Periapsis Falling Slowly For No Reason?

[UndyingWar]

So I'm making my first space station, and I'm in orbit around Kerbin at 100km apoapsis and periapsis. The problem is that they both are slowly falling for no reason whatsoever. I have no thrusters or engines on board, so it shouldn't be changing.

 

Is this anything to be concerned about?

Any help would be greatly appreciated.

 

EDIT: I noticed that when I timewarp, it stops changing. Also, when I get to the periapsis, it rises, and the apoapsis drops. And when I get to the apoapsis, it rises, and the periapsis drops.

Edited February 11, 2016 by UndyingWar

[HebaruSan]

By how much? I believe that when a craft is active, the game takes it "off rails", so instead of having a constant Ap and Pe and calculating position solely based on time, the acceleration due to gravity is calculated to modify the velocity and then the position in each physics frame, so some small variation due to rounding errors is common. Are you seeing more than a few dozen meters of change over time?

[UndyingWar]

@HebaruSan it's changing about about 1m every 2-3 seconds. I haven't watched it for too long yet. I'm kind of just hoping/assuming it will stay somewhere around 100km over time.

[Guest]

I think that's normal. Just random fluctuations in the game's formulas.

[Benoit Hage]

I noticed it on a space station of mine once. If you don't play stock, I'm curious to know about your mod list.

 

[UndyingWar]

@Beniot Hage I have like 45 mods or something

[ZooNamedGames]

As others have said, it's just errors in the calculations system, and your station is fine. Your station will remain in orbit safely. I will note, your mods should not have an effect on your orbit unless you have the orbital decay mod, which even then only works during warp (IIRC). 

These changes can come from calculation errors or even possibly the flexing of your spacecraft, you will see this change happen often with large interplanetary transfer vehicles, since the mass of the vehicle flexing causes the calculations vary. 

Hope this helped.

[Benoit Hage]

If I'm right, this happened to me when I try to integrate some of the realism overhaul suite mods into the stock game ( without Real Solar System). But that was a long time ago... And it only happened on a space station, no other ship. I usually don't play with a lot of mod. If you post your list, I can tell you which one I used.

Maybe post in the support thread. Someone might have a solution.

[ZooNamedGames]

2 minutes ago, Benoit Hage said:

If I'm right, this happened to me when I try to integrate some of the realism overhaul suite mods into the stock game ( without Real Solar System). But that was a long time ago... And it only happened on a space station, no other ship. I usually don't play with a lot of mod. If you post your list, I can tell you which one I used.

Maybe post in the support thread. Someone might have a solution.

RO on its own (so I believe) does not have any effect on your orbit, that should be the same as stock, granted, your altitude and FAR may have been a minor factor. That is unless you also had orbital decay installed.

His problem sounds like a basic one that doesnt require support assistance, but thats his choice as to whether or not he wants to report it.

[Benoit Hage]

3 minutes ago, ZooNamedGames said:

As others have said, it's just errors in the calculations system, and your station is fine. Your station will remain in orbit safely. I will note, your mods should not have an effect on your orbit unless you have the orbital decay mod, which even then only works during warp (IIRC). 

These changes can come from calculation errors or even possibly the flexing of your spacecraft, you will see this change happen often with large interplanetary transfer vehicles, since the mass of the vehicle flexing causes the calculations vary. 

Hope this helped.

I agree that there is some degree of variation, especially when the ship is using SAS. But that time it was really different. The drop of altitude was constant. I also had a bug once where the apoapsis would increased indefinitely.

[ZooNamedGames]

1 minute ago, Benoit Hage said:

I agree that there is some degree of variation, especially when the ship is using SAS. But that time it was really different. The drop of altitude was constant. I also had a bug once where the apoapsis would increased indefinitely.

could be a physics bug (ie Kraken). RO has been known to incur them, but thats just my thought, I may be wrong, your the one who was playing. *puts hands up*

[Benoit Hage]

Quote

That is unless you also had orbital decay installed.

That is exactly how it felt. Like the orbit was decaying. Yet I didn't had the mod installed. I ended up reinstalling the game without looking more into the problem...

@UndyingWar how much is your periapsis falling? Just a few meters and then it get back to its original height, or is it a constant drop?

[ZooNamedGames]

1 minute ago, Benoit Hage said:

That is exactly how it felt. Like the orbit was decaying. Yet I didn't had the mod installed. I ended up reinstalling the game without looking more into the problem...

@UndyingWar how much is your periapsis falling? Just a few meters and then it get back to its original height, or is it a constant drop?

Might be Kraken...

[bewing]

This will happen to any craft that has focus, in the stock game. When craft are not in focus, the game forces them to move on a perfect elliptical orbit. When a craft is in focus, unfortunately, the game tries to calculate the orbit from the current velocity vector, with numerical errors. It conserves energy, however. So, as said above, your craft cannot fall out of orbit. But a small amount energy will shift from the periapsis to the apoapsis and back again. If I could modify the program I could fix this -- but if you want it to stop happening to your space station, then you need to shift focus to something else.

 

Edited February 12, 2016 by bewing

[FullMetalMachinist]

Also, the rounding errors, and therefore the drifting effect, is more pronounced the closer to a perfect circle your orbit is. So for things like space stations that you want in nice, tidy, circular orbits you'll notice this more than something that's in a highly eccentric orbit. This may be the reason that you never noticed it before, but I promise it's normal and has been around for a while. 

[ZooNamedGames]

1 hour ago, FullMetalMachinist said:

Also, the rounding errors, and therefore the drifting effect, is more pronounced the closer to a perfect circle your orbit is. So for things like space stations that you want in nice, tidy, circular orbits you'll notice this more than something that's in a highly eccentric orbit. This may be the reason that you never noticed it before, but I promise it's normal and has been around for a while. 

Fair point, and you and can see this for yourself by using hyperedit and putting yourself into a perfect orbit. It wont last long, even with a 1 part count ship, it will drift.

[Choctofliatrio2.0]

2 hours ago, Benoit Hage said:

I also had a bug once where the apoapsis would increased indefinitely.

That's the opposite of a problem. You call it a bug, I call it an amazing opportunity to explore the solar system with no Delta V

[ZooNamedGames]

29 minutes ago, Choctofliatrio2.0 said:

That's the opposite of a problem. You call it a bug, I call it an amazing opportunity to explore the solar system with no Delta V

Thats known as kraken drives, they were very common in older versions of KSP, and although newer versions kill some of them, they do still exist.

[Kerbart]

Yes. As a result, do not attempt a Kerbal rescue mission from a lower orbit. I thought hilariously that if I entered a 70.5×70.5 orbit I could approach the Kerbal from that direction. I then had to watch in agony how my orbit deteriorated relatively rapidly into a 69.9×70.5 orbit. As I was waiting to get out of the atmosphere and passing through my periapsis it deteriorated of course to 69×69 and worse from there. And of course I couldn't jump to rails to “fix” the rounding error...

[ZooNamedGames]

3 minutes ago, Kerbart said:

Yes. As a result, do not attempt a Kerbal rescue mission from a lower orbit. I thought hilariously that if I entered a 70.5×70.5 orbit I could approach the Kerbal from that direction. I then had to watch in agony how my orbit deteriorated relatively rapidly into a 69.9×70.5 orbit. As I was waiting to get out of the atmosphere and passing through my periapsis it deteriorated of course to 69×69 and worse from there. And of course I couldn't jump to rails to “fix” the rounding error...

Vernors or otherwise small propulsion can offset the error, or at least act as an emergency solution. They also can push you back into space for short durations. 

[UndyingWar]

2 hours ago, Benoit Hage said:

That is exactly how it felt. Like the orbit was decaying. Yet I didn't had the mod installed. I ended up reinstalling the game without looking more into the problem...

@UndyingWar how much is your periapsis falling? Just a few meters and then it get back to its original height, or is it a constant drop?

No actually, I think it's good. I probably was worrying over nothing. It's not a big enough change to effect anything, anytime soon.

[Mitchz95]

In my current save, one of my space stations randomly dropped from a 210x200km orbit to 175x135 in between scene changes. I have no idea what caused it.

[Brainlord Mesomorph]

IFAIK, the point (and lines) of map view refer to the CoM of the ship. If the ship is big and wobbly (space station, hello) then the COM is moving a tiny bit within the ship.

I think that's what causes wobbly Pe and Ap points. 

[Tex_NL]

23 minutes ago, Brainlord Mesomorph said:

IFAIK, the point (and lines) of map view refer to the CoM of the ship. If the ship is big and wobbly (space station, hello) then the COM is moving a tiny bit within the ship.

I think that's what causes wobbly Pe and Ap points. 

Close, but not quite correct.

All objects will rotate around its CoM however the orbital path is calculated from the root part. If the root and CoM are very close together as they are in small ships there will be very little to no wobble when a craft rotates. (Even a stationary craft in orbit will rotate once every orbit relative to the parent body.) In large ships relative to the CoM and parent body the root is changing in altitude and velocity which will result in changes in Pe and Ap. The further the CoM and root are apart the larger the individual differences will be. In low orbit the wobble will usually be only a few metres but on interplanetary encounters you could completely miss (or hit ) a planet/moon.
To minimize wobble try to design your large craft with the root and CoM as close together as possible. Starting the build with the fuel tank should suffice.

Back to the OP issue.
Make a quicksave for safety, write down Ap and Pe. Do not timewarp or switch to another craft. Go do the dishes, take a shower, do your homework, watch a movie. Just leave you craft running for a few orbits.
After a few orbits compare Ap and Pe to what you wrote down. If there is no significant change you're simply dealing with orbital wobble as described above. If there are significant changes you could be dealing with phantom forces caused by things like part clipping.

[Brainlord Mesomorph]

Hey Tex,

Oh, that explains a lot. esp. given my big ships, and that always make the root part the CM or probe core(!)

Given the re-root tool in the SPH and VAB, it would be easy to root any ship on its CoM.

However most of my big stuff is assembled in orbit. So I have no idea where my root part(s) are.