Why did the Apollo capsule have seats if the astronauts never were sitting in it?

Question

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).

Answer 1

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.

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.

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.

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.

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

Answer 2

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.

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.