[huthuthike]

You can't just make something free floating bigger and bigger without introducing new issues. In particular, as objects get larger in zero gravity, it becomes harder to dampen oscillations. The lighter the object is the more problematic these oscillations become. You also run into issues with thermal expansion.

[giantrobot]

And torsion effects when trying to move structures or reorient the whole structure.

[pyinstallwoes]

Can you expand the oscillations part? I find that interesting.

[adhesive_wombat]

Basically, things that are bolted tightly onto 73 million million million tonnes of rock tend not to flop around very much. It's a near-perfect "momentum sponge".

Things in virtual freefall that can flex (and everything can) do so in response to forces (e.g. thrusting, but also heat stresses, say), and will continue to do so if they start unless you take care to damp them and dump them into heat. There's nowhere for the vibration to "go" unless you design one in. Sometimes the structure of the craft itself has enough damping for practical purposes, especially when you take care to isolate large vibration sources (the ISS has a Sorbothane damper for the treadmill, for example), but when your big floppy (i.e. light) mirror surface has to stay put on a nanometre scale, it's not so simple.

It's a bit like the difference between a tuning fork glued down flat to a table and one hanging from a string.

[pyinstallwoes]

By any chance do you know the history of these solutions and what what went before understanding this or was it already calculated and known far in advance of needing to account for it?

In the sense of building things that last in space.

[adhesive_wombat]

I don't know specifically. I imagine that a lot of the concepts came from naval architecture (sloshing of fuel, water and cargo has sunk many ships through the ages, for example, as well as hull resonances called "springing") then aviation (e.g. "flutter", where the wings oscillate, has destroyed planes) and space and missiles (again with the fuel sloshing, and other modes like pogo oscillation where the vibration feeds back into the engines and self-reinforces). Some concept of it also in civil engineering: the Tacoma Narrows bridge is the canonical example.

Fundamentally they're all somewhat similar in that there's a flexible and/or sloshing thing that doesn't have a huge mass that it's rigidly connected to. Spacecraft deployed in space usually have smaller forces on them (no air or water and the hard acceleration is done) but are also much flimsier due to being ultra-light. Telescopes are even worse as even a tiny vibration can ruin the usefulness of the optical paths.

Vibration of telescopes is an issue on Earth as well, e.g.: https://opg.optica.org/ao/abstract.cfm?uri=ao-53-21-4651

[jpollock]

Look into "Chaos Theory", "Control Theory" and "Damping".

In particular, consider how you would damp an undesired movement by a satellite. A naive approach would be to apply thrust in the opposite direction. However, the control can't be exact, leading to thrust -> thrust <- over and over, eventually to the measurement limit of the thruster's control.

With a large mass, it's replaced with a spring, and converted to heat.

https://en.wikipedia.org/wiki/Damping

[pyinstallwoes]

Hm, sounds like a fun oscillator algorithm to try for a synthesizer!

[chaps]

Objects have a resonant frequency and will jiggle in strange ways. The larger the object, the larger the possible jiggles. The larger the jiggles the larger the destruction.

[pyinstallwoes]

Are you saying any object in space will naturally be resonating with itself at some frequency or that by proximity and interaction to another object it may cause resonance on another and therefore cause it to jiggle in strange ways.

Thanks!

[chaps]

The latter. Basically [1] but not as exaggerated. Think of things like screws shaking loose over time which leads to structural failure. Plus, no easy means to release that energy like you would when you're attached to a planet.

[1] https://www.youtube.com/watch?v=XggxeuFDaDU

[huthuthike]

Ok, so basically anything that has any forces on it will end up with oscillations. On Earth it tends to not matter, because those oscillations travel from the object down into the ground, which is really good at damping them because the ground is, uh, pretty big. Even for things not attached to the ground, the atmosphere inherently damps oscillations, because as objects move back and forth in a fluid, a low pressure zone is created behind the object, which pulls it away from the direction it is moving. Also, rigid objects do much better with oscillations, because the entire structure has to move (more mass moving means less movement when the same amount of force is exerted on it). Floppy objects do worse, because one section can start oscillating on its own without transferring that motion to the entire body.

In space, there is nothing to damp the oscillations. They will just continue without active features of the craft to damp them. If they continue unabated a section may reach a resonant frequency, which can quickly cause failure. Even if it doesn't, those vibrations can cause cyclic loading failures, or just affect the stationkeeping of the craft or it's usefulness in gathering scientific data.

To make a spacecraft resistant to oscillations requires devices like gyroscopes or friction dampers, or long weighted booms which decrease the magnitude of oscillations. Making the craft rigid helps, but the larger it is, the less rigid it will be. And to make it more rigid, or to include more anti-oscillation devices, means more weight. That's an important limiting factor when you need to get the object up into orbit.

One misunderstanding some people have is to think that there are no external forces on free-floating structures. This is untrue. Most importantly, they are all affected by the solar wind, which is a generally constant pressure pushing the object away from the sun. Of course they will also be affected by gravity, and if close enough to the earth they will interact with the atmosphere. (There's not really a clean cutoff to where our atmosphere ends and space begins.) As a result, spacecraft have to perform some amount of stationkeeping maneuvers, which involves applying a force on one section of the craft. That itself will cause further oscillations, because the force can never be transferred perfectly to the entire body. (Imagine pushing a piece of paper in the air with a single finger. Yes, you can get the paper to move in a direction, but you cannot get all segments of the paper to move in exactly the same manner when exerting force at only one point.)

So forces on spacecraft are inherently unavoidable, and oscillations happen any time a force is applied. Oscillations are challenging to control in a free-floating vacuum environment, and become more problematic the larger a craft is. This results in fundamental issues with operating very large spacecraft. That's not to say it is impossible. But in space it's not a simple solution to say "just build it bigger."

[pyinstallwoes]

Thank you very much for that detailed reply. Lots of interesting things to contemplate further based on your descriptions. What incredible complexity to balance things! It makes me wonder if there's systems that are engineered to handle the balancing in a positive-feedback, almost cybernetic way in consideration of all the inputs and outputs that influence each other.

So many things to learn about... thanks!

Also, your description of using the ground to dampen oscillations has very similar implications to electricity and ground... not a coincidence?