DOS-7K No. 2: A failed Salyut
On July 29, 1972, the USSR attempted to launch a second copy of the Salyut space station, however, its UR-500K (Proton) rocket failed to reach orbit, so the mission was never publicly announced.
DOS-2 mission at a glance:
In accordance with established practice, the Soviet space program funded the construction of at least two flight-worthy vehicles and the Salyut space station project was not an exception. The construction of the second Salyut, identified in the design paperwork as DOS-7K No. 2 or simply DOS-2, proceeded at Khrunichev Rocket and Space Plant, RKZ, Moscow with around a year-long lag relative to the original.
The second Salyut was practically identical to its predecessor, but, in May 1971, Vasily Mishin, Designer General at the TsKBEM design bureau in charge of the project, made some notes about possible spacewalks aboard the planned DOS-2. Although the forward compartment on DOS could be configured to serve as an airlock, spacewalks had not been attempted either on DOS-1 nor would be tried aboard several subsequent Soviet space stations until the launch of Salyut-6 in 1977.
Change of plans
Following the Soyuz-11 disaster in June 1971, the 7K-T transport ship was grounded for safety upgrades, while the first Salyut could not be sustained in orbit for new expeditions due to its dwindling resources.
By early August 1971, a new plan was drafted to launch a duplicate station, then still under construction, in the first quarter of 1972. Three of four modified 7K-T transport vehicles were expected to be allocated for delivering crews to the DOS-2 station throughout 1972. Like its predecessor, DOS-2 was designed to support up to three month-long expeditions lasting a total of 90 days.
It would be followed by the DOS-3 station in the fourth quarter of 1972, which would also operate for a year with the support of three or four 7K-T crew transports. Finally, DOS-4 would be launched at the end of 1973 to host up to four expeditions arriving aboard a new 7KS crew ship variant, featuring the Lira rendezvous system replacing the older Igla system.
However, the unpiloted test flight of the 7K-T variant, modified after the Soyuz-11 tragedy, only reached orbit in June 1972. By that time, specialists at the Tyuratam launch site were putting the finishing touches on the DOS-2 space station. In the footsteps of its predecessor, the fresh station was expected to be publicly announced as Salyut-2 and the surviving visuals confirm that the name was actually painted on the vehicle.
Simultaneously, two 7K-T transport vehicles – No. 34 and 35 – were prepared for launch in Tyuratam.
Because the upgrades of the 7K-T transport ship after the Soyuz-11 accident required reducing the number of crew members from three to two, planned expeditions to the second DOS station were sized accordingly. Four pairs of cosmonauts were officially approved for the DOS-2 training on Oct. 10, 1971:
Kubasov, who was excluded from the Soyuz-11 mission on the eve of the faithful launch in 1971, was able to return to active training, after it was concluded that his lung issue had been the result of an allergic reaction to a chemical used to spray bushes near the Kosmonavt hotel in Tyuratam. Four other cosmonauts were moved to the DOS-2 program from the Kontakt project, which had just been canceled after several years of hopes to test an alternative docking hardware for lunar expeditions. At the same time, Petr Kolodin and A. Voronov, who were members of original three-person expeditions trained for the original Salyut, got no flight assignments in the re-configured crews and they would never make it to space.
The active cosmonaut training for the DOS-2 project was conducted from October 1971 to July 1972. (231)
DOS-2 fails at launch
The UR-500K rocket with the DOS-7K No. 2 station lifted off from Pad 23 at Site 81 in Baikonur on July 29, 1972. The first stage of the launch vehicle performed normally and separated as planned, but the second stage failed during its firing at T+181.9 seconds in flight. The stricken vehicle hit the ground 536 kilometers downrange from the launch site.
The investigation showed that the automated stabilization system of the second stage had malfunctioned due to a short circuit in its pitch and yaw channels. It was caused by an elastic deformation of the housing during operation in vacuum and was attributed to a design flaw. In the aftermath of the failure, the instrument underwent redesign and new testing. (997)
As usual for failed missions that had never reached orbit, the USSR made no public announcement about what was supposed to be the Salyut-2 space station, thus leaving this designation for a subsequent mission.
Leonov (right) and Kubasov working inside a full-scale simulator of the Salyut space station.
The DOS-7K No. 2 space station.