The Soyuz 7K-LOK, a heavily-modified derivative of the 7K-OK, was to have been the Soviet equivalent of the Apollo CSM, taking two cosmonauts to lunar orbit (from where one would transfer to the LK lander, land on the moon, and return to orbit) and back. (Due to the failure of the N1 launch vehicle, the 7K-LOK never went to the moon, and was never flown manned.)

Although the 7K-OK used solar panels for power generation (as would all future Soyuzes except for the battery-powered 7K-T space-station ferry), the 7K-LOK instead used fuel cells (like the Apollo CSM) - why?


I don't have definite knowledge, but my guess is that the basic Soyuz needed to support long duration missions, while the Soviet lunar mission profile was sharply limited in time.

Fuel cells consume hydrogen and oxygen to run, while solar panels provide power for an essentially unlimited duration, but are heavy for the power they produce. Thus, there's some break-even point in mission duration -- shorter missions will be lighter if you use fuel cells, but longer ones would require enough hydrogen and oxygen tankage that solar cells would be lighter.

I believe the Soviet lunar landing mission profile had to be similar to Apollo 11: a short stay on the surface, a single surface EVA, no lingering in lunar orbit to do experiments (as the later Apollo flights did). The N-1 couldn't put as much mass into a translunar trajectory as the Saturn V, so every part of the mission would be stripped down to the bare essentials. Thus, a 7K-LOK mission would be only 8 days, as opposed to 7K-OK flights such as the 18-day Soyuz 9.

If we assume the mass-break-even duration for the Soviet fuel cell and solar technologies of that time was, say, 10 days, then going to fuel cells for the 7K-LOK would be a mass savings, and that mission design needed all the mass savings it could get.

The major hole in this explanation is the Zond/Soyuz 7K-L1. Being designed for lunar flyby missions, it should have faced similar tradeoffs, but it retained the Soyuz solar panels. It may be that such a major change to the electrical system wasn't attempted for Zond due to time constraints.

  • 1
    $\begingroup$ Is this the first mission that they flew a fuel cell? $\endgroup$ Jul 12 '19 at 20:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.