Rendezvous Concept for Circumlunar Gemini (1965)

Graphic representation of a circumlunar journey. Image credit: Martin Marietta Corporation

On 18 August 1965, U.S. Representative Olin Teague of Texas, chair of the House Subcommittee on NASA Oversight and an ally of President Lyndon Baines Johnson, wrote a letter to NASA Administrator James Webb. "Much discussion is now taking place," the veteran Congressman wrote, "on the possibility of a circumlunar flight using a Gemini system prior to the Apollo lunar landing." Teague asked Webb for his opinion of the desirability of such a mission.

It was not the first time a piloted Gemini flight around the Moon on a free-return path — that is, without injection into lunar orbit — had been discussed. In late 1961, when Gemini was still called "Mercury Mark II" and NASA had yet to approve it as a formal program, a circumlunar flight had been proposed as one of its program objectives. The program was approved on 7 December 1961 and named Gemini the following month, but without the circumlunar flight. 

Gemini was envisioned as an experience-building bridge between relatively simple one-man Mercury flights and complex Apollo lunar landing missions. Use of rendezvous to accomplish President John F. Kennedy's objective of a man on the Moon by 1970 already seemed likely in early 1962. Rendezvous might take place in Earth orbit, lunar orbit, or both, and its challenges seemed daunting to many NASA planners. Gemini thus became seen as a rendezvous demonstrator.

In the spring of 1964, NASA Associate Administrator for Manned Space Flight George Mueller sought to pay McDonnell Aircraft Corporation, makers of the Mercury and Gemini spacecraft, to study a Gemini circumlunar mission. He saw the contractor study as an insurance policy; if Apollo suffered a major technical setback, or if the Russians looked set to carry out a piloted lunar flight, then circumlunar Gemini might salvage U.S. prestige. On 8 June 1964, however, NASA Associate Administrator Robert Seamans informed Mueller that Webb would authorize only in-house studies; NASA would not signal to its contractors a possible expansion or re-direction of Gemini.

Circumlunar Gemini came to Teague's attention 14 months later because astronaut Charles "Pete" Conrad, slated to serve as Gemini V pilot, was much taken with the concept. His enthusiasm led the Houston, Texas-based NASA Manned Spacecraft Center (MSC), home base of the astronauts, to study a circumlunar Gemini mission in collaboration with McDonnell and another major Gemini contractor — Martin Marietta Corporation, which manufactured the Gemini launch vehicle, the Gemini-Titan. Martin Marietta produced a report on the joint study in July 1965. 

The report lacked an MSC contract number and the Headquarters ban on NASA funding for contractor studies of circumlunar Gemini remained in effect. The companies apparently donated their time and expertise. 

The circumlunar Gemini mission described in the July 1965 report was scheduled to take place in June 1967, assuming a program go-ahead in September 1965. Use of existing or near-term planned hardware "building blocks" with minimal alteration would make the tight schedule possible. The building blocks included the Gemini-Titan and its larger cousin, the Titan IIIC launch vehicle, a modified Titan IIIC transtage upper stage, and a modified Gemini spacecraft. 

The Gemini-Titan was a Titan II Intercontinental Ballistic Missile modified to carry the two-person Gemini spacecraft. Modifications aimed mainly at improving safety. Among these were addition of backup systems and a Malfunction Detection System (MDS) that enabled the crew to monitor launch vehicle performance during ascent to low-Earth orbit. The Gemini-Titan, which launched from Pad 19 at Cape Canaveral Air Force Station (CCAFS), Florida, measured about 10 feet (three meters) in diameter and stood 107.65 feet (32.8 meters) tall with a Gemini spacecraft on top.

Gemini III launch, 23 March 1965. Image credit: NASA

By July 1965, the Gemini-Titan had flown four times. The Gemini I mission (8 April 1964) saw the two-stage rocket launch a simplified Gemini spacecraft into low-Earth orbit. Ballast replaced many missing Gemini spacecraft systems to give it a realistic weight and mass distribution. The spacecraft reentered and burned up as planned on 12 April 1964. 

Gemini II (19 January 1965) was a suborbital Gemini-Titan flight which ended with the first Gemini spacecraft splashdown and recovery. The third Gemini-Titan launched Gemini III, the first piloted Gemini spacecraft, on 23 March 1965. Mercury veteran Gus Grissom and rookie astronaut John Young orbited Earth three times before splashing down in the Atlantic Ocean.

Gemini IV (3-7 June 1965) saw James McDivitt and Ed White use their Gemini-Titan rocket for something other than ascent to low-Earth orbit. They tried unsuccessfully to approach and fly formation with its second stage, expending much more propellant than expected and, it appeared, confirming that the rendezvous maneuvers required in the Apollo Lunar-Orbit Rendezvous mission plan posed a significant challenge.

The first Titan IIIC rocket to fly stands on Launch Pad 40 at Cape Canaveral Air Force Station, 23 May 1965. Image credit: U.S. Air Force

The Titan IIIC launch vehicle was new in July 1965; its successful first launch had taken place on 18 June 1965. The 137-foot-tall (41.75-meter-tall) U.S. Air Force rocket comprised four stages. Stage 0 was a pair of Solid Rocket Motors (SRMs) that ignited simultaneously at liftoff. Each was about 10 feet (three meters) in diameter and 85 feet (25.9 meters) tall. 

The Titan IIIC SRMs were attached to the sides of a two-stage core closely resembling the Gemini-Titan rocket. The core stages, which burned Aerozine 50 fuel and nitrogen tetroxide oxidizer, were designated Stage 1 and Stage 2. Stage 1 ignited 105 seconds after liftoff, just before Stage 0 separation. It included a thermal shield to protect its engine assembly during Stage 0 operation, attachment points for Stage 0, and strengthened structure. It measured 10 feet (three meters) in diameter and 71 feet (21.6 meters) tall. Stage 2, 10 feet (three meters) in diameter and 37 feet (11.27 meters) tall, included strengthened structure and an extended interstage adapter to accommodate Stage 3.

A weather-beaten Titan IIIC transtage with a conical payload fairing arrives at NASA Johnson Space Center (JSC) in 2016. The old upper stage, destined for analysis by NASA orbital debris scientists, was transferred to NASA JSC after it was spotted in the aircraft "boneyard" at Davis-Monthan Air Force Base in Tucson, Arizona. Image credit: NASA

Stage 3, the fourth stage of the Titan IIIC, was the transtage, a restartable upper stage for boosting payloads from low-Earth orbit to higher orbits, including geosynchronous orbits. It measured about 10 feet (three meters) in diameter and 15 feet (4.6 meters) tall. Immediately after Stage 2 shutdown, retro-rockets ignited on Stage 2 to slow it, and Stage 3 slid along rails within the Stage 2 interstage adapter to ensure smooth separation.

The Titan IIIC transtage, with a pair of 8000-pound-thrust engines burning Aerozine 50 fuel and nitrogen tetroxide oxidizer, formed the basis of the most heavily modified circumlunar Gemini building block, the Modified Transtage (also called Transtage 2). The Martin Marietta/McDonnell/NASA MSC team sought to trim its weight so that it could boost an 8000-pound (3630-kilogram) Gemini spacecraft out of low-Earth orbit on a circumlunar path. They did this in part by relying on the Stage 3 Transtage attitude control system, telemetry system, and batteries. Removing these from the Modified Transtage reduced its weight.

They also added a Target Docking Adapter (TDA) borrowed from the Gemini Agena Target Vehicle (GATV), which at the time of their study had yet to fly. The GATV was, as its name implies, based on the Agena upper stage; in addition to giving Gemini crews a rendezvous and docking target, it would provide auxiliary propulsion for large orbit changes.

Gemini VI viewed from Gemini VII, 16 December 1965. Image credit: NASA

Cutaway of a Gemini spacecraft. Image credit: NASA

The final building block was, of course, the Gemini spacecraft. It comprised the Reentry Module and Adapter Module. The latter included the Equipment Module and the Retro Module. 

The Reentry Module included a pressurized cockpit with forward-facing windows, a blunt nose housing parachutes, attitude control thrusters, and rendezvous equipment, and a heat shield to protect it during Earth atmosphere reentry. The Gemini heat shield would be made sturdier and thicker to withstand the high-speed atmosphere reentry at the end of the circumlunar mission.

The Retro Module included solid-propellant deorbit rockets; these would be retained during the circumlunar Gemini mission to enable abort late in Gemini-Titan ascent to low-Earth orbit and to permit emergency reentry in the event that the mission could not depart low-Earth orbit. The Equipment Module, the broadest part of the Adapter Module, included the Orbit Attitude and Maneuvering System (OAMS) propulsion system and electricity-producing fuel cells.

The circumlunar Gemini flight program would begin with a Titan IIIC-launched heat shield qualification test without a crew in early February 1967. The Stage 3 transtage with attached stripped-down 5000-pound (2270-kilogram) Gemini would slide free of the Stage 2 stage at an altitude of about 100 nautical miles (185 kilometers) about 700 nautical miles (1295 kilometers) downrange from CCAFS. 

The transtage engine would fire for a short time, then the transtage-Gemini combination would coast to an altitude of about 150 nautical miles (280 kilometers) about 1500 nautical miles (2800 kilometers) downrange of the launch site. The transtage would then ignite for a second time, lofting the Gemini to an altitude of about 160 nautical miles (295 kilometers) about 2500 nautical miles (4630 kilometers) downrange before pitching over to drive the Gemini into the atmosphere. 

Transtage burnout and Gemini separation would take place about 3800 nautical miles (7040 kilometers) downrange at an altitude of about 120 nautical miles (220 kilometers). The modified Gemini would cast off its two-part Adapter Module and turn so its beefed-up heat shield faced in its direction of motion. Reentry at lunar-return speed of 36,000 feet (10,970 meters) per second would begin at 65 nautical miles (120 kilometers) of altitude about 4300 nautical miles (7960 kilometers) downrange, over the Atlantic Ocean near the space tracking facilities on Ascension Island. Splashdown and Reentry Module recovery would occur about 4600 nautical miles (8520 kilometers) downrange of CCAFS.

An Earth-orbital dress-rehearsal for the circumlunar flight would follow in mid-April 1967. The mission would test the rapid-fire launch, rendezvous, docking, and low-Earth orbit departure procedure McDonnell, Martin Marietta, and NASA MSC had selected for their circumlunar mission.

The test mission would begin with a Titan IIIC with a Modified Transtage and a Gemini-Titan with a Gemini spacecraft with two astronauts on board poised for launch on their respective pads. NASA would count down the two launches simultaneously. The Titan IIIC, with a shorter countdown, would reach launch (T) minus 30 seconds, then would be placed in a countdown hold. The Gemini-Titan would, meanwhile, count down to T minus six minutes and also be placed in a hold. 

After thorough system checkouts, the Gemini-Titan countdown would resume; 90 seconds later, the Titan IIIC countdown would restart, with T minus zero and liftoff taking place as the Gemini-Titan countdown reached T minus four minutes. Four minutes later, the Gemini-Titan countdown would reach T minus zero and liftoff would take place.

If all went as planned, the Gemini spacecraft would inject into an orbit 100 nautical miles (185 kilometers) above the Earth and separate from its Gemini-Titan second stage very near the Titan IIIC transtage and attached Modified Transtage. Ideally, rendezvous would occur at the moment of Gemini injection into low-Earth orbit. Launch dispersions were to be expected, however. The Martin Marietta/McDonnell/NASA MSC team was confident, however, that the Gemini spacecraft could inject into low-Earth orbit with its rendezvous target in range of its nose-mounted rendezvous radar.

The Gemini spacecraft and Titan IIIC transtage/Modified Transtage combination would orbit Earth every 90 minutes. One orbit after launch, the Gemini would be close enough to its target to begin a leisurely two-orbit "closure & docking" phase. Its slow pace would, it was hoped, conserve OAMS propellants. 

At the end of the closure & docking phase, the crew would insert their spacecraft's nose into the TDA on the front of the Modified Transtage. An electrical umbilical protruding from the nose would link to a receptacle in the TDA, enabling the astronauts to monitor and control the Modified Transtage. An external display panel on the TDA would also provide the astronauts with information on Modified Transtage systems.

A look inside the shrouds reveals a Titan IIIC transtage and, above it, the conceptual Modified Transtage for the circumlunar Gemini mission. A = streamlined payload fairing; B = Target Docking Adapter (TDA); C = TDA transition structure; D = payload fairing separation plane; E = Modified Transtage; F = Modified Transtage separation plane; G = Titan IIIC transtage/Stage 3; H = Titan IIIC transtage/Stage 3 separation plane. Image credit: Martin Marietta

Gemini docked with Modified Transtage. A = Gemini spacecraft; B = Target Docking Adapter (TDA) support structure; C = external status display panel visible to Gemini crew; D = TDA docking cone; E = Gemini electrical umbilical and TDA receptacle; F = TDA transition structure; G = Modified Transtage. Image credit: Martin Marietta.

Events would then occur rapidly. As the Gemini/Modified Transtage/Titan IIIC transtage stack orbited into the proper position to begin flight to the Moon, the crew would fire explosive bolts, severing links between the two transtages, then would ignite OAMS thrusters to pull the Modified Transtage clear of the Titan IIIC transtage. This would cause propellants in the Modified Transtage to settle toward its engines, permitting ignition.

With that, the April 1967 test would complete its main objectives. The astronauts on board the Gemini spacecraft would not ignite the Modified Transtage engines; instead, they would soon separate from the Modified Transtage and return to Earth. When time came for the actual circumlunar flight to begin in June 1967, however, the crew on board the docked Gemini would ignite the twin Modified Transtage engines within five minutes of separation from the Titan IIIC transtage, beginning the Trans-Lunar Injection (TLI) maneuver.

The Modified Transtage engines would fire for six minutes and 40 seconds, expending 22,565 pounds (10,235 kilograms) of propellants. At the start of the TLI burn, the crew would feel acceleration equal to 0.6 Earth gravities. Because they would face the Modified Transtage, they would feel as though they were falling out of their seats toward the Gemini spacecraft nose (straps would, of course, hold them firmly in place). Acceleration would mount up as the Modified Transtage expended its propellants and became lighter, reaching a maximum of five Earth gravities just before the engines shut down.

The astronauts would undock from the Modified Transtage, turn their Gemini spacecraft around, and fire the OAMS engines to move away. They would then settle in for a trip around the Moon.

The Martin Marietta/McDonnell/NASA MSC report contained few details on what the astronauts would do during their circumlunar voyage, apart from using the OAMS thrusters to carry out four course correction maneuvers. The first would take place during the period between three and 10 hours after TLI, the second and third  40,000 nautical miles (74,080 kilometers) before and after passing the Moon, respectively, and the fourth between five and 10 hours before Earth atmosphere reentry. Propulsive velocity change during the course-correction burns would total between 170 feet (51.8 meters) per second and 230 feet (70 meters) per second.

Other possible mission objectives included testing the worldwide tracking and communications system ahead of its use during Apollo lunar landing missions and lunar photography as the circumlunar Gemini passed over areas of the Moon lit by the Sun. The Martin Marietta/McDonnell/NASA MSC team estimated that about a third of the lunar farside hemisphere would be in sunlight as the spacecraft passed over it.

Flight time, maximum distance from Earth, and lunar passage distance depended on many factors and could be highly variable. For a circumlunar mission that would pass the Moon when it was near perigee and that would perform a splashdown near Cape Kennedy in daylight, the mission would last 143 hours (five days, 23 hours), would reach a distance of 221,700 miles (356,790 kilometers) from Earth, and would pass within between 660 nautical miles (1220 kilometers) and 1300 nautical miles (2410 kilometers) of the lunar surface. For a daylight splashdown near Hawaii when the Moon was near apogee, the equivalent numbers were 172 hours (seven days, four hours); 253,363 miles (407,748 kilometers); and between 800 nautical miles (1480 kilometers) and 1330 nautical miles (2460 kilometers).

Gordon Cooper (left) and Charles Conrad: the crew of Gemini V. Image credit: NASA

This post began with U.S. Representative Olin Teague's query to NASA Administrator James Webb. Three days after the date on Teague's letter, astronaut Pete Conrad, whose enthusiasm for a circumlunar Gemini flight had helped to inspire the Martin Marietta/McDonnell/NASA MSC study, reached orbit with Gordon Cooper on board Gemini V (21-29 August 1965). They doubled the voyage duration of Gemini IV and broke the world record for time in space (seven days, 23 hours). It was the first time the U.S. held that record — and it demonstrated that a human could live in space long enough to carry out a circumlunar voyage.

On 23 August 1965, while Gemini V orbited the Earth, Webb testified before the U.S. Senate Committee on Aeronautical and Space Sciences, chaired by Clinton P. Anderson of New Mexico, another ally of President Johnson. During his testimony, which marked the start of a three-day hearing on NASA's future, Webb reviewed work accomplished in the Apollo Program and sought support for an Apollo-derived post-Apollo space program. 

Without prompting, Webb briefly mentioned the circumlunar Gemini mission concept. If his aim was to elicit senatorial comment, he failed; the assembled Senators did not take the bait. The mission concept received no further mention during the three-day hearing.

On 10 September 1965, Webb responded to Teague. He explained that "insertion in our program of a circumlunar flight, using the Gemini system, would require major resources." Webb told Teague that "we are proceeding with many complex developmental, test, and operational efforts with too thin a margin of resources," adding that "if additional funds were available. . .it would be in the national interest to use these in the Apollo program." Webb included a copy of his Senate testimony with his letter.

At the end of September, Webb ordered his communications with Teague to be forwarded to Robert Gilruth, director of NASA MSC, Wernher von Braun, director of the NASA Marshall Space Flight Center, and Kurt Debus, director of NASA Kennedy Space Center, Florida. In an accompanying memorandum, Robert Freitag, director of Manned Space Flight Field Center Development at NASA Headquarters, explained that "this indicates NASA's position on possible circumlunar Gemini flights."

Sources

Rendezvous Concept for Circumlunar Flyby in 1967, Martin Marietta, July 1965

Letter, Olyn Teague to James Webb, 18 August 1965

National Goals for the Post-Apollo Period: Hearing on Alternative Goals for the National Space Program Following the Manned Lunar Landing, U.S. Senate Committee on Aeronautical and Space Sciences, 23-25 August 1965, U.S. Government Printing Office, 1965, p. 22

Letter with attachment, James Webb to Olyn Teague, 10 September 1965

Memorandum with attachment, Robert Freitag to various, 30 September 1965

Project Gemini: A Chronology, SP-4002, J. Grimwood, B. Hacker, and P. Vorzimmer, NASA, 1969, p. 153

On The Shoulders of Titans: A History of Project Gemini, SP-4203, B. Hacker and J. Grimwood, NASA, 1977, pp. 73-74, 200-201, 354

More Information

Around the Moon in 80 Hours (1958)

Gemini on the Moon (1961)

Space Station Gemini (1962)

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