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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?)

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    $\begingroup$ Do you really mean "structural integrity"? Things can have unwanted or even uncomfortable vibrations without losing their structural integrity. Things can also vibrate, and those vibrations can slowly decay due to structural damping and internal friction without being "unstable". I think this is going to be an interesting question, but the terminology may have to be adjusted a little bit first. $\endgroup$ – uhoh Feb 17 '17 at 5:45
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    $\begingroup$ @uhoh I guess I specifically mean "structural stresses" especially on interconnecting parts like the docking adapters, which add up to the overall health of 'structural integrity'. I was thinking that if a re-boost was not throttled or induced significant torque or stress, it might affect the overall lifespan of the station, and also be a consideration for any future large inter-planetary habitats. $\endgroup$ – IT Bear Feb 17 '17 at 19:24
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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.

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    $\begingroup$ Good answer in general. I would add that there is literally an entire team of people whose job it is to analyze exactly this. Thruster firings, different flight attitudes, thermal conditions, docking loads, vibrations due to crew activity, EVA loads, and any unexpected excursions. $\endgroup$ – Tristan Feb 17 '17 at 16:53
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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.

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    $\begingroup$ I just wanted to say that second paper you linked to is fantastic, written in very plain english and very easy to follow. The "Leak Before Burst" metric is especially interesting. So is, "dedicated test articles would be refurbished to become flight articles themselves later in the program. [...] Therefore, [the program] required that no ultimate loads testing be performed on any of this hardware and that test loads applied to these elements induce no permanent detrimental deformation." $\endgroup$ – IT Bear Feb 19 '17 at 8:39

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