The Gemini program was designed as a bridge between the Mercury
and Apollo programs, primarily to test equipment and mission
procedures in Earth orbit and to train astronauts and ground crews for future Apollo
missions. The general objectives of the program included: long duration flights in excess
of of the requirements of a lunar landing mission; rendezvous and docking of two vehicles
in Earth orbit; the development of operational proficiency of both flight and ground
crews; the conduct of experiments in space; extravehicular operations; active control of
reentry flight path to achieve a precise landing point; and onboard orbital navigation.
Each Gemini mission carried two astronauts into Earth orbit for periods ranging from 5
hours to 14 days. The program consisted of 10 crewed launches, 2 uncrewed launches, and 7
target vehicles, at a total cost of approximately 1,280 million dollars.
After the prototype Gemini 1, which lacked most of the systems of the Gemini spacecraft, beginning with Gemini 2 the complete Gemini capsule was flown.
Capsule
The Gemini spacecraft was a cone-shaped capsule consisting of two components, a reentry
module and an adaptor module. The adaptor module made up the base of the spacecraft. It
was a truncated cone 228.6 cm high, 304.8 cm in diameter at the base and 228.6 cm at the
upper end where it attached to the base of the reentry module. The re-entry module
consisted of a truncated cone which decreased in diameter from 228.6 cm at the base to
98.2 cm, topped by a short cylinder of the same diameter and then another truncated cone
decreasing to a diameter of 74.6 cm at the flat top. The reentry module was 345.0 cm high,
giving a total height of 573.6 cm for the Gemini spacecraft.
The adaptor module was an externally skinned, stringer framed structure, with magnesium
stringers and an aluminum alloy frame. The adaptor was composed of two parts, an equipment
section at the base and a retrorocket section at the top. The equipment section held fuel
and propulsion systems and was isolated from the retrorocket section by a fiber-glass
sandwich honeycomb blast shield. The retrorocket section held the re-entry rockets for the
capsule.
The reentry module consisted mainly of the pressurized cabin which held the two Gemini
astronauts. Separating the reentry module from the retrorocket section of the adaptor at
its base was a curved silicone elastomer ablative heat shield. The module was composed
predominantly of titanium and nickle-alloy with beryllium shingles. At the narrow top of
the module was the cylindrical reentry control system section and above this the
rendezvous and recovery section which holds the reentry parachutes. The cabin held two
seats equipped with emergency ejection devices, instrument panels, life support equipment,
and equipment stowage compartments in a total pressurized volume of about 2.25 cubic
meters. Two large hatches with small windows could be opened outward, one positioned above
each seat.
Control, Propulsion, and Power
Attitude control was effected by two translation-maneuver hand controllers, an attitude
controller, redundant horizon sensor sytems, and reentry control electronics, with
guidance provided via an inertial measuring unit and radar system. The orbital attitude
and maneuver system used a hypergolic propellant combination of monomethylhydrazine and
nitrogen tetroxide supplied to the engines by a helium system pressurized at 2800 psi. Two
95 lb translation thrusters and eight 23 lb attitude thrusters were mounted along the
bottom rim of the adaptor, and two 79 lb and 4 95 lb thrusters were mounted at the front
of the adaptor. Power was supplied by 3 silver-zinc batteries to a 22- to 30-volt DC
two-wire system. During reentry and post-landing power was supplied by four 45 amp-hr
silver-zinc batteries.
Communications
Voice communications were performed at 296.9 MHz with an output power of 3 W. A backup
transmitter-receiver at 15.016 MHz with an output power of 5 W was also available. Two
antenna systems consisting of quarter-wave monopoles were used. Telemetry was transmitted
via three systems, one for real time telemetry, one for recorder playback, and a spare.
Each system was frequency-modulated with a minimum power of 2 W. Spacecraft tracking
consisted of two C-band radar transponders and an acquisition-aid beacon. One transponder
is mounted in the adaptor with a peak power output of 600 W to a slot antenna on the
bottom of the adaptor. The other is in the reentry section, delivering 1000 W to three
helical antennas mounted at 120 degree intervals just forward of the hatches. The
acquisition-aid beacon was mounted on the adaptor and had a power of 250 mW.
Reentry
At the time of reentry, the spacecraft would be maneuvered to the appropriate
orientation and equipment adaptor section would be detached and jettisoned, exposing the
retro rocket module. The retro rockets consisted of four spherical-case polysulfide ammonium
perchlorate solid-propellant motors (Star-13E (TE-M-385)) mounted near the center of the reentry adaptor module,
each with 11,070 N thrust. They would fire fired sequentially at 5.5 second intervals to initiate the spacecraft reentry into the
atmosphere, with attitude being maintained by a reentry control system of 16 engines, each
with 5.2 N thrust. The retrorockets could also be salvo fired in conjunction with a launch abort scenario above 15,000 feet to separate the spacecraft from the Titan II Launch vehicle. The retrorocket module would then be jettisonned, exposing the heat
shield at the base of the reentry module. Along with the ablative heat shield, thermal
protection during reentry was provided by thin Rene 41 radiative shingles at the base of
the module and beryllium shingles at the top. Beneath the shingles was a layer of MIN-K
insulation and thermoflex blankets. At an altitude of roughly 15,000 meters the astronauts
would deploy a 2.4 meter drogue chute from the rendezvous and recovery section. At 3230
meters altitude the crew releases the drogue which extracts the 5.5 meter pilot parachute.
The rendezvous and recovery section is released 2.5 seconds later, deploying the 25.6
meter main ring-sail parachute which is stored in the bottom of the section. The
spacecraft is then rotated from a nose-up to a 35 degree angle for water landing. At this
point a recovery beacon is activated, transmitting via an HF whip antenna mounted near the
front of the reentry module.
Crews
