Project Gemini

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 easy to repair.[12]

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. A Titan II booster explosion had a smaller blast effect and flame than on the cryogenically fueled Atlas and Saturn. Ejection 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 to Earth inside the spacecraft, which would separate from the launch vehicle.[13]

The main proponent of using ejection seats was Chamberlin, who had never liked the Mercury escape tower and wished to use a simpler alternative that would also reduce weight. He reviewed several films of Atlas and Titan II ICBM failures, which he used to estimate the approximate size of a fireball produced by an exploding launch vehicle and from this he gauged that the Titan II would produce a much smaller explosion, thus the spacecraft could get away with ejection seats.

Maxime Faget, the designer of the Mercury LES, was on the other hand less-than-enthusiastic about this setup. Aside from the possibility of the ejection seats seriously injuring the astronauts, they would also only be usable for about 40 seconds after liftoff, by which point the booster would be attaining Mach 1 speed and ejection would no longer be possible. He was also concerned about the astronauts being launched through the Titan's exhaust plume if they ejected in-flight and later added, "The best thing about Gemini was that they never had to make an escape."[14]

The Gemini ejection system was never tested with the Gemini cabin pressurized with pure oxygen, as it was prior to launch. In January 1967, the fatal Apollo 1 fire demonstrated that pressurizing a spacecraft with pure oxygen created an extremely dangerous fire hazard.[15] In a 1997 oral history, astronaut Thomas P. Stafford commented on the Gemini 6 launch abort in December 1965, when he and command pilot Wally Schirra nearly ejected from the spacecraft:

So it turns out what we would have seen, had we had to do that, would have been two Roman candles going out, because we were 15 or 16 psi, pure oxygen, soaking in that for an hour and a half. You remember the tragic fire we had at the Cape. (...) Jesus, with that fire going off and that, it would have burned the suits. Everything was soaked in oxygen. So thank God. That was another thing: NASA never tested it under the conditions that they would have had if they would have had to eject. They did have some tests at China Lake where they had a simulated mock-up of Gemini capsule, but what they did is fill it full of nitrogen. They didn't have it filled full of oxygen in the sled test they had.[16]

Gemini was the first astronaut-carrying spacecraft to include an onboard computer, the Gemini Guidance Computer, to facilitate management and control of mission maneuvers. This 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). Its core memory had 4096 addresses, each containing a 39-bit word composed of three 13-bit "syllables". All numeric data was 26-bit two's-complement integers (sometimes used as fixed-point numbers), either stored in the first two syllables of a word or in the accumulator. Instructions (always with a 4-bit opcode and 9 bits of operand) could go in any syllable.[17][18][19][20]

Unlike Mercury, Gemini used in-flight radar and an artificial horizon, similar to those used in the aviation industry.[17] Like Mercury, Gemini used a joystick to give the astronauts manual control of yaw, pitch, and roll. Gemini added control of the spacecraft's translation (forward, backward, up, down, and sideways) with a pair of T-shaped handles (one for each crew member). Translation control enabled rendezvous and docking, and crew control of the flight path. The same controller types were also used in the Apollo spacecraft.[9]

Gemini paraglider during tests at Edwards Air Force Base in August 1964.

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.[21] 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.

The adapter module in turn was separated into a Retro module and an Equipment module.

The Retro module contained four solid-fuel TE-M-385 Star-13E retrorockets, each spherical in shape except for its rocket nozzle, which were structurally attached to two beams that reached across the diameter of the retro module, crossing at right angles in the center.[22] Re-entry began with the retrorockets firing one at a time. Abort procedures at certain periods during lift-off would cause them to fire at the same time, thrusting the Descent module away from the Titan rocket.

Gemini was equipped with an Orbit Attitude and Maneuvering System (OAMS), containing sixteen thrusters for translation control in all three perpendicular axes (forward/backward, left/right, up/down), in addition to attitude control (pitch, yaw, and roll angle orientation) as in Mercury. Translation control allowed changing orbital inclination and altitude, necessary to perform space rendezvous with other craft, and docking with the Agena Target Vehicle (ATV), with its own rocket engine which could be used to perform greater orbit changes.

Early short-duration missions had their electrical power supplied by batteries; later endurance missions used the first fuel cells in crewed 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 US crewed space program clearly began showing its superiority over that of the Soviet Union with long duration flight, rendezvous, and extravehicular capability.[note 4] 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 crewed lunar program.

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 became the backup crew for Shepard and Stafford. Grissom and Borman then 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. 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, coincidentally 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 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.

• Gemini 1: Intentionally disintegrated upon re-entry to the atmosphere
• Gemini 2: Air Force Space and Missile Museum, Cape Canaveral Air Force Station, Florida
• Gemini III: Grissom Memorial, Spring Mill State Park, Mitchell, Indiana
• Gemini IV: National Air and Space Museum, Washington, D.C.
• Gemini V: Johnson Space Center, NASA, Houston, Texas
• Gemini VI: Stafford Air & Space Museum, Weatherford, Oklahoma
• Gemini VII: Steven F. Udvar-Hazy Center, Chantilly, Virginia
• Gemini VIII: Armstrong Air and Space Museum, Wapakoneta, Ohio
• Gemini IX: Kennedy Space Center, NASA, Merritt Island, Florida
• Gemini X: Kansas Cosmosphere and Space Center, Hutchinson, Kansas
• Gemini XI: California Museum of Science and Industry, Los Angeles, California
• Gemini XII: Adler Planetarium, Chicago, Illinois

Gemini TTV-1 paraglider capsule

• Gemini 3A: St. Louis Science Center, St. Louis, Missouri.
• Gemini MOL-B: National Museum of the United States Air Force, Wright-Patterson Air Force Base, Dayton, Ohio
• Gemini Trainer: Discovery Center, Fresno, California
• Gemini Trainer: U.S. Space & Rocket Center, Huntsville, Alabama
• Gemini Trainer: Kentucky Science Center, Louisville, Kentucky
• 6165, GATV: National Air and Space Museum, Washington, D.C. (not on display)[30]
• El Kabong: Kalamazoo Air Museum, Kalamazoo, Michigan
• Gemini Trainer: Kalamazoo Air Museum, Kalamazoo, Michigan
• TTV-2: National Space Centre, Leicester, UK
• Trainer: Pate Museum of Transportation, Fort Worth, Texas
• MSC 313: Private residence, San Jose, California
• Rogallo Test Vehicle: White Sands Space Harbor, White Sands, New Mexico
• TTV-1: Steven F. Udvar-Hazy Center, Chantilly, Virginia
• unnamed: Air Force Space and Missile Museum, Cape Canaveral Air Force Station, Florida
• unnamed: Air Force Space and Missile Museum, Cape Canaveral Air Force Station, Florida
• Ingress/Egress Trainer: U.S. Space & Rocket Center, Huntsville, Alabama
• MSC-307: USS Hornet Museum, former NAS Alameda, Alameda, California

• 
  Gemini 2 at Air Force Space and Missile Museum in 2006
• 
  Gemini III at Grissom Memorial in 2011
• 
  Gemini IV at National Air and Space Museum in 2009
• 
  Gemini V at Johnson Space Center in 2011
• 
  Gemini VI-A at Stafford Air & Space Museum in 2011
• 
  Gemini VII at Steven F. Udvar-Hazy Center in 2009
• 
  Gemini VIII at Armstrong Air and Space Museum in 2010
• 
  Gemini IX-A at Kennedy Space Center in 2011
• 
  Gemini X at Kansas Cosmosphere and Space Center in 2010
• 
  Gemini XI at California Museum of Science and Industry in 2013
• 
  Gemini XII at Adler Planetarium in 2010

McDonnell Aircraft, the main contractor for Mercury and Gemini, was also one of the original bidders on the prime contract for Apollo, but lost out to North American Aviation. McDonnell later sought to extend the Gemini program by proposing a derivative which could be used to fly a cislunar mission and even achieve a crewed lunar landing earlier and at less cost than Apollo, but these proposals were rejected by NASA.

A range of applications were considered for Advanced Gemini missions, including military flights, space station crew and logistics delivery, and lunar flights. The Lunar proposals ranged from reusing the docking systems developed for the Agena Target Vehicle on more powerful upper stages such as the Centaur, which could propel the spacecraft to the Moon, to complete modifications of the Gemini to enable it to land on the lunar surface. Its applications would have ranged from crewed lunar flybys before Apollo was ready, to providing emergency shelters or rescue for stranded Apollo crews, or even replacing the Apollo program.

Some of the Advanced Gemini proposals used "off-the-shelf" Gemini spacecraft, unmodified from the original program, while others featured modifications to allow the spacecraft to carry more crew, dock with space stations, visit the Moon, and perform other mission objectives. Other modifications considered included the addition of wings or a parasail to the spacecraft, in order to enable it to make a horizontal landing.

Big Gemini (or "Big G") was another proposal by McDonnell Douglas made in August 1969. It was intended to provide large-capacity, all-purpose access to space, including missions that ultimately used Apollo or the Space Shuttle.

The study was performed to generate a preliminary definition of a logistic spacecraft derived from Gemini that would be used to resupply an orbiting space station. Land-landing at a preselected site and refurbishment and reuse were design requirements. Two baseline spacecraft were defined: a nine-man minimum modification version of the Gemini B called Min-Mod Big G and a 12-man advanced concept, having the same exterior geometry but with new, state-of-the-art subsystems, called Advanced Big G. Three launch vehicles-Saturn IB, Titan IIIM, and Saturn INT-20 (S-IC/S-IVB) were investigated for use with the spacecraft.

The Air Force had an interest in the Gemini system, and decided to use its own modification of the spacecraft as the crew vehicle for the Manned Orbital Laboratory. To this end, the Gemini 2 spacecraft was refurbished and flown again atop a mockup of the MOL, sent into space by a Titan IIIC. This was the first time a spacecraft went into space twice.

The USAF also had the notion of adapting the Gemini spacecraft for military applications, such as crude observation of the ground (no specialized reconnaissance camera could be carried) and practicing making rendezvous with suspicious satellites. This project was called Blue Gemini. The USAF did not like the fact that Gemini would have to be recovered by the US Navy, so they intended for Blue Gemini eventually to use the airfoil and land on three skids, carried over from the original design of Gemini.

At first some within NASA welcomed sharing of the cost with the USAF, but it was later agreed that NASA was better off operating Gemini by itself. Blue Gemini was canceled in 1963 by Secretary of Defense Robert McNamara, who decided the NASA Gemini flights could conduct necessary military experiments. MOL was canceled by Secretary of Defense Melvin Laird in 1969, when it was determined that uncrewed spy satellites could perform the same functions much more cost-effectively.

 This article incorporates public domain material from websites or documents of the National Aeronautics and Space Administration.

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