Soyuz change-overs

[Ben E]

With the recent arrival of a third Soyuz at the ISS, I have a question: why is it necessary to deorbit one half of a six-person crew and then send up a replacement half two weeks later? This means, in effect, that for a month of each year (or possibly more), the crew will drop to six to three (or even two, in Suraev and Williams' case in December), then back to six, then back down to three, then back up to six etc. If the last mission has just demonstrated that three Soyuzes can dock and they can execute a smooth changeover, why are we not seeing a PERMANENT, all-year-round six-person crew?

I read with interest the plan for next year:

From early Dec until Dec 23, there will be 2 crew (Suraev, Williams)
From Dec 23 until ?mid-Mar, there will be 5 crew (Suraev, Williams, Kotov, Noguchi and Creamer)
From ?mid-March until early April, there will be 3 crew (Kotov, Noguchi and Creamer)
From early April until late May, there will be 6 crew (Kotov, Noguchi, Creamer, Skvortsov, Kornienko and Caldwell)
From late May until ?early June, there will be 3 crew (Skvortsov, Kornienko and Caldwell)
From ?early June, there will be 6 crew (Skvortsov, Kornienko, Caldwell, Walker, Wheelock and Yurchikhin)

Surely by reducing the crew by half at three-monthly intervals, the ISS must be losing scientific productivity? If Soyuz TMA-16 has shown that they can support changeovers, to ensure a permanent six-crew capability, why aren't they doing it?

[davidboyd]

I was wondering the same thing.

My guess is that one of the three Russian docking ports will have a Progress or ATV docked to it, so one Soyuz will have to leave before another can take its place.

[anik]

If the last mission has just demonstrated that three Soyuzes can dock and they can execute a smooth changeover, why are we not seeing a PERMANENT, all-year-round six-person crew?

When we have three ports on Russian segment, it is better to do indirect crew rotation, because one Soyuz will be on Zarya and one Soyuz will be on Pirs, and Progress or ATV will be on Zvezda, providing reboost/deboost capability.

Today's situation with three Soyuzes on Russian segment is bad, because if we will need to perform the debris avoidance maneuver we use Soyuz TMA-16 spacecraft. Also nine crewmembers on ISS during rotation period is big load for ECLSS. But this situation has appeared due to flight of spaceflight participant and due to postponing of MRM-2 launch from August 15 to November 10.

When we have four ports on Russian segment, it is better to do indirect crew rotation, because one Soyuz will be on Zarya and one Soyuz will be on MRM-2, and Progress or ATV will be on Zvezda, providing reboost/deboost capability, and Progress will be on Pirs, providing efficient roll control for ISS.

[Targeteer]

But aren't there are more docking ports on the way?  One in November on a Soyuz and another next year on the Shuttle?  My guess is logistics support is also involved. Less crew means less of everything being used.  I agree that portraying the Station has having a "permanent 6 person crew" is not accurate.

[hop]

But aren't there are more docking ports on the way?

MRM2 (to be flown on progress M-MIM2 in November) is the only one that provides an additional port. MRM1 (to be flown on the shuttle) will just act as an extender on the FGB docking port that is already used by Soyuz.

And as anik says 9 pushes the ISS ECLSS hard too. Shuttle missions don't suffer this as much, because unlike Soyuz, the Shuttle ECLSS runs for the duration of it's stay.