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Somewhere I read the shuttle fuel tank kept structural integrity only thanks to internal pressure of the fuel, a'la inflated balloon; empty it would collapse under own weight; and the walls were quite thin, a BB pellet could easily thwart a mission punching a hole. How much truth is to that?

A perfect answer would provide a full crossection of the wall, air on one side, liquid hydrogen on the other, materials and distances in between. But a good summary on 'material design' of the shuttle - materials used, their estimated thickness, will suffice.

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    $\begingroup$ There's a somewhat amusing KSC wildlife anecdote that woodpeckers bored holes in the STS-70 ET insulation foam and NASA later had to install balloons resembling owls around the launchpad (can you spot at least two of these on the photo?) to scare them away and used around the clock contingent of human woodpecker spotters. Can you imagine what woodpeckers would do to it if Space Shuttle was using balloon tanks? $\endgroup$ – TildalWave Nov 16 '14 at 14:41
  • $\begingroup$ @TildalWave: Not much different than what they did. I'm pretty sure balloon tank is only fully pressurized before the launch; otherwise the pressure just keeps its shape. If the woodpeckers got through the shell of a full shuttle fuel tank, that would be equally disastrous as with balloon one. $\endgroup$ – SF. Nov 16 '14 at 23:19
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You might be confusing it with the balloon tanks used by the original Atlas and the Centaur upper stage. The Shuttle external tank was not a balloon tank - it did not need to be pressurized to be structurally stable under its own weight - but the pressurization did contribute to its ability to withstand flight loads, as is the case for almost all liquid-propellant rocket tanks. The mechanism for this strength-enhancement is that the pressure provides a local restoring force for small deformations in the skin, resisting the effect of buckling.

The Super Light-Weight Tank (SLWT) used in the majority of Shuttle missions has a fairly simple basic structure. In the barrel sections, 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 a thin (<0.02") layer of epoxy primer, then approx 0.5" of mysterious grey substance and then ~2" of orange foam insulation.

ET cross section engineering sample

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  • $\begingroup$ 2.5mm of metal, that's it?! That seems crazy thin. $\endgroup$ – radex Aug 13 '15 at 11:30
  • $\begingroup$ That's not that thin for a structural alloy, and the loads on it aren't that high. It would also still be enough to stop a BB (actually that would likely be stopped by the foam) $\endgroup$ – pericynthion Aug 13 '15 at 19:56
  • $\begingroup$ @pericynthion Could you, please, add the reference to the statement that there was "0.5" of epoxy" and also reference for the photo. After some discussions in comments to this question space.stackexchange.com/questions/40221/…, there are some doubts in authenticity of this information. $\endgroup$ – LeoS Dec 3 '19 at 23:05
  • $\begingroup$ @LeoS I can barely recall writing this answer 5 years ago and have no idea where I got the information on that "epoxy" layer. Definitely an argument for providing references at the time of writing! I agree half an inch of epoxy seems unlikely to serve a useful purpose but there is clearly some material there that's different from the bulk of the foam insulation. Maybe it's another kind of foam that is still fairly lightweight but adheres well to both the metal and the orange foam? I'm only speculating. I'll be happy to edit the answer if someone can find a definitive reference. $\endgroup$ – pericynthion Dec 4 '19 at 1:36
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    $\begingroup$ "...mysterious grey substance..."? Me want! $\endgroup$ – Stu Smith Dec 4 '19 at 1:58
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Due to the discussion around this question, I've done some more digging to find out whether or not the epoxy layer shown in pericynthion's answer can be considered a structural part of the tank.

In many NASA documents, the epoxy layer isn't mentioned:

NASA applied two basic types of Thermal Protection System materials to the ET. One type was a low-density, rigid, closed-cell foam. This foam was sprayed on the majority of the tank’s “acreage”—larger areas such as the liquid hydrogen and liquid oxygen tanks as well as the intertank—also referred to as the tank “sidewalls.”
The other major component was a composite ablator material (a heat shield material designed to burn away) made of silicone resins and cork.

NASA Facts - External Tank Thermal Protection System doesn't mention it either:

There are two basic Thermal Protection Systems on the External Tank: One is low-density, closed-cell foam; the other is a denser composite material called ablator, made of silicone resins and cork. An ablator is a material that dissipates heat by eroding.
The closed-cell foam used on the tank acreage is a Spray-On- Foam-Insulation often referred to by its acronym as SOFI (pro- nounced sō-FEE). The composite material is Super Lightweight Ablator, known as SLA (pronounced slaw).
The External Tank uses ablators on areas that are subjected to extreme heat, such as the aft dome near the engine exhaust and on protuberances that are exposed to aerodynamic heating, such as the cable trays.

I have found references that mention an epoxy primer.

The application of the (epoxy) primer, used largely as an adhesive for foam insulation,...

It seems unlikely to me that you'd need a layer of primer that's 12 mm thick, but I'm no expert in this area.

The Thermal Protection Systems document contains this graphic:

Foam application

which lists a resin/cork combination as the ablator, used in high heating areas. From the graphic this ablator appears to be used instead of SOFI foam, not underneath it.
The resin/cork material was applied in sheets. This material caused some problems before the first launch, with panels coming off the tank as it contracted when it was filled with cryogenics.

Provisionally, I'd have to conclude that the epoxy layer is not a structural part of the tank.

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  • $\begingroup$ Have a look at the "Liquid Hydrogen Tank Dome" entry. It lists three components. The ablator (SLA-561), BX-265 which is used about everywhere else, probably the insulation foam, and NCFI 24-124. I tried to look up this material but data is exceedingly scarce. It seems to be extremely lightweight (density about 1% of aluminium and 10% of other foams), and from what I could make of some graphs, about as durable as other foams. Thus, a layer thick enough could provide some structural durability without compromising the weight - but as I said, there was no solid data. $\endgroup$ – SF. Dec 29 '15 at 18:26
  • $\begingroup$ ...also, don't dismiss a material of low tensile strength outright, but compare it with its density first. A 1mm^2 steel wire has worse tensile strength than a meter wide column of styrofoam! $\endgroup$ – SF. Dec 29 '15 at 18:29
  • $\begingroup$ The foam is so weak compared to aluminum that it provides negligible structural support. $\endgroup$ – Brian Lynch Dec 29 '15 at 22:06
  • $\begingroup$ @BrianLynch: source? The ultimate tensile strength of aluminium is far, far from the buckling force. And the layer of the foam is much thicker than the layer of the aluminium. $\endgroup$ – SF. Dec 30 '15 at 5:43
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    $\begingroup$ @SF: The buckling load for a 2.5 mm thick aluminum cylinder with 8.4 m diameter and 46.9 m length is 182.6 MN (the weight of the empty tank on Earth is 0.26 MN). Buckling is definitely not an issue for the Shuttle ET! As for the foam, here is a paper showing the strength to be at maximum 0.79 MPa, compared to aluminum at 290 MPa (from here). The stiffness is even worse in relative terms -- 70 GPa for aluminum and about 0.027 GPa for the foam. $\endgroup$ – Brian Lynch Dec 30 '15 at 8:54

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