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Why is that segment of the ISS truss called S0, not P0, C (for center) or just 0?

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    $\begingroup$ When it was Space Station Freedom, S0 on the truss was originally called M1, flanked by P1 and S1 either side. $\endgroup$ Oct 7, 2023 at 19:11
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    $\begingroup$ Anecdotally, because there is no "C" designation for center. But that just begs the question of why not. $\endgroup$ Oct 7, 2023 at 19:23
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    $\begingroup$ I hope M in M-1 didn't mean 'middle'.... $\endgroup$ Oct 7, 2023 at 19:25
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    $\begingroup$ @blobbymcblobby the ISS On-Orbit Assembly, Modeling, and Mass Properties Data Book confirms what you say about M1 and says it was the "middeck truss section". i.imgur.com/kApFNAE.png $\endgroup$ Oct 7, 2023 at 19:29
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    $\begingroup$ I guess we have a name before, but not a why it changed... But I guess many things changed when it went from SSF to ISS... $\endgroup$ Oct 7, 2023 at 19:39

2 Answers 2

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Not a final answer as it lacks a 'why' but:

Why is that segment of the ISS truss called S0, not P0, C (for center) or just 0?

It was originally neither.

The truss originates from before the ISS, when the program for Space Station Freedom was at the (design, re-design, re-design again) fabrication stage in the early 1990s.

Post-change from deployable lattice works to pre-integrated truss structure (interestingly the Soviets encountered the same issue and made the same change to Mir-2, after having erected deployable truss lattice works on Salyut 7 and Mir; the US had done so from the cargo bay of the Space Shuttle), the middle truss, looking much like it does now, was called M-1, M1 or (as Organic Marble confirms in his comment) - "middeck truss section".

By that notion, although not center, middeck or M, would have been a pretty good indicator of where this sat in the truss plan, with P1 and S1 either side. There was no '0'

(Note: P - port, S - starboard)

enter image description here

Construction of trusses aside, assembly of the Space Station Freedom (and then ISS') truss was always going to be assembled, by the Space Shuttle, outwards-in from Solar Array, starboard side first to the middeck, and then followed by the addition of the port side truss segments. So S- side trusses always had priority.

enter image description here

  • Mission Build 1 - (integrated truss segment) S4, S3, inboard solar arrays, mobile transporter, UBM (docking system that was replaced by PMA on ISS post-Mir experience)
  • Mission Build 2 - S2 (propulsion module that got replaced by Russian module capability when it became ISS)
  • Mission Build 3 - S1
  • Mission Build 4 - (integrated truss segment) M1
  • P1 does not get assembled until flight (Mission Build) 8.

One, lazy, observation, is that the Space Shuttle Orbiter already had a 'middeck' that was extensively referred to in experiments and missions prior to this. For a segment that was being assembled in space called middeck, by something that already had a middeck, might be seen as a reason to rename to avoid any chance of confusion.

As i say, only a partial answer - SSF was so long ago that many documents are no longer available and reasons may well be lost to time.

Graphical representation of a later version (June 1993) of assembly build (MB1-4) shown above:

enter image description here

links:

Space Station Freedom Assembly Sequence: History, Methods, And Status jstor.org/stable/44733150

Space Station Redesign Team: Final report to the Advisory Committee on the Redesign of the Space Station https://ntrs.nasa.gov/api/citations/19930018390/downloads/19930018390.pdf

Space Station Redesign Team Final Report to the Advisory Committee on the Redesign of the Space Station https://ntrs.nasa.gov/citations/19940006208

Dynamics of on-orbit construction process https://ntrs.nasa.gov/citations/19930017217

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    $\begingroup$ Nice! What document is that screenshot from? $\endgroup$ Oct 8, 2023 at 0:19
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    $\begingroup$ Space Station Freedom Assembly Sequence: History, Methods, And Status jstor.org/stable/44733150 $\endgroup$ Oct 8, 2023 at 0:46
  • $\begingroup$ Remember when the main truss of the US space station plans was pressurized. $\endgroup$ Oct 8, 2023 at 5:25
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    $\begingroup$ @AntiElonGuy no, don't remember that. Link? $\endgroup$ Oct 8, 2023 at 11:50
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    $\begingroup$ Weeeeell, bit of a mistype: I was referring to the modular space station. astronautix.com/m/modularizedspacestation.html $\endgroup$ Oct 8, 2023 at 23:06
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Wikipedia knows that it comes from starboard (S) and port (P) side:

The P1 and S1 trusses (also called the Port and Starboard Side Thermal Radiator Trusses).

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    $\begingroup$ But this does not explain why this segment counts as Starboard and not as Port. $\endgroup$
    – EarthAndMoon
    Oct 7, 2023 at 17:02
  • $\begingroup$ One point to note was from Space Station Freedom assembly days (after transition from deployable truss to pre-integrated truss structure), Starboard (as in S1, S2, S3, etc) was always done first, followed by M1, and then portside (P1) truss elements followed, and the ISS ITS followed some of the assembly plan. $\endgroup$ Oct 7, 2023 at 20:41

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