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Crewed spacecraft for travel to/from Earth, and much speculation for the SpaceX Starship, have seats with the astronauts upper body level in an eyeballs in orientation. This is because this is the orientation human tolerate acceleration best.

However it is also the case that the astronauts upper legs are vertical like a typical chair rotated 90 degrees, rather than horizontal like a bed. Is there any reason for this posture other than the small size of some spacecraft and varying acceleration direction of the Space Shuttle?

I have found a NASA report Seating Considerations for Spaceflight: The Human to Machine Interface but it doesn't go into leg orientation.

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    $\begingroup$ Highly related: space.stackexchange.com/q/46746/6944 $\endgroup$ Dec 27 '20 at 1:31
  • $\begingroup$ It seems this question is more about the seated position as posture than seats as furniture. I have altered the title to better fit that. If this is incorrect, feel free to revert the change. $\endgroup$
    – DrSheldon
    Jul 23 at 12:32
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The primary reason for placing astronauts in a seated posture instead of a standing posture is that it is the best posture to tolerate higher accelerations (and for a greater period of time). The seated position places the legs forward of the rest of the body. When the spacecraft accelerates in the forward direction, this displaces blood out of the legs and into the brain. In contrast, a standing position leaves the legs at the same level as the head, preventing the displacement of blood to the brain.

The results can be seen in the following graph from The Physiological Basis for Spacecraft Environmental Limits, p. 81:

posture vs tolerance graph

The best tolerance is situation A, where the astronaut is immersed in water; however, this is impractical for a spacecraft. The next best tolerance is B, the seated position with forward acceleration, as is actually used on spacecraft. C and D are standing postures. The worst tolerance is F, standing headfirst.

Some less-important advantages of the seated posture:

  • A seated human is slightly more compact than when standing. The most compact form is the fetal position; your mother's womb was your first "space capsule"!
  • The two surfaces of the seat are better at restraining the astronaut in place than the single surface of a standing couch.
  • When performing tests and training on Earth, seats are more ergonomic than a standing position.
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    $\begingroup$ And astronauts (mainly being fighter jocks) are used to handling hi g forces while sitting in a cockpit. It seems obvious to let them continue doing what works best for them. $\endgroup$
    – Snow
    Jul 23 at 13:39
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    $\begingroup$ Good data. Bad, bad graphing technique. They use similar line indictors, including for crossing lines. utterly arbitrary orientation on the directional diagrams, different scaling of diagrams (unless D is intended for children, compared to B which is almost twice as tall). And , although they do label it a such, an effectively arbitrary and non-representational scaling on the time axis. It is hard to believe such shoddy presentation comes from a 1979 formal NASA research paper, my uni shot down my papers for much less egregious things. $\endgroup$
    – PcMan
    Jul 23 at 17:29

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