This is the question that I should have asked here. The space shuttle and the two solid rocket boosters (SRBs) are mechanically attached to the giant tank.

SRB's, shuttle, and tank all experience forces due to drag, and due to gravity. SRBs and shuttle also experience forces due to thrust.

There are also mechanical forces between the SRBs and tank, and between shuttle and tank. My question here is about these forces.

While under flight, the SRBs are lifting the tank, so the forces of the SRBs on the tank are positive. Near the end of their propulsive flight, they separate from the tank - are they still lifting at this moment?

The shuttle's three main engines burn LOX & LH2 from the tank. The shuttle sometimes carries a very heavy payload, but sometimes it does not - meaning the gravitational force on the shuttle can vary substantially from one flight to the next. The shuttle's main engines support the weight of the shuttle. But do they always share in the lifting of the tank? Or when the shuttle has a very heavy payload, is the tank (+SRBs) actually pulling the shuttle?

Ideally, I'd like to see if there is a plot of SRB-tank, and shuttle-tank force (magnitude and sign) as a function of time, for small payload mass and maximum payload mass (Hubble?).

Basically what-pushes-what vs time?

enter image description here above: STS-79 (NASA) from here

enter image description here above: STS-51f - Spacelab (NASA) from here

enter image description here above: STS-132 (NASA) from here

enter image description here above: STS-117 (NASA) from here

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    $\begingroup$ There are plots related to what you seek in this document ntrs.nasa.gov/api/citations/19770023219/downloads/… But only for first stage, and in such form (load indicators) that it would take an enormous effort to write it up as an answer. To get a flavor, here is the definition of the load indicator locations imgur.com/FJufdOZ and here is the plot for the fwd attach z-axis load indicator FT01 imgur.com/fkO5j5S $\endgroup$ May 12, 2021 at 2:13
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    $\begingroup$ @OrganicMarble oh this is already extremely helpful. This is exactly what I needed to see i.stack.imgur.com/fzSOF.png $\endgroup$
    – uhoh
    May 12, 2021 at 2:46

2 Answers 2


The three SSMEs provide 568 tons of thrust, the Orbiter has a weight in the region of 100 tons incl payload. The Orbiter always exerts a positive force on the tank from liftoff to MECO, no matter what the payload (0-16 tons in practice).

The SRBs are jettisoned when their thrust drops to 100,000 lbf/45 tons, so by then their thrust/weight ratio is below 1 and they are being pulled along by the Orbiter. Thrust dropoff is very rapid, so this occurs for just a few seconds.

  • $\begingroup$ OK this is really helpful! Somewhere out there, there must be some kind of plot of these vs time. $\endgroup$
    – uhoh
    Jul 10, 2016 at 8:22
  • $\begingroup$ @uhoh -- Somewhere out there is a plot of force vs time. Click on Hobbes' link, and there it is. $\endgroup$ Jul 10, 2016 at 9:35
  • $\begingroup$ @DavidHammen there is a plot of something, but not what I've asked for. Please read my question again. "Ideally I'd like to see if there is a plot of SRB-tank, and shuttle-tank force (magnitude and sign) as a function of time, for small payload mass and maximum payload mass (Hubble?)" $\endgroup$
    – uhoh
    Jul 10, 2016 at 10:13

To provide a little more clarity on what's pushing what as a supplement to @Hobbes's answer:

Thrust loads from the SRBs go through their forward SRB/ET attach fittings (pictured), which connect to a thrust beam that runs through the ET intertank, between the hydrogen tank at the aft end and the oxygen tank at the forward end.

SRB forward skirt with ET attach point circled in red

Thrust loads from the SSMEs go through the aft ET/Orbiter attach fittings at the bottom of the hydrogen tank.

  • $\begingroup$ Thanks for your answer and revisiting this I realize I no longer really understand what pushes what from (at)Hobbes' answer. In an image there I see the SRBs connected to the shuttle and to my non-rocket-scientist eyes it looks like the SRB is attached directly the Shuttle and "pulls up" on it directly i.stack.imgur.com/KGetr.jpg But "Thrust loads from the SRBs go through their forward SRB/ET attach fittings, which connect to a thrust beam that runs through the ET intertank..." sounds like the SRBs "pull up" on the external tank, as does the Shuttle. $\endgroup$
    – uhoh
    May 11, 2021 at 23:37
  • $\begingroup$ Should I ask "What are the functions of this thing?" as a new question, where "this thing" is the coupling between SRB and shuttle, labeled in the image in my previous comment? $\endgroup$
    – uhoh
    May 11, 2021 at 23:38
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    $\begingroup$ @uhoh the picture you're showing is the structural frame that attaches the ET to the orbiter. To make it clear as mud, the SRBs pull up on the ET from the middle -- they push up on the oxygen tank and pull on the hydrogen tank. The orbiter pushes up on the ET from the bottom. The aft attachments on the SRBs serve only to keep them pointed in the right direction, as does the bipod at the forward part of the orbiter. $\endgroup$
    – Tristan
    May 12, 2021 at 0:46
  • $\begingroup$ Okay that really helps quite a lot actually! The coupling with the arrow I drew isn't simply a point-to-point orbiter-to-SRB connection; it's just not a good viewing angle to see what all is going on there and while the thrust beam isn't seen (since the camera didn't have X-ray vision) it's really one of the "main players" we need to appreciate. Thanks for your time and patience with me :-) $\endgroup$
    – uhoh
    May 12, 2021 at 1:05

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