How do orbital boosts affect the structural integrity of the ISS?

Question

I've become curious of this question since my time on KSP, and searches have not rendered much.

In the simulator, building large stations even without the micro atmosphere being simulated, can cause them to become unstable from even the slightest rotational torque, and start "wiggling" about.

It seems a little exaggerated in the game, but something tells me their structural stiffness and bendiness values may not be so far off, perhaps?

So as they were building the ISS, and the station's length expanded until the solar arrays were installed, were the re-boosts accounting for the extra torque and off-center of thrust, perhaps with throttling?

I just want to see if structural integrity was ever considered for re-boosts at the station's current size, if there's a paper maybe?

Extra is for info on off-center center-of-thrust and center-of-mass considerations and compensations. (Active systems would add an additional torque, was this considered?)

Answer 1

Yes. It is considered (sorry I don't know exactly how much, but probably a LOT - they track the torque on individual bolts during construction for example). They had an accident a while ago where a thruster started firing and oscillating wildly, causing the whole station to wobble and shake, a bit like they do in KSP. Although the forces applied exceeded design limitations, later analysis showed that no real damage had been done. Still, must have been brown trousers time for everyone involved.

Onboard footage of the incident:

Here you can listen to Suni Williams talk about the exercise bike. Note that she mentions it is not directly coupled to the walls, because the motions of the cyclist would put energy into the structure, and cause the solar arrays to vibrate up and down:

So if they are paying attention to the vibrations of the exercise bike, they are surely spending a lot of time calculating all the forces and stresses and oscillations involved in a re-boost.

Another interesting source of strain on the structure, which is absolutely not modelled in KSP, is that the ISS is so large, the different ends or corners of it are in significantly different orbits, and so want to drift relative to each other. This induces regular, slow compression and expansion on the structure (and no doubt causes a few creaks and pops).

Following a docking, both vehicles are put into a free drift for a while, to prevent unnecessary vibration, torque and stress. The docked vehicle can gently rock back and forth and dissipate energy through friction before being slowly pulled in and firmly latched.

Sorry I don't have specific data on the re-boost, but the attention paid to the structure is VERY detailed, as seen above, and so no doubt the re-boost gets appropriate attention.

Answer 2

This paper describes tests done in 2010, where thrusters on the ISS were fired specifically to analyze the resulting vibrations and compare them with the theoretical models.

Modal analyses, model validations and correlations are performed for the different configurations of the International Space Station (ISS). Three Dedicated Thruster Firings (DTF) tests were conducted during ISS Stage ULF4; this paper will focus on the analysis and results of the DTF S4-1A, which occurred on October 11, 2010. The objective of this analysis is to validate and correlate analytical models used to verify the ISS critical interface dynamic loads.

Another paper on structural verification in general:

This paper is intended to give the reader an overview of some of the key decisions made during the structural verification planning for the elements of the U.S. On-Orbit Segment (USOS) as well as to summarize the many structural tests and structural analyses that were performed on its major elements.