Why is my orbit changing by itself?

[airelibre]

http://imgur.com/a/waOCK

I have a single part in orbit as you can see here, but its periapsis and apoapsis change by themselves. Is this a known bug?

[FenrirWolf]

You're brushing the edge of Kerbin's atmosphere.

[maltesh]

http://imgur.com/a/waOCK

I have a single part in orbit as you can see here, but its periapsis and apoapsis change by themselves. Is this a known bug?

When in Physical Warp, KSP does a stepwise physics simulation, where it works out where your spacecraft is, calculates the gravity and other forces on your craft, assumes that all such forces will be constant over the next timestep, and moves the spacecraft appropriately. After the timestep, the new velocity and position are used to project where your spacecraft would be if it went into a perfect Keplerian elliptical orbit based on its current position and velocity.

But since your spacecraft is not actually /traveling/ a perfect Keplerian Elliptical orbit, but instead making a lot of tiny jumps that /approximate/ one, the projected orbit (and thus the projected apoapses and periapses) will change from timestep to timestep.

It's not a big deal, it's just an unavoidable consequence of how the physics engine works. If you want to actually /travel/ a perfect Keplerian orbit, you'll need to kick up to 5x time acceleration, (colloqually refered to as being "on rails") and you'll see that apoapse and periapse don't budge when that happens, but you're also unable to make your spacecraft /do/ anything.

[Sirrobert]

You're brushing the edge of Kerbin's atmosphere.

Look again. The altimeter at the top reads 179km, and 150km. Kerbins atomsphere ends at 70km

[Leszek]

Look again. The altimeter at the top reads 179km, and 150km. Kerbins atomsphere ends at 70km

He is right, look at KER on the side there, the PE is 69ish.

[Sirrobert]

He is right, look at KER on the side there, the PE is 69ish.

Look at the Mission time. Those 2 pictures are 3 minutes apart, not an entire orbit

[Taki117]

When in Physical Warp, KSP does a stepwise physics simulation, where it works out where your spacecraft is, calculates the gravity and other forces on your craft, assumes that all such forces will be constant over the next timestep, and moves the spacecraft appropriately. After the timestep, the new velocity and position are used to project where your spacecraft would be if it went into a perfect Keplerian elliptical orbit based on its current position and velocity.

But since your spacecraft is not actually /traveling/ a perfect Keplerian Elliptical orbit, but instead making a lot of tiny jumps that /approximate/ one, the projected orbit (and thus the projected apoapses and periapses) will change from timestep to timestep.

It's not a big deal, it's just an unavoidable consequence of how the physics engine works. If you want to actually /travel/ a perfect Keplerian orbit, you'll need to kick up to 5x time acceleration, (colloqually refered to as being "on rails") and you'll see that apoapse and periapse don't budge when that happens, but you're also unable to make your spacecraft /do/ anything.

This is the most accurate explanation.

[airelibre]

When in Physical Warp, KSP does a stepwise physics simulation, where it works out where your spacecraft is, calculates the gravity and other forces on your craft, assumes that all such forces will be constant over the next timestep, and moves the spacecraft appropriately. After the timestep, the new velocity and position are used to project where your spacecraft would be if it went into a perfect Keplerian elliptical orbit based on its current position and velocity.

But since your spacecraft is not actually /traveling/ a perfect Keplerian Elliptical orbit, but instead making a lot of tiny jumps that /approximate/ one, the projected orbit (and thus the projected apoapses and periapses) will change from timestep to timestep.

It's not a big deal, it's just an unavoidable consequence of how the physics engine works. If you want to actually /travel/ a perfect Keplerian orbit, you'll need to kick up to 5x time acceleration, (colloqually refered to as being "on rails") and you'll see that apoapse and periapse don't budge when that happens, but you're also unable to make your spacecraft /do/ anything.

Thank you for the thorough explanation. I let it run on 4x acceleration all night and the changes seem to have cancelled out, because when I came back both the ap and pe had dropped a little (143km and 67km). Not sure if they would be lower without this artefact.

[Taki117]

Thank you for the thorough explanation. I let it run on 4x acceleration all night and the changes seem to have cancelled out, because when I came back both the ap and pe had dropped a little (143km and 67km). Not sure if they would be lower without this artefact.

They would be because you were still skimming the atmosphere, however, the initial changing orbit was due to the physics warp.

[airelibre]

They would be because you were still skimming the atmosphere, however, the initial changing orbit was due to the physics warp.

No, I mean if the game calculated things differently, and I didn't get this phantom rising periapsis, I wonder if my resulting ap and pe would be the same, or, if it slowed down my lithobraking at all.

[Sirrobert]

No, I mean if the game calculated things differently, and I didn't get this phantom rising periapsis, I wonder if my resulting ap and pe would be the same, or, if it slowed down my lithobraking at all.

Yes but if you left it on a full night in this orbit, it would have skimmed the atmosphere on each pass, which explains the differences

[airelibre]

Yes but if you left it on a full night in this orbit, it would have skimmed the atmosphere on each pass, which explains the differences

I know why there is a difference, obviously I wouldn't leave it running all night if I thought it wasn't skimming the atmosphere.

My point is, my orbit went from 69,000 by 192,000, to 67,000 by 143,000, BUT, would it be lower if the apoapsis and periapsis weren't rising gradually without any influence on my part, due to the way the game works (as explained above)? Say for example 65,000 by 130,000, or even lower (I thought I would wake up to find it destroyed after crashing into the ground after leaving it for so long).

[panzer1b]

There are a few possiblilities in general.

One as mentioned before is the physics warp being unreliable for accuracy purposes. Its a nice tool if you dont care 100% about perfect physics and want to go faster, but it will knock off or add a small amount of velocity/altitude here and there if used. Another possiblity if that you have eneterted atmosphere if any planet that has one. This means that you are feeling drag, which will lower your speed and altitude. Another possiblity is that you left throttle at a very low setting that you did not notice. Ive had this happen, as some of the engines do not create any sound or effects when they are at very close to 0 throttle, but still give some thrust. One more possibility is that you could be experiencing a krakendrive effect. While it is an actual engine that has been made multiple times, many clipped parts can often end up causing anything from minor to server acceleration/torque. If possible, check that there is minimal clipping, and make absolutely sure no moving parts are clipped (landing gear, wheels, ect).

[Hannu]

There is also some bug which changes orbits occasionally. It is very rare event, maybe about one ship of hundred see it. I do not know how to demonstrate it reliably or is it due to KSP or some mod. It effects most often when ship is on trajectory from outer planet to inner planet. Effect is probably around tens of meters per second. It can not be immediately seen on map view, if I fly other crafts, but when I try to execute deep space maneuver, I notice that I have not encounter and it may take hundreds of meters per second to fix it. I try to fly optimal ships on optimal trajectories, so typically it means failed mission.

It may be some error in SOI detection. I installed Kerbal Construction Time couple of weeks ago and when I warped time with KCT, once it interrupted acceleration and showed message that some craft, which had left Kerbin's SOI couple of hours ago had detected SOI change. That craft went to Eve, so its velocity referenced to Kerbin was low and it was still near Kerbin. When I had correction maneuver about one hundred days later, I noticed that just that craft was on wrong orbit. Magnitude of the error suggested, that it may be due to false SOI changes.

I warp always manually to SOI and run transition at low warp factor, when I insert my craft to interplanetary trajectory. After SOI crossing I check changes in orbit, use MechJeb to calculate midcourse correction, and put alarm to Kerbal Alarm Clock. Typically the maneuver is after tens or hundreds of days and I go to make other things with other crafts until KAC alarms. So the bug can not be real SOI crossing at too large time warp.

[GoSlash27]

panzerb has mentioned the most common culprit; phantom acceleration forces due to part clipping.

I've noticed it a lot more since .90.

Best,

-Slashy

[Aethon]

And it's quite unfortunate. We get theses great new build widgets to make more streamlined vessels, and using them (offset tool) causes ships to accelerate.

[airelibre]

You guys, ahem, if you look at the "craft" from the first post, you will see that it is a single piece, so no clipping or leaving the engines on! And again, losing speed due to the drag from the air was the point, I had the piece marginally in the atmosphere and I wanted it to drop down over the course of a night, but I believe that its orbit didn't drop as much as it should have​ because of the constant increase in periapsis and apoapsis which can only come from physics inaccuracies or a bug.

[numerobis]

I've noticed this often; atmosphere has nothing to do with it. For one, it happens outside the atmosphere. For another, the apoapsis rises, which wouldn't happen if you were losing energy to drag -- indeed, the apoapsis rises more that periapsis decreases, so you're gaining energy! Not enough to write home about though.

Another fun one: on 1x physical time, check the Pe. Speed up to 2x. Check the Pe. Slow down to 1x. There's often (always?) a difference. About 20m in a recent multi-pass aerobrake, which meant that if I warped on a pass, upon switching back from 2x to 1x so I could then go on rails, I'd regain much of the energy that I had burned off in drag.

[Richmountain112]

You may have accidentally made a kraken drive.

[Streetwind]

@Richmountain112 Dude... this thread is almost eight years old... please don't do that.

[Vanamonde]

On 12/18/2022 at 12:00 PM, Richmountain112 said:

You may have accidentally made a kraken drive.

Thank you for being helpful.