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Whilst looking for information regarding Shuttle's External Tank insulation, I came across this answer to this question Structural composition of shuttle liquid fuel tank wall.

The answer states that for Super Light-Weight Tank (the third version of it):

the innermost layer is approx 0.1" thickness of Al-Li alloy. The thickness varies along the length of the tank due to varying hydrostatic loads. Outside this there is approx 0.5" of epoxy and then ~2" of foam insulation.

The other answer (to the same question) contains some discussion to the role of epoxy layer, but without definitive conclusion.

So, to me this is still unanswered question: what was the primary role for this epoxy layer (and why is it so thick)?

From one side, it indeed seems way too thick to serve as adhesive primer for the foam (paint with thickness of an order of magnitude smaller was discarded for weight benefits...); from the other side, mechanical strenght of epoxy (without any kind of embedded internal honeycomb) would not be high enough to serve the structural purpose.

A mid-layer to "smoothen" different thermal expansion rates between aluminum alloy and the foam, maybe?

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    $\begingroup$ Just a wild guess but if the foam is foamed with pretty much anything but helium that gas would condensate on the liquid hydrogen tank possibly collapsing the foam. The epoxy layer might just be enough to get above liquid nitrogen temperature. $\endgroup$ – Christoph Dec 3 '19 at 15:11
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    $\begingroup$ @Christoph I would recommend to look up the boiling temperatures of hydrogen and helium, compare them, think about it and then delete your comment. $\endgroup$ – Uwe Dec 3 '19 at 21:57
  • $\begingroup$ The foam did not use helium as the blowing agent. $\endgroup$ – Organic Marble Dec 3 '19 at 22:34
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A Technical History of the Space Shuttle External Tank refers to the epoxy layer only as a "primer". The epoxy also had anti-corrosion properties. The document states

BX 250, a foam which did not require a heated substrate and only RH below 40%, was used. Because foam would not stick to bare aluminum, the tank was covered with an epoxy primer. The MPTA used the first generation primer which, while providing a suitable under layer for foam adhesion, provided minimal corrosion protection. The combination of non-protecting primer and weather exposure far beyond predictions resulted in massive corrosion problems. This was aggravated by the LH2 which froze any gas at the tank surfaces causing negative pressure relative to ambient. This caused the outside air to be cryopumped through microscopic cracks in the foam. As the moisture in the outside air was drawn through this foam, it leached chlorides from the foam blowing agent which resulted in a chlorine rich liquid at the metal surface. Since Al 2219 is prone to surface corrosion, the ET propellant tanks suffered extreme corrosion, in one case 87% of the thickness of the aluminum. Martin developed a new primer with chromates replacing some of the inert filler materials.

(Emphasis mine)

Acronymology:

  • MPTA - Main Propulsion Test Article, an ET + Orbiter boattail + SSMEs test system
  • RH - Relative Humidity

Also note that the "0.5 inch of epoxy" in the linked answer is unreferenced, as is the image provided. The document I link shows a much thinner epoxy coat, 0.01 inches. The massive layer in that answer appears to be an error.

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  • $\begingroup$ The source you quote, on page 3, mentions that "original ET concept" had epoxy layer 50 times thinner: "0.010-inch epoxy primer". If they used that configuration for the MPTA, all quoted text makes perfect sense: thin epoxy layer failed to stop corrosive reactants penetrating onto metal structure. The paper doesn't mention an increase of epoxy layer thickness, but states only compositional changes (page 3): "After MPTA, strontium chromate and carbon black were added to the primer which changed it from yellow to green and increased its corrosion resistance." $\endgroup$ – LeoS Dec 3 '19 at 22:09
  • $\begingroup$ @LeoS I agree, I just added an additional edit stating that I think the 1/2 inch was an error. $\endgroup$ – Organic Marble Dec 3 '19 at 22:10
  • $\begingroup$ So, perhaps, the thickness increase was required to serve the secondary (but not less important) role of either adding to thermal insulation, as suggested in @Christoph comment, to increase the temperature on the outer epoxy layer; or decreasing the reactants' ability to penetrate the epoxy layer; or both. Still, 0.5inch of epoxy seems to be a lot of weight penalty to solve the corrosion problem. Thus, the other possibility, as per OrganicMarble answer's update, the 0.5" epoxy layer might be an error and didn't represent the true design. $\endgroup$ – LeoS Dec 3 '19 at 22:18
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    $\begingroup$ @LeoS I am not convinced that there was a thickness increase. I'd like to see a reference stating such a large thickness. The original answer has no reference for the claimed thickness. $\endgroup$ – Organic Marble Dec 3 '19 at 22:19
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    $\begingroup$ Agree, the 0.5" epoxy layer might be wrong information and therefore didn't represent the true design. $\endgroup$ – LeoS Dec 3 '19 at 22:24

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