23 July 2017

Plush Bug, Economy Bug, Shoestring Bug (1961)

President John F. Kennedy, flanked by Vice President Lyndon Baines Johnson (right) and NASA officials, addresses employees of the Manned Spacecraft Center (MSC) in Houston, Texas, on 12 September 1962. The Apollo lunar lander mockup in this image is a North American Aviation design. Image credit: John F. Kennedy Library/NASA
Following President John F. Kennedy's 25 May 1961 "moon speech" before a joint session of Congress, NASA re-directed Project Apollo. When first conceived, 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.

NASA planners hoped that eventually - by about 1970 - Apollo 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, many asked the obvious question: 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. Both modes included several variants.

In the EOR mode, one or more propulsion stages and a piloted moonship (and sometimes tankers for filling the propulsion stages with propellants) were brought together in Earth orbit. The propulsion stage or stages were then 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 mission modes would be 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 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.

John Houbolt at the chalkboard. He points toward an "LEV" (Lunar Excursion Vehicle); this would become the Apollo Lunar Module. Image credit: NASA
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 otherwise 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 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 orbiting Apollo for transport to Earth. Of the three designs, the Shoestring Bug would come closest to fulfilling the strict letter of President Kennedy's mandate - 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's dry mass would total 2234 pounds; it would transport up to 100 pounds of rock samples from the moon's surface 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. Plush Bug dry mass would total 3957 pounds; it 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: an Apollo with two Shoestring Bugs. If the first Shoestring Bug became trapped on the lunar surface, then the second would be used to mount a rescue.

John C. Houbolt's concept drawing of a Lunar Orbit Rendezvous spacecraft with twin Shoestring Bugs. The stack depicted would reach Earth parking orbit atop a Saturn C-3 rocket. Weights (in pounds) are for "wet" hardware (that is, loaded with propellants and expendables). Image credit: NASA 
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. The missions, flown between February and August 1963, would enable doctors to gather basic data on human performance in space.

NASA would launch 15 Surveyor automated soft-landing missions to the moon 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).

Astronauts would practice rendezvous and docking using maneuverable Mercury Mark II spacecraft launched into Earth orbit atop modified Titan missiles. The two-seater Mercury Mark II, which was renamed Gemini in January 1962, would dock with separately launched unmanned Agena upper stage target vehicles during five missions spanning October 1963 to June 1964.

Six manned Mercury Mark II missions between August 1964 and June 1965 would see astronauts practice docking with Bug landers in Earth orbit. For each mission, the Bug and the Mercury Mark II would be launched together on a Saturn C-1 rocket. Saturn C-1 was envisioned as 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 spanning September 1964 through August 1965. A pair of Saturn C-1-launched manned Apollo/Bug test missions in Earth orbit would follow in September-October 1965, laying the groundwork for four piloted 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 occur 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 if it 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. This would in effect 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 burn all of its propellants to 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.

LaRC noted that the mission could follow "a free return circumlunar trajectory" that would enable the Apollo/Bug stack to swing around the moon and return directly to Earth without additional propulsion. This would come in handy if the Apollo/Bug stack suffered a propulsion malfunction after Earth-orbit departure.

Assuming, however, that all occurred as planned, the astronauts in the Apollo/Bug stack would turn the small propulsion module toward its direction of motion as it looped behind the moon. Over the center of the lunar Farside hemisphere, they would ignite the module so that the moon's gravity could capture the Apollo/Bug stack into lunar orbit.

LaRC recommended a 50-mile-high circular orbit over the moon's equator, "especially if the exploration time on the lunar surface [was] to be of the order of a week." A spacecraft in such an orbit would pass over all points on the moon's equator every two hours. This meant that every two hours the Bug would have an opportunity to descend to a specific equatorial target landing site or to lift off from an equatorial site and perform a 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 could take place. 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 maneuverability - would be capable of being throttled and gimbaled (that is, pivoted for steering).

Halfway around the moon from the selected landing site - that is, out of view of Earth, over the Farside - 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.

All of LaRC's Bug designs would employ a single pair of engines for descent and ascent. If landing proved impossible, the pilot could throttle up the engines and abort back to orbit. Assuming that all went as planned, the Bug would gently settle on the surface at the target landing site as its pilot throttled back to zero.

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.

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 blinking 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. 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). On 7 November 1962, NASA awarded the LEM contract to Grumman Aircraft Engineering Corporation in Bethpage, Long Island, New York. The Grumman design featured separate descent and ascent stages.

Grumman's winning November 1962 LEM concept featured five landing legs, two docking ports, and large curved glass windows. The design would evolve rapidly as the company and NASA confronted the challenges of landing a man on the moon. Image credit: NASA
Early LM designs took as their design inspiration 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 reduced the LM's mass; large, curved, multi-pane glass windows were, it was found, 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 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; rather, most Apollo systems ended up heavier than at first estimated because no one had built piloted lunar spacecraft before.

20 July 1969: The Apollo 11 LM Eagle pirouetted in lunar orbit so that Command Module Pilot Michael Collins, alone on board the Command Module Columbia, could inspect and photograph it. At Eagle's controls were Apollo 11 Commander Neil Armstrong and Lunar Module Pilot Edwin Aldrin. Less than two hours after Collins took this photograph, Eagle touched down on the moon's Sea of Tranquility, an ancient dusty plain pocked by impact craters. Image credit: NASA
In preparation for the first manned moon landing, the NASA carried out robotic Ranger, Surveyor, and Lunar Orbiter missions. They mainly focused on equatorial and near-equatorial areas of the lunar Nearside hemisphere. Three Rangers imaged small areas close-up as the fell toward destructive impact, five Surveyors soft-landed and imaged, analyzed, and dug into the lunar surface, and five Lunar Orbiters imaged large regions and small candidate LM landing sites. Meanwhile, 10 two-man Gemini missions in Earth orbit gave astronauts rendezvous and spacewalk practice.

The LM reached space for the first time atop a Saturn IB (an uprated Saturn C-1 variant) during the unmanned Apollo 5 mission in January 1968. Apollo 7, also launched on a Saturn IB, was a piloted CSM test in Earth orbit.

Apollo missions 8 through 17 each launched on a three-stage Saturn V rocket; originally designated Saturn C-5, the Saturn V was more powerful than the Saturn C-3 and Saturn C-4 rockets described in LaRC's report to the Golovin Committee, which were never built. Apollo 8 was a CSM-only piloted lunar orbital flight. Apollo 9 was a piloted test of the CSM and LM in Earth orbit. Apollo 10, which included a CSM and an LM, was a lunar-orbital dress rehearsal for the first lunar landing attempt scheduled for Apollo 11. Astronauts used the LOR mode 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 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. Odyssey's propulsion, electricity generation, and life support systems were all compromised. Fortunately, they had an undamaged spacecraft at their disposal.

The Apollo 13 crew used LM Aquarius's single descent engine and navigation aids to change their course to a free-return path and speed their docked CSM and LM spacecraft back to Earth. They relied on the LM's life support system to provide oxygen and scrub exhaled carbon dioxide from their cabin air. 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 Via Rendezvous," NASA Langley Research Center, presentation materials, 19 April 1961

Manned Lunar Landing Through Use of Lunar-Orbit Rendezvous, NASA Langley Research Center, 31 October 1961

The Apollo Spacecraft: A Chronology, NASA SP-4009, The NASA Historical Series, I. Ertel and M. Morse, Vol. I, pp. 81-202

Enchanted Rendezvous: John C. Houbolt and the Genesis of the Lunar-Orbit Rendezvous Concept, Monographs in Aerospace History #4, J. Hansen, NASA History Office, December 1995

More Information

Starfish and Apollo (1962)

The Spacewalks That Never Were: Gemini Extravehicular Planning Group (1965)

If an Apollo Lunar Module Crashed on the Moon, Could NASA Investigate the Cause? (1967)

"Still Under Active Consideration": Five Proposed Earth-Orbital Apollo Missions for the 1970s (1971)


  1. In the drawing the functions of lunar orbit insertion (LOI) and earth transfer insertion are performed by two distinct propulsion stages. This would open the possibility to use the LOI stage for orbit ejection and deceleration of the lunar lander similar to the scheme intended by the Soviets. Was this option ever studied by Houbolt's team?

    1. Buck:

      That's a really good question. Houbolt's group and other groups in NASA looked at all kinds of variations on the LOR theme. As evidence of this - the LOR spacecraft design in the drawing is not the same as the one described in the LOR report. So, it's possible, though I didn't run across anything like the N-1/L3 lunar landing scheme among the LaRC plans.

      One notion, quite speculative - the STG folks - the nucleus around which MSC formed - messed around with a "crasher" design in this period as part of their Direct Ascent studies. That was similar to the N-1/L3 scheme. They wanted to make the lunar lander shorter so it'd be more stable on the lunar surface and so the astronaut would not have to descend/climb a 50-foot ladder. They weren't too keen on it, so it's possible that their disdain could have influenced Houbolt & Co. not to look into anything like it. Again, that's speculative - I'm quite likely to see a comment here in a few hours pointing to something exactly like the N-1/L3 scheme. :-)



I like hearing from my readers. No rules except the obvious ones - please keep it civil and on topic.