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In the Wikipedia article about Spaceplane there's a (rare and cool) shot of the Space Shuttle from one of the SBRs immediately after it was released. The web page for the image says:

Space Shuttle Discovery seen with her external fuel tank as she climbs into orbit 3 seconds after solid rocket booster separation on STS-121, July 4, 2006. Photo from video from the right aft SRB camera.

It looks like there are three circular areas at the "bottom" end of the tank. Two appear bright and reflective (and overlap) and a third appears dark.

What are these areas called and what are their functions?

enter image description here

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    $\begingroup$ When I was working on shuttle main propulsion system simulation long, long before the 107 disaster, I always wanted a good photo of STS in 2nd stage, when just the SSMEs are running and those distracting SRBs are gone. Never really thought it would happen though. Not to mention video! $\endgroup$
    – Organic Marble
    Commented May 28, 2018 at 2:03

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It is an access hatch used during construction and maintenance.

image Credit: NASA-KSC

image Credit: NASA

This part got at least some media coverage during the scrubbing of STS-121, when a Engine Cutoff (ECO) sensor, a fuel gauge, mounted behind that cover, inside the Liquid Hydrogen (LH2) tank, malfunctioned, causing that launch to be delayed, while the sensors were replaced. This is where the first image is from.

Here and here are articles about the scrub.

image Credit: NASA

Some of those hatches/holes are covered with the insulation foam during manufacturing.

image Credit: Lawrence Tsuei

Some pictures of a test article that are probably not representative of the actual tank but illustrate the covers quite nicely, that are underneath the insulation foam.

image Credit: Project Habu (with more images)

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  • $\begingroup$ Wow, very informative, and colorful answer! $\endgroup$
    – uhoh
    Commented May 26, 2018 at 15:35
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    $\begingroup$ Ah, yeah. Apparently a common solution for tanks that need to be welded from inside in case you don't intend to use the lost welder method. $\endgroup$
    – SF.
    Commented May 26, 2018 at 16:40
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    $\begingroup$ That picture from "project habu" is not representative of the aft end of a flight LH2 tank. It's an early ground test article. What you show may be similar to the forward dome end of a flight LH2 tank, but it isn't the aft dome of one for sure. That said, that's a really cool web site, thanks for the link. $\endgroup$
    – Organic Marble
    Commented May 26, 2018 at 21:14
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    $\begingroup$ @OrganicMarble that explains why it confused me. Though, is it just me that thinks it looks like an R2 unit that's fallen over? $\endgroup$
    – Kaithar
    Commented May 27, 2018 at 3:54
  • $\begingroup$ TLDR: the tank has three belly buttons. $\endgroup$
    – 300D7309EF17
    Commented May 30, 2018 at 20:59
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This part of the External Tank is called the "LH2 Tank aft dome". There are really two large circular penetrations on it. They are the ones offset from the center of the tank.

One is the access hatch/manhole. (this description is from the LO2 tank part of the linked document. Further down it says the LH2 "manhole fitting was similar to those on both the LO2 tank domes and the LH2 tank forward dome")

A 45”-diameter manhole fitting, welded to the dome cap, provided a 36”-diameter clear access to the tank interior. The fitting featured ninety-two attachment points for mating to the machined aluminum manhole cover. The fifty-seven pound manhole cover measured 40.32” in overall diameter and 0.185” thick at the center. It provided an interfacing sealing surface with the manhole fitting, as well as a means for making a leak check of the primary seal.

The other is the "siphon plate hole". It's where this large device is mounted and was inserted during manufacture.

Here is an inside view of the aft part of the LH2 tank, manhole at the bottom, siphon at the top.

enter image description here

The 3rd circular feature in the center is an artifact of the manufacturing process and of the way the insulating foam was applied.

Here is a picture I took of an ET on the launch pad, stacked for flight. The manhole is at the top and the siphon plate is at the bottom.

enter image description here

Sources: HAER report, personal photo, External Tank - legacy of success

BONUS Something to think about: Why wasn't the siphon in the geometric center of the tank bottom? Mouseover for answer.

Because of the side mounting of the tank to the Orbiter with the engines mounted on the Orbiter, the acceleration vector was tilted away from the centerline of the tank.

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  • $\begingroup$ Thanks! So the "siphon plate hole" might be used only during initial assembly, and the access hatch is likely the only thing used regularly, at least on this end? $\endgroup$
    – uhoh
    Commented May 26, 2018 at 16:12
  • $\begingroup$ I'd have to look but I think the manhole was also for assembly. The tank was shipped from Michoud all foamed up and ready to shoot; if they had to access the inside of the LH2 tank after shipping it would have been a big deal. $\endgroup$
    – Organic Marble
    Commented May 26, 2018 at 16:15
  • $\begingroup$ In this image nasa.gov/images/content/105045main_et-3-lg.jpg you can see a tank arriving and it's already all sealed up. $\endgroup$
    – Organic Marble
    Commented May 26, 2018 at 16:17
  • $\begingroup$ I'm guessing the different coloured bit closer to axis is from a rotisserie style coating process? $\endgroup$
    – Kaithar
    Commented May 27, 2018 at 3:57
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    $\begingroup$ I think so, and the spot in the center when the "axle" attached was later foamed manually. $\endgroup$
    – Organic Marble
    Commented May 27, 2018 at 4:02

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