Question about apoapsis decay.

[Olack]

Hello, I am a very new player so excuse me if this is a dumb question but I find the following behavior a bit weird. When I get into an orbit, way out of the atmosphere, let's say 500km apoapsis and 300km periapsis, then it s my understanding that the orbit should stay like that forever. If there is no atmospheric drag (outside of 70km), then the orbit should stay like this indefinitely since there is no orbital decay implemented. However, when I look at apoapsis, then it is slowly decreasing. When I say slowly, I mean like ULTRA slowly, maybe 1m every 10 seconds. However, it seems to happen only at normal time speed, and not when I have time accelerated. And it happens when I keep the mouse cursor on the apoapsis for a while, the last 2 digits of the apoapsis flicker a little bit and then it decreases by 1m. Then it stays like that. If I take the mouse cursor off the apoapsis and put it back there again, then the same thing happens again and 1m goes down again. This behavior looks like a bug to me, but I thought I should ask here for any other possible explanations first. Does anyone else experience this behavior?

Thanks!

[Pecan]

You are correct that it shouldn't happen and it's probably down to rounding-errors. Unless you somehow have a little thrust from engines or RCS then, yes, your orbit should stay the way it is forever. In real-life orbits decay and are altered because the other celestial bodies have a constant but varying effect ... best to just write-off these minor fluctuations in KSP as 'other stuff' like that and plan to make small orbital-adjustments every few months if it's important enough.

[DeepOdyssey]

There are phantom forces in the game, as a result of SAS simulation. It just apply torque on your ship, and it can sometimes generate thrust, if the torques on both side of CoM are uneven.

[allmhuran]

My guess is that it's probably due to the rotation, flexing, or other wobbling of the craft.

The orbital path of any vessel has to be determined from some particular spot on a craft, since the craft is not a point in space and therefore different parts of the vessel are actually at slightly different orbital altitudes, which means they have slightly different orbits.

The game probably uses the primary control part (command pod or probe) to calculate your orbit. So if your craft is rotating, then the altitude of that part is changing and therefore the orbit will also change very very very very slightly.

In rails warp the orbit is calculated in a deterministic manner and no longer takes this kind of thing into account.

On big ships that bend and flex a great deal more this can be very noticable when looking at solar orbits, intercepts, etc. At regular speed your orbital path may jump around, but go into rails warp and everything becomes stable.

[MarvinKitFox]

Your ship is either rotating, or wobbling.

The fact that the "orbit change" vanishes when entering timewarp is a pretty good sign of this.

Try: switch SAS and RCS and engines off, then timewarp to stop the wobbles, un-timewarp. It should now be steady(er)

[Olack]

It still seems to happen despite switching SAS and RCS on and off again and time warping. So maybe it's a rounding error or something. Well, thanks in any case, it doesnt affect me in any way at all, I was just curious.

[cantab]

I encountered this once before. The change was on the periapsis for a Mun flyby, and the "decay" was very consistent regardless of how the craft was oriented, totally different to the wobble that results from flexible ships. (It was a tiny probe so pretty rigid anyway).