Phantom Forces?

[Streetwind]

Okay, here's a funny story.

Just before dinnertime, I was checking the arrival times of all my current vessels in flight. Then I jumped back to the ship with the shortest time left (a couple hours).

I set the camera for a pretty perspective with Kerbin in the background, then went downstairs to cook and eat. I did not touch any ship-controlling button whatsoever. SAS was off, no RCS installed, and the ship completely stationary.

Roughly one hour later, I come back to a ship that's spinning wildly around all three axes. What is happening here? o_O

[Kasuha]

Yes, there are phantom forces. Especially if your SAS is off, you are moving at less than 750 m/s and/or are in the planet's rotational reference frame (usually below 100 km).

Turning SAS on usually solves these problems.

[Claw]

Do you have a cat? They can be evil.

(Seriously, I don't know. I haven't seen this yet.)

[Streetwind]

No, I don't have a cat. No, I was not under 100km to the planet - I was 1 hour out from a Mun intercept, about 9-10 million km above Kerbin. However yes, I was moving slower than 750 m/s.

Very interesting. I also find it curious that no other spacecraft of mine, many of which flew nearly identical routes, ever exhibited this. It was so strong, I could sit and watch it happen - zero out everything perfectly, then turn SAS off, and within 10-20 seconds it would be spinning randomly again.

Could this maybe be the result of accidental part clipping, since it seems to be craft specific?

[Vanamonde]

Make sure your joystick is calibrated.

[Streetwind]

Keyboard & mouse here.

[Kasuha]

Could this maybe be the result of accidental part clipping, since it seems to be craft specific?

I believe it is not related to part clipping but it may depend on how exactly are your ship's parts connected to each other. In totally unpredictable way, because it's caused by arithmetic errors which are somewhere on 8th decimal place.

[Phafor]

Kasuha is right there. It's probably not because of part clipping

[helaeon]

Sometimes the ship for whatever reason, is not perfectly symmetrical (even if symmetry is on). SAS cancels out those effects right away.

Little bits (or a lot) of part wobble can do this too, the SAS is usually able to cancel those out before they multiply on each other. If you come out of non-physics time warp and don't touch the controls does the same thing happen in the same period of time? Standard time warp stabilizes the craft, so if its stabilized then physics turned on and you don't touch it and it remains stable there's probably a part in there somewhere wobbling.

[Streetwind]

So I figured it out. It didn't like me having struts attached to modular girders, for some reason. As soon as I removed those struts and attached them to somewhere else, the issue immediately stopped. When previously, the craft would have spun itself enough to make a full rotation within half a minute of stopping time warp, now it stays perfectly still. Watched it for 5 minutes and it didn't even budge a degree out of line.

[Kerbart]

I have a station completely made from stock parts that spins like "crazy". Makes docking quite the challenge.