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Zvezda, the third ISS module, (I think it was Zarya (FGB), Node 1, and then Zvezda) known as the Service Module was actually built as a spare for Mir and was originally to be the core of Mir-2. But with the merging of the US and Russian station plans, it became the key building block of the initial ISS.

As a Mir spare, the docking module had 5 ports. 6 if you count its connection to the Service Module itself. As delivered to the ISS it only has nadir (Earth facing) and zenith (space facing) ports, and of course the port it uses to dock to Zarya.

Why did they remove two docking ports? It seems like the flexibility would have been useful, and it was already built that way.

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While I can't speak with authority on the motivation behind design decisions made years ago, it's worth pointing out that port and starboard docking ports on Zvezda would create conflicts with the Zvezda solar arrays, the FGB solar arrays, the HRS radiators, and the solar array wings. In fact, if you look throughout the station, the only docking and berthing ports are either forward, zenith, nadir, or aft.

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  • $\begingroup$ Which seems like why the further docking ports will be extended out below the plane of the current line of Russian modules, so that makes sense. $\endgroup$
    – geoffc
    Commented Nov 15, 2013 at 18:06
  • $\begingroup$ Except for Node 3, Columbus, Kibo which use side ports. But they are on the 'wrong side' of the radiators. Also the Russian solar arrays are mostly retracted these days anyway. $\endgroup$
    – geoffc
    Commented Nov 15, 2013 at 18:16

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