How the Gemini Spacecraft Worked

A Titan II rocket carrying the Gemini VI capsule launches. See more Gemini images.
Image courtesy NASA

Between NASA's Project Mercury, which launched the first American astronauts into space, and Project Apollo, which landed men on the moon, there was Project Gemini. On May 5, 1961, Alan B. Shepard Jr. became the first American in space. Twenty days later, President John F. Kennedy addressed Congress and announced the goal of landing a man on the moon before the end of the decade.

NASA had a long way to go from Project Mercury. The Mercury spacecraft could hold only one astronaut and had limited capabilities. NASA designed the craft for suborbital and orbital flights. The longest Mercury mission lasted less than a day and a half. In order to make a trip to the moon, NASA would have to create a spacecraft that could stay in space for more than a week.


On top of that, the complex trip to the moon and back would require more than one pilot. The Apollo spacecraft would need to be much larger than the Mercury vehicle. After performing some calculations, NASA engineers determined that it made more sense to find a way to enable the craft to dock with other structures in space. That way, part of the craft could detach from the rest, land on the moon, launch from the moon into a lunar orbit, and rendezvous and dock with the rest of the spacecraft.

NASA scientists decided they needed to create a project to span between Mercury and Apollo. They had to test how humans handle prolonged space travel. The spacecraft would have to be able to dock with another object in space. The new capsule also needed to have more maneuverability than the Mercury spacecraft. Engineers based their design on the Mercury capsule, but made it larger so that two astronauts could travel together. A NASA employee came up with the name Gemini, named after the twin constellation.

What happened in the Gemini project, and why were docking maneuvers so important? Keep reading to find out. ­­


Overview of the Project Gemini

A distant view of the Gemini VI launch.
Image courtesy NASA

The Gemini Project included 12 flights, two of which were unmanned. NASA intended these flights to test the effects of prolonged space travel on humans. Spacewalks became an important part of several Gemini missions, so NASA devoted a lot of time and effort into improving the design of space suits. Earlier versions of the suits were meant as emergency backup systems only. As such, they weren't very flexible or comfortable.

All the astronauts in the Gemini program returned to Earth safely. The Gemini missions included:


  • Gemini I and II, the two unmanned missions, which tested the spacecraft's systems and compatibility with the Titan II launch vehicle
  • Gemini III with the two-man crew of Virgil "Gus" Grissom and John Young. Grissom gave the spacecraft the nickname "Molly Brown." It was the only spacecraft in the project to receive a nickname. Young also made a unique contribution. He smuggled contraband on board the spacecraft: a corned beef sandwich, which he returned to a pocket in his space suit once he realized crumbs could get into the instrument panels.
  • Gemini IV saw the first American extravehicular activity (EVA), also known as a spacewalk. Edward White took a 22-minute space stroll during the mission.
  • Gemini V was the first Gemini flight that used fuel cells as a power source. Earlier spacecraft relied on battery power.
  • Gemini VI had the odd distinction of launching out of order. That's because the unmanned vehicle with which the Gemini VI should have docked exploded during its launch. NASA decided to delay the Gemini VI launch. They launched Gemini VII on schedule and then launched Gemini VI days later to rendezvous with it. The two spacecraft met and flew in formation for several hours.
  • Gemini VIII ended early due to a malfunctioning thruster that caused the spacecraft to rotate once a second.
  • Gemini IX was supposed to dock with an unmanned vessel, but an obstruction in the target vessel's docking mechanism prevented the maneuver.
  • Gemini X had two successful docking attempts with two different unmanned vessels, proving that vehicles could dock together in space.
  • Gemini XI flew in a higher orbit than any previous manned spacecraft and also relied entirely on computer guidance during re-entry.
  • Gemini XII, the final mission in the program, included the longest spacewalks in the program. Edwin "Buzz" Aldrin accumulated more than five hours in space over three spacewalks.

What was the launch vehicle for the Gemini Project like? Find out on the next page.


The Titan Launch Vehicle

The Gemini capsule on top of the Titan II launch vehicle.
Image courtesy NASA

During Project Mercury, NASA relied on two different launch vehicles: a Redstone launch vehicle for suborbital flights and an Atlas vehicle for orbital ones. Because the Gemini capsule was larger and heavier than the Mercury capsule, NASA had to find a more powerful launch vehicle.

After considering several candidates, NASA decided to use a modified intercontinental ballistic missile (ICBM) made by Martin Marietta (we know the company as Lockheed Martin today). It was called the Titan II ICBM.


The Titan II and Gemini capsule together stood 108 feet tall (33 meters). The Titan II used Aerozine-50, a 50-50 mixture of hydrazine and unsymetrical­-dimethyl hydrazine, as fuel. For an oxidizer (an agent that allows fuel to burn), it used nitrogen tetroxide. The oxidizer and hydrazine are hypergolic agents, which means that when you mix the two together, they ignite.

The Titan II had two sections, or stages, that separated at a specific point in the launch. The first stage was the Titan 2-1, and the second was Titan 2-2. Titan 2-1 contained two Aerojet LR-87-7 rocket engines and produced 430,000 pounds (1,913,500 newtons) of thrust. Titan 2-2 had one Aerojet LR-91-7 rocket engine. It could provide up to 100,000 pounds (445,000 newtons) of thrust.

Just before launch, NASA would combine the fuel and oxidizer in the first stage of the Titan II launch vehicle. Upon mixing, the fuel ignited and the vehicle and Gemini capsule rocketed into the atmosphere. After about two and a half minutes, the Titan 2-1 would shut down, having consumed its fuel. At that time, the Titan 2-2 engine would fire and the first stage would separate from the rest of the vehicle and plunge into the ocean. Once in orbit, the Gemini capsule jettisoned the second stage.

NASA modified the Titan II extensively to act as a launch vehicle. Engineers added a malfunction detection system that would warn the crew if something went wrong before or during a launch. They also reinforced the rocket's electrical and hydraulic systems, providing backups in case the primary systems failed. Other modifications included adding monitoring devices so that NASA could track the flight of the rocket during launch.

While the Titan II wasn't designed to return to Earth, it remained useful even after it used up all its fuel. That's because the astronauts practiced flying in formation with the spent Titan 2-2 stage, giving them valuable experience with piloting the Gemini capsule in space.

So what made the Gemini capsule tick? Keep reading to find out.


The Gemini Capsule

An artist's concept of the Gemini spacecraft.
Image courtesy NASA

The Mercury capsule could hold only one astronaut, so NASA had to design a larger spacecraft in order to send a two-man crew up into space. It based much of the capsules design off the Mercury spacecraft, but didn't double the size. Instead, NASA engineers increased the interior space by about 50 percent. That made things a little cramped for the astronauts sitting inside. What's more, the astronauts couldn't get up and move around in the capsule -- they were confined to their seats.

The capsule was shaped like a cone and was 18.6 feet (5.67 meters) tall. At its base, the capsule's diameter was 10 feet (3.05 meters) wide. It weighed a hefty 8,490 pounds (3,851 kilograms).


The only exception to this situation was when an astronaut went on a spacewalk. At that time, both astronauts pressurized their space suits. One would open the hatch above his chair to exit the vehicle (unlike the Mercury capsule, the Gemini spacecraft had two hatches). Once outside the capsule, he could stretch his legs while his crewmate stayed inside the vessel to pilot the ship.

NASA had to do more than just make a larger version of the Mercury capsule. The Mercury's maneuverability in space was extremely limited, while the Gemini would need to be able to dock with another vehicle. To that end, engineers built and installed a retrograde section containing eight thrusters (small rocket engines). This section attached to the bottom of the Gemini capsule. Besides housing the thrusters, this section also contained a tank of drinking water, an oxygen tank, a coolant pump system, fuel tanks, an electrical power system and a communications system. The retrograde section remained with the Gemini spacecraft until re-entry, whereupon the spacecraft jettisoned the section into space.

Before Gemini V, the Gemini spacecraft used batteries to supply electrical power. Gemini V was the first spacecraft to use fuel cells to generate power. Fuel cells use hydrogen and oxygen to generate electricity. One of the benefits of the fuel cell system is that the byproduct of generating electricity is water. Later, on the Apollo spacecraft, NASA would create a system that could reclaim water generated by the fuel cells and use it as drinking water.

A fish-eye view of the interior of the Gemini capsule.
Image courtesy NASA

Inside the capsule, the astronauts' view consisted of two windows and several displays and control panels. The spacecraft's computer analyzed data gathered from various sensors and calculated the correct trajectory and power needed to achieve mission goals. The capsule also contained the spacecraft's radar system, re-entry and attitude control system and a parachute landing system. While the astronauts could pilot the Gemini spacecraft while in orbit, the computer system controlled many of the maneuvers by sending commands directly to the appropriate systems.

NASA designed the Gemini capsule to dock with other structures while in space. What did they use as a docking vessel? Keep reading to find out.


Docking the Gemini Spacecraft

The Gemini Agena Target Vehicle
Image courtesy NASA

In order to practice docking maneuvers in space, NASA needed to provide a structure with which the Gemini capsule could dock. The solution was a modified Agena second rocket stage. Normally, the Agena would act as part of a launch vehicle for a spacecraft. NASA modified it so that it could also become a docking vessel. Engineers designed a docking collar that fit on the top end of the rocket stage and modified the rocket engine so that it could restart after shutting down.

Using an Atlas rocket as the first stage, NASA launched the newly dubbed Gemini Agena Target Vehicle (GATV) into orbit. Using a radio-controlled computer system, NASA ground control could maneuver the Agena into the proper orbit and alignment to await docking from a Gemini capsule.


The GATV had a Model 8247 rocket engine mounted on a gimbal, which means it could tilt in different directions. By tilting the rocket engine, NASA could control in which direction the vessel moved. It used unsymmetrical dimethylhydrazine (UDMH) for fuel and inhibited red fuming nitric acid (IRFNA) as an oxidizer.

When docked with the Gemini capsule, the astronauts could use the GATV's engine to provide extra thrust and move into higher orbits. Together, the two vehicles could move all the way to the edge of the Van Allen Belt, a region of radiation within 4,000 miles of the Earth's surface [source: NASA].

­NASA designed the GATV's docking collar to fit around and latch to the end of the Gemini capsule. Once NASA and the astronauts aligned the two vessels in the same orbital plane, they carefully maneuvered the Gemini spacecraft so that the end entered the docking collar of the GATV. Once docked, the astronauts could check the GATV's systems on the Gemini-ATV Status Panel (ASP).

The first spacecraft to dock successfully with a GATV was Gemini VIII in March 1966 -- for 30 minutes. In July 1966, the Gemini X docked with two different GATVs during its mission. The success meant that NASA met the Gemini Project's most important mission objective. It also meant that landing a man on the moon before the end of the decade was feasible. The Apollo mission could proceed as intended.

NASA originally intended the Gemini to land on solid ground, but later decided to land only in the water. What made them change their minds? Find out in the next section.


Gemini Re-entry

In this shot of the Gemini VI and VII rendezvous, the two spacecraft were 29 feet (9 meters) apart.
Image courtesy NASA

During the early planning stages of Project Gemini, NASA explored the possibility of designing the capsule so that it could touch down on land. The Mercury capsules could land safely only in water. In order to make it possible to touch down on land, NASA tried to design a spacecraft with fixed or retracting wings, to turn the spacecraft into a paraglider. While engineers made some progress toward this goal, the paraglider wings never deployed fast enough to be effective. NASA eventually scrapped the idea in 1964.

While initially disappointing, the switch to a water landing system was probably for the best. On early Gemini flights, astronauts manually controlled much of the spacecraft's maneuvers during re-entry. Despite their best efforts, they usually landed many miles away from their target landing zone. Even Gemini XI, which relied on the spacecraft's computer system for an automatic re-entry, landed 2.65 nautical miles (4.9 kilometers) away from the intended landing zone. While one stretch of water in the middle of the Pacific Ocean is much like another, it takes a great deal of precision to touch down safely on a specific section of land.


Just before re-entry, the Gemini capsule would jettison the retrograde section, leaving only the cone-shaped spacecraft holding the crew. In most cases, the astronauts used the capsule's controls to maneuver it so that the large, blunt end faced the Earth. This was where NASA installed the Gemini's heat shield.

The tip of the Gemini capsule contained a parachute system. Small explosives deployed the parachutes, which helped slow the capsule's descent. The capsule then would make a big splash in the ocean and float until a rescue ship could retrieve the vehicle and astronauts.

Gemini X splashes down into the ocean.
Image courtesy NASA

Cynics might say that NASA's main motive for the Gemini spacecraft was to keep space exploration in the public eye during the years between Project Mercury and Project Apollo. While that may have played a role, NASA also used Project Gemini to gather important information and prove that vehicles in space could dock together. Without this experience, it's doubtful that NASA could have succeeded in achieving Kennedy's vision.

To learn more about space exploration and other topics, blast off to the links on the next page.


Lots More Information

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More Great Links

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