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The news that Boeing will bid the CST-100 as a Commercial Resupply Services 2 (CRS-2) vehicle raises a question: will they be using a docking port, to a PMA or berthing to a CBM port?

The problem is that NASA wants two open ports when you attempt to connect. So there is primary CBM (earth facing Node 2), and backup CBM (sky facing Node 2). They are rearranging the station so that the up-facing CBM will get PMA-3 + NDS interface attached and will be PMA/docking backup. And MPLM will be moved from Node 1 earth facing onto Node 3 (forward facing port), freeing up the CBM as backup.

You can see a lot more about this and the reconfiguration in this question and its answers.

So if a crew vehicle is on station using PMA-2 (forward facing Node-2) then could a cargo dock to PMA-3? And would that mean the cargo vehicle has no backup if an issue with PMA-3 occurs?

This is true if SpaceX bids Dragon V2 for cargo as well. NASA did allow in the RFP for PMA docking for cargo, but did not seem to address this issue.

The crew vehicle also serves as lifeboat so will stay on station 6 months or longer. Would that then block cargo vehicles from using PMA-3, or will this restriction be relaxed?

On the Russian side this is less of an issue since there are 4 possible ports, and one is almost always a Progress, so they could dump that if a port fails to allow the new vehicle to dock.

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  • $\begingroup$ Cargo will continue to berth with the CBMs while crew will be on the PMAs $\endgroup$ – tl8 Jun 29 '15 at 21:56
  • $\begingroup$ @tl8 That is not for certain. Cargo Dragon is said to continue using CBM, while Crew Dragon will use PMA. But CST-100 is PMA for crew, and PMA for cargo. The Cargo contract allows for PMA docking of cargo. Just not both winners. $\endgroup$ – geoffc Jun 29 '15 at 22:01
  • $\begingroup$ I guess winner takes all then. $\endgroup$ – tl8 Jun 29 '15 at 23:09
  • $\begingroup$ @tl8 The point is, if current policy is to have a backup port available, how does that change as they start seeing more visiting vehicles at the same time. One possible answer is that they will not allow a second vehicle. $\endgroup$ – geoffc Jun 30 '15 at 0:08
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The winners of the Commercial Crew were announced after this question was asked. At that point, both winners, Boeing's CST-100 and SpaceX's Dragon V2 will use the PMA based docking ports. (There will be in 'insert', the IDA, International Docking Adapter between the vehicle and the older PMA's.)

The cargo resuppliers, Orbital Science's Cygnus and SpaceX Dragon use the CBM docking ports.

With the Visiting Vehicle schedule for 2016 looking very busy, it looks like there will be several times when both providers will have vehicles at the station for cargo and using both CBM ports. With the initial SpaceX return to flight schedule, NASA was showing video of the Cygnus and Dragon docked to the station at the same time.

That still leaves outstanding the issue of policy for the PMA's, when dealing with manned flights.

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As you stated, there will be 3 ports on the American side. It seems very likely (Although I can't find it written anywhere) that the plan will be to alternate ISS resupply missions at one dedicated port, alternate crewed missions at the other port, and keep the 3rd port as a backup in case there are problems. Thus, there will be a crew port, a cargo port, and a backup port (Although the ports might be changed from time to time).

It does seem like the American side is definitely short on ports, but I haven't heard any plans to bring new ones to the station. It seems like they will be able to manage (Although barely) with the moving around of ports as is currently planned.

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    $\begingroup$ 4 ports. 2 CBM, 2 PMA. $\endgroup$ – geoffc Jan 6 '16 at 20:34

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