Project Gemini

Project Gemini was the second human spaceflight program of NASA, the civilian space agency of the United States government. It was conducted between projects Mercury and Apollo, with ten manned flights occurring in 1965 and 1966.

Its objective was to develop space travel techniques in support of Apollo, which had the goal of landing men on the Moon. Gemini achieved missions long enough for a trip to the Moon and back, perfected extra-vehicular activity (working outside a spacecraft), and orbital maneuvers necessary to achieve rendezvous and docking. All Gemini flights were launched from Cape Canaveral, Florida using the Titan II Gemini launch vehicle ("GLV")1.

1The only Gemini spacecraft not on a Titan II was the re-flight of Gemini 2 for a Manned Orbital Laboratory test in 1966, which used a Titan IIIC

Edward White during spacewalk, Gemini 4, June 1965 (NASA)


Program objectives

After the existing Apollo program was chartered by President John F. Kennedy on May 25, 1961 to land men on the Moon, it became evident to NASA officials that a follow-on to the Mercury program was required to develop certain spaceflight capabilities in support of Apollo.

Originally introduced on December 7 as Mercury Mark II, it was re-christened Project Gemini on January 3, 1962, from the fact that the spacecraft would hold two crewmen, seated abreast, as gemini in Latin means "twins" or "double". Gemini is also the name of the third constellation of the Zodiac and its twin stars, Castor and Pollux.

The major objectives1 were :

To demonstrate endurance of humans and equipment to spaceflight for extended periods, at least eight days required for a Moon landing, to a maximum of two weeks

To effect rendezvous and docking with another vehicle, and to maneuver the combined spacecraft using the propulsion system of the target vehicle

To demonstrate Extra-Vehicular Activity (EVA), or space-"walks" outside the protection of the spacecraft, and to evaluate the astronauts’ ability to perform tasks there

To provide the astronauts with zero-gravity, rendezvous, and docking experience required for Apollo

1Gemini Program. Venice, CA : Revell, Inc. 1965. p. 1. pamphlet included with 1/24 scale model of Gemini spacecraft ; based on official records of NASA.

Atlas Agena target vehicle liftoff for Gemini 11 from Pad 14 (NASA)



James A. "Jim" Chamberlin was a Canadian aerodynamicist who contributed to the design of the Canadian Avro Arrow, NASA’s Gemini spacecraft and the Apollo program. In addition to his pioneering air and space efforts, he is often cited as an example of Canadian brain drain to the U.S.

Virgil Ivan Grissom, better known as Gus Grissom, was one of the original NASA Project Mercury astronauts, test pilot, mechanical engineer, and a United States Air Force pilot. He was the second American to fly in space, and the first member of the NASA Astronaut Corps to fly in space twice

Günter F. Wendt was a German-American engineer noted for his work in the U.S. manned spaceflight program. An employee of McDonnell Aircraft, he was in charge of the spacecraft close-out crews at the launch pads for the entire Mercury and Gemini programs, and the manned phase of the Apollo program.

The two-man Gemini capsule was designed by a Canadian, Jim Chamberlin, formerly the chief aerodynamicist on the Avro Arrow fighter interceptor program with Avro Canada. Chamberlin joined NASA along with 25 senior Avro engineers after cancellation of the Arrow program, and became head of the U.S. Space Task Group’s engineering division in charge of Gemini. The prime contractor was McDonnell Aircraft, which had also been the prime contractor for the Project Mercury capsule.

In addition, astronaut Gus Grissom was heavily involved in the development and design of the Gemini spacecraft (the other Mercury astronauts dubbed the Gemini spacecraft the "Gusmobile")1. Grissom writes in his posthumous 1968 book Gemini ! that the realization of Project Mercury’s end and the unlikelihood of his having another flight in that program prompted him to focus all of his efforts on the upcoming Gemini program.

The Gemini program was managed by the Manned Spacecraft Center, Houston, Texas, under direction of the Office of Manned Space Flight, NASA Headquarters, Washington, D.C. Dr. George E. Mueller, Associate Administrator of NASA for Manned Space Flight, served as acting director of the Gemini program. William C. Schneider, Deputy Director of Manned Space Flight for Mission Operations, served as mission director on all Gemini flights beginning with Gemini 6A.

Guenter Wendt was a McDonnell engineer who supervised launch preparations for both the Mercury and Gemini programs and would go on to do the same for the manned section of the Apollo program. His team was responsible for completion of the complex pad close-out procedures just prior to spacecraft launch, and he was the last person the astronauts would see prior to closing the hatch. The astronauts appreciated his taking absolute authority over, and responsibility for, the condition of the spacecraft and developed a good-humored rapport with him2.

1"The Unsinkable Gusmobile". http://amyshirateitel.com. Retrieved 6 July 2012.

2Farmer, Gene ; Dora Jane Hamblin (1970). First On the Moon : A Voyage With Neil Armstrong, Michael Collins and Edwin E. Aldrin, Jr. Boston : Little, Brown and Co. pp. 51–54. Library of Congress 76-103950.

Armstrong and Scott with Hatches Open (NASA)



NASA selected McDonnell Aircraft, which had been the prime contractor for the Project Mercury capsule, to build the Gemini capsule in 1961 and the first capsule was delivered in 1963. The spacecraft was 19 feet (5.8 m) long and 10 feet (3.0 m) wide with a launch weight of 8,490 pounds (3,850 kg). The Gemini capsule first flew with a crew on March 23, 19651.

Unlike Mercury, which was equipped only with an attitude control system to change its pitch, yaw, and roll orientation in space, the Gemini spacecraft was equipped with an Orbit Attitude and Maneuvering System (OAMS) which also performed translation in all three perpendicular axes (forward/backward, left/right, up/down) in addition to attitude control. This was used to alter its orbital inclination and altitude, and to perform rendezvous and docking with the Agena Target Vehicle (ATV). The ATV had its own rocket engine which could be used to perform larger altitude changes.

A cutaway illustration of the Gemini spacecraft

Also unlike Mercury, and the later Apollo spacecraft, Gemini’s emergency launch escape system did not use an escape tower powered by a solid-fuel rocket, but instead used aircraft-style ejection seats. The tower was heavy and complicated, and NASA engineers reasoned that they could do away with it as the Titan II’s hypergolic propellants would burn immediately on contact, thus a booster explosion would be much smaller than on the cryogenically fueled Atlas and Saturn, and ejector seats were sufficient to separate the astronauts from a malfunctioning launch vehicle. At higher altitudes where the ejection seats could not be used, the astronauts would return inside the spacecraft, which would separate from the launch vehicle.

A major difference between the Gemini and Mercury spacecraft design was that all Mercury systems other than the re-entry rockets were situated within the capsule, most of which were accessed through the astronaut’s hatchway. In contrast, Gemini housed power, propulsion, and life support systems in a detachable Equipment Module located behind the Reentry Module, which made it similar to the Apollo Command/Service Module design. Many components in the capsule itself were reachable through their own small access doors. Unlike Mercury, Gemini used completely solid-state electronics and its modular design made it very easy to repair.

Gemini spacecraft - Typical interior arrangement
From Project Gemini Familiarization Manual

Gemini was the first manned spacecraft to include an onboard computer, the Gemini Guidance Computer2, to facilitate management and control of mission maneuvers. Unlike Mercury, the Gemini used in-flight radar and an artificial horizon—devices similar to those used in the aviation industry.

The original intention for Gemini was to land on solid ground instead of at sea, using a Rogallo wing rather than a parachute, with the crew seated upright controlling the forward motion of the craft. To facilitate this, the airfoil did not attach just to the nose of the craft, but to an additional attachment point for balance near the heat shield. This cord was covered by a strip of metal which ran between the twin hatches. This design was ultimately dropped, and parachutes were used to make a sea landing as in Mercury. The capsule was suspended at an angle closer to horizontal, so that a side of the heat shield contacted the water first. This eliminated the need for the landing bag cushion used in the Mercury capsule.

Early short-duration missions had their electrical power supplied by batteries ; later endurance missions used the first fuel cells in manned spacecraft.

Gemini was in some regards more advanced than Apollo because the latter program began almost a year earlier. It became known as a "pilot’s spacecraft" due to its assortment of jet fighter-like features, in no small part due to Gus Grissom’s influence over the design, and it was at this point where the American manned space program clearly began showing its superiority over that of the Soviet Union with long duration flight, rendezvous, and extravehicular capability3. The Soviet Union during this period was developing the Soyuz spacecraft intended to take cosmonauts to the Moon, but political and technical problems began to get in the way, leading to the ultimate end of their manned lunar program.

1"McDonnell Gemini Spacecraft". Retrieved 30 March 2011.

2"Computer, Guidance, Gemini 8". Retrieved 23 December 2005.

3During the two years of the Gemini program, the Soviets made no manned flights, and despite achieving the first EVA, did no more EVAs until January 1969.

The Augmented Target Docking Adapter (ATDA) as seen from the Gemini 9 spacecraft (NASA)

The Titan rocket that launched the Manned Orbiting Laboratory’s unmanned test flight was a much more robust booster than the Titan II used throughout NASA’s Gemini program.


Launch vehicle

Gemini Profiles
Original poster : Reubenbarton

The Titan II had debuted in 1962 as the Air Force’s second-generation ICBM to replace the Atlas. By using hypergolic fuels, it could be stored for long periods of time and be easily readied for launch in addition to being a simpler design with fewer components, the only caveat being that the propellant mix (nitrogen tetroxide and hydrazine) was extremely toxic compared to the Atlas’s liquid oxygen/RP-1. However, the Titan had considerable difficulty being man-rated due to early problems with pogo oscillation.

The Titan II rocket that carried the Gemini spacecraft was guided by its own (separate) ASC-15 guidance computer. The Gemini Guidance Computer, sometimes called the Gemini Spacecraft On-Board Computer (OBC), was very similar to the Saturn Launch Vehicle Digital Computer. The Gemini Guidance Computer weighed 58.98 pounds (26.75 kg). The core memory had 4096 addresses, each containing a 39-bit word, composed of three 13-bit "syllables".


Crew selection

Deke Slayton
Donald Kent Slayton (March 1, 1924 – June 13, 1993), (Maj, USAF), better known as Deke Slayton, was an American World War II pilot, aeronautical engineer, test pilot, one of the original NASA Mercury Seven astronauts, and a NASA administrator.

Deke Slayton, as director of flight crew operations, had the main role in the choice of crews for the Gemini program. With Gemini it became a procedure that each flight had a primary crew and backup crew, and that the backup crew would rotate to primary crew status three flights later. Slayton also intended for first choice of mission commands to be given to the four remaining active astronauts of the Mercury Seven : Alan Shepard, Grissom, Cooper, and Schirra. (John Glenn had retired from NASA in January 1964 and Scott Carpenter, who was blamed by some in NASA management for the problematic reentry of Aurora 7, was on leave to participate in the Navy’s SEALAB project and was grounded from flight in July 1964 due to an arm injury sustained in a motorbike accident. Slayton himself continued to be grounded due to a heart problem.)

In late 1963, Slayton selected Shepard and Stafford for Gemini 3, McDivitt and White for Gemini 4, and Schirra and Young for Gemini 5 (which was to be the first Agena rendezvous mission). The backup crew for Gemini 3 was Grissom and Borman, who were also slated for Gemini 6, to be the first long-duration mission. Finally Conrad and Lovell were assigned as the backup crew for Gemini 4.

Delays in the production of the Agena Target Vehicle caused the first rearrangement of the crew rotation. The Schirra and Young mission was bumped to Gemini 6 and they now were the backup crew for Shepard and Stafford. Grissom and Borman now had their long-duration mission assigned to Gemini 5.

The second rearrangement occurred when Shepard developed Ménière’s disease, an inner ear problem. Grissom was then moved to command Gemini 3. Slayton felt that Young was a better personality match with Grissom and switched Stafford and Young. Finally, Slayton tapped Cooper to command the long-duration Gemini 5. Again for reasons of compatibility, he moved Conrad from backup commander of Gemini 4 to pilot of Gemini 5, and Borman to backup command of Gemini 4. Finally he assigned Armstrong and Elliot See to be the backup crew for Gemini 5.

Apollo 40th Anniversary - Neil Armstrong

The third rearrangement of crew assignment occurred when Slayton felt that See wasn’t up to the physical demands of EVA on Gemini 8. He reassigned See to be the prime commander of Gemini 9 and put Scott as pilot of Gemini 8 and Charles Bassett as the pilot of Gemini 9.

The fourth and final rearrangement of the Gemini crew assignment occurred after the deaths of See and Bassett when their trainer jet crashed, coincidently into a McDonnell building which held their Gemini 9 capsule in St. Louis. The backup crew of Stafford and Cernan was then moved up to the new prime crew of the re-designated Gemini 9A. Lovell and Aldrin were moved from being the backup crew of Gemini 10 to be the backup crew of Gemini 9. This cleared the way through the crew rotation for Lovell and Aldrin to become the prime crew of Gemini 12.

Along with the deaths of Grissom, White, and Roger Chaffee in the fire of Apollo 1, this final arrangement helped determine the makeup of the first seven Apollo crews, and who would be in position for a chance to be the first to walk on the Moon.

Edward H. White II, pilot of the Gemini 4 spacecraft, floats in the zero gravity of space with an earth limb backdrop (NASA)