Apollo CM surfaces

[Dalhousie]

I have a question about the surface of Apollo CMs.

For example, Apollo 9, during flight it was surfaced by what appears to be aluminium like other CMs, e.g. http://all-that-is-interesting.com/astronaut-david-scotts-out-of-this-world-job

In post splash down photos of most CMs  the foil appears partly burnt away, leaving remnants of what appears to be aluminium and gold foil over a rusty-grey coloured substrate, e.g. Apollo 11 http://cdnph.upi.com/sv/b/upi/UPI-52581247858565/2009/1/8946b1a943b062fcc725242b20b8b6bd/Apollo-splashdown-timetable.jpg

More recent photos of the Apollo 9 CM on display appear to show a surface layer of white material which has partly flaked away, exposing the more familiar aluminium and gold foil remnants over the rust-coloured substrate, e.g http://46blyz.com/an-apollo-9-anniversary/

Something similar seems to be visible on Apollo 8's CM after recovery, e.g http://upload.wikimedia.org/wikipedia/commons/f/fa/Ap8-S68-56310.jpg . As preserved however, Apollo 8 seems to be stripped back to the rusty substrate, e.g. http://history.nasa.gov/ap08fj/pics/cm-port.jpg

Confusingly, the Skylab 4 CM was clearly painted or coated white, e.g. http://www.astronautix.com/graphics/0/10076348.jpg and post recovery the Akylab 3 CM just shows a degraded white coating http://upload.wikimedia.org/wikipedia/commons/d/d0/Skylab_3_Apollo_Command_Module.JPG

Can someone explain what is happening here?

[Bob B.]

Below is the composition of the hull:



The Mylar is the mirror-like surface that you see in the Apollo 9 in-orbit photo. The rust color that you see in the Apollo 8 museum photo is the heat shield.  Most of the other photos show the surface layers partly burned or torn away.  I'm not sure about the Skylab photos.

[JayUtah]

Sklyab CMs used white paint instead of aluminized Mylar as in the Apollo CMs.  This was deemed better protection against the Skylab mission's thermal cycling profile.

[ka9q]

I wonder what those thermal environment differences were.

Offhand, I can think of a few:

While both had to withstand frequent eclipses on a 90-120 minute cycle, the earth's albedo is considerably higher than the moon's. The moon, however, is much hotter at the subsolar point so its far IR contribution is much higher. This was the main reason for adding a water boiler as backup to the CSM's radiators.

The lunar CSMs had to withstand deep space away from both the earth and the moon, although they were in a BBQ roll during this time.

The Skylab CSMs spent most of their time docked to Skylab and had to follow its attitude changes.

The Skylab CSMs exhausted their fuel cell reactants before each stay was over, so their environmental control systems were presumably shut down at that time and reasonable temperatures had to be passively maintained. Batteries were added to the SM for powering the trip home.

What else?