Periapsis Falling Slowly For No Reason?

[Tex_NL]

Some time ago there was a post explaining how craft types determined what would be the root of a docked ship. I can't find that post at the moment but I do remember some of the rules:

Base and station are high in the hierarchy, ship and lander in the middle and probe and debris are low. The craft with the highest hierarchy will become the core after docking. The name and type of that section will be inherited and the root part of that section will become the root part of the final craft after docking. So if a lander docks to a station the station keeps its name and will be the root. Now if you would change the station to be a probe before docking the lander would keep its name and would be the root.
All this will be useful when building large craft and wanting to keep control over the root part. Simply call your interplanetary craft a station for the time being and call all parts you dock to it ships or probes.

Edited February 13, 2016 by Tex_NL

[FullMetalMachinist]

3 hours ago, Tex_NL said:

 however the orbital path is calculated from the root part.

IIRC this is only true while the ship is off rails and the game is constantly updating the orbit based on physics calculations. Once you time warp and go on rails, it switches to the CoM and uses the double precision floats. This is why you can have a very wobbly orbit and when you time warp it makes up its mind and stays put. 

[Tex_NL]

1 minute ago, FullMetalMachinist said:

IIRC this is only true while the ship is off rails and the game is constantly updating the orbit based on physics calculations. Once you time warp and go on rails, it switches to the CoM and uses the double precision floats. This is why you can have a very wobbly orbit and when you time warp it makes up its mind and stays put. 

Does not matter what part the system uses once you go 'on rails'. The reason you no longer get a wobble 'on rails' is because of the lack of physics. Everything is calculated as if the entire craft was a single point with a certain amount of mass and speed.

[Glaran K'erman]

20 minutes ago, Tex_NL said:

Does not matter what part the system uses once you go 'on rails'. The reason you no longer get a wobble 'on rails' is because of the lack of physics. Everything is calculated as if the entire craft was a single point with a certain amount of mass and speed.

Actually doesn't really matter at all what or where your root part is, you will always have inaccuracies. All these problems including the OP's stem from issues with floating point calculations. See here for how this can mess things up worse.