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All manned spacecraft have had seats in an "on-back" angle for launch. What is the benefit of this seating position for a manned spacecraft launch?

This a example of the "on-back" seating angle

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  • $\begingroup$ The answers are fine, but the real benefit of tolerating high-g environment better is much better concentration which is critical for taking any manual actions in an emergency. $\endgroup$ – Deer Hunter Dec 17 '13 at 20:39
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The human body tolerates G-forces best if they're perpendicular to the spine.
When you apply a G-force along the spine, the blood will gravitate towards the feet, leaving the head starved of blood, which induces loss of consciousness. Having the head at the lowest position helps to prevent these blackouts.
3G is tolerable even if you're facing down, by the way. The seating position in capsules is chosen to support the body during landing (the forces in reentry and landing are larger than during launch).

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let us take the shuttle

"Space Shuttle (takeoff): 29 m/s2"

which means 29/9.8 $/approx$ 3 In such a case the force acting on the body is would be tremanderous so if your seating position is in opposite direction the force is strong enough to crush your ribs(chest) and the broken bones from the ribs can even penetrate leading to death or it can causebreathing problem and even cardioc attack.

If the seating is on-back" angle for launch the much of the force during the accleration would be transfered to your back side of the body to the chair so less force felt by your heart

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    $\begingroup$ Fast-jet pilots pull 9g in a more-or-less vertical seats. 3g will not crush your ribs. $\endgroup$ – Pete Kirkham Dec 17 '13 at 13:22
  • $\begingroup$ An alternative way to put it would be to say a 70kilogram astronaut would encounter the exertion of carrying an extra 140kilograms (+; $\endgroup$ – Everyone Dec 18 '13 at 16:24
  • $\begingroup$ @PeteKirkham while jet seats are vertical, jets don't usually accelerate up. the engine provides thrust forward, hence the seat is faced forward for the same reasons. presumably that 9g acceleration vector would not push along the pilot's spine. $\endgroup$ – Florian Castellane Nov 22 '18 at 7:06
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    $\begingroup$ @Florian, there is no jet that can provide anywhere near 9g acceleration from the engine. Acceleration from the wing is vertical. $\endgroup$ – prl Nov 22 '18 at 19:06
  • $\begingroup$ @FlorianCastellane the 9g comes from lift from the wings in tight turns. Some jets can 'stand on their tails' - i.e. they can provide 1g of thrust, but the force due to turning is much higher. $\endgroup$ – Pete Kirkham Nov 22 '18 at 20:26
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Interestingly, the Soyuz capsule has seats that change orientation at landing time.

They are mounted appropriate for absorbing G-Loads the best at launch. Then seconds before landing, the seats change orientation to prepare for better shock absorption of landing.

Apollo's seats had shock absorbers for landing, and required a certain depths for 'travel' which was an issue they has to consider when they designed the Skylab rescue Apollo for the worst case of a disabled Apollo at Skylab. They would need to carry at least 4 people, (one pilot up, 3 down, more likely 2 up so they could work as a team and three down) which meant 2 rows of seats, which the travel requirement caused issues with making it all fit.

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