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?
above: STS-79 (NASA) from here
above: STS-51f - Spacelab (NASA) from here
above: STS-132 (NASA) from here
above: STS-117 (NASA) from here