Soyuz 6

Soyuz 6 (Russian: Союз 6, Union 6) was part of a joint mission with Soyuz 7 and Soyuz 8 that saw three Soyuz spacecraft in orbit together at the same time, carrying a total of seven cosmonauts. The crew of Georgi Shonin and Valeri Kubasov were meant to take high-quality movie photography of the Soyuz 7 and Soyuz 8 docking, but the rendezvous systems on all three spacecraft failed.

It is still not known exactly what the actual problem was, but it is often quoted as being a helium pressurization integrity test. The version of Soyuz 7K-OK spacecraft used for the missions carried a torus-shaped docking electronics equipment housing surrounding the motor assembly on the back of the service module, which is thought to have been pressurised with helium to provide a benign environment for the electronics. It was then jettisoned after docking to lower the mass of the spacecraft for reentry. Due to unstable temperature, disparity between the frequencies of the transmitters and receivers, which were stabilized by special quartz resonators, occurred. The piezocrystals were supposed to be in thermostats at a strictly constant temperature.