Soyuz 6 launched October 11, 1969, the first an ambitious three spacecraft mission. Soyuz 6, 7 and 8 would rendezvous with Soyuz 6 providing spectacular film of the Soyuz 7 and 8 docking. However this never took place as none of the spacecraft were able to dock due to the failure of all their rendezvous systems.

It is still not quite known exactly what the actual problem was but it is often quoted as being a helium pressurisation integrity test. The version of Soyuz spacecraft used for the missions (7K-OK) carried a torus shaped docking electronics equipment housing surrounding the motor assembly on the back of the service module. This is thought to have been pressurised with helium to provide a nice benign environment for the electronics. It was then jettisoned after docking to lower the mass of the spacecraft for reentry. What went wrong with the electronics on all three spacecraft is still not known.

On board Soyuz 6 was Georgi Shonin and Valeri Kubasov each on their first spaceflights. They used the callsign Antey (or 'Antaeus' en anglais).

Even though the rendezvous failed the crew were able to complete some of their objectives. They investigated welding in orbit and a vacumn by using the Vulkan welding furnace in the depressurized orbital module.

They tested three methods: using an electron beam, a low pressure plasma arc and a consumable electrode. The apparatus was designed at the E. O. Paton Electric Welding Institute, Kiev, Ukraine. The weld quality was said to be in no way inferior to that of Earth based welds.

After 80 orbits of the Earth they landed on October 16, 1969, 180 km northwest of Karaganda, Kazahkstan.

Sources (various posts)