Rotation issues with docking.

[jbdenney]

RUnnin 1.8.1. I am having an issue with Docking. In orbit around Minmus.

I can do the close approach getting with in 500M before I start trying to close to dock.

My docking target has a Porbobodyne OKTO mounted on it so the SAS will only hold set position. The issue is that the docking port is on the side of the target and not the end (there is a docking port Jr.on the nose but that is the wrong size). I can get to with in 400M of the target but the target is slowly rotating on its Main axis. If I go into the target and set my docking ship as its target I can point the Target Port at my Docking Ship but when I transfer my control to the Docking Ship the Docking target does not hold its orientation and starts rotating. My Docking ship ends up doing the back and forth dance where I i keep trying to fly past the target and then turn and repeat. If I do get close to the target the port has rotated out of alignment and I cannot circle the target to get the alignment right.

I am trying to use the Docking Port Alignment Indicator mod but it is giving me fits. Not sure if I understand how to use the HUD indicators.

All of the tutorials make it look so simple but I have been trying to do this dock for a week now with no luck. I have managed to dock nose to nose but this is driving me nuts and I am getting to the point I have to be able to dock.

Any ideas of what I am doing wrong?

 

Bruce

[VoidSquid]

Not sure if I really understand your problem here, but have you checked out this guide?

I don't understand why you'd change the active vessel in the process, the target should keep it's orientation constant, but that's about it. And you don't need both vessel's docking ports to target each other.

[Superfluous J]

You can right click the docking port an "Control from here." Then aim that port in the direction you want.

[James M]

If it were me I'd do as 5th said at first but lets just say for the sake of the argument the probe has no control. If that's the case, line yourself up generally with where the docking port will be and head towards that location. Then go in slowly and let the craft bump into the one you're controlling to cancel out most (If not all) of it's rotation. Just make sure no solar panels, antennas, or radiators are in the way to get broken. Once it's fairly still try docking again, but always remember to aim for where your target will be.

Also don't be afraid to cancel rotations using your kerbals as bumpers. They can take it! 

[Zhetaan]

10 hours ago, jbdenney said:

Any ideas of what I am doing wrong?

I have a couple of ideas, but more importantly, I have a couple of solutions to go with them.  I put them in spoilers because, in my search to give you a complete answer, I ended with four answers and would spare you an intimidating wall of text.  I also have instructions on how to use the Docking Port Alignment Indicator and explanations for what each of the indicators means.

You mentioned that you are in orbit of Minmus.  I don't know your altitude, but typical Minmus low orbit is between ten and twenty kilometres, which gives orbital periods between approximately forty-five minutes and one hour.  That is not quite so fast as low Kerbin orbit, but it still means between six and eight degrees per minute.

When you set a vessel's SAS to stability assist (what you called 'set position'), it fixes the orientation in a non-rotating reference frame.  The problem is that orbit, at least from the perspective of a vessel in orbit, is not a non-rotating reference frame.  During each orbit, the vessel will hold its orientation fixed relative to the stars, but because you are in orbit, it will appear to rotate away from you relative to your perception of directions such as down.  The reality is that the direction of down changes (relative to that non-rotating reference frame), but you're trained to assume that it doesn't (by your lifetime of experience with down being a reliable direction and likely equivalent experience of not standing on your head), and what you are seeing as an unhelpful rotation is in fact related to the rate of change of down.

That's the problem--or, I should say, that is what I believe to be the most likely problem given what you have said so far.  Here's my first solution:

Spoiler

Point the docking port in a direction such that this pseudo-rotation doesn't matter.  In other words, align the docking port axis with the axis of the vessel's rotation.  The straightforward way to do that is to use the part action window (also known as the 'right-click menu') to control the docking port (right-click it and select 'control from here'), and then rotate it so it faces in the normal () direction (or anti-normal (), which works just as well), use stability assist to hold it in place, and then, instead of flying past and trying to circle the target, you approach from 'above' or 'below' the plane of the orbit to find a docking port that, albeit still rotating, is much easier to approach.  This may be tricky if the target docking port axis is significantly distant from the centre of mass.

For a second solution:

Spoiler

Consider simply waiting for the target vessel to rotate back to face you.  Alternatively, turn it so that it faces somewhat away from your approaching vessel and take advantage of its rotation so that instead of beginning perfectly aligned and rotating away from you before you get there, it begins rotated away from you and ends perfectly aligned when you arrive.  Divide 360 by your orbital period to get the number of degrees per minute that it rotates, multiply by the number of minutes that it typically takes you to approach, and turn it by that many degrees--I know that Docking Port Alignment Indicator has a rotational alignment readout, though it may take you a couple of tries to turn the vessel in the correct axis.

For a third solution:

Spoiler

Consider changing your approach.  By this I refer to your procedure of closing distance between the vessels, not just your intellectual exercise in solving the problem.  Getting within 400 metres of the target is absolutely phenomenal for a rendezvous, but perhaps you should try to get the vessels to within 50 metres before you consider rotating the target to align the docking port.  To do that, set the navball to target mode, zero your relative velocity, and then thrust towards your target.  Normally, that won't work for docking, but it is a passable means of reducing your separation when you are already close to your target.  I suggest that you use RCS rather than the main engines for this (RCS uses H and N for forward and reverse thrust, if you didn't already know that) because RCS 4-way block thrusters can thrust in reverse without you needing to rotate your vessel (and without blasting the target with engine exhaust). If that doesn't work, then consider 20 metres.  I suspect that you approach docking with a relatively slow closing speed--that's good and will save paint and broken parts--but it also leaves more time for mis-alignments to happen.  Consider closing first and then aligning.

For a fourth solution:

Spoiler

I am merely making a guess, but your mention of circling the target suggests to me that you are approaching docking with a minimum of RCS use, or at least without much translational control.  I suggest that, provided that your vessel has both reaction wheels and RCS, you use the reaction wheels for rotation and the RCS thrusters for translation.  For your information, the translational controls follow somewhat the same pattern as the rotational ones, but using IJKL instead of WASD on the standard layout, with H and N for forward and reverse in the same vein as you use Q and E for rotation on the longitudinal axis.  L, for example, will move your vessel so that it strafes right--do please keep in mind that your camera orientation may make it so that it doesn't appear to be the correct direction:  right is determined by the orientation of the controlling part, which is usually a pod or a probe core.  You may want to change the camera setting to LOCKED (press V repeatedly until you get to this setting) to aid your approach.  This way, instead of circling the target in a way that takes a long time, you can translate to the side as you approach such that when you reach it, you follow the rotation of the docking port and can simply rotate to face it, again using translational controls to keep yourself from overshooting.  It helps in these cases to also switch on Fine Control Mode (Caps Lock, and the orange pitch, yaw, and roll indicators in the lower left corner will turn blue to show that you are using Fine Control) because Fine Control Mode automatically adjusts RCS output to prevent unwanted rotation while you translate from side to side.

Lastly, I will warn you that Docking Port Alignment Indicator, while an informative tool, also presents a lot of information in a small window and can be difficult to interpret without experience.  There are many different indicators in the main window and they are all related to one another, but they tell different things.

Spoiler

On the screen, you have:

• The white crosshairs, which are permanently fixed to the direction of your controlling part.  If that part is a docking port (and it should be), then they reflect the direction that the port currently faces.
• The orange target indicator, which tells you whether your vessel is aligned with the plane of the target docking port.  If you are significantly misaligned (such as pointing in the opposite direction), then the orange marker may be missing, and instead there will be an orange arrow at the edge of the indicator that points in the orange marker's direction.  Simply rotate your vessel until the marker appears.
• The green crosshairs, which tell you whether you are aligned with the axis of the target docking port.
• The yellow prograde (or retrograde) indicator, which tells you your travel vector towards (or away from) the target docking port.
• The curved indicator at the top with an orange alignment marker and degrees readout (R°), which tells your roll alignment.  This is used when you want to make ISS clones or other stations where you want to get the orientation of the solar panel modules just right, in cases where you're using ports that won't dock outside of specific rotational alignments (Konstruction welding ports can be like this), or when you're simply obsessive about getting all of your alignments perfect in every direction.  This indicator changes mainly when you use Q and E to roll your vessel.
• Three numerical indicators at the bottom:  Dist, CDst, and CVel.
  • Dist tells your straight-line distance between your controlling port and the target port, in the sense that if you went EVA, that's how far you'd need to go to get to the other port.
  • CDst tells your closing distance, which is the distance to the target port's plane.  Geometrically, if you imagine a right triangle where your docking port is at one acute vertex and the other port is at the other acute vertex, Dist is the length of the hypotenuse and CDst is the length of the leg to the right angle.  If Dist and CDst are significantly different, then you're not aligned.
  • CVel is your closing velocity, which is the rate at which you are approaching the target port's plane.  Like the difference between Dist and CDst, this may be different from your vessel's velocity.  Unlike the other two, any discrepancy between vessel velocity and this is unimportant in docking.  So long as you are aligned, a closing velocity under three tenths of a metre per second will give good results (unless there is a different problem, such as having one port on backwards or mismatched sizes).

To use it correctly (and there is more than one way, but this is one of them), once you are within approximately fifty to one hundred metres from the target and have used the navball target mode to set your relative velocity to zero:

1. Ensure that you set your docking port to 'Control from Here' and select the target docking port as the target ('Set as Target'--this does not require you to switch vessels, but you must be fairly close; DPAI also has a function to cycle through available target docking ports should you prefer to use that).
2. Next, align the orange target marker with the white crosshairs.  This sets the plane of your docking port parallel to the plane of the target port--in other words, it ensures that the ports are properly faced and will not meet in a 'V'.
3. Then use translational controls to approach (beginning with H to move forwards at a few tenths of a metre per second).  The yellow prograde marker should look like a prograde marker ():  if it looks like a retrograde marker (), then you are actually drifting away from the target.  As you use the translational controls, note that they move the prograde marker about on the DPAI screen.
4. You'll want to move it in the direction of the green crosshairs.  This will bring the green crosshairs towards the white crosshairs; think of the yellow prograde marker as a magnet that draws in the green crosshairs.  When they are aligned, then it means that your vessel is aligned with the axis of the target--that is to say, your vessel is directly in front of the target docking port.  If the green crosshairs are aligned and the orange marker is not, then you are aligned with the target port but not pointing at it (which can result in meeting in a 'T').
   • I get the best results when I set my approach to something rather slow (usually less than three tenths of a metre per second) and keep the yellow prograde marker approximately halfway to the green crosshairs.  As the green crosshairs get closer to the white, I keep moving the yellow marker closer and closer to the white crosshairs to ensure that I consistently reduce my translational velocity during the approach.  That forestalls the possibility of overshooting the alignment and needing to correct.
   • If the green crosshairs ever turn red (this is usually accompanied by the orange marker disappearing, the yellow marker turning into a retrograde marker, much confusion, and possibly much cursing), then that means you have overshot your target port.  Press N for reverse thrust.  Do this quickly; if you are still translating, then you can crash if you don't back away a bit.
5. When the orange marker, yellow marker, green crosshairs, and white crosshairs are all aligned, the docking ports are also aligned.  Press H for more forward thrust (but keep your CVel at less than three tenths), and watch the Dist and CDst values drop to zero as you dock.

I hope that helps.  If it doesn't, then please feel free to return.  We're all interested in helping players have fun and avoid unnecessary frustrations.  Good luck!

[XLjedi]

Well...  as far as a station or any item rotating in orbit in this game goes...  That can be fixed with a quick time accel/decel clickety-click. 

Then just dock with the stationary installation.  Everything in this game gets reset to motionless every time a save game is loaded anyway.  If there's an issue with your installation inexplicably rotating on its own.  That may be a larger problem with the installation or mods you've got.

[jbdenney]

Just FYI

1 -  I have seen that tutorial and was using it as a reference.

2 - I am using the control from here function.

3 - My target has Station keeping SAS - Probodobodyne OKTO.

Actually it may not be the Target that is rotation but I cannot seem to get to Zero relative velocity, and keep it there, with the Docking Ship. I can get to Zero relative but after a few seconds it will start to creep up and that may be making go into a constant target miss.

 

[jbdenney]

Zhetaan - Thanks for the explanation - especially the DPAI information. I now know I was trying to misuse some of the indicators. I will spend some time working on this since I have just been told I have to spend the next two weeks working form home. Luckily for my job that is not a problem.:)

 

[Zhetaan]

52 minutes ago, jbdenney said:

Actually it may not be the Target that is rotation but I cannot seem to get to Zero relative velocity, and keep it there, with the Docking Ship. I can get to Zero relative but after a few seconds it will start to creep up and that may be making go into a constant target miss.

That is also to be expected.  The only way for you to have and to maintain zero relative velocity is to be in exactly the same orbit as your target.  The obvious solution to that is to be physically connected to the target as in the case of docking, but if you had an orbit that was identical in every way except that you were in a different place on it, then it would remain at zero in that case, too.  However, that also would mean that you would maintain that separation, whatever its value (even if it were zero, again as in the case of docking), and once you began to approach, you'd again be subject to this orbital drift.

There are tricks to minimise the drift, but they vary in helpfulness with your proximity to your target.  For example, if you are approaching the target from behind, as you would if descending from a higher orbit, then aligning your orbital prograde marker () with your target marker () will help to ensure that when you zero your target-relative velocity, you will be in nearly the same orbit.  However, that only works when you are very close to your target, and even then is an approximation subject to eventual drift.  On the other hand, the drift is periodic; provided that you and the target have the same orbital period, you will always return to the same relative position no matter how far you drift, and you will do this reliably on every orbit.

The other common trick to avoid orbital drift, and this works when approaching from any direction, is to close quickly, but incrementally.  The idea is that as you make successive approaches, correcting the drift can be done with the braking burn.  To do it in KSP, zero your relative velocity, burn towards your target (), and then, while keeping the navball in target mode, set SAS to retrograde ().  Do not under any circumstances orient your vessel to antitarget ().  When you achieve your desired separation (closer is better--until you crash, so don't get that close), burn retrograde to re-zero the relative velocity.  This solves for both your excess speed and the accumulated drift in one burn.

So long as you are within physics range (that's somewhere between two and three kilometres), you can zero your relative velocity, point towards your target, and burn.  A rendezvous burn from two thousand metres may drift enough that you completely miss the target by a hundred metres or more--especially in a low or fast orbit.  In that case, brake at your point of closest approach (using the navball's relative velocity readout and retrograde marker for the speed and direction, and the map view's target distance readout for range) and do it again.  It wastes a bit of fuel, but that bit of fuel is from an aptly-named manoeuvring reserve.  Docking is not about conserving fuel; it's about conserving rocket parts.  Since you can reliably get to within four hundred metres, you will have a much easier time of it.

This drift is the reason why nearly every docking tutorial begins with a review of rendezvous; when you are more than a few tens of metres from the target, any attempt to approach the target directly will result in enough drift that you will miss.  This is a fundamental consequence of the way orbital mechanics works:  they had the same problem with Gemini 4.  The reason is because even though you begin dead in space relative to one another, you are both following curved trajectories around another body, and any difference in those trajectories will result in a variable relative velocity.  If you were moving in a straight line, then pointing at the target and giving a tiny amount of impulse would always result in a dock, but you never are moving in a straight line when you are in orbit.

This is the reason why docking is considered among the most difficult of skills to learn.  But you can learn it; eventually, what is now awkward and frustrating will become second nature and intuitive.

[jbdenney]

I finally got docked. With much thanks to Zhetaan. He gave me the little bits of info I needed.

I aligned the docking target control port to the Normal alignment and once I was within 100m of the target I put the Docking controlling port into the Anti-Normal. The one glitch I had with this is that I forgot to set both the target and the Docking ship NAV ball to Orbit instead of Target. This got me pretty well aligned. The other thing I found is that on final approach I have to stop chasing the indicators on the DPAI and go to MK-1 Eyeball. I got this to work by going into LOCKED camera mode and looking straight up the nozzle and watching the position of the Target Icon. I could see when I was drifting out of alignment and correct the translation. It made me nuts not being able to actually see the distance from docking and having to rely on the distance number on the Target Icon.

I noticed my main issue was knowing the relative translation speeds between the ports. Trying to eyeball when I had reached zero up/down and left/right relative translation was very interesting. Trying to know when to counter my UP translation with DOWN thrust was very trial and error. Going to fine RCS control was absolutely a necessity.

The other thing that mad me nuts was no Depth of Field. I have that in every flying program I have ever played. Having a 3D view with good DOF would be a major plus - at least until the final 20 meters. Having the distance number is all well and good but being able to eyeball to back up the numbers would help the "muscle" response.

But all is good - at least until the next time I have to spend 30 minutes going 3.2 million kilometers and then spend 2 hours going the last 100 meters.

 

Thanks all.