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People often talk about docking and berthing. They both seem to be connecting a spacecraft to the ISS, yet seem to be very different. Why?

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FYI, Wikipedia has a brief overview page on this topic: Docking and berthing of spacecraft – Basil Bourque Jan 23 at 23:44
up vote 15 down vote accepted

The terms docking and berthing have a nautical origin. Smaller ships come into port under their own authority and dock. Large ships instead are berthed. They come to a stop outside of the port, relinquish control to the port authority, and are towed into port by tug boats.

Docking with the International Space Station is essentially a controlled collision with the station. This controlled collision has to be extremely precise in order to be nondestructive. Because the space station is an extremely expensive resource, approving a vehicle that plans to dock with the station is something done with extreme care (and extreme expense).

Berthing opened up the door to automated operations by the Japanese HTV, SpaceX's Dragon, and Orbital's Cygnus. With berthing, the designers of a vehicle need to prove to NASA that their vehicle can safely approach the station, that it can hold position and attitude in well-designed berthing box, that it recognizes and responds to problems along the way, and that it relinquishes control to the ISS upon command. That the ports used for berthing are larger than the ports used for docking is a side benefit.

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The basic difference between docking and berthing methods is as described by @OrganicMarble and the linked article, that berthing involves the robot arm and docking does not.

The reasons why one is used rather than the other was discussed by the NASA PRO in a recent webcast. Obviously, if the vehicle is not capable of docking it must be berthed; however it must be equipped with a berthing connector and a robot arm connector. These things must then be designed and built-in.

One of the reasons why one is selected in the design rather than the other is cargo size. The berthing and docking ports on the ISS are of different shapes and dimension and can thus be used to transfer different volumes of cargo. In particular the Common Berthing Mechanism (CBM) used by the H-II Transfer Vehicle (HTV) and the Dragon spacecraft have a square opening of 1.3m (50") which can transfer whole equipment racks. By contrast the Russian and new US docking port is a circular port of 0.8m (31"), which is too small for racks.

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Is there reason why docking port are small? – Ian Ringrose Jan 23 at 23:18
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Straight to the point +1. – hellyale Jan 23 at 23:20
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@IanRingrose The Soviet/Russian docking port was the first such port for docking to a space station, dating back to 1971. It was as big as was needed then; never intended to haul the kind of gear used now in the ISS station. – Basil Bourque Jan 23 at 23:49
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@BasilBourque, but the new "standard" docking poor seems to still be very small. – Ian Ringrose Jan 24 at 22:26

Berthing is when the vehicle is grappled by the robot arm and moved to a port. Docking is when it flies in on its own.

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The link is literally the first thing that comes up when you google "docking and berthing", BTW. – Organic Marble Jan 23 at 18:39
    
So why is docking needed for maned vehicles? Why can a berthed vehicle be designed to be able to leave without needed the robot arm? – Ian Ringrose Jan 23 at 18:49
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@IanRingrose -- It's not. The Shuttle docked with the Station, as does the Soyuz. The Dragon V2 will also dock rather than berth. (The unmanned Dragon V1 berths with the station.) – David Hammen Jan 24 at 1:22
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Manned vehicles need to be able to leave on their own in an emergency without the use of the robot arm. – Organic Marble Jan 24 at 2:05

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