In another question, this came up:
For example, if a Falcon 9 launches 20 tons of fuel to dock in LEO, how can the same upper stage be used to launch 40 tons of fuel to the same orbit? Doesn't it need to be larger and maybe sturdier to support the much heavier payload?
And I realized I'm not sure how structural strain on upper stages actually works.
Sitting on the ground, the second stage structure is supporting the weight of that 20 or 40 ton payload under Earth gravity, which we can express as a force in Newtons.
Once out of the atmosphere and staged, though, the operative force is that of the second stage engine - some 95 tons for the Merlin Vac at peak thrust - so is that the force on the structure of the second stage? Or assuming the stage plus payload is accelerating together uniformly, does the force need to be pro-rated between the stage mass and payload mass? Does that mean the effective weight of the payload increases as propellant is used?
How would structural strain on the second stage during the early part of ascent be calculated, with aerodynamic drag opposing the thrust from the first stage engines?