Long duration orbital station of the second generation Salyut-4
The Salyut station was a new word in space technology, however the technical solutions implemented during its development quickly ceased to satisfy the developers. Some of them imposed certain constraints on efficiency of the station use, significantly limited the time of its operation. One of the disadvantages of this kind was a rigid installation of solar arrays (SA) on its body. For SA pointing to the Sun the corresponding turn of the whole station with its subsequent spin-up was required to keep the maximum SA lighting, in this case “this spin-up” shall be periodically repeated because of instability of the rotation itself that resulted in significant propellant consumption. Such a mode confined the conduct of research and reduced the total operation time of the station in orbit.
The basic structural change in the second generation station as compared with the first Salyut OS was introduction of three SA each of which could rotate around its longitudinal axis. To compensate the mass increase required for introduction of such SA the number of tanks of the propulsion system was decreased and to reduce the propellant needs for the orbit correction its altitude was increased to 350 km. Superefficient attitude control system Kaskad and experimental navigation system Delta were installed for the first time on the station and for the first time an experimental loop with heat pipes was used in thermal control system (TCS), all this proved to be very perspective for the subsequent generations of orbital stations. Also work on creating the closed cycle of the crew supply with water began, for which purpose system of water regeneration from condensate (SWR-C) was installed onboard the station; using this system moisture released by the crew during the vital activity was collected, cleared and became suitable for drinking. The new science hardware was installed, namely X-ray spectrometer–telescope, mirror X-ray telescope (XRT-4), infrared spectrometer-telescope (IRST-R) and other scientific devices. The assured operation life of the station in orbit was prolonged from 90 to 180 days.
The first of the second generation stations LDOS No. 3 (Kosmos557) was injected into orbit on May 11, 1973. For orbit correction the attitude control system of the station was used, however because of the failure of the ionic flux sensors of the motion control system in the phase beyond the radio visibility zone the full fuel depletion of the correcting propulsion system of the long duration orbital station occurred that excluded a capability of its normal in-orbit operation. The station ceased its existence in May 1973. It was especially offensive to the work participants as on May 14, 1973 the first U.S. orbital station Skylab was injected into orbit; it was used during the forced break in Salyut OS flights.
In 1972–1973 engineers and workers of TsKBEM, TsKBM (F), ZEM and ZIKh manufactured new station of the second generation Long Duration Orbital Station – 4 (article 17Ê No. 124). By the science hardware structure and content it was similar to LDOS3 lost earlier. On December 26, 1974 the station was launched and after injection it was called Salyut4. It was more lucky than all its predecessors: two expeditions of the duration of 28 and 63 days successfully worked onboard the Salyut-4 OS.
The Salyut-4 Orbital Station on the assembly trolley
One of the tasks of the Salyut-4 station operation was work to support longer in-orbit operation of transport vehicles. To verify the Soyuz MTV service life the Soyuz-20 vehicle with no crew docked to the Salyut-4 station upon completion of its operation in the manned mode; the vehicle operated in orbit as a part of the complex for 97 days. On February 3, 1977 on command from Earth the Salyut-4 station was de-orbited and ceased its existence in the water area of the Pacific Ocean.
Work on the Salyut-4 station was highly appreciated by the country leaders. Chief Designer Yu.P. Semenov and assembler of the Soyuz vehicles, ZEM, V.I. Morozov were awarded a title of Hero of Socialist Labour. Some detail characteristic of that time is of interest. When approving candidates in the staff of the Central Committee of the CPSU, as it was done, there was a strong objection; it turned out that V.I. Morozov was not a member of the CPSU. But TsKBEM and ZEM leaders were not going to depart from the letter of the law. They had to solve the problem of admitting V.I. Morozov as a candidate member of the CPSU within two days. The objection of the Central Committee staff was withdrawn.