How can I determine what forces are acting on a ship?

[icedown]

I have a station in a low orbit of the mun. Even with all of the engines and torque wheels disabled, it pitches back and forth constantly, as well as constantly changing it's orbit. Is there any way to find out what is causing this? The motion doesn't seem to come from a particular spot, as the changes in rotation don't appear to be uniform. eg it will swing back and forth on the same axis without rotating. KER shows 0 Thrust, 0 accel, but a thrust offset angle of 0.3 and thrust torque of 5.7kNm.

Edited May 19, 2015 by icedown

[FleshJeb]

That might be the phantom acceleration bug. Does it persist after you shutdown and restart KSP?

[icedown]

Yes, I've tried that several times. It persists with docking/undocking as well

[Dilli]

Large ships / stations can pick up an oscillation due to flex between the joints which SAS aggravates, it can make the orbit appear to change constantly.

One solution is turn SAS off, warp time a bit, then return to normal time with sas off. The craft should then be still and your orbit should be constant.

[icedown]

It is rotating as well, this happens with all SAS, and reaction wheels disabled.

[TORCHWOOD]

There's an option on the debug menu under physics. You can show aero forces and thrust/drag forces. This might help determining the issue, but beyond that, I can't really help.

[match]

This happens to many of my longish stations with lots of mass in lower orbits. My theory is that gravity is acting on individual parts and if the CoM is not the center of the vessel, then the vessel starts to tumble and translates the movement into smallish orbital changes that eventually accumulate into larger ones.

Try putting it into a higher orbit and see if this remedies it, or at least subdues it.