The Apollo capsule had three lying seats on its bottom, but the astronauts actually never sat in the capsule: during launch, reentry and in the ocean the astronauts were lying down, while in space they were weightless. When the astronauts fasten their seat belts in space you may call that 'sitting' but technically that's no sitting because they're weightless, it's tying themselves to the seat.

Why wasn't the Apollo spacecraft made so that the astronauts would simply lie on the bottom of the capsule during launch and reentry, without their legs up? They would just need to be tied to the bottom somehow (to a mattress perhaps).

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    $\begingroup$ The space behind the couches was for storage, and it gave them space to spread out. $\endgroup$ – GdD Sep 25 '20 at 7:15
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    $\begingroup$ Do not have a source handy yet but the couches were also part of the shock absorption system during landing. $\endgroup$ – GremlinWranger Sep 25 '20 at 7:20
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    $\begingroup$ most likely because legs-up is optimal for handling the G loads $\endgroup$ – JCRM Sep 25 '20 at 8:41
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    $\begingroup$ The question contains a false premise. As it states, itself, the astronauts frequently sat in the seats. So never sat is wrong. $\endgroup$ – Oscar Bravo Sep 25 '20 at 15:51
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    $\begingroup$ @Giovanni You concocted a definition of sitting that excluded everything a normal person would consider sitting. Therefore, you proved your own premise. Anyway, as has been explained by the other answers, it was a silly question. $\endgroup$ – Oscar Bravo Sep 27 '20 at 10:24

The water impact deceleration was short but it could be strong. Vertical speed with only two parachutes was 36 foot/sec or 11 m/s, horizontal wind speed during a storm could be more. Apollo 8 wind speed was 32 ft/s or 19 knots or 9.75 m/s.

excerpt from Apollo  Operations  Handbook

There were attenuator struts to reduce the landing shock. When the capsule hit the water, the attenuators extended and the couches moved down. The space under the couches was needed to reduce the impact deceleration to the astronauts.

If the astronauts would simply lie on the bottom of the capsule, there would be no shock absorption at all during splash down.

I marked the attenuators with red arrows indicating their extension caused by the impact.

figure 1-13 from Apollo  Operations  Handbook

part of figure 1-15 from Apollo  Operations  Handbook

The blue arrow indicates the direction of horizontal movement caused by wind. An impact into a wave would move the couches in this direction. Foots and legs will be supported by the footpans and legpans of the couches for both the vertical and horizontal components of the impact velocity.

The "legs up" position for high g-loads was successfully tested in many experiments with rocket slides and centrifuges before. It prevents a blood shift from head and torso to the legs. This position was used for all rocket engine burns of the Saturn V and the Service Module, during launch, reentry, parachute deployment and splashdown.

excerpt from Apollo  Operations  Handbook

Source: Apollo Operations Handbook

Some numbers about the neccessary length of an attenuator: landing speed $ v = 15 m/s $, constant deceleration $ a = 150 m/s^2 $ or about 15 g.

$$ s = \frac{v^2} {2a} = \frac{15^2} {2*150} \frac{(m/s)^2} {m/s^2} = \frac{225} {300} \frac{m^2/s^2} {m/s^2} = 0.75 m $$

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    $\begingroup$ While landing had the greatest potential for injury, it is also worth mentioning that the astronauts also used the couches during launch, TDE, and mid-course corrections. This positioned the astronauts to see the instruments and reach the controls. There also was enough acceleration that you wouldn't want astronauts floating loose in the cabin during these times. $\endgroup$ – DrSheldon Sep 25 '20 at 11:28
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    $\begingroup$ @DrSheldon reaching the controls is another key point - if they were lying on the base of the capsule, you'd need to bring all the control units down in front of them - which would then mean they'd be in the middle of the capsule volume and make it much more complicated to use the space when in orbit. $\endgroup$ – Andrew Sep 25 '20 at 11:47
  • $\begingroup$ But the attenuator struts should work only during landing but not during launch. Reaching controls should not be impaired during flight. $\endgroup$ – Uwe Sep 25 '20 at 12:35
  • $\begingroup$ The capsule was designed to make a water landing without injury, and a landing on land survivable, possibly without injury. Impacting on land was a serious consideration, and a large part of what the struts were about. $\endgroup$ – Wayne Conrad Sep 26 '20 at 0:04
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    $\begingroup$ It's not just about protecting the astronauts - it's also about keeping the mass distribution even, so that the rocket doesn't need to abuse RCS to keep its angle right because it's unbalanced, and the capsule keeps the optimal angle of attack during descent. $\endgroup$ – SF. Sep 29 '20 at 15:20

Why wasn't the Apollo spacecraft made so that the astronauts would simply lie on the bottom of the capsule during launch and reentry, without their legs up? They would just need to be tied to the bottom somehow (to a mattress perhaps).

Actually, it sort of was, later.

While the standard and ideal configuration had three seats suspended from shock absorbing struts for landing, there was an alternate 5-seat "rescue" configuration prepared for the Skylab missions - two crew for the rescue mission, plus the three being rescued.

In that configuration the lockers were removed and replaced by two additional seats, which would not have had space for shock absorbing struts. It's tempting to conclude that getting banged up on landing beats running out of consumables in orbit, but perhaps the absorbing system was over-designed to begin with: an article at wikipedia claims:

The biggest risk in a rescue was the three upper seats "stroking" or collapsing onto the two lower seats in a rough landing, but no stroking occurred in previous missions.

The accompanying graphic to that article shows that the extra seats do have their knees raised somewhat, but not including the feet, and not as much as in the ordinary seating. Hey, it's a rescue, pragmatic compromises happen.

Skylab rescue

In terms of head-vs-feet the extra seats were also oriented 180 degrees opposite the usual ones. It's tempting to think that means they'd be upside down in the lifting reentry profile, but it's turns out that it's actually the traditional seats that are head-down during re-entry.

Apollo re-entry

  • $\begingroup$ "overdesigned" may be the wrong word. It was more than needed for the nominal case, but might have been more important if everything else wasn't perfect. E.g., were they built to tolerate a single chute failure? $\endgroup$ – fectin Sep 28 '20 at 14:12

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