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Gemini spacecraft

Gemini 7 spacecraft in orbit as seen by Gemini 6A
Manufacturer	McDonnell Aircraft
Country of origin	United States
Operator	NASA
Applications	Manned orbital flight
Specifications
Launch mass	7,100 to 8,350 pounds (3,220 to 3,790 kg)
Crew capacity	2
Dimensions	18 feet 5 inches (5.61 m) long 10 feet (3.0 m) diameter
Volume	90 cubic feet (2.5 m3)
Power	2 General Electric fuel cells
Batteries	5 silver-zinc batteries for re-entry
Regime	Low Earth orbit
Design life	14 days
Production
Status	Retired
Built	12
Launched	12
Failed	0
Lost	0
Maiden launch	April 8, 1964
Last launch	November 11, 1966
Last retirement	November 15, 1966
Related spacecraft
Flown with	Agena Docking Target
Configuration
Gemini spacecraft diagram ← Mercury Apollo →

Mass
Crew compartment:	5,910 pounds (2,680 kg)
Retro module:	1,303 pounds (591 kg)
Equipment module:	1,277 pounds (579 kg)
Total:	8,490 pounds (3,850 kg)
Rocket engines
Reentry Control System (MMH/N2O4):	16 x 25 pounds-force (110 N)
OAMS (attitude) (MMH/N2O4):	8 x 25 pounds-force (110 N)
OAMS (maneuvering) (MMH/N2O4):	6 x100 pounds-force (440 N) transverse/fwd 2 x85 pounds-force (380 N) aft
Performance
Max delta-v:	8,490 feet per second (2,590 m/s)

The Gemini spacecraft was a two-man spacecraft used for the United States Project Gemini, which followed the one-man Project Mercury and supported the Apollo program goal of landing men on the Moon by the end of the 1960's. It was built for NASA by McDonnell Aircraft. It was launched into low Earth orbit by the Titan II launch vehicle, a derivative of the US Air Force Titan II missile. Two unmanned test flights were made, followed by 10 manned missions, in 1965 and 1966. Gemini demonstrated the capability of keeping two astronauts in orbit for up to 14 days, and was the world's first manned spacecraft capable of changing its orbit in space, and rendezvous and dock with another craft. Its hatches were designed to be opened and closed in space, allowing astronauts to perform extravehicular activity.

The Gemini consisted of three segments: the crew compartment (capsule) with seats for two and equipment needed for re-entry and splashdown; a retro module which housed four rockets used to take the craft out of orbit; and an equipment module that provided propulsion, electrical power and storage for various consumables required during a mission. The equipment and retro modules were cast off and allowed to burn up in the atmosphere before the crew compartment re-entered and brought the crew home.

The Gemini program used an Agena Target Vehicle, a modified version of the Agena D rocket stage, to practice docking techniques and for propulsion to raise its orbit to as high as 739.2 nautical miles (1,369.0 km).

The United States Air Force chose a modified Gemini (Gemini B) as the crew launch and re-entry vehicle for its Manned Orbiting Laboratory (MOL). An Air Force astronaut corps was selected, and MOL made one unmanned test flight in 1966 reusing one of the unmanned Gemini capsules. But MOL was cancelled in 1969 by the administration of President Richard M. Nixon.

In August 1969, McDonnell (then McDonnell Douglas) proposed a Big Gemini derivative of the spacecraft as a reusable 9-12 person shuttle vehicle for a space station. This proposal was never adopted.

Re-entry module

Construction

The Gemini re-entry module resembled its Mercury capsule predecessor, with 50% greater internal volume. One innovation over Mercury, and even the Apollo Command Module, was the use of modular packages of major components, which could be easily removed and replaced through external service panels, without replacing or disturbing other systems already checked out.

The truncated cone-shaped module was 39 inches (99 cm) in diameter at the top and 90 inches (230 cm) in diameter at the bottom. Ring-frame stabilized axial stringers were made of titanium and magnesium. The inner pressure shell was made of a fusion-welded titanium frame attached to side panels, with fore and aft bulkheads made of doubled 0.010 inches (0.25 mm) titanium sheets, the outer one of which was beaded for stiffness. Like Mercury, the outer wall was covered with overlapping, beaded 0.016 inches (0.41 mm) René 41 shingles. A layer of insulation thermally isolated the outer skin from the pressure compartment.

The top (forward) end was skinned with unbeaded beryllium shingles, capped with a plastic / fiberglass laminated nose fairing. The aft heat shield was composed of two five-ply face plates of resin-impregnated glass cloth separated by a 0.65-inch (1.7 cm) thick fiberglass honeycomb core, with a second such honeycomb core bonded to the outside (convex) side, filled with poured and air-hardened Dow-Corning DC-325 organic compound as the ablative material. A Fibrite ring surrounded the heat shield.

Earth landing system

An 18-foot (5.5 m) diameter ringsail drogue parachute was deployed with a mortar at an altitude of 60,000 feet, followed by an 84-foot (26 m) diameter ringsail main parachute which opened at 10,000 feet. Impact velocity was 30 feet per second (9.1 m/s). The capsule was suspended from the main parachute at a slight angle, so that its bottom end would strike the water first, and it would end up floating horizontally rather than vertically. This eliminated the need to suspend the heat shield from a landing bag to cushion the shock of splashdown, as was used on the Mercury capsule.

Gemini was originally designed to use a Rogallo wing to make a controlled glide on land, using a three-skid landing gear which deployed from the nose and aft side of the capsule, but development problems caused this approach to be abandoned.

Re-entry Control System

Sixteen 25-pound-force (110 N)} thrust engines mounted in the nose section fired in groups of four to provide pitch, yaw, and roll control during re-entry. The thrusters were fed by two mono-methyl hydrazine fuel tanks, two nitrogen tetroxide oxidizer tanks, and two helium pressurant tanks.

Hatches

Two large hatches, one on each side, allowed relatively easy ingress and egress for the two astronauts. They would lock open in the event of a launch abort, or other emergency escape. Each hatch contained a forward-facing observation window. The Command Pilot's left-hand window had two outer panes of 96 percent silicate glass, and an inner pane of tempered alumino-silicate glass. The Pilot's right-hand window was designed for maximum optical clarity for observation and photography, and replaced the alumino-silicate inner pane with a thickened silica glass pane, yielding greater than 99 percent optical transmission.

The hatches initially proved troublesome for opening and closing in space during the Gemini 4 mission, on which the first American EVA was performed.

Crew escape system

Unlike Mercury, Gemini did not use an escape tower. Instead, the crew were seated in military aircraft-style ejection seats, each with its own parachute. The system was usable during launch emergencies up to altitudes in excess of 60,000 feet, or after re-entry.

Environmental control system

The cabin was pressurized with 100% pure oxygen, which a dual pressure regulating valve kept between 5 to 5.3 pounds per square inch (0.34 to 0.37 bar) above ambient pressure at all times. This meant the prelaunch-to-liftoff pressure was slightly above atmospheric, and the on-orbit pressure kept a cabin altitude between 25,700 to 27,000 feet (7,800 to 8,200 m). Oxygen was supplied from a tank in the equipment module, which also fed the electrical power system's fuel cells.

The astronaut's spacesuits were automatically pressurized with oxygen at 3.7 pounds per square inch (0.26 bar) when the cabin was depressurized for extravehicular activity.

Specifications

• Crew: 3
• Crew cabin volume: 90 cubic feet (2.5 m³)
• Length: 10 feet 11 inches (3.33 m)
• Diameter @ heatshield: 7 feet 6 inches (2.29 m)
• Diameter @ top: 3 feet 3 inches (0.99 m)
• Mass: 1,300 pounds (590 kg)
• RCS: sixteen 25 lbf (110 N) thrusters, firing in groups of four
• RCS propellants: MMH/N₂O₄
• CO₂ scrubber: lithium hydroxide
• Electric system batteries: five silver-zinc batteries
• Parachutes: one18 feet (5.5 m) ringsail drogue; one 84 feet (26 m) ringsail main

Adapter module

The adapter module was a 7-foot-6-inch (2.29 m) long truncated cone, which provided a smooth aerodynamic transition from the 10-foot (3.0 m) wide Titan II GLV second stage to the 90-inch (2.3 m) wide re-entry module. It contained the Orbit Attitude and Maneuvering System (OAMS), and was divided into two sections: an equipment module which contained the on-orbit consumables and electrical power system, and a retro module which contained the retrorockets used to take the re-entry module out of orbit.

Construction

The adaptor module was made of circular aluminum alloy frames, T-shaped extruded magnesium alloy stringers, and magnesium skin. The outer skin served as a space radiator to dump waste heat from the environmental control and electrical power systems; it was painted white and contained tubes threaded through the stringers, through which ethylene glycol coolant was circulated. To shield the contents of the equipment module from solar radiation, the aft end of the adaptor was covered with glass fiber cloth, to which a coating of gold was vapor-applied.

Orbit Attitude and Maneuvering System

Main article: Orbit Attitude and Maneuvering System

Gemini OAMS and RCS

Attitude control was acheved by firing pairs of eight 25-pound-force (110 N) thrusters located around the circumference of the equipment module at the extreme aft end. Lateral translation control was provided by four 100-pound-force (440 N) thrusters around the circumference at the forward end of the retro module (close to the spacecraft's center of mass). Two forward-pointing 85-pound-force (380 N) thrusters (changed from the original spec of 100 pounds-force (440 N)) at the same location, provided aft translation, and two 100-pound-force (440 N) thrusters located in the aft end of the equipment module provided forward thrust, which could be used to change the craft's orbit.

All sixteen thrusters used ablative cooling, and were fed by hypergolic propellants, mono-methyl hydrazine and nitrogen tetroxide, helium pressure-fed from tanks stored in the equipment module.

Equipment module

Electrical power system

Two General Electric fuel cells combined liquid hydrogen with liquid oxygen to produce electrical power for up to fourteen days, with drinkable water as a byproduct. The reactants were stored in two cryogenic tanks, and the output water was added to the potable tank. The fuel cells were first carried on the unmanned Gemini 1, but failed and were shut off before launch. Conventional batteries were used for the next three missions (the longest of which lasted four days). The fuel cells were next carried on Gemini 5, but again gave out and were shut down before the end of the eight-day mission. The remaining seven missions had no problems with the fuel cells, and the full duration was demonstrated on Gemini 7.

Retro module

The retro module contained four spherical solid-fuel rocket motors mounted on two crossed I beams. After the equipment module was jettisoned, these rockets were fired in sequence with a slight overlap. Each one produced 2,500 pounds-force (11,000 N) of thrust for 5.5 seconds.

Specifications

• Length: 7 feet 6 inches (2.29 m)
• Diameter, base: 10 feet (3.0 m)
• Diameter, top: 7 feet 6 inches (2.29 m)
• Mass: 2,450 pounds (1,110 kg)
• OAMS thrust:
  • +X (fwd) translation: two x 100 pounds-force (440 N)
  • -X (aft) translation: two x 85 pounds-force (380 N)
  • Y/Z (lateral) translation: one x 100 pounds-force (440 N) in each direction
  • Rotation: two x 25 pounds-force (110 N) in each axis
• OAMS propellants: mono-methyl hydrazine / nitrogen tetroxide
• Spacecraft delta-v: 9,200 feet per second (2,800 m/s)
• Electrical System: two fuel cells
• Retrorockets: four x 2,500 pounds-force (11,000 N) solid rocket motors

References

Bibliography

• Gatland, Kenneth (1976). Manned Spacecraft (Second ed.). New York: Macmillan.
• Hacker, Barton C.; Grimwood, James M. (1977). On the Shoulders of Titans: A History of Project Gemini. NASA SP-4203. Washington, D.C.: National Aeronautics and Space Administration. Retrieved 2015-01-02.

Category:Crewed spacecraft Category:Human spaceflight