Soviet Space Stations: Salyut 6

Cutaway of Salyut 6 with Soyuz and Progress (source)

Salyut 6 was the first real workhorse of the Soviet space stations. It had more visits than any other station up to that point, and provided some real experience for the modular space stations that followed.

The Design

Salyut 6 cutaway (source)

Salyut 6 pioneered several design changes for the following Salyut stations. The aft end was reworked, adding another docking port and new engines paralleling the port (similar to Almaz). This change also enabled propellant transfer by the newly developed Progress freighters, allowing it to have a much longer useful lifespan. Progress freighters could only dock at the rear, as that port had propellant transfer lines set up. The trio of solar panels were the same as those used on Salyut 4.

Scientific equipment included the BST-1M multispectra telescope (the conical area in the large working compartment), which could only be operated on the night side of Earth, and worked in infrared, ultraviolet, and submillimeter waves. Another instrument was MKF-6M multispectral camera, as well as KATE-140 mapping camera, both used for Earth sciences.

The Mission
Salyut 6 launched in September 1977, and the first expedition followed 10 days later. Soyuz 25, however, failed to latch after docking to the front port. Battery power was limited to two days in the 7K-T, and they didn't have enough fuel to try the aft port, so they aborted. Future missions required at least one space veteran in order to reduce pilot error as a factor.

EO-1 (The First Long-Term Expedition)

Soyuz 26 docked at aft port

Soyuz 26 launched in December and stayed for 96 days, a new record after Skylab. They docked at the aft port, and did an EVA from the transfer compartment at the nose (which was the first time they'd done an EVA from a Salyut space station). The front port was totally fine, so they repressurized the airlock as demonstrated in Salyut 5. The rest of the expedition was spent testing the navigation system and boosting the station's orbit using Soyuz 26's engines.

Soyuz 27 arrives at front port

Soyuz 27 launched in January and docked at the front port. This was the first time three spacecraft docked together that were all independently launched, and also the first spacecraft swap when they left only five days later in the Soyuz 26 spacecraft, leaving the original crew on-board for a longer-duration stay. The reason they took the other spacecraft was to free up the aft port for the upcoming Progress freighter, and they didn't yet feel comfortable moving it.

Visiting crew leaves in Soyuz 26, freeing aft port for Progress 1

Progress 1 launched about five days later, while the first expedition was still aboard. The Progress 7K-TG was an unmanned freighter, very similar to the Soyuz 7K-T that served as the main space station ferry up to this point. The primary difference was that the orbital module was used as pressurized storage, while the unshielded descent module was heavily modified as a propellant storage tank for refilling the station. Progress 1 docked to the aft port of Salyut 6, and offloaded 1000 kg of propellant, and 2300 kg of food, parts, and scientific equipment; in return, it was loaded up with waste from the station to be disposed on reentry. It undocked from the station a few weeks later and deorbited in February 1978.

Progress 1 docked to Salyut 6

Soyuz 28 launched in March and brought the first Czechoslovakian astronaut, the first who wasn't an American or Soviet, and marked the beginning of Interkosmos, a Soviet program to bring several allied foreigners to space. Like Soyuz 27, they simply visited the resident crew for a week before departing. A few days later, the Soyuz 26 crew also returned to Earth, taking with them the other Soyuz spacecraft. It took them two weeks to fully recovered from their record stay in space.

EO-2
The second long-term expedition launched in Soyuz 29 on 15 June, 1978 and set a new record of 139 days in space. They docked to the front port and de-mothballed Salyut 6 after its three months of inactivity. They also installed a Splav-01 furnace in the intermediate compartment so it could function in vacuum.

Soyuz 30 (aft port) went up a few weeks later with the second Interkosmos astronaut, this time from Poland. They stayed for about a week, studying life sciences and aurora borealis.

A few days later, Progress 2 arrived, replenishing their air supply and performing the first automated fuel transfer.

Progress 3 launched in mid-August 1978, followed by Soyuz 31 a couple weeks after, another Interkosmos flight with an East German onboard. The Interkosmos crew left in Soyuz 29 (front port), and the resident crew swapped the fresh Soyuz 31 spacecraft from the aft port to the front port by rotating the station 180 degrees. This was the first time they swapped ports (see Soyuz 26) and became a common occurrence thereafter.

Progress 4 launched in October, and in November the second resident crew left in Soyuz 31. However, they had just discovered a leak in the main station propulsion system, which if left unfixed would have corroded vital parts of the engines and fuel lines.

EO-3

Soyuz 32 brought the third long-duration crew in February 1979, and they stayed for 175 days, a new record. A couple weeks later Progress 5 arrived, and they began repairs of the propulsion system. They consolidated fuel in the good tanks, then spun the station around to purge the leaked fuel from the station, then sealed off the damaged tank. This completely fixed the propulsion system (aside from one sealed-off tank).

Soyuz 33 launched in April but the main engine failed and it reentered safely. This was supposed to replace Soyuz 32 as the crew's ride home, and sparked revisions in the design of future Soyuz ferries to improve reliability.

Progress 6 arrived in May with supplies and boosted the station's orbit, and in June, Soyuz 34 was launched unmanned to replace the crew's main spacecraft. It docked at the aft port,  then Soyuz 32 was loaded up with experiments to return to Earth and deorbited unmanned. Then Soyuz 34 was switched to the front port with another 180 degree flip of the station.

Deploying the KRT-10

Stamp commemorating construction of KRT-10 radio telescope on Salyut 6 (source)

In late June, Progress 7 brought a new major component, the KRT-10 radio telescope. They began assembling it in the aft compartment and the orbital module of Progress 7.  When it was ready, Progress 7 was undocked and the 10 meter antenna was unfurled from the aft port of Salyut 6. Unfortunately a few weeks later it failed to separate as planned, and was blocking both the main engines and the Progress resupply port. The crew performed an EVA to remove it, finding that some of the station's insulation had been damaged in the process. The crew left a week later in Soyuz 34.

While the station was empty for several months, a new spacecraft was tested. Soyuz T-1 launched in December and was the first flight test of the Soyuz-T, sporting brand new electronics hoping to fix the frequent docking problems earlier Soyuz suffered. It also carried solar panels again, allowing for 11 days of independent flight. The propulsion module was overhauled, with the main engines and reaction control systems now using the same fuel system. This saved weight and was more reliable. Finally, the descent module was made big enough to support a crew of three (for the first time since Soyuz 11) in space suits. Soyuz T-1 remained docked to the station for three months unmanned, then demonstrated a new technique for reentry by jettisoning the orbital module before making the deorbit burn. This reduced the fuel requirement and gave more flexibility in reentry maneuvering as well as cargo return and became the standard for the next eight years.

EO-4

Progress 8 launched in March 1980 in preparation for the fourth resident crew, which arrived in April in Soyuz 35 and stayed for 185 days.