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Author Topic: What colour should the James Webb Space Telescope sunshield be painted?  (Read 1610 times)

Offline CliffordK

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When looking at the JWST sunshield, it is organized in 5 layers of silicon coated Kapton.



It appears as if both the top and bottom of each layer are reflective, with NASA assuming that any heat (IR) will get  between the layers, and essentially bounce to the side, and radiate out into space.

Is the top layer, towards the telescope reflective?  So that incident light could reflect off of the heat shield and into the telescope?  I suppose that is ok as long as the actual sensor is protected as the light hitting the mirrors would be reflected elsewhere.



It would seem that such tightly spaced heatshields would tend to cause reabsorption of the energy, rather than reflecting it laterally into space.

So, I would suggest that rather than silvering both sides, the sun-side should be silvered, and the telescope side should be black.

I tried to do some simple calculations.  I don't have the numbers for the foil that they use.  I was thinking about 10% absorption which is probably high, but should be representative. 

1st layer absorbs 100W (both sides black, except first layer).
Each successive layer absorbs 50% of previous and next layer.
Layer 1 asymptote at 167W.
Layer 5 asymptote at 33W

1st layer absorbs 100W (Both sides same, 60% sent out to space via sides).
Each successive layer absorbs 40% of previous and next layer.
Layer 1 asymptote at 125W
Layer 5 asymptote at 5.9W

1st layer absorbs 100W (Both sides same, 80% sent out to space via sides).
Each successive layer absorbs 20% of previous and next layer.
Layer 1 asymptote at 104W
Layer 5 asymptote at 0.189W

1st layer absorbs 100W  (different colors/reflectivity, 0% reflected to space between layers)
Each successive layer absorbs 20% of previous layer, 80% of next layer.
Layer 1 asymptote at 125W
Layer 5 asymptote at 0.37W

1st layer absorbs 100W  (different colors/reflectivity, 2.5% reflected to space between layers)
Each successive layer absorbs 17.5% of previous layer, 80% of next layer.
Layer 1 asymptote at 120W
Layer 5 asymptote at 0.188W  (just the 2.5% difference made this better than the double silvered).

1st layer absorbs 100W (different colors/reflectivity, 0% reflected to space between layers)
Each successive layer absorbs 10% of previous layer, 90% of next layer.
Layer 1 asymptote at 111W
Layer 5 asymptote at 0.015W

Anyway, by bouncing the light back and forth as in the NASA diagram, one has to hope most of the heat gets pushed out the side.  But, if the path of light gets bounced, say 5 to 10 times, it would seem like more energy would have to get absorbed by the layers.

However, if the design is to try to bounce the heat back towards the first heatshield, and radiate it back towards the Earth and sun.  The first heatshield would naturally heat up slightly more, but it doesn't take much of a difference to cause it to be more efficient by painting the satellite side black, and trying to force the absorption back towards the sun side.

It wouldn't take much to test this with the high-tech tools they have at NASA, but perhaps I could do reasonably well with a hot lamp, tinfoil, 2 layers or 3 layers, and some black and silver spray paint.
« Last Edit: 03/02/2012 21:58:47 by chris »


 

Offline wolfekeeper

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It wouldn't take much to test this with the high-tech tools they have at NASA, but perhaps I could do reasonably well with a hot lamp, tinfoil, 2 layers or 3 layers, and some black and silver spray paint.
I didn't read the rest of your post very carefully, but this bit is definitely wrong. These kinds of multilayer insulation systems only work in a very, very hard vacuum indeed, even outgasing makes a very big difference for several days after launch.
 

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