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Landing with a Soyuz capsule is often compared to being in a car accident1, it's pretty violent on touch-down. On the other hand, I imagine the Space Shuttle's touch-down to be a little like a rough plane landing.

So I was wondering, which space craft has or had the most gentle touch-down? And how about the forces on reentry per se? Are there capsules/crafts where the crew experience less deceleration forces compared to other or are these forces experienced by the crew more or less the same for all of them?

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Of crewed spacecraft, the space shuttle had the gentlest landing. No other crewed spacecraft had rubber tires, shock absorbers, and a "flare" at touchdown. (Mercury had an airbag.) The shuttle's maximum allowable vertical descent rate at touchdown was 2.75 meters per second, and this article suggests that the goal was closer to 1 m/s.

Soyuz descends at ~7 m/s under parachute, but a solid rocket fires at about 1 second prior to touchdown to further slow the capsule to 1.5 m/s -- but the touchdown rocket itself is a bit of a jolt, and there's no landing gear to absorb the impact force.

Apollo splashed down at 10 m/s if all three parachutes were functioning normally. Apollo 15 came down a little faster as one of its parachutes was fouled. The capsule hung from the chutes at a slight angle, to help spread the impact force over time by hitting on an edge instead of "belly-flopping" on the broad heat shield. The nature of splashdown led to some variability in crew comfort; Apollo 12 hit a rising wave on splashdown and a camera was jolted out of its mount, concussing lunar module pilot Alan Bean.

Gemini and Mercury were somewhat similar. Mercury had an inflatable airbag between the heat shield and the capsule. Gemini hung at a much steeper angle than Apollo for an edge-first entry.

As for reentry, shuttle again was the most comfortable; the heat resistant tiles allowed it to spend more time in reentry than the ablative heat shields of the small capsules, decelerating at a gentler rate, and the maximum g-force was around 3g.

Soyuz, Apollo, and Gemini all use/used lifting reentry. The center of gravity of the capsule is offset from the geometric center, so the heat shield hits the airstream at a slight angle, producing lift, which keeps the capsule from descending too quickly. Without the lift, the capsule would fall quickly into denser and denser air before it has a chance to bleed off much of its orbital speed, and the peak deceleration is much higher.

When Soyuz' lifting entry works properly, it subjects the crew to about 4.5g maximum. When it doesn't work properly, if for example the reentry module doesn't separate from the service module (as has happened a couple of times), the capsule doesn't produce significant lift, and it goes into an 8g ballistic reentry -- very uncomfortable but not likely to cause permanent injury.

Apollo capsules peaked at about 6g on reentry.

Mercury was brutal: there was no lift, and it came down like a cannonball until the air stopped it; the peak exceeded 11g on the suborbital flights but was closer to 8g on the orbital flights, which descended less steeply. On the bright side, at 11g deceleration the reentry is over very quickly!

I assume Shenzhou is very similar to Soyuz in descent characteristics, but I've never seen any statistics for it.

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    $\begingroup$ If Dreamchaser gets a crewed version in the next 10 years, I bet it will take the crown, with only 1.5 Gs on reentry. Of course, it's current non-existence precludes it from being an answer to this question. $\endgroup$ – ORcoder Feb 18 '18 at 17:48
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    $\begingroup$ You could stand up and walk around in the Orbiter during entry. True Story. books.google.com/… $\endgroup$ – Organic Marble Feb 18 '18 at 17:57
  • $\begingroup$ The Apollo CM had a LOAD-ATTENUATION SYSTEM. It was a Aluminum-honeycomb-core load attenuator attached to the crew couches. You may call it a shock absorber too. $\endgroup$ – Uwe Feb 18 '18 at 18:10
  • $\begingroup$ Also the dragon 2 capsule is able to propulsively land using it's launch abort system but it won't. $\endgroup$ – Dragongeek Feb 18 '18 at 21:49
  • $\begingroup$ Apollo 15 had a very hard splashdown, about 15 g at least, see page 15 (21 of the pdf) of this paper. This was estimated from the state of the Apollo Couch Energy Absorbers after recovery of the capsule. $\endgroup$ – Uwe Feb 19 '18 at 15:21

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