# Why is the shuttle's external tank's metal skin painted the same dark color as the foam insulation?

The last flight-qualified space shuttle external tank in existence is now in Los Angeles and moved to its new home over the weekend. I noticed that what appears to be the metal skin under the insulation is also dark orange/rust colored, very similar to the insulation. I'm guessing it's paint on the metal.

As explained in this answer the dark orange/rust color of the foam insulation is it's natural color, and it darkens after exposure to the sun.

In the first photo (found in Twitter) you can see the metal tank exposed through the funny-shaped hole in the insulation. Not identical colors, but very similar. There's a good video in the article also.

I'm wondering if the metal skin was painted a similar color for aesthetic reasons, or for another reason. If it were white underneath, then "missing foam" would be much more apparent. That could be considered a good thing or a bad thing.

Photos are circa Saturday, May 21, 2016 in Los Angeles, California. First two from here.

This one is from the NPR story

• Possible duplicate of Why do the Shuttle's external tanks appear to be rusted? – user10509 May 22 '16 at 17:00
• @JanDoggen ah, didn't see that one, I've modified the question to focus on the color of the metal skin under the foam being painted to match the foam's color. Thanks! – uhoh May 22 '16 at 23:57
• @JanDoggen I think this no longer qualifies as a duplicate candidate, based on the edited question and the new information in the answer, including the PPG Aerospace Coatings Color Experts reference. Is the This question may already have an answer here banner at the top still necessary? – uhoh May 23 '16 at 4:04
• Looks fuzzy! I'm surprised that the museum didn't replace the missing foam, if they were going to paint it. – ikrase May 23 '20 at 10:11

tl;dr - the parts at the rear of ET-94 where the foam was removed were painted orange for display.

The foam was not dyed but started out a light cream color. It slowly turned orange when exposed to light. Here is a picture of foam that was trimmed off during the stringer crack problem on STS-133. You can see the internal foam is lighter, and the metal is natural color.

Some areas of the tank had other thermal protection materials applied, like Super Light Ablator (SLA). A good picture showing the different materials is found in the CAIB report page 51. The area in the odd-shaped hole in your picture either had one of these other materials applied, or, most likely, had the foam shaved down for some reason to a lesser thickness, and the remaining foam turned orange. This tank - ET-94 - was used extensively for investigation by the CAIB, I would guess that is where the odd-shaped hole originated. However, this fact sheet states that the aft dome had SLA applied, so it may be that.

You can see the variance in the foam color in this amazing picture of four ETs at Michoud, when production was humming along.

Edit: In the left background of this image taken at Michoud, you can see an unfinished aft dome with some foam applied. The metal below is clearly unpainted.

This article confirms that the "odd shaped hole" was caused by CAIB work, and says "work was done to return it to its original appearance"...I believe this is implying that the hole in ET-94 tank was painted orange for display.

Edit 2: And the smoking gun: This article talks about the paint used to touch up the tank for display and transport. The paint "was tinted to match the burnt-orange foam by PPG aerospace coatings color experts"

• I seem to remember that the first couple of shuttle flights had external tanks painted white. I think they quit doing that because the paint job added basically useless weight. – Howard Miller May 22 '16 at 22:49
• Great information - and photos! In the first photo of the unusually shaped hole (and the mentioned video) it really looks like the metal skin itself, painted orange. Are you saying that's not paint on metal? Note I've had to edit the question, and focus on the color of that surface. Please keep this useful information, but help me understand if that is indeed painted metal, and why it's painted to match. – uhoh May 23 '16 at 0:03
• Great information - and photos! In the first and especially the second photo of the unusually shaped hole (and the mentioned video) it really looks like the metal skin itself, painted orange. Are you saying that's not paint on metal? Note I've had to edit the question, and focus on the color of that surface. Please keep this useful information, but help me understand if that is indeed painted metal, and why it's painted to match. – uhoh May 23 '16 at 0:16
• @uhoh see edits. I found confirmation that it was painted for display / transport. – Organic Marble May 23 '16 at 1:28
• Great! You always come through with the facts. PPG Aerospace Coatings Color Experts no less! Much appreciated. – uhoh May 23 '16 at 4:06