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There have many instances where the ISS only has 3 crew members, often for many weeks. This places significant strain on workload and productivity. Why isn't a new crew launched quicker?

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  • $\begingroup$ I can't say for sure, but I think it's probably money money money mooooneeey. $\endgroup$ – GdD May 14 at 15:18
  • $\begingroup$ Because the US decided to spend the money needed on a white elephant, the SLS. $\endgroup$ – Martin Schröder May 15 at 7:09
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There are probably several reasons and it is a mix of them all.

At some levels your question is somewhat specific on why they do indirect handoffs, where one crew of three remains, one crew of three departs, and then later one crew of three joins the station.

On Mir with only a single active crew (for the most part) they would do direct handoffs, where the second Soyuz would dock with three new crew members and co-habitate for a few days to a week and then the old crew would take the old Soyuz back.

Sometimes for long duration missions they would launch a fresh Soyuz with a crew (Often selling a seat to foreign nations or a tourist) who would stay for a few days to a week and return in the old Soyuz, giving the active crew another 200+ days of life on the Soyuz.

Usually this is a decision based on scheduling issues. There are many components.

The Soyuz vehicle itself has a lifetime in space. (Dragon Crew and Starliner will also have similar lifetimes). As they get close to that date, they need to leave.

Rarely do they go right to the wire, since you never know when an emergency could come up and you might need some extra time before leaving.

You need to have a new vehicle and crew available to launch, which has its own basket of scheduling problems. (actual hardware, launch site, launch window, good time to dock vs bad times to dock). You might be waiting for some piece of equipment to bring up. There might be other ops in progress that interfere on the station.

In some cases it is a cost savings measure. Need fewer Progress launches of food/water/supplies for a crew of three vs six. Need fewer Soyuz vehicles.

There was a point where I think the crew on ISS was down to 2, the bare minimum for that reason.

On the US side, this is going to go to 4 per launch and the Russian side will continue to launch 3.

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    $\begingroup$ It's easy to forget that from the end of the Shuttle era until (hopefully) two weeks from now, the Soyuz was the only vehicle bringing humans to the ISS, which put a strict limit on how many people could be brought to (and by extension from) the station. This was not only a bottleneck, but also, for all its reliability, a single-point-of-failure. (See Soyuz MS-10.) $\endgroup$ – Jörg W Mittag May 14 at 18:18
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    $\begingroup$ @JörgWMittag Agreed - note I called out Soyuz limit of 3 and US limit of 4 (going forward). Shuttle is ancient history now. :) $\endgroup$ – geoffc May 14 at 19:12
  • $\begingroup$ @JörgWMittag Agreed, but they could have agreed so start more Soyuz. $\endgroup$ – Martin Schröder May 15 at 7:11
  • $\begingroup$ Negative, I was not inquiring on the handoffs but why the station operates on minimal crew for extensive periods. Doubt they're very productive considering it needs a minimum of 5 for station keeping alone. $\endgroup$ – Eloy Falco May 16 at 4:34
  • $\begingroup$ @Eloy Falco, I'm curious about your statement that crew of 5 is needed to maintain ISS. Where can I read more about it? $\endgroup$ – Lesser Hedgehog May 17 at 2:18

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