The two craft of the Soyuz 7K-OKS generation were modified from the original Soyuz 7K-OK. The new "probe and drogue" docking mechanism, which was first used by these two missions, featured an internal docking hatch that allowed for the first time internal transfer between Soviet spacecraft. This "probe and drogue" docking mechanism introduced with Soyuz 7K-OKS is still in use today at the ISS. The external toroidal fuel tank, a holdover from the original lunar mission models of the Soyuz, was dropped from the 7K-OKS since it was unneeded for Earth orbital flights.
On its maiden flight, the 7KT-OK successfully launched into earth orbit, but failed to dock completely with the Salyut 1 space station. Upon reentry, the spacecraft encountered problems with toxic fumes.
This generation of Soyuz spacecraft is notable for the first successful manning of the first space station Salyut 1 by Soyuz 11 – this success was however overshadowed by the death of the crew, who were killed when the capsule depressurized during the re-entry phase.