Station Rotating in Minmus Orbit

[something]

Got a station in Minmus orbit at an altitude between 20km/22km PE/AP. That station is experiencing some weird phantom forces and starts to rotate once you focus on it. Also its periapsis is oscillating, although decreasing in the long term. (trim is off)

Heard that there is a threshold at which physics change from a reference frame which is centered on the celestial body (a co-moving rotating reference frame) to a reference frame which is not moving with the celestial body. At Kerbin that height is 100km which is why it is a bad idea to put a station at that altitude as it will experience such phantom forces. The station is at least rotating around two different axes at two different speeds.

Now, I am interested in Minmus: At which altitude does the change of the reference frames occur there? Any chance it is 20-22 km?

Well, or did anybody else solve this problem anyhow?

[Snark]

Aside from possible rotating-frame issues that you mention... is there any possibility that you've accidentally set "trim" on your station?

Lengthy explanation below, but the TL;DR is:  try pressing Alt+X and see if that fixes your station and gets rid of the "phantom" rotation. 

Here's the deal with trim, which may or may not be relevant to your situation:

"Trim" is a feature that was added to the game to help with airplane flying.  The basic idea is that when you set trim, you put your ship into a state where it continually tries to adjust its pitch upward or downward-- the idea is that if you have a plane that tends to nose-down, for example, setting the trim to pitch up can make it easier to make long-distance sustained flights.

However, trim is a notorious source of confusion and pain for players, for several reasons:

• Most players have no idea that the feature even exists.
• It applies to spacecraft in outer space, via reaction wheels, which (IMHO) is completely useless and pointless for a feature that was intended for airplanes in atmospheric flight.
• It's easy to accidentally trigger the feature, without realizing that you've done it (it's activated via Alt plus one of the WASD keys, so it's easy to accidentally do that if you're piloting your ship and are a little slow to let go of the Alt key when doing something else).
• There's no visual indication that the feature is active.  (Or, indeed, that it even exists.)
• Once it's active for a craft, it stays active, including when saved and re-loaded (so restarting KSP doesn't fix the problem).
• Once the feature is active, there's no obvious way to turn it off, unless you happen to know the magic incantation Alt+X.

...Personally, I wish they'd just remove the feature entirely.  In the 2+ years that I've been playing KSP, I've never once activated it on purpose (admittedly, I'm not an airplane guy), but I have activated it accidentally several times... and every single time I do it accidentally, I end up tearing my hair out over it until I discover (again) that "ohhhhh, it's trim."  (The accidents happen infrequently enough that I always forget about this feature by the time the next incident occurs.)

But at the very least, even if they do keep the feature... I really wish they'd give it an explicit UI so that people who don't know and don't care about trim (i.e. the large majority of KSP players, I'd guess) won't get bitten by this.  For example:  a small graphical UI that pops up when the feature is activated, and goes away when it's not.  The feature would have a dot or crosshairs or something, whose 2D position would indicate the trim setting.  There'd be a visually obvious "cancel" button on the feature.  That way, when someone accidentally triggers it, the scenario would go like:  "Hey, what's this thing that just popped up?  And hey, my ship is acting funny.  Well, I'll push this button here... okay, it went away.  Don't know what it was, but my ship stopped spinning, so whatever."

But I digress.    Anyway, the existence of the "trim" feature is something worth knowing about, even if it turns out not to be the problem in your particular case.

[FullMetalMachinist]

1 hour ago, Snark said:

There's no visual indication that the feature is active.  (Or, indeed, that it even exists.)

This isn't quite true, though it is hard to see sometimes. 

When trim is set, the little orange markers in the bottom left hand corner of the screen that show your control inputs will move in the direction of the trim. You can see this effect easily if you really lay on the trim for a long time in a single direction. 

Having said that, if you accidentally set trim, then those markers will have moved only a pixel or two, so it's very hard to notice, but it is there. 

[Snark]

21 minutes ago, FullMetalMachinist said:

This isn't quite true, though it is hard to see sometimes. 

When trim is set, the little orange markers in the bottom left hand corner of the screen that show your control inputs will move in the direction of the trim. You can see this effect easily if you really lay on the trim for a long time in a single direction.

Yeah, I was wondering if someone was going to mention that. 

But I stand by my original position, for a few reasons:

First, those little indicators are so useless that my eyeballs have long since trained themselves to not even see them.  I mean, seriously, what's the point?  If I'm pressing the "W" key to pitch down, it's not as if I need a little control-needle indicator to tell me that I'm pressing the "W" key.  I suppose they might be useful for debugging purposes, if I were trying to author an autopilot mod or something, but for normal usage, they're simply of no practical use to me.  The only time I ever look at them is to tell whether I'm in "fine control" mode or not (they change color, red for normal or cyan for fine).  The actual position of the needles is something that I never look at, and my eyeballs are very strongly trained to ignore.  I could stare at them for hours and not notice anything amiss.

Second, if there's only a little bit of trim (which is generally the case when it's triggered accidentally), the deflection is going to be minor enough that it's not super noticeable even if you were looking right at the needles.

Third, even if I did happen to notice the position of the needles... I would still, as a player, have no idea of why they're off center, or how they got that way, or how to stop it.

So... yeah, as far as I'm concerned, there's no visible way to tell that anything happened.  It may be that technically there's a way, but it's so subtle that it might as well not be there, at least for me.

[Shadowmage]

On Tuesday, August 09, 2016 at 1:06 PM, something said:

Got a station in Minmus orbit at an altitude between 20km/22km PE/AP. That station is experiencing some weird phantom forces and starts to rotate once you focus on it. Also its periapsis is oscillating, although decreasing in the long term. (trim is off)

Heard that there is a threshold at which physics change from a reference frame which is centered on the celestial body (a co-moving rotating reference frame) to a reference frame which is not moving with the celestial body. At Kerbin that height is 100km which is why it is a bad idea to put a station at that altitude as it will experience such phantom forces. The station is at least rotating around two different axes at two different speeds.

Now, I am interested in Minmus: At which altitude does the change of the reference frames occur there? Any chance it is 20-22 km?

Well, or did anybody else solve this problem anyhow?

I notice the same things with stations in low minmus orbit.  Can't remember if it is fixed above 50km or 100km... but a higher orbit will result in -much- more stable stations.  Some detailed investigation would likely find the answer quickly (set up an elliptic orbit like 25x125, and just watch your ap/pe as you go from pe->ap... when they quit wobbling you've found the stability altitude).

I can only -hope- that you don't need a >100km orbit for stability around minmus, as most of my minmus stations (in previous versions at least) were generally around 30-50km.

[OHara]

How much rotation and how fast?

I had a satellite in a 7km--56km elliptical orbit, so I let it run over lunch, and saw only 0.1°/s per after a half-orbit (which is less than I would have expected just from tidal forces -- I'll have to make a tidal-force demonstrating satellite sometime).  

Ap and Pe do wander around, up to 300m, which is consistent with the dev-note descriptions of the effects of roundoff.

I don't know where KSP might switch to surface-referenced coordinates on Minmus, but the time-warp limits are at 12km and 24km.

(Note that the original post mentioned that trim was checked, and zero)

[something]

Trim is off. I actually tried to compensate the rotation by adjusting trim but that does not work so I deactivated it again.

Don't know how fast the station rotates as it is hard to count when two axes are actively rotated around. I would say at least 360deg within a minute. As I cannot access the savegame for the weekend I can't tell the orbital period but 20-22km Ap/Pe should give you an order if magnitude.

Will try to move it to a higher orbit and then let's see..

 

 

[Kerbal101]

I seriously wonder if this bug is related.

[TheDestroyer111]

3 hours ago, Snark said:

Spoiler

 

Aside from possible rotating-frame issues that you mention... is there any possibility that you've accidentally set "trim" on your station?

Lengthy explanation below, but the TL;DR is:  try pressing Alt+X and see if that fixes your station and gets rid of the "phantom" rotation. 

Here's the deal with trim, which may or may not be relevant to your situation:

"Trim" is a feature that was added to the game to help with airplane flying.  The basic idea is that when you set trim, you put your ship into a state where it continually tries to adjust its pitch upward or downward-- the idea is that if you have a plane that tends to nose-down, for example, setting the trim to pitch up can make it easier to make long-distance sustained flights.

However, trim is a notorious source of confusion and pain for players, for several reasons:

• Most players have no idea that the feature even exists.
• It applies to spacecraft in outer space, via reaction wheels, which (IMHO) is completely useless and pointless for a feature that was intended for airplanes in atmospheric flight.
• It's easy to accidentally trigger the feature, without realizing that you've done it (it's activated via Alt plus one of the WASD keys, so it's easy to accidentally do that if you're piloting your ship and are a little slow to let go of the Alt key when doing something else).
• There's no visual indication that the feature is active.  (Or, indeed, that it even exists.)
• Once it's active for a craft, it stays active, including when saved and re-loaded (so restarting KSP doesn't fix the problem).
• Once the feature is active, there's no obvious way to turn it off, unless you happen to know the magic incantation Alt+X.

...Personally, I wish they'd just remove the feature entirely.  In the 2+ years that I've been playing KSP, I've never once activated it on purpose (admittedly, I'm not an airplane guy), but I have activated it accidentally several times... and every single time I do it accidentally, I end up tearing my hair out over it until I discover (again) that "ohhhhh, it's trim."  (The accidents happen infrequently enough that I always forget about this feature by the time the next incident occurs.)

But at the very least, even if they do keep the feature... I really wish they'd give it an explicit UI so that people who don't know and don't care about trim (i.e. the large majority of KSP players, I'd guess) won't get bitten by this.  For example:  a small graphical UI that pops up when the feature is activated, and goes away when it's not.  The feature would have a dot or crosshairs or something, whose 2D position would indicate the trim setting.  There'd be a visually obvious "cancel" button on the feature.  That way, when someone accidentally triggers it, the scenario would go like:  "Hey, what's this thing that just popped up?  And hey, my ship is acting funny.  Well, I'll push this button here... okay, it went away.  Don't know what it was, but my ship stopped spinning, so whatever."

But I digress.    Anyway, the existence of the "trim" feature is something worth knowing about, even if it turns out not to be the problem in your particular case.

 

 

NO NO DONT DELETE TRIM! It's essential for actual plane flying! And for visual identification of it, I think it should work like with time warp (a window opens telling you what happened, and you can press ok or press dont say this again) so that people who use it a lot don't constantly get annoyed by some stupid windows while people who don't know can still know about it. Another good solution, probably much better one, is simply telling people about this in one of the in-game tutorials.

29 minutes ago, something said:

Trim is off. I actually tried to compensate the rotation by adjusting trim but that does not work so I deactivated it again.

Don't know how fast the station rotates as it is hard to count when two axes are actively rotated around. I would say at least 360deg within a minute. As I cannot access the savegame for the weekend I can't tell the orbital period but 20-22km Ap/Pe should give you an order if magnitude.

Will try to move it to a higher orbit and then let's see..

 

 

Also, 360 degree per minute is probably the rate of rotation caused simply the orbit going around minimus... gonna check that in ksp...

Nope. The source of rotation must be different. The orbital period is well over an hour. Hmm... Do you see any joints bending and flexing when you load the craft?

[something]

No there's no flexing of the station upon loading - I have to say Squad improved that issue in the latest versions. Also activating SAS or MJ does stop the station from rotating, but doesn't really fix the decreasing orbital height.

It could, however, be the bug that @Kerbal101 pointed out - gotta check that once I am back.

[Tyko]

On 8/11/2016 at 0:01 PM, Shadowmage said:

I notice the same things with stations in low minmus orbit.  Can't remember if it is fixed above 50km or 100km... but a higher orbit will result in -much- more stable stations.  Some detailed investigation would likely find the answer quickly (set up an elliptic orbit like 25x125, and just watch your ap/pe as you go from pe->ap... when they quit wobbling you've found the stability altitude).

I can only -hope- that you don't need a >100km orbit for stability around minmus, as most of my minmus stations (in previous versions at least) were generally around 30-50km.

My orbits are jittery below 100km. I just use a 125km parking orbit for stations/motherships to avoid trying to dock with those jitters.

[fourfa]

@Snark, can you just delete the trim key binding in your save? I don't have my game comp with me to try it..,

[Zargg]

OMG....thank you Snark!