Soyuz 7K-OKS

The Soyuz 7K-OKS was a crewed Soviet spacecraft built to ferry cosmonauts to the Salyut 1 space station. It was a modification of the earlier Soyuz 7K-OK that added a lightweight docking system and an internal crew transfer tunnel, so that cosmonauts could pass from the ship into the station. The complete vehicle had a gross mass of about 6,790 kilograms, carried a crew of three, offered roughly nine cubic metres of habitable volume and could store supplies for up to 35 days in orbit. It was made up of a descent capsule, a living or orbital module and a service module with the KTDU-35 engine. Only two were ever flown, in April and June 1971. Soyuz 10 could not dock with Salyut 1 despite several attempts, and Soyuz 11 was successful until the return to Earth, when a valve between the orbital and descent modules jarred open and vented the cabin atmosphere, killing the three crew members, who flew without spacesuits. After that accident the Soyuz underwent a complete safety redesign to the Soyuz 7K-T configuration.