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The ISS has three (four?) types of docking ports. The Russian side has one standard with two types (active/passive?) and the US side has either a PMA (soon to be updated with the IDA to the NDS standard) or a CBM port.

The PMA (Pressurized Mating Adapter) with the IDA (International Docking Adapter) allow for vehicles to dock and undock without intervention and thus are designed/designated for crew vehicles. The IDA brings the PMA to the NDS (Nasa Docking Standard) that CST-100, Dragon V2, and possibly Dream Chaser cargo will match.

The CBM (Common Berthing Module) is meant for attaching larger modules like the various ISS Modules (Unity, Harmony, Tranquility, Columbus, Kibo, PLM, Quest Airlock, Cupula, BEAM, etc). Thus it is not really meant for unattended operation.

Thus a critique of using a Dragon (cargo/v1 whatever you wish to call it) for crew, even if all other concerns were met, is that it cannot unberth on its own in case of an emergency.

So what exactly keeps it from unberthing on it's own?

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There are a list of things that have to happen during an unberth operation.

The two CBM ports (one on the vehicle, one on the station module) have hatches that have to be opened and closed. There is a small narrow vestibule in between the two hatches once the two are berthed.

That is, the docking ring of each side mates, and once that is airtight, there is a gap (about a foot wide) between the outside of the ISS and vehicle hatches. Both those hatches need to be opened to allow passage through.

There are bolts that are driven (16, driven in sets of 8) to hold the structures together in the rings.

Once the vehicle is detached, then the CanadArm2 is used to move the visiting vehicle far enough away that its use of thrusters won't impinge on the station in a harmful fashion. (Big solar panels and big radiators they would like you to avoid getting dirty with hypergolics or their combustion products).

The CanadArm2 can be operated from the ground, so that is likely the least of the set of problems.

The biggest problem is likely that once the crew departing are inside their vehicle, they cannot close the ISS side inner hatch and seal it air tight. Thus leaving on their own would likely vent the station to space.

In case of a truly bad, station is going down situation, a CBM berthed vehicle could likely leave in a way that left the ISS unihabitable if the berthing ring bolts could be driven remotely.

Sealing off just that ISS Module would not really help, since the open CBM ports used for visiting vehicles are either on Node 1 Unity which is the transit point between the US and Russian segments, or else on Harmony (Node 2) which node connecting Columbus and Kibo to the station. Closing off either of those modules on boths its docking ports, would also leave the station mostly unusable.

A really good reference to the CBM ports is in the answer to this question.

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