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A number of times now, the cargo vehicles (Dragon, Cygnus, or HTV) seem to have arrived while the station was between crew switchovers.

With indirect crew handovers, three crew depart in a Soyuz, reducing ISS Crew size from 6 to 3, and a week or two later another Soyuz launches with a crew of 3.

What I have noticed is that a couple of times now, cargo missions have docked, while running a crew of 3 only. Since the cargo vehicles implies additional work. (They only stay docked about a month, on the US side, since with only one primary CBM port available (Harmony nadir. Harmony zenith is secondary) See Will the ISS need more docking ports for details.)

With only a month on station, there is a fair bit of cargo to unload, store somewhere (Tracked of course, else no one will be able to find anything), then refill the vehicle with downmass (Dragon only) or trash (Cygnus, HTV, Progress, ATV). There are requirements to balance the mass on reloading, so it is non-trivial.

You would think, targeting 6 person crew compliments would make more sense.

But is there a reason? Perhaps the schedule is so tight on the Visiting Vehicle Schedule they cannot 'waste' the two weeks when the crew is reduced?

Harmony, where the CBM for cargo docking resides, is really central to the station, (you must go through Harmony to get to Columbus (ESA module), Kibo (JAXA module) and rest of the station. So maybe a crew of three is easier, since there are fewer people in the way? Or moving cargo disturbs fewer people or experiments?

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    $\begingroup$ I'm not sure of the answer, but can absolutely confirm that the Visiting Vehicle Schedule is super tight, and constraining to many mission operations including launches (i.e. launches have been delayed, sometimes for months, because there isn't a suitable schedule opening) $\endgroup$ – pericynthion Sep 11 '14 at 6:55
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While I don't have a complete answer I thought I'd share what I've found in the hope that someone can come up with something better.

I can find nothing online about exactly how and why the visiting vehicle schedule is constructed (Although this could simply be my poor google-fu skills). One thing that is clear is just how tightly packed the schedule is. Here is an ISS vehicle schedule from NASA covering expeditions 10-41 (10/16/04-09/25/14). And Here is a schedule until mid 2020 complied from various sources.

I also came across a good graphic that shows the visiting vehicle schedule from the beginning of the station to mid 2013. enter image description here Source.

There have been numerous instances where a launch has had to be delayed because a vehicle lost its slot in the schedule. perhaps the most interesting is when ATV Jules Verne had to wait on-orbit for STS-123 to leave the station before docking.

Probably the most useful document I found is "Operations Planning for the International Space Station" by the ESA Directorate for Technical and Operational Support. It details how operational plans are constructed, but doesn't contain enough detail to directly answer the question.

Finally there's this document that, while it doesn't help answer the question, does include a huge amount of detail about the resouces available on ISS.

In conclusion I think it is very unlikely that these missions were targeted for a time when the station was half-crewed, I think it's far more likely that it's just a product of coincidence and a full schedule. That said I can't find a definitive statement confirming this.

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  • $\begingroup$ That is the best explanation I have seen so far. I assumed it was coincidence but had no proof to back that up. However this comes closest. $\endgroup$ – geoffc Jun 29 '15 at 12:30
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    $\begingroup$ Any value for this naive argument? When there are 6 crews, two docking ports are used by Soyuz return crafts, only one when there are 3 crews aboard. So there is one more port potentially available. $\endgroup$ – mins Jun 29 '15 at 13:06
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    $\begingroup$ @mins That's true for Progress and ATV, but not Dragon, HTV, and Cygnus. They all have to share CBM ports. $\endgroup$ – ForgeMonkey Jun 29 '15 at 13:08
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    $\begingroup$ @mins The Russian side and US side are not compatible at all. ATV docks on Russian side. Shuttle, HTV, Cygnus, Dragon uses US side. Future manned crew will use PMA + IDA (IDA-1 lost in CRS-7 explosion). $\endgroup$ – geoffc Jun 29 '15 at 15:20

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