Gemini 6A

Description

Gemini 6A was the fifth crewed Earth-orbiting spacecraft of the Gemini series, having been launched after Gemini 7, with the intent of making rendezvous with Gemini 7 in Earth orbit. The astronauts on the 26 hour mission were Walter Schirra and Thomas Stafford. The mission priorities were to demonstrate on-time launch procedures, closed-loop rendezvous capabilities, and stationkeeping techniques with Gemini 7. Other objectives were to evaluate the spacecraft reentry guidance capabilities, and conduct spacecraft systems tests and four experiments. This mission was originally designated Gemini 6 and scheduled for launch on 25 October but was cancelled when the Agena target vehicle failed to go into orbit an hour earlier.

Mission Profile

Gemini 6A was scheduled to launch on 12 December 1965, but the launch was aborted one second after engine ignition because an electrical umbilical separated prematurely. This was the first time an astronaut mission was aborted after ignition start. The mission launched successfully from Complex 19 on 15 December at 8:37:26 a.m. EST (13:37:26.471 UT) and was inserted into a 161.0 x 259.4 km orbit at 8:43:25. At this point Gemini 6A trailed Gemini 7 by about 1900 km. Four major thruster burns were performed starting at 9:11 to catch up to Gemini 7. First radar lock indicated a distance of 396 km. Two more major thruster burns preceded the final braking maneuver at 2:27 p.m. EST. Rendezvous was technically achieved and stationkeeping begun at 2:33 with the two Gemini spacecraft in zero relative motion at a distance of 110 meters. Stationkeeping maneuvers involving the spacecraft circling each other and approaching and backing off continued for 5 hours 19 minutes over three and a half orbits. During the maneuvers, all four astronauts on both spacecraft took turns in the formation flying activities and photographs were taken from both spacecraft. This marked the first time two spacecraft were maneuvered with respect to each other by their crews. At the end of stationkeeping Gemini 6 fired thrusters to move to a position roughly 50 km away from Gemini 7 for drifting flight during the sleep period.

Near the end of the 15th revolution the retrorockets were fired at 9:53:24 a.m. EST on 16 December and splashdown occurred at 10:28:50 at 23.58 N, 67.83 W only 13 km from the target. This was the first successful controlled reentry to a predetermined point in the U.S. manned spaceflight program. The spacecraft and crew were brought aboard the aircraft carrier U.S.S. Wasp at 11:32 a.m. The recovery and rendezvous section of the spacecraft splashed down in the same area and was retrieved, this was the first time the service section was recovered. Total mission elapsed time was 25:51:24.

All primary mission objectives were achieved. The only major malfunction was the failure of the delayed time telemetry tape recorder at 20 hours 55 minutes into the mission, resulting in loss of the last 4:20 of delayed time telemetry. The crew conducted three scientific experiments -- (1) synoptic terrain photography, (2) synoptic weather photography, and (3) dim light photography. The fourth experiment, measurement of radiation in spacecraft, was only partly completed.

Spacecraft and Subsystems

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 retrorocket module. The retrorockets consisted of four spherical-case polysulfide ammonium perchlorate solid-propellant motors mounted near the center of the reentry adaptor module, each with 11,070 N thrust. They would fire 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 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.

Gemini Program

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.