Salyut 6

Salyut 6, launched on a Proton 8K82K rocket on 29 September 1977,[5] 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.[1]

Salyut 6 with Progress (P) and Soyuz (F) spacecraft

The most important feature on Salyut 6, however, 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. However 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, whilst 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.[1] 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.[6]

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.[1]

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 unsymmetrical dimethylhydrazine and nitrogen tetroxide,[3] 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.[1] 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.

Salyut 6's propulsion system experienced a serious malfunction during the second crew residency in 1978 and was not usable again for the remainder of the station's lifespan. As a consequence, it was limited to firing its attitude-control thrusters, and visiting spacecraft had to perform orbital adjustments. After each crew residency ended, it was necessary for Progress and TKS spacecraft to boost the station into a high orbit, so it wouldn't decay until the next residency began.

To enable spacewalks, Salyut 6 was equipped with an inward-opening EVA hatch on the side of the forward transfer compartment, which could be used as an airlock in a similar way to the system used on Salyut 4. This compartment contained two new semi-rigid spacesuits, which allowed much greater flexibility than earlier suits and could be donned within five minutes in case of an emergency. Finally, the station offered considerable improvements in living conditions over previous outposts, with machinery being soundproofed, the crews being provided with designated "cots" for sleeping and the equipping of the station with a shower and extensive gymnasium.[1]

The original Progress variant, which was first used to resupply Salyut 6 in 1978.

Salyut 6 was primarily supported by the crewed Soyuz spacecraft, which carried out crew rotations and would also have been used in the event of an emergency evacuation. The ferries docked automatically to the station, making use of the new Igla automatic docking system, and were used by departing crews to return to Earth at the end of their flight.[7]

The station was the first to be able to be resupplied by the newly developed uncrewed Progress freighters, although they could only dock at the rear port, as the front port lacked the plumbing used to refuel the propulsion system. The freighters docked automatically to the station via the Igla, and were then opened and emptied by the cosmonauts on board, whilst transfer of fuel to the station took place automatically under supervision from the ground.[1][7]

In addition to the Soyuz and Progress spacecraft, after the final crew had left, Salyut 6 was visited by an experimental transport logistics spacecraft called Kosmos 1267 in 1982. The transport logistics spacecraft, known as the TKS, was originally designed for the Almaz programme, and proved that large modules could dock automatically with space stations, a major step toward the construction of multimodular stations such as Mir and the International Space Station.[8][9]

The station received 16 cosmonaut crews, including six long-duration crews, with the longest expedition lasting 185 days. Resident crew missions were identified with an EO prefix, whilst short-duration missions were identified with EP.

1. On 10 December 1977 the first resident crew, Yuri Romanenko and Georgy Grechko, arrived on Soyuz 26 and remained aboard Salyut 6 for 96 days.
2. On 15 June 1978, Vladimir Kovalyonok and Aleksandr Ivanchenkov (Soyuz 29) arrived and remained on board for 140 days.
3. Vladimir Lyakhov and Valery Ryumin (Soyuz 32) arrived on 25 February 1979 and stayed 175 days.
4. On 9 April 1980 Leonid Popov and Valery Ryumin (Soyuz 35) arrived for the longest stay on Salyut 6, 185 days. While aboard, on 19 July 1980, they sent their greetings to the Olympians and wished them happy starts in the live communication between the station and the Central Lenin Stadium, where the opening ceremony of the 1980 Summer Olympics was held. They appeared on the stadium's scoreboard and their voices were translated via loud speakers.
5. A repair mission, consisting of Leonid Kizim, Oleg Makarov, and Gennady Strekalov (Soyuz T-3) worked on the space station for 12 days starting on 27 November 1980.
6. On 12 March 1981 the last resident crew, Vladimir Kovalyonok and Viktor Savinykh, arrived and stayed for 75 days.

• Timeline of longest spaceflights