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I struck out with my previous question so I'll try again.

What are these large circular 4-vane structures on the sides of NOAA-19 - the satellite that "fell down"?

They remind me of adjustable air vents but I have a feeling that's not quite right.

You can see them from another angle here.

enter image description here

Spacecraft rotated for better view:

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enter image description here

uhoh
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1 Answers1

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They are thermal control pinwheels. Reference: figure on page 3 of this document .

enter image description here

Note: odd how the diagram shows 4 devices, but the note says 3. In the photos there are 3 on one side, and 4 on another.

"Elements of Space Technology for Aerospace Engineers", page 302, describes thermal control pinwheels as follows:

Pinwheel louvers are similar, but in place of rectangular blades incorporate vanes (typically four) in the form of sectors of a circle that together cover one-half of the area under the pinwheel. They are also rotated by bimetal actuators. Depending on the position of the sectors, they will cover (uncover) areas that have high ratios of solar absorptivity to emissivity and uncover (cover) areas that have a low such ratio.

And, there is a cross-section: enter image description here

Organic Marble
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    They are some kind of radiator. I am not sure if the quad blades rotate or not. Looking for reference. – Organic Marble May 17 '16 at 12:07
  • These might be the covers, with darker (high emissivity) radiators hidden underneath? – uhoh May 17 '16 at 12:17
  • Possible, and they could rotate to tweak the output. But haven't found confirmation yet. – Organic Marble May 17 '16 at 12:21
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    @uhoh Looks like it's what I thought, and they do rotate. Edited answer. – Organic Marble May 17 '16 at 12:40
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    Bravo - thank you for that!! Extra credit for a speedy conclusion. I am puzzled, usually absorptivity and emissivity are similar - something back absorbs light and (thermally) radiates light well, something white or metallic is usually poor at both. I'm not sure what's going on here. Maybe the key is wavelength - solar radiation (~5000K) is mostly near IR and visible, while "ambient temperature emissivity" (~300K) would be more like 10 to 20 microns. So whatever they use is dark in the visible, but shiny (low $\epsilon$) in IR. – uhoh May 17 '16 at 14:22
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    There may be more info in the linked document, I just skimmed. Key to the google success was "pinwheel", I never would have come up with that. – Organic Marble May 17 '16 at 14:27
  • @uhoh, the key is direction, not wavelength: you aim the vents at deep space, which acts as a 3K blackbody, rather than at the Sun, which acts as a 5500K blackbody. – Mark May 17 '16 at 18:20
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    @Mark The top of the linked page in the book Elements of Space Technology by R. X. Meyer it says "If this surface* is provided with a high ratio of* solar radiation absorptivity to ambient temperature emissivity, control over a wide range of temperatures can be obtained." I think I'm interpreting it correctly - let me know if I'm missing something. – uhoh May 18 '16 at 04:38
  • I couldn't get the link in your first sentence to work. I found this https://www.nasa.gov/pdf/111742main_noaa_n_booklet.pdf and is the diagram saying that there are (15) pinwheels? – uhoh Jun 21 '16 at 13:36
  • Just in case it's not clear, these gizmos are for thermal control. Electronics are mounted inside the structure, under the pinwheels. If the electronics get too hot, then the pinwheel rotates so the high emissivity/low absorptivity surface is uncovered. That surface ten radiates the excess heat and lowers the temperatures. If things then cool off too much, then the pinwheel rotates back the other way - like zipping up your jacket. – John McCarthy Jan 31 '22 at 01:39
  • @JohnMcCarthy my answer literally starts with "These are thermal control pinwheels." – Organic Marble Jan 31 '22 at 01:57
  • Organic Marble: Sorry if I offended you. I wanted to expand on your answer a bit, and I could have started off without the "just in case". I'll be more careful. – John McCarthy Feb 01 '22 at 20:09