoestring Bug became trapped on the lunar surface, then the second Shoestring Bug would be used to mount a rescue.
As with the other proposed Apollo modes, many preparatory and precursor missions would lead up to LaRC’s LOR moon landing. LaRC’s proposed five-year “Master Flight Plan” would begin with 11 Ranger automated lunar rough-landing missions before October 1963. These would help engineers and scientists characterize the lunar surface so that they could design the Bug, lunar surface space suits, and surface exploration equipment. Meanwhile, NASA would fly four Mercury manned Earth-orbital missions, each completing 18 Earth orbits, to gather basic data on human performance in space (February-August 1963).
NASA would fly 15 Surveyor automated lunar soft-landing missions between August 1963 and March 1966. Meanwhile, back on Earth, the space agency would drop Apollo reentry vehicles from aircraft 20 times between September 1963 and June 1964 to test glide characteristics, parachutes, and land landing systems (the LaRC engineers assumed a landing on U.S. soil, not a splashdown at sea).
Astronauts would practice rendezvous and docking on board maneuverable Mercury Mark II spacecraft launched into Earth orbit atop modified Titan missiles. The two-seater Mercury Mark II, renamed Gemini in January 1962, would dock with separately launched unmanned Agena target vehicles during five missions spanning from October 1963 to June 1964.
Six manned Mercury Mark II missions flown between August 1964 and June 1965 would see astronauts practice docking with Bug landers in Earth orbit. The Bugs and Mercury Mark IIs would be launched together on Saturn C-1 rockets. Saturn C-1 would be the first NASA launch vehicle designed specifically for piloted spaceflight. The Mercury Mark II/Bug Saturn C-1 missions would overlap with eight Saturn C-1-launched Apollo suborbital and Earth-orbital flight tests (September 1964-August 1965). A pair of Saturn C-1-launched manned Apollo/Bug test missions in Earth orbit (September-October 1965) would follow, laying the groundwork for four manned Apollo high-elliptical Earth-orbit and circumlunar/lunar-orbit missions between November 1965 and February 1966.
LaRC suggested that the high-elliptical and circumlunar/lunar-orbit missions, each of which would leave Earth on a Saturn C-3 or C-4 rocket, might be converted into manned lunar landing attempts if necessary; for example, if the Soviet Union were believed to be on the verge of launching its first piloted lunar landing attempt. Assuming, however, that they were not turned into landing missions, then the first four manned lunar landing attempts of the Apollo Program would take place between March and June 1966.
LaRC’s LOR missions would begin with launch on a Saturn C-3 or C-4 from Cape Canaveral, Florida. The LaRC team noted that a direct flight to the moon from a fixed site on the Earth’s surface could begin only during a short period each month, particularly if the mission sought to land at a specific lunar landing site. To circumvent this limitation, LaRC’s Apollo mothership/Bug lander stack would enter low-Earth parking orbit before setting out for the moon. In effect, this would give the mission a mobile launch site, providing planners with “complete freedom” in selecting lunar landing mission start time.
The first and largest of the two Apollo propulsion modules would push the Apollo mothership with its small propulsion module and a Bug lander out of Earth orbit, then would separate. LaRC opted for an Earth-moon transfer lasting from 2.5 to three days. It noted that the mission could follow “a free return circumlunar trajectory, which would be utilized if a. . .malfunction prohibited going into lunar orbit.” That is, in the event of propulsion failure, the Apollo/Bug stack could swing around the moon and return directly to Earth without additional propulsion after it left Earth orbit.
Assuming, however, that all worked as planned, the Apollo mothership’s small propulsion module would fire over the far side of the moon to slow itself, the Apollo, and the Bug so that the moon’s gravity could capture them into orbit. LaRC recommended a 50-mile-high circular orbit over the moon’s equator, “especially if the exploration time on the lunar surface is to be of the order of a week.” Spacecraft in such an orbit would pass over all sites on the moon’s equator every two hours. This meant that every two hours the Bug would have the opportunity to descend to a specific equatorial target landing site or lift off from that site and rendezvous with the orbiting Apollo.
If, on the other hand, the Apollo entered an orbit inclined relative to the lunar equator so that the Bug could descend to a non-equatorial landing site, the moon’s slow rotation would gradually take the site out of the Apollo’s orbital plane (that is, the mothership would no longer pass over the Bug landing site). The Bug or the Apollo (or both) would then need to expend propellants to perform a plane-change maneuver to match orbits before rendezvous and docking. LaRC noted, however, that the plane change necessary after a one-day stay at a non-equatorial site would be “insignificant.”
LaRC proposed that the moon landing occur close to local lunar midnight under a full Earth, “thus avoiding the bright glare and black shadows of the sunlit side.” Prior to Bug separation, the crew would examine the moon from orbit to make their final landing site selection. One or two astronauts would then enter the Bug, undock, and fire its engines briefly to move away from the Apollo. This would prevent the Apollo from being enveloped in the Bug’s engine plume when the more powerful “lunar letdown” maneuver began. The Bug’s engines – LaRC recommended two for redundancy and improved maneuvering – would be capable of being throttled and gimbaled (that is, pivoted for steering).
Halfway around the moon from the selected landing site, the Bug pilot would fire the engines to slow the lander by 60 feet per second. This would nudge its orbit so that it would intersect the surface at the landing site. The Bug would then coast for an hour, steadily losing altitude. Bug and Apollo would remain in visual and radio contact throughout the descent.
About 100 miles from the landing site, the Bug pilot would fire the twin engines to reduce speed, then would commence landing maneuvers. The Bug would gradually tip so that it would reach the landing site with its engines and landing leg footpads pointed down. The pilot would then have one minute of hover time to choose a safe spot for final letdown. Since it was a single-stage vehicle, LaRC’s Bug would use the same engines for descent and ascent. If a landing proved impossible, the pilot could throttle up the engines and abort back to orbit.
Following a period of surface activity, the astronaut or astronauts would prepare the Bug for liftoff. Just before liftoff, the orbiting Apollo mothership would climb into view above the Bug’s horizon. The Bug pilot would spot it visually and with radar, then would ignite the Bug’s engines. The lander would climb 10 miles high at 0.5 gravities of acceleration, then would coast along an arcing course for up to 33 minutes. Meanwhile, the Apollo spacecraft would orbit over the landing site and pull ahead of the Bug. The lander would thus approach its mothership from below and behind.
A gyroscope-equipped “inertial attitude reference” would provide guidance data to the Bug pilot; if electronic aids failed, however, he could complete rendezvous and docking using visual cues. The Bug pilot would start homing on the Apollo’s light beacon about 250 feet out. Docking would take place over the lunar night hemisphere to avoid sun glare and improve beacon visibility. After docking, the astronaut or astronauts would transfer to the Apollo and cast off the Bug.
The Apollo mothership’s small propulsion module would ignite for a second time to push it out of lunar orbit, then would be cast off. LaRC reported that studies of optimum Earth-return trajectories for accomplishing land landings in the U.S. were in progress.
NASA formally adopted LOR in July 1962. In preparation for the first manned moon landing, the U.S. civilian space agency successfully carried out three robotic Ranger imaging missions, five robotic Surveyor soft-landing missions, five robotic Lunar Orbiter missions, ten manned Gemini missions (most of which featured some form of rendezvous practice), two manned Apollo Earth-orbital missions (Apollos 7 and 9), and two manned Apollo lunar-orbital missions (Apollos 8 and 10).
The Apollo spacecraft became known as the Command and Service Module (CSM) and the Bug was designated the Lunar Excursion Module (LEM – later changed to Lunar Module, or LM). Grumman in Bethpage, Long Island, New York won the LM contract with a design featuring separate descent and ascent stages.
Early LM designs (image at top of post) took as their model small helicopters, so featured curved surfaces and large windows; as the LM design evolved, however, it became faceted and asymmetrical, with small triangular windows. These changes were made to reduce the LM’s mass; large, curved, multi-pane glass windows would, it was found, be heavier square centimeter for square centimeter than the LM’s metal skin.
No LM was as light as the heaviest LaRC Bug – the Apollo 11 LM Eagle had a dry mass of 9271 pounds, or about 2.5 times the dry mass of the Plush Bug. It is unlikely that Houbolt and his colleagues knowingly low-balled their mass estimates; most Apollo systems ended up being heavier than originally estimated because no one had built piloted lunar spacecraft before.
The LM first reached space atop a Saturn IB (as the Saturn C-1 became known) during the unmanned Apollo 5 mission in January 1968. LOR lunar missions launched atop three-stage Saturn V rockets, which were more powerful than the Saturn C-3 and Saturn C-4 rockets invoked in LaRC’s report to the Golovin Committee. Astronauts used LOR to land successfully on the moon six times between July 1969 and December 1972 (Apollos 11, 12, 14, 15, 16, and 17).
In April 1970, the LM Aquarius served as a lifeboat for Apollo 13 astronauts James Lovell, Fred Haise, and Jack Swigert after an oxygen tank explosion crippled the CSM Odyssey en route to the moon. The mothership’s propulsion, electrical power generation, and life support systems were all compromised. Its crew used Aquarius‘s single descent engine to change course to a free-return path and speed their docked CSM and LM spacecraft back to Earth. Had an EOR or Direct-Ascent Apollo spacecraft suffered a similar mishap, its crew would almost certainly have perished through asphyxiation, collision with the moon (if moon-bound), or uncontrolled Earth-atmosphere reentry (if Earth-bound).
Manned Lunar Landing Through Use of Lunar-Orbit Rendezvous, NASA Langley Research Center, 31 October 1961.
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