Soyuz 11 was the first successful visit to the world’s first space station, Salyut 1. However the mission ended in disaster when the crew capsule depressurised during preparations for re-entry, killing the three-man crew. This accident resulted in the first and to date only astronaut deaths to occur in space (not in high atmosphere). The cosmonauts aboard Soyuz 11 were Vladislav Volkov, Georgi Dobrovolski and Viktor Patsayev.
Shortly after Soyuz 11 undocked from Salyut 1 and made an initial retro fire, communication was lost with the crew far earlier than normal. The capsule descended and was recovered on June 29, 1971 23:17 GMT. When the hatch was opened it was discovered that the crew was dead.
After an apparently normal re-entry of the capsule of the Soyuz 11 mission, the recovery team opened the capsule to find the crew dead. It quickly became apparent that they had suffocated. The fault was traced to a breathing ventilation valve, located between the orbital module and the descent module, that had been jolted open as the descent module separated from the service module. The two were held together by explosive bolts designed to fire sequentially, but in fact, they fired simultaneously. The force of this caused the internal mechanism of the pressure equalization valve to loosen a seal that was usually discarded later, and normally allowed automatic adjustment of the cabin pressure. The valve opened in space, and the gradual loss of pressure was fatal within seconds. The valve was located beneath the cosmonaut’s couches, and was impossible to locate and block before the air was lost. Flight recorder data from the single cosmonaut outfitted with biomedical sensors showed death occurred within 40 seconds of pressure loss. By 935 seconds after the retrofire, the cabin pressure was zero.
The Soyuz spacecraft was extensively redesigned after this incident to carry only two cosmonauts. The extra room meant that the crew could wear space suits during launch and landing. A Soyuz capsule would not hold three cosmonauts again until the Soyuz-T redesign in 1980, which freed enough space for three cosmonauts in lightweight pressure suits to travel in the capsule.
- Science: Triumph and Tragedy of Soyuz 11 (time.com)