Wobbling Orbits

[Delta_8930]

My apoapsis and perapsis always wobble and change for no reason (when the craft is not under any thrust). 

Can anybody explain why?

[sal_vager]

Sorry @Delta_8930 but this is how the game is.

[Delta_8930]

Will the orbits eventually decay or will they move slightly above and below the original apoapsis? Is this a KSP bug or not?

[Daveroski]

Yes it's a bug and not it's not a bug.

Yes I know that you were confused enough before I answered now I just made matters worse.

The thing is even in real life, no orbit is absolutely stable.

Some reasons why;

Gravity is a bit different in different parts of the orbit. Usually a little greater where there are mountains or deposits of heavy metals such as iron.

The atmosphere actually goes far beyond even the moon and is gradually being blown away by the sun. Of course the Electromagnetic field protects it somewhat and the loss is very small. It is also being recreated by plankton and such but the point is, it's out there.

The atmosphere from any planet will affect anything else in its orbit over time.

All our space junk will eventually fall back to earth because of these gravity fluctuations and the very slight reduction in orbital velocity cause by the atmosphere.

If you get a high enough orbit, it will be stable for longer but it will not have any of the benefits of the protection from our electromagnetic shield and the movement of other bodies in the system will affect it over time.

The moon, for example, is getting bored of our company and is gradually breaking away from us. It was quite a bit closer and appeared larger in the sky when the dinosaurs were around.

The moon also affects the orbits of items in orbit around the earth. Just as it pulls on the seas it also pulls on things in orbit. Just a little because they are so much less massive than an ocean.

So you see the answer is no it is not a bug if the intent was to show orbital drift (Albeit in an extremely overpowered manner)

And the answer is yes it is a bug considering how great the discrepancy is, I doubt it was intended to be so vigorous.

It could also be argued that the 'bug' if indeed we accept that it is one, is not the fault of squad or the game but of the game engine Unity which is a virtual environment with it's own limits and faults.

If you read this far, then you are now as wise about it as some of us are.

 

Edited February 28, 2016 by Daveroski
typo

[nobodyhasthis2]

5 hours ago, Delta_8930 said:

Will the orbits eventually decay or will they move slightly above and below the original apoapsis? Is this a KSP bug or not?

It is a bug. No orbits do not eventually decay if periapsis above 70km. Unless this is modded in.

We could talk realism. It is a game.and it really is a bug. Change scene and come back and the wobble goes away. 

Edited February 28, 2016 by nobodyhasthis2

[Delta_8930]

6 minutes ago, nobodyhasthis2 said:

It is a bug. No orbits do not eventually decay if periapsis above 70km. Unless this is modded in.

We could talk realism. It is a game.and it really is a bug. Change scene and come back and the wobble goes away. 

Yeah, that's what I tried to do. I don't have the orbit decay mod. Even if I change scenes the wobble still comes back. 

I was in a circular orbit of approximately 8000km and the orbit decayed to 7940km. 

[Tex_NL]

I wonder how many times I've already explained this behaviour here on the forum.

No. orbits will not decay over time. Even an orbit that dips into an atmosphere will not decay as long as you will not switch to that craft. Only active craft in an atmosphere will decay. (With the exception for craft that go very deep into the atmosphere. Any unfocussed craft that gets below roughly 23km on Kerbin gets deleted. Other planets have different cut off altitudes.)

Orbits wobble for two reasons.

1. Floating point errors. It is simply not possible to calculate anything into finite detail.
2. All craft rotate round their centre of mass (CoM) but obits are calculated from the root part. When the root and CoM are very close together the orbit will be steady but if the root and CoM are a significant distance apart any amount rotation will change the root parts velocity causing changes in the orbital path.
   There is very little you can do about it, this is just the way the game functions. All you can do is to design your craft better and make sure the CoM and root are as close together as possible.

[Tex_NL]

2 hours ago, Daveroski said:

Yes it's a bug and not it's not a bug.

Yes I know that you were confused enough before I answered now I just made matters worse.

[mega snip]

If you read this far, then you are now as wise about it as some of us are.

 

As correct as this information may be it has NOTHING to do with what is happening in-game and does NOT answer the OP question!

There actually is a way in which orbits can decay in the stock, unmodded game: phantom forces due to part clipping.

[Daveroski]

Ok Mr Everlasting know-it-all, I stand corrected. You must be right.

 

[John FX]

3 hours ago, Delta_8930 said:

Will the orbits eventually decay or will they move slightly above and below the original apoapsis? Is this a KSP bug or not?

No your orbit will not decay as long as you are above the atmosphere, even if your orbit dips into the atmosphere your orbit will not decay if you are on rails. Your orbit will only `wobble` when you are controlling your craft and not on rails.