There is a section of the Space Shuttle's external tank that appears to have large corrugations. I spotted them and saw how deep they are in a photo in this answer, a cropped section of which is shown below.

I could speculate about thermal or aerodynamic functions, but luckily there's a much better way to find out, so I'll just ask...

Question: What is their function? Why only this section?

below: Cropped from image found in this answer. Credit: NASA

Space Shuttle external fuel tank (NASA)

cropped from Space Shuttle Atlantis launches on STS-132

above: Cropped from Space Shuttle Atlantis launches on STS-132 below: Cropped from STS122 Atlantis.

cropped from STS122 Atlantis

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    $\begingroup$ they're the stiffening stringers on the intertank. $\endgroup$ – user20636 May 23 '19 at 10:58
  • $\begingroup$ Interesting that in the top picture Atlantis has no plugs in the RCS nozzles, but in the bottom picture it does. I wonder why. $\endgroup$ – Moo May 23 '19 at 21:26
  • $\begingroup$ @Moo perhaps that's worth asking as a new question? $\endgroup$ – uhoh May 23 '19 at 21:40
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    $\begingroup$ @Moo in the top picture, the vehicle has already lifted off, and the paper covers have blown off. There's already a question about it. space.stackexchange.com/questions/33828/… $\endgroup$ – Organic Marble May 23 '19 at 23:04
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    $\begingroup$ @Moo They are just supposed to keep water out of the jets while the vehicle is sitting on the pad. $\endgroup$ – Organic Marble May 23 '19 at 23:28

That's the intertank - the cylinder that connected the bottom of the LO2 tank to the top of the LH2 tank.

enter image description here

It didn't contain propellant, but did contain the forward interface with the Solid Rocket Boosters, and was built for lightness and strength, with skin-stringer construction. The ribs you see were the stringers.

The intertank is a steel / aluminum semimonocoque cylindrical structure with flanges on each end for joining the liquid oxygen and liquid hydrogen tanks. The intertank houses ET instrumentation components and provides an umbilical plate that interfaces with the ground facility arm for purge gas supply, hazardous gas detection and hydrogen gas boiloff during ground operations. It consists of mechanically joined skin, stringers and machined panels of aluminum alloy. The intertank is vented during flight. The intertank contains the forward SRB-ET attach thrust beam and fittings that distribute the SRB loads to the liquid oxygen and liquid hydrogen tanks. The intertank is 270 inches long, 331 inches in diameter and weighs 12,100 pounds.

This shows how the three sections fit together:

enter image description here

During the launch campaign for STS-133, some of these stringers caused a launch scrub - they were made from substandard material and cracked.

enter image description here enter image description here


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    $\begingroup$ Where is the rubber band that connects the two tanks, pulling them sung into it?(humor) It looks to me like it was just applied to the outside of one long cylinder; so both my guesses were way off, as usual. $\endgroup$ – uhoh May 23 '19 at 11:25
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    $\begingroup$ Great answer! I suppose the next question is why do the tanks NOT have the stringers for stiffness? Balloon support from internal pressure? $\endgroup$ – Greg May 23 '19 at 20:07
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    $\begingroup$ @Greg I think so. Shuttle Flight Rule A5-155 mentions in its rationale that ET structural failure can occur if the pressure in the tank drops too low. archive.org/details/flight_rules_generic/page/n1091 The LH2 tank at least did have internal stringers, maybe machined in. spaceflight101.net/uploads/6/4/0/6/6406961/9757423.jpg $\endgroup$ – Organic Marble May 23 '19 at 20:16
  • $\begingroup$ "they were made from substandard material and cracked" - What material were they made from? $\endgroup$ – SSight3 May 23 '19 at 21:45
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    $\begingroup$ I'm giving up a lot of free answers here :) They were made of Aluminum 2024. en.wikipedia.org/wiki/2024_aluminium_alloy $\endgroup$ – Organic Marble May 23 '19 at 21:51

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