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Discovery Launch
Source: Discovery Launch Captured by Multiple Cameras, NASA, YouTube; edited

A more clickbaity but accurate description of my question would be:

  • How did the Space Shuttle / External Tank not shear off?

The view above always fascinated me. Note how once the ET is jettisoned, where it connects is revealed, and it is a small connector.

During lift-off I always thought of the SRBs carrying the ET, and the Shuttle carrying itself, but once the SRBs are jettisoned, and the acceleration is coming only from the Shuttle (thrust-line is now parallel more or less to the ET's vertical axis), I can only wonder how is that small connector possible; how is it designed to withstand such a shear force, yet remain with minimal footprint on the Shuttle underside / heat shield?

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    $\begingroup$ Note that the much of the ET volume is liquid hydrogen, with a density about 1/14 that of water, and once the SRBs separate, it's burned about 1/4 of its contents, so it is somewhat less massive than it appears at a glance. A good question, though! $\endgroup$
    – Russell Borogove
    Sep 11, 2019 at 22:46
  • $\begingroup$ The other connections are at least somewhat more substantial $\endgroup$
    – CourageousPotato
    Sep 12, 2019 at 1:21
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    $\begingroup$ different but related: Shear forces between Shuttle, tank, and boosters - what pushes what? $\endgroup$
    – uhoh
    Sep 12, 2019 at 2:30

1 Answer 1

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There were three attach points. The forward bipod that you show in your question, and two aft attach points. At each attach point a large bolt with an explosive nut held the tank and Orbiter together. Large umbilical door openings in the aft of the Orbiter let the aft bolts pass through and also had all the fluid and electrical connections. After separation tile-covered doors closed over these openings.

Read all about it here.

The 1982 Press Manual has some detailed drawings of the bolts and associated fittings.

enter image description here

Here is a picture of some of the bolts being prepped for flight.

enter image description here

(Personal photo)

Also, as kindly pointed out by Tristan,

all of the +/-X shear load between the orbiter and ET came through the aft attach points and that the bipod fitting at the front, with the spherical bearing, only reacted Y/Z loads

This is confirmed by an early analysis paper Engineering analysis division internal note. OFT-1 margin assessment

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    $\begingroup$ That helped me find this paper. If I'm reading it right, the Orbiter/ET forward attachment experienced ~500 kN in two directions. That's equivalent to the thrust of a Boeing 777's engine, which is also bolt mounted to the wing. So while it's a big number, I suppose indeed a bolt can do it. (I think I have overestimated the thrust of the SSMEs.) $\endgroup$
    – ymb1
    Sep 12, 2019 at 2:54
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    $\begingroup$ They were big bolts. $\endgroup$
    – Organic Marble
    Sep 12, 2019 at 3:44
  • $\begingroup$ SSME's produced ~400,000 lbf of thrust for a combined 1.2M lbf will all 3 firing at 104%. The thrust of each individual SSME varied slightly based on the number of cores (basically picture a collection of ~ 100 straws through which the propellant mixture flowed) that were operative for each engine. $\endgroup$
    – David C. Rankin
    Sep 13, 2019 at 5:39
  • $\begingroup$ A reasonably SSME Diagram shows the main injector cores feeding the main combustion chamber. $\endgroup$
    – David C. Rankin
    Sep 13, 2019 at 5:50
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    $\begingroup$ I don't really have a public reference other than basically an examination of the design, which is partially evident in the first picture of your answer. Basically, the bipod can rotate forward and aft via hinges on the ET and the spherical bearing on the orbiter. This is to accommodate the change in length of the ET due to thermal contraction when it is filled with cryos (by up to 7 inches across its full length per nasa.gov/centers/marshall/about/star/et_11.html). A necessary consequence is that it cannot react any loads in that direction $\endgroup$
    – Tristan
    May 11, 2021 at 20:29

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