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Salyut 6

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Salyut 6

Salyut 6 (Russian: Салют 6, lit.'Salute 6') was a Soviet orbital space station, the eighth station of the Salyut programme, and alternatively known DOS-5 as it was the fifth of the Durable Orbital Station series of civilian space stations. It was launched on 29 September 1977 by a Proton-K rocket. Salyut 6 was the first space station to receive large numbers of crewed and uncrewed spacecraft for human habitation, crew transfer, international participation and resupply, establishing precedents for station life and operations which were enhanced on Mir and the International Space Station.

Salyut 6 was the first "second generation" space station, representing a major breakthrough in capabilities and operational success. In addition to a new propulsion system and its primary scientific instrument—the BST-1M multispectral telescope—the station had two docking ports, allowing two craft to visit simultaneously. This feature made it possible for humans to remain aboard for several months. Six long-term resident crews were supported by ten short-term visiting crews who typically arrived in newer Soyuz craft and departed in older craft, leaving the newer craft available to the resident crew as a return vehicle, thereby extending the resident crew's stay past the design life of the Soyuz. Short-term visiting crews routinely included international cosmonauts from Warsaw Pact countries participating in the Soviet Union's Intercosmos programme. These cosmonauts were the first spacefarers from countries other than the Soviet Union or the United States. Salyut 6 was visited and resupplied by twelve uncrewed Progress spacecraft including Progress 1, the first instance of the series. Additionally, Salyut 6 was visited by the first instances of the new Soyuz-T spacecraft.

The success of Salyut 6 contrasted with the programme's earlier failures and limited successes. The early history of the programme was plagued by the fatalities of Soyuz 11 and three launched stations which quickly failed. Earlier successful stations received few crews, limited to several weeks' habitation by the design life of their Soyuz craft and the presence of a single docking port per station; unsuccessful docking was also common. Salyut 6 on the other hand routinely received successful dockings of crewed and uncrewed craft, although the first visiting craft Soyuz 25 and later Soyuz 33 failed to dock with the station.

From 1977 to 1981, the station was occupied by human crews during six separate, discontinuous intervals, each coterminous with the presence of a resident crew who were first-in, last-out while support crew visited. Between each of these intervals Salyut 6 was vacant, although it was visited by Soyuz T-1 and Kosmos 1267 during its periods of vacancy. Following the launch of successor Salyut 7, Salyut 6 was de-orbited on 29 July 1982, almost five years after its own launch.

Salyut 6, launched on a Proton-K rocket on 29 September 1977, marked the switch from engineering development stations to routine operations, and united the most effective elements from each of the previous stations. Its navigation system, made up of the Delta semi-automatic computer to depict the station's orbit and the Kaskad system to control its orientation, was based on that used on Salyut 4, as was its power system, which consisted of a trio of steerable solar panels, together producing a peak of 4 kilowatts of power over 51 m2. The station's thermal regulation systems, which made use of a sophisticated arrangement of insulation and radiators, was also derived from that used on Salyut 4. In addition, Salyut 6 made use of environmental systems first used on Salyut 3, and controlled its orientation using gyrodynes first tested on that station.

The most important feature on Salyut 6 was the addition of a second docking port on the aft end of the station, which allowed two spacecraft to be docked at once. This enabled resident crews to receive shorter, visiting expeditions whilst they remained on board, and for crew transfers to take place between the Soyuz craft. Typically, visiting crews returned to Earth in the older Soyuz which the resident crew had flown to the station, leaving a newer craft available to the resident crew as a return vehicle. This procedure allowed resident crews to remain aboard Salyut 6 past the three-month lifespan of the first-generation Soyuz 7K vehicle. Salyut 6 was not occupied continuously, or "handed off" from one expedition to another. Rather, the six resident crews took up occupancy in the station and were visited by various Soyuz support missions and Progress supply tugs, and upon each resident crew's departure, Salyut 6 was vacant. The first long-duration crew to visit the station broke a long-standing endurance record set on board the American Skylab station, staying 96 days in orbit; the longest expedition lasted 185 days. Most of the visiting expeditions were flown as part of the Intercosmos programme, with non-Soviet cosmonauts visiting the station. Vladimír Remek of Czechoslovakia, the first space traveller not from the US or USSR, visited Salyut 6 in 1978, and the station hosted cosmonauts from Hungary, Poland, Romania, Cuba, Mongolia, Vietnam, and East Germany. Georgi Ivanov, a Bulgarian, was a member of the crew of Soyuz 33, which failed to dock with the station. The failure implicated the resident crew's Soyuz 32, with the result that a modified Soyuz 34 was launched to them without a crew, as a return vehicle.

The rearward of the two ports was fitted with plumbing to allow the station to be refueled by uncrewed Progress spacecraft. These freighters, which brought supplies and extra equipment to keep the station replenished, helped ensure that the crew were always able to carry out useful scientific work aboard the station. In all, twelve Progress flights delivered over 20 tonnes of equipment, supplies and fuel.

The addition of the extra docking port caused the adoption of the Almaz-derived twin-chamber propulsion system first used on Salyut 3 and 5, with the two engine nozzles – each producing 2.9 kilonewtons of thrust – mounted peripherally on either side of the aft port. Salyut 6 introduced a unified propulsion system, with both the engines and the station's control thrusters running on N2O4 / UDMH, drawn from a common set of pressurized tanks, allowing the refueling capabilities of the visiting Progress tankers to be exploited to the maximal effect. The entire engine and fuel storage assembly was contained within an unpressurized bay at the rear of the station, which had the same diameter as the main pressurized compartment. However, the replacement of the Soyuz engine used on previous stations with the bay meant that the station kept an overall length similar to its predecessors. The main engines could not be fired if the rear docking port was in use, hence any orbital maneuvers during this time had to be performed by the visiting spacecraft.

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