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Source: nasa.gov; modified

What happened to 2 of the 3 radiators on P6 (solar array) when P6 was moved from Z1 (the top) to its final position on the port side?

Initially I thought 2 were removed and placed on P1 and S1, making the total there 4, with additional 2 later on for the final 6 we see today. One of the sources of confusion is the view after STS-120 (when P6 was moved) where 4 radiators can be seen on P1 and S1 (2 more than before, and 2 fewer on P6), as shown below.

But after checking the ISS guide (PDF; 37 MB), I'm not sure if they're interchangeable, as two types of radiators (I think) are mentioned: Heat Rejection Subsystem (P1 and S2?) and Photovoltaic Radiators (solar arrays?).

Why did P6 not work with just one radiator before and with however many required on P1 and S1?

Information and work on the radiators is sparse on Wikipedia. For example it's mentioned in passing on the STS-120 page, simple stating:

P6 aft radiator shroud installations [...] S1 radiator configuration.

Which doesn't tell much really. For good measure I watched the STS-120 Post Flight Presentation. Again, little to none on the work done on the radiators.

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Source: wikimedia.org; modified

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tl;dr the P6 truss had a special cooling system installed on it to provide thermal control of the proto-ISS until the full external thermal system was installed. This system incorporated two special radiators that were eventually stowed.

For a spacecraft that had to provide all the normal functions (GNC, life support, thermal regulation, propulsion, etc.) through a protracted, piece-meal assembly in space, the ISS had surprisingly few systems dedicated just to the assembly phase which ended up not being used in the final configuration.

One such, however, was the Early External Thermal Control System.

Since the US Lab becomes operational before the permanent External Thermal Control System (ETCS) is assembled, a temporary external cooling system is needed. External cooling from the Russian segment is not possible because there are no operational interfaces between the USOS and the ROS thermal systems. Instead, a modified version of the Photovoltaic Thermal Control System (PVTCS) called the Early External Thermal Control System (EETCS) acts as a temporary thermal system. The EETCS is needed until the components of the permanent ETCS are launched and activated. Once the permanent ETCS becomes operational on Flight 12A.1, the EETCS will be deactivated.

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ISS Familiarization Manual

Each segment of the truss which carries a solar array has a PhotoVoltatic Thermal Control System (PVTCS) with an associated radiator to cool the electrical equipment associated with the array. P6, the first array-carrying truss segment to be brought up, had that radiator along with the EETCS radiators, as shown here.

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The PVTCS radiators are still used in today's ISS configuration along with the External Thermal Control System radiators which supplanted the EETCS.

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  • $\begingroup$ Many thanks. Did it indeed become fully operational on Flight 12A.1 (STS-116)? Reason for asking is after STS-120, the ETCS had 4/6, not all 6, or were Expedition 16 EVAs involved when they also moved the freshly delivered Node 2? $\endgroup$
    – ymb1
    Aug 7, 2019 at 16:56
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    $\begingroup$ Having trouble finding details, but it looks like the EETCS radiators were still deployed when STS-116 undocked with half of the P6 array retracted. spaceflight.nasa.gov/gallery/images/shuttle/sts-116/hires/… But when STS-117 undocked, only the PVTCS radiator on P6 was still deployed. spaceflight.nasa.gov/gallery/images/shuttle/sts-117/hires/… $\endgroup$
    – Organic Marble
    Aug 7, 2019 at 17:54
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    $\begingroup$ The STS-120 press kit says P1 and S1 are to be configured so that ground can deploy the rest of the radiators. Since the press kit is published before the flight, I confirmed the actions took place by checking photos from STS-122 before docking. $\endgroup$
    – ymb1
    Aug 8, 2019 at 12:35

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