Working on an alternate-history SF story that involves an orbital rescue in 1975 requiring use of the Soyuz 18 capsule. This had room for carrying up to 3 cosmonauts to orbit but only carried 2 to the Soviet Salyut 4 station in 1975. If they tried to return to Earth in it with 3 people instead of 2, would that have been possible? I assume they didn't launch from Earth with an empty 3rd seat; why waste the mass? But would there have been room for a 3rd person on return, assuming they cobble together some kind of cushion/seat? Anyone know how the actual interior of Soyuz 18 was configured? Thanks all.
No. At least, not easily. Soyuz 18 used a variant of the Soyuz capsule (the 7K-ST) that could only handle two crew. After Soyuz 11 tragedy, the spacecraft was redesigned so that the space that would be taken up by another crew member was instead taken up by life support. So there would be a major redesign of the spacecraft required in order to do that (which happened when the Soyuz-T was designed).