[Curiosity:] Why do unpiloted vehicles rotate?

[Vanamonde]

As soon as you switch away from personally piloting a vehicle, it picks up little random rotations. Why? In real life, escaping gas, imperfect vacuum in the vicinity of a planet, tidal effects, and a number of other things can exert an off-center force on a vehicle. But I don\'t believe KSP simulates any of those things, so what\'s causing it in the game? For that matter, how come these objects always seem to spin gradually? Shouldn\'t the rotation be either speeding up or entirely absent? There\'s the reaction, but where\'s the force?

(Oops! Meant to post this in 'general.' Sorry mods!)

[Endeavour]

I think the ship remains in the same orientation to the background, so it appears to rotate relative to the planet(?) But I\'m not sure.

[Cykyrios]

There are indeed some 'phantom forces' that make the ships randomly rotate, other than the usual keeping-orientation-relative-to-stars: you can notice it most when you practice docking, as it\'s almost impossible to stay aligned for more than 5 seconds. Some times the target ship spins to the right, others to the left, and along all 3 axes.

I don\'t know the cause of these either, but I\'d like to know as well.

[vexx32]

The slow, gradual rotation isn\'t actually rotation at all. It\'s the movement of the craft giving the appearance of rotation as it orbits a massive body, which you would logically take your reference point from. Any other rotation is, yes, a bug caused by some other things. The ones Cykyrios describes, I believe, are the effect of the ASAS module affecting other ships in the vicinity as well as your own. They are planned to be fixed in future, though.

[Divaglio3578]

Or it could be the fact that you have not actually cancelled out rotation, but rather reduced it to a minimal level, unnoticeable over a short amount of time, but obvious as you continue to observe the object

[Vanamonde]

not actually cancelled out rotation

Not in this instance, anyway. While practicing rendezvous I used ASAS (which it seems may be a factor?) to stabilize one ship, and it stayed that way for a while, then I hopped to the other. The first started off stable as I had left it, then developed up a random spin. Definitely not residual.

The slow, gradual rotation isn\'t actually rotation at all

Right. That\'s actually the camera POV rotating.

On the subject of rotations that may be glitches, is it me or do rockets built using tri-couplers seem to wallow around a lot? Trying to yaw you get unintentional pitch and roll, for example. Setting control surfaces or RCS thrusters at 120 angles seem to have similar issues, so I\'ve taken to avoiding trilateral symmetry altogether because they\'re a nuisance to steer.

[Cykyrios]

I think the issue with the tricoupler comes from physics, and is normal: if you try to turn right, as you have two tanks or whatever at the 'bottom' and only one at the 'top' of your rocket, moving the two will be more difficult, and the rocket will start rotating along an other axis as well.

Or it could be the fact that you have not actually cancelled out rotation, but rather reduced it to a minimal level, unnoticeable over a short amount of time, but obvious as you continue to observe the object

As Vanamonde said, it\'s very unlikely. In my docking video tutorial, I had to edit and delete 2-3 minutes of trying to get the alignment right. I then proceeded to regularly switch between the two ships, keeping ASAS on before each change, and even then the target kept rotating randomly.

[Vanamonde]

Yes, the distribution of mass with the tricouplers did seem to be the problem. I guess I was hoping it was a glitch instead because I didn\'t want to avoid using these things that group the engines so tidily and securely. I hope they\'ll eventually introduce bi- and quad-couplers to get around the problem, but in the meantime I build kind of bulky rockets on a 4-surround-1 symmetry. They fly straight, but are hard to rotate with all that mass so far from the centerline.

[MoonHeadJohn]

I\'ve noticed this a couple of times on some of my ships when I use modded parts. Not sure whether it\'s down to the parts themselves or the coding itself.

On my course at uni I learned that when you use a data type such as a float, it\'s never a whole number. Say for example you want it to equal 3, instead it will equal 2.99999. Could just be a random number which has been thrown up by one of these data values, or simply a trace amount of force which can\'t be gotten rid of so easily which causes these spins to come out of nowhere.

Just an idea which popped up into my head, could be completely wrong.