I was watching the redocking the other night and they kept mentioning that it can redock autonomously. If so, why is the crew there? Is it there as a backup in case the autonomous system fails? Is it so in case something happens and the capsule has to be deorbited, there aren't more people left on the ISS than can be evacuated in an emergency with the available vehicles?
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1$\begingroup$ Failure would be grim. $\endgroup$– Jon CusterCommented May 3 at 13:28
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1$\begingroup$ different but related and potentially helpful: Has the ISS ever not had a lifeboat? and Have space station occupants always had a return vehicle ready? $\endgroup$– uhohCommented May 5 at 16:46
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2$\begingroup$ @JonCuster Depends on the fault - might not want to have anyone on board in SOME situations! $\endgroup$– MikeBCommented May 6 at 7:44
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2 Answers
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If the capsule were to fail to autonomously redock successfully, the crew would not be able to use it to return to Earth. Therefore, the crew rides in the capsule so that if does not redock autonomously, they can either debug the problem or, if they fail to resolve it, simply return to Earth ahead of schedule.
The Dragon capsule [is used as] a lifeboat to escape the space station in an emergency. NASA did not want the astronauts on the space station without an escape pod if the Crew Dragon failed to redock with the complex. (ref)
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9$\begingroup$ This is the same reason this is done similarly by the Soyuz capsules as well. $\endgroup$– geoffcCommented May 3 at 10:54
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2$\begingroup$ Safety is a factor, in case of station evacuation, but money is the other aspect, as they'd have to send up a uncrewed/minimally-crewed rescue mission (see Soyuz MS-22/23). $\endgroup$ Commented May 3 at 18:36
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4$\begingroup$ Safety-critical activities can often end up resembling the question about the man with the cabbage, the goat, and the wolf. $\endgroup$– ikraseCommented May 5 at 4:27
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Every ISS crew member must always have a seat on a lifeboat available at any time. This is dictated by ISS Flight Rules, as confirmed in a comment by former Operations Controller Doresoom. Unfortunately, unlike Shuttle, the Flight Rules are not published.
If you undock Crew Dragon without its crew in it, then its crew will have no lifeboat available during that time, thus violating Flight Rules.
There was an interesting situation in 2023 where those lifeboat rules were tested: the Soyuz MS-22 capsule had sprung a leak in its coolant system, and there was concern that the astronauts would be subjected to high temperatures during the return flight. Therefore, it was decided to push the Soyuz MS-23 mission forward and turn it into an uncrewed mission.
However, this meant that cosmonauts Sergey Prokopyev and Dmitry Petelin, and astronaut Frank Rubio would have had no lifeboat for two months until the arrival of Soyuz MS-23.
NASA decided that Frank Rubio's seat liner would be un-installed from Soyuz MS-22 and jerry-rigged into Crew-5's capsule Endurance. While Crew Dragon always flies with a crew of at most 4, it was originally designed for 7, therefore, its environmental control and life-support systems would easily be able to handle a crew of 5; this also means there is enough space on the cargo pallet behind the four seats, where the other three seats would have been. Crew Dragon's water landing is fairly benign, so not having a proper seat for Frank Rubio was deemed acceptable – remember, we are only talking about an emergency return, where different Flight Rules apply and NASA is willing to accept more risk.
Roskosmos decided that Soyuz MS-22 was probably safe enough for an emergency return, especially with the reduced heat load of only 2 cosmonauts, so it served as lifeboat for Sergey Prokopyev and Dmitry Petelin.
In the end, none of that was needed. After the arrival of Soyuz MS-23, the three seat liners were moved from Endurance and Soyuz MS-22 to Soyuz MS-23, at which point, the normal lifeboat situation was re-established.
Soyuz MS-22 returned uncrewed. To make its return as easy and fast as possible, the ISS was pitched 90° and the Soyuz's thrusters were fired longer than normal, reducing the return journey to only 55 minutes. The temperature reached 50°C, which was lower than the worst-case estimates, and would have been survivable.
answered May 4 at 9:47
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1$\begingroup$ I didn't know about the MS-22 return details, thanks. My impression is that a nominal Soyuz landing is quite a jolt but not exactly violent, I have always assumed that all of the strictness with custom seat liners is a precaution in case the retro rocket fails to ignite at touchdown. I'm not sure how New Shepard and Starliner mitigate that contingency, perhaps with more robust shock absorbers for the seats. $\endgroup$ Commented May 4 at 11:14
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1$\begingroup$ Can confirm, it is indeed an ISS flight rule that no crew member is cut off from their return vehicle. $\endgroup$– DoresoomCommented Aug 22 at 13:43