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The radiators look like a highly stretched paper accordion. Is it just because of how they decomposed, or is there a deeper reason?

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    $\begingroup$ What could '… how they decomposed…' mean, please? $\endgroup$ Apr 10, 2023 at 21:15

3 Answers 3

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From the horse's (Vought) mouth, the angle created between each panel and its corresponding scissor beam (20°) is to "provide structural stiffness":

The deployment is achieved by rotating the first panel 80° relative to its initial stowed position and rotating the first scissors beam 80° in the opposite direction. The scissors mechanism rotates alternate panels in equal angles but opposite direction until an angle of approximately 160° exists between adjacent panels. The angle between each panel and its corresponding scissors beam at the pivot pin is approximately 20° when deployed. This 20° angle provide the structural stiffness for a minimum resonant frequency of 0.16 Hz.

— Oren, John A., and Harold R. Howell. "Space station heat rejection subsystem radiator assembly design and development." SAE transactions (1995): 1086-1095. p. 7.

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    $\begingroup$ Wow, that's a superb document. I have the console handbook for the External Thermal Control System and it looks like the radiator section was largely taken from this paper...but they left out the part you quoted. $\endgroup$ Mar 28, 2023 at 14:00
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    $\begingroup$ Thanks for finding that and validating my guess / hazy memory of being told about that at some point. $\endgroup$
    – Erin Anne
    Mar 28, 2023 at 20:57
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The radiators are jointed because they were launched folded up into a flat stack.

This screenshot from the official NASA visualization tool DOUG shows one of the radiators partially retracted.

enter image description here

This picture of STS-113 delivering a truss section shows the radiators folded flat.

enter image description here

This prelaunch photo shows a better view of the folded radiator panels.

enter image description here

(last two images are NASA photos)

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    $\begingroup$ I wonder if there's a reason they don't deploy fully flat. They wouldn't particularly need to for the cold side, since The Black covers more than a hemisphere. Maybe that's reason enough. $\endgroup$
    – Erin Anne
    Mar 27, 2023 at 19:19
  • $\begingroup$ @ErinAnne I pondered the same thing; wonder if it has to do with hinge design. The Russian segment radiators look similar as far back as Mir, so maybe there is a deeper reason. Good followup question? $\endgroup$ Mar 27, 2023 at 19:21
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    $\begingroup$ not to imply that your answer is not an answer to the question, because I think it is as stated, but I think as-put OP's question would cover why they haven't deployed flat too $\endgroup$
    – Erin Anne
    Mar 27, 2023 at 19:50
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    $\begingroup$ I suspect the answer is "corrugation makes them more vibration-resistant;" while trying to find a source I was looking through Design Considerations for Lightweight Space Radiators Based on Fabrication and Test Experience with a Carbon-Carbon Composite Prototype Heat Pipe, saw a "gas turboalternator" in Figure 14 and stopped to look and hey this has a notional megawatt-level lunar nuclear power plant design in it, lol $\endgroup$
    – Erin Anne
    Mar 27, 2023 at 20:18
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    $\begingroup$ Isn't it simply that the construction would be structurally stronger? There's not much benefit from making them perfectly flat, while both strength and design constraints would make the flat configuration heavier and more complex. Not to mention that it would be much harder to fold them back up again, which is something you definitely want as a feature. $\endgroup$
    – Luaan
    Mar 29, 2023 at 6:58
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This is a scissor mechanism, as used in lifts and pantographs. A fundamental property of the scissor mechanism geometry is that it cannot "open completely" (90* rotation of members) due to joint conflicts. The radiator could be designed to pack flat, or to deploy flat. But not both.

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    $\begingroup$ Interesting! Looks like they chose the "pack flat" option. +1, but would love a reference for further reading. $\endgroup$ Mar 27, 2023 at 23:49
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    $\begingroup$ @OrganicMarble ... I think the choice was governed by utility. A few degrees of not-flat-packed doubles the volume. But a few degrees of not-open-completely has almost no effect on radiation. I suspect a cos vs sin relationship but I'm too lazy to derive a formula. $\endgroup$
    – Woody
    Mar 28, 2023 at 0:30
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    $\begingroup$ Also the flatter you open it, the less stiff against bending the scissor mechanism becomes. I'm not sure how much of a concern bending can be in space, though (there will be some drag and the radiator is quite a big sail, so it could play a role, but then I think it basically flies edge-on, so maybe not). $\endgroup$
    – TooTea
    Mar 28, 2023 at 6:49
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    $\begingroup$ If the panels were not tied to the scissor arms, but rather only to the pivot points, then they could have locked completely flat, with the scissors keeping them under a constant slight tension. But in that case, they would not have been able to easily and reliably fold up again, and I imagine that's a super useful feature for repairs. $\endgroup$ Mar 28, 2023 at 7:15
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    $\begingroup$ @DewiMorgan it would even be possible for the panels to lock flat while being tied to the scissor arms. It just requires making the outer scissor booms slightly longer than the width of the panels, so that they zig-zag a bit even when the panels are flat. Only, they would take up slightly more space when stowed away then, and the tiny increase in radiation efficiency would not make up for this disadvantage. $\endgroup$ Mar 29, 2023 at 12:40

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