Is it normal for objects in Munar orbit to constantly rotate so I can't dock?

[Dunbaratu]

I don't know if this is a bug or if it's correct behavior, because this is the first time I tried docking in Munar orbit instead of Kerbin orbit where I'd done lots of docking before.

I'm having a heck of a hard time docking ships in orbit around the Mun because whichever ship I'm currently not controlling always starts rotating on its own, even if its rotation had been stilled either by SAS or by using the cheat of stopping rotation via a brief time warp. Regardless of how its spin was stopped, it will start spinning again on its own when I leave control of it and use "[" and "]" to switch to the other ship. The spin is very very slow - maybe at a rate between about 0.5 RPM and 1.0 RPM or thereabouts, but it's JUST enough to make docking an infuriating pain because docking is such a slow gentle operation that tiny rotation rate is still enough to mess it up. I line up on the docking port from just a few meters away, and start drifting gently toward it at about 0.3 m/s, and by the time I get there the target craft has rotated far enough that the docking port is at too sharp and angle to connect. So far the only way I've been able to do it is to constantly swap control back and forth with "[" and "]" to keep re-correcting the rotations of both craft to point toward each other, but since that makes the game forget the targeting (I have to re-target the clamps each time I switch) it's really fiddly.

Is this normal behavior? Am I missing something? In Kerbin orbit, the target ships did NOT start rotating all on their own. The only other difference I can think of is that since this is the Mun, I'm also using much smaller ships by the time I've dropped all the stages to get here.

Is there something about the tighter, smaller orbit of the Mun that makes this happen? I am doing this at an orbit with an altitude of only about 7000 m, which is possible on the barren Mun but not Kerbal. I started thinking about things like tidal forces causing swinging, but I don't think they're implemented in the game so its probably not that. I thought maybe it's an illusion due to the object orbiting the Mun, i.e. it's one revolution per orbit as the object stays fixed going in a big circle around, but the rotation rate is too fast for it to be that. If it was that it would be one rotation every 10 minutes or so, not, every minute.

What am I doing wrong here?

For disclosure, I have these mods installed:

Crew Manifest.

That's it. I removed most of the other mods with the most recent update, and then re-installed Crew Manifest.

Edited June 5, 2013 by Steven Mading
(set to [Answered])

[triffid_hunter]

is it a glitch inducing rotation against the fixed stars, or simply the navball rotating due to your ship remaining stationary vs the fixed stars but moving relative to the moon, and your craft's respective orbits pulling them out of alignment?

if the latter, point your docking ports parallel to your orbit's axis which is north-south for an equatorial orbit. Then the apparent rotation will simply result in the docking port rolling rather than moving away. Also the orbits will be cocentric and simply cause your crafts to move closer and further away along the docking axis rather than shifting around.

MechJeb's Smart A.S.S. has Orbit/NML+ and NML- for this. I always lock my space stations and other dockables to that orientation.

If all else fails, try docking in a higher, slower orbit.

hope this helps, post your results for anyone else who comes across this thread

[Specialist290]

It's normal behavior, and yes, the fact that your orbit around the Mun is much tighter is definitely contributing to it; the drift also happens in Kerbin orbit, but it's much less noticeable precisely because you're moving around the edge of a much larger orbit.

That said, one thing that helps keep your ship stable is to point your target craft in either the "normal" or "antinormal" direction -- perpendicular to the direction of your orbit, but parallel to the Mun's surface. In an equatorial orbit, that would be either north (0 degrees on the Navball, lined up right in the horizon) or south (180 degrees, same), but for a significant inclination off that it gets tricky (though you can cheat a little bit by plotting a maneuver node along one of the purple vectors, then pointing in that direction).

[Vanamonde]

The ships are not rotating, but your perspective is. Consider the following bit of stunning computer graphics.

Your ships (yellow arrows) are facing the same direction the whole time, but as they go around the world, their positions shift relative to each other, making them appear to rotate as you look from one to the other. The reason it's more noticeable around Mun is that your orbit, although slower in velocity than around Kerbin, is a smaller circle and therefore you are covering a larger angle in the same amount of time.

[Dunbaratu]

This seems to work well. Thanks for the advice. On a typical east-west oriented orbit, when I point straight north or south, docking becomes possible.

Now the only question I can think of is... why? What's the physics behind this? I'd understand if it was rotating at one rotation per orbit, but it's going faster than that so that can't be it. (Besides if it was just appearing to rotate once per orbit because it's not tide-locked, then why doesn't my craft experience the same effect? Anyway, it works now that I followed the advice.

[Dunbaratu]

Your ships (yellow arrows) are facing the same direction the whole time, but as they go around the world, their positions shift relative to each other, making them appear to rotate as you look from one to the other. The reason it's more noticeable around Mun is that your orbit, although slower in velocity than around Kerbin, is a smaller circle and therefore you are covering a larger angle in the same amount of time.

If you re-read my initial post, you see that I mentioned this possibility already, but the fact that it's rotating more quickly than one revolution per orbit doesn't seem to fit this explanation.

[Dunbaratu]

(On a purely forum-mechanics question - how do I set this thread subject's prefix to "[answered]" now? I tried clicking on the edit button on the original post, but the subject and prefix doesn't appear to be editable.)

[Dunbaratu]

Looks like I spoke too soon. The problem is still there no matter how I rotate the craft, and I'm starting to suspect it's a bug with that one craft rather than being universal. It doesn't happen with other spaceships I have. This specific craft is composed of exactly 8 parts:

1 three-man capsule.

1 senior docking port , on the pointy end of the capsule.

6 struts to strengthen the connection between the docking port and the capsule - that were there because this was once a piece of a larger rocket and I wanted to strengthen that connection.

That's it - that's the entire craft. It's a broken off piece of a larger vessel that met with doom, but it's the piece with the crew capsule and I'm trying to rescue them.

I'm trying to dock with it with another craft that has a senior docking port and then carry it home.

For some unknown reason THAT particular craft is rotating entirely on its own, and I know for a fact it's not the effect Vanamonde is talking about because the craft is rotating not just relative to the mun, but also relative to the stationary starfield background, AND furthermore I just noticed that it sometimes turns a different direction. It's behaving as if there was someone pressing "W" for a while, then pressing "A" for a while, then pressing "D" for a while, etc even though my hands are completely off the keyboard. I'd show it but the only video capture software I have is freeware FRAPS with a 30 second limitation and that's not enough to show the effect well. (And I'm not paying for it just to use the software once. I'd have to have more other uses for it too before I do that.)

[triffid_hunter]

As far as I know, that can happen sometimes if you have clipped parts. occasionally, the VAB will place multiple parts in the same place if you leave symmetry on when placing longitudally. you can check your persistence file to see if that's happened. the format is human readable but rather verbose. There may be other ways it can happen.

I vaguely recall there being some bug relating to rotating frames of reference that was fixed in one of the recent patches.. I presume you're using 0.20.2?

[Atanar]

Looks like you experienced the same as in this video:

Try exiting and reloading your save file.

[georgTF]

Sadly i have suffered my fuel depot picking up rotation very quickly and becoming insanely difficult to dock with. Even when i kill rotation by quickly engaging/disengaging time warp, my depot starts spinning again. Far too much to be a side effect of orbiting... In fact, after killing rotation with timewarping, i notice "gravity" slightly tugging the craft and spinning it up. It then continues until i can't chase the docking port anymore.

It may be a consequence of unintentional clipping. One depot certainly did have a probe core attached inside a large ASAS so that may be the problem.

One other thing i've noticed is the altitude. Orbiting the Mun at 15km was much worse than 45km. Any idea why? It may be gravity or orbit, but really that sort of thing should not happen at all. It's really miserable to have to lug a huge pile of space crap into a proper orbit, only to find that it's nigh on impossible to dock with it.

[Scrogdog]

(On a purely forum-mechanics question - how do I set this thread subject's prefix to "[answered]" now? I tried clicking on the edit button on the original post, but the subject and prefix doesn't appear to be editable.)

Press the "Go Advanced" button while editing and you'll find a drop down that will do that.

[Eric S]

I've seen two causes for phantom forces causing this kind of rotation.

The first is as mentioned, part clipping.

The second is more annoying since it's harder to avoid. Sometimes, struts just do that. The first time I encountered this and it wasn't clipping was when I was doing a mothership to Jool type mission, and the phantom rotation started up in LKO. I eventually found that by changing the position of the struts that the phantom rotation would go away. So I did a shakedown cruise to the Mun and Minmus, and the phantom rotation returned in Minmus orbit. I deleted all the struts and put them back in approximately the same place and went on the shakedown cruise again, and had no phantom force problems.

The second time was on a grand tour mission, where there was no phantom force problems in LKO, the Mun, or Minmus (yes, because of precisely this issue, those are now required stops on any mothership-style mission that I do). However, it started up in Moho orbit, the first stop of the grand tour. This time, I did more thorough testing. I created a new drive section, hyperedited it into Moho orbit, and it started spinning. I stripped off all of the struts in the drive section, hyperedited that into Moho orbit, and there was no spinning, but the craft wasn't stable enough to launch.

I then returned to the original design and stripped off the bottom-most 8 struts (out of 16 struts total, if I remember correctly), hyperedited that into Moho orbit, and there was no spinning. Several attempts at replacing the eight struts all resulted in spinning. So I went back to the reduced strut drive section, found it stable enough to launch, flew it out to Moho where it picked up the mission sections and completed the mission. The drive section with the problem is still in Moho orbit in that save file. In copies of that save file, I've both hyperedited the drive section back to LKO (and low Minmus orbit), and just flown it back, and the phantom forces only appear in Moho orbit. Maybe someday I'll take that drive section on it's own grand tour to see if there are any other planets/moons that cause that particular one to have phantom forces.

[AnalogAddict]

Make sure your target craft is perfectly still before you swap, with SAS turned off. For instance, if you swap crafts just as SAS was making a slight adjustment then the craft will continue it's rotation in the direction of the correction maneuver.

By swapping crafts, you have effectively disabled SAS thus allowing rotation to continue if any rotation were applied or induced by last second SAS adjustments.

Frustrating, yes, but I've experienced this issue many times and no matter how many times I dock, it all comes back to plain patience.

[Dunbaratu]

It repeats no matter how much I quit and reload. I eventually decided to "cheat" and edit the capsule out of the persistence file, after EVA'sing the crew to a different capsule. I figure it's a cheat, but it's okay because it's a cheat to avoid a bug that's not my fault.

And yes, I'm on 0.20.2

[Rosarium]

It repeats no matter how much I quit and reload. I eventually decided to "cheat" and edit the capsule out of the persistence file, after EVA'sing the crew to a different capsule. I figure it's a cheat, but it's okay because it's a cheat to avoid a bug that's not my fault.

And yes, I'm on 0.20.2

Its your fault.... According to your dats you dont sve an ASAS on it so it will be never stable

[SDIR]

Could you have the trim on at any point? If I recall properly, the trim remains on after quick saves and loads. If the trim is enabled, then the craft will spin no matter whether you have ASAS on or off. Reset trim by pressing Alt + x

Edited June 5, 2013 by SDIR

[Dunbaratu]

Its your fault.... According to your dats you dont sve an ASAS on it so it will be never stable

First off, I stated this is a broken piece of a larger craft, so you don't in fact know if it had ASAS or not.

Secondly, the purpose of ASAS is to stop rotation that is already going, not to avoid STARTING rotation in the first place that magically comes from nowhere.

Thirdly, ASAS is useless without RCS or atmospheric canards, or vectored thrust engines. It does NOT use the torque wheels of the capsule, which is all this piece has. So it would have to be SAS, not ASAS.

So you're wrong on three counts.

[Dunbaratu]

I've made another version of the capsule where I removed the struts and then put them back again - this magically made the rotations stop so I think that was it.

[Dunbaratu]

Sadly i have suffered my fuel depot picking up rotation very quickly and becoming insanely difficult to dock with. Even when i kill rotation by quickly engaging/disengaging time warp, my depot starts spinning again. Far too much to be a side effect of orbiting... In fact, after killing rotation with timewarping, i notice "gravity" slightly tugging the craft and spinning it up. It then continues until i can't chase the docking port anymore.

It may be a consequence of unintentional clipping. One depot certainly did have a probe core attached inside a large ASAS so that may be the problem.

One other thing i've noticed is the altitude. Orbiting the Mun at 15km was much worse than 45km. Any idea why? It may be gravity or orbit, but really that sort of thing should not happen at all. It's really miserable to have to lug a huge pile of space crap into a proper orbit, only to find that it's nigh on impossible to dock with it.

The effect that changes with altitude is the one where it rotates once around on the axis of orbit per orbit. The clipping effect doesn't seem to be altitude related.

[Dunbaratu]

Press the "Go Advanced" button while editing and you'll find a drop down that will do that.

Thank you. Done.

[Brenok]

It does NOT use the torque wheels of the capsule, which is all this piece has. So it would have to be SAS, not ASAS.

Yes, it does. If you're in a fastly rotating pod in vacuum, hitting "t" will stop it almost immediatly.

[jheriko]

I have had a bug like this when I got a landing leg stuck inside a solar panel. I debugged the problem by mistake by deploying the landing gear and noting that the problem went away, but returned if I raised the gear. On closer inspection one of my three landing legs was occasionally 'jittering'. The result I had was slow rotation which gradually accelerated - but when I say slow I mean, say 30s or so before the lander was tumbling like mad... I lowered the gear and decided to land anyway, which was lucky in hindsight.

This sounds only vaguely like your problem though.

The other thing I've seen which can cause similar things is having uneven distribution of fuel, or parts in general. If you get it badly wrong enough ASAS will compound things instead of making them better as well..

EDIT: not sure how but i seemed to miss the entire middle page of replies :I

[triffid_hunter]

Thirdly, ASAS is useless without RCS or atmospheric canards, or vectored thrust engines. It does NOT use the torque wheels of the capsule, which is all this piece has.

umm yep ASAS actually does use the torque wheels.. have a pod with an ASAS module, turn SAS on then point it in a different direction and it'll go back to original heading. SAS only stops rotation, ASAS maintains a heading.

[Dunbaratu]

The other thing I've seen which can cause similar things is having uneven distribution of fuel, or parts in general. If you get it badly wrong enough ASAS will compound things instead of making them better as well..

But unevenly distributed weight should only cause rotational accelerations when under thrust. This is a problem that occurred without any engines powered up (or even present).