The second Soviet crewed space tragedy occurred just four years after the loss of Col. Komarov on Soyuz 1.
Soyuz 11 was launched on June 6 1971 with the goal ferrying cosmonauts G. Dobrovolsky, V. Patsayev, and V. Volkov to the Salyut 1 space station.
Soyuz 1 successfully docked with the station on June 7, and the crew began a mission to set a then record duration of 23 days (22 on the station). All the while the crew conducted biological, astrophysical, and geophysical experiments, and gave daily broadcasts to the Soviet people every night on their progress in this strange new weightless world. This made the following events even more tragic.
On June 29, after what seemed to be a smooth undocking and nominal retrofiring, communication with the reentering capsule was lost just before the normal blackout period. During this time a valve, which had malfunctioned at the time of separation from the orbital module, allowed all the air to escape from the descent module capsule.
The crew did not have spacesuits, or partial pressure suits because the reentry module design lacked sufficient room for the three cosmonauts to wear them. The crew was unable to close the valve manually. They were found dead after the capsule completed its automated touchdown sequence.
The crewed Soyuz program was again grounded, this time for nearly two and a half years. By the time Soyuz 12 was flown in September 27, 1973 the Soyuz spacecraft was not only reengineered with redundant backup systems, but cosmonaut crews were now required to wear partial pressure suits, a practice which still continues to this day.
Until the Soyuz-T variant became available, the number of cosmonauts was restricted to two in order to accommodate the bulky suits.