Orbital rendevous above Minmus

[Loren Pechtel]

This has been driving me nuts.  It's simply too sensitive.  I can't match orbits, Mechjeb can't match orbits.  The targeting jiggles all over the place (precision errors, perhaps??) and can drive it nuts.  Even RCS is far too powerful--I've watched it jiggle my orbit my several kilometers each way--due to alternating use of RCS jets while trying to use SAS.  I've got two kerbels to rescue and there's something in a very, very low orbit that I don't even know what it is.

[Temstar]

First, turn on Cap Lock, this turns on precision control so you can use RCS more accurately, it also gives RCS thrusters thrust compensation depending on their distance from CoM, if you have more than one set of RCS that should eliminate unwanted rotation while translating.

Then, don't worry so much about getting a 0.0km close encounter. You just have to get within oh say 5km and that's good enough. Minmus orbital velocity is so low that once you get close you can just cancel out your relative velocity to target and then point and shoot straight at it without worrying about orbital mechanics.

Edited February 25, 2016 by Temstar

[Snark]

6 minutes ago, Loren Pechtel said:

This has been driving me nuts.  It's simply too sensitive.  I can't match orbits, Mechjeb can't match orbits.  The targeting jiggles all over the place (precision errors, perhaps??) and can drive it nuts.  Even RCS is far too powerful--I've watched it jiggle my orbit my several kilometers each way--due to alternating use of RCS jets while trying to use SAS.  I've got two kerbels to rescue and there's something in a very, very low orbit that I don't even know what it is.

Minmus orbital intercepts are actually really easy to do-- low dV requirements, everything's moving in slow motion.  However, depending on what your ship design is and so forth, it's possible there could be some other issue getting in the way and screwing things up for you.

First, can you post some screenshots so we have some idea what you're talking about?  e.g. a screenshot of your orbiter, and a screenshot of your map view showing your orbit and the orbit of the kerbal you need to rescue.

As far as RCS is concerned:  It shouldn't be wildly jittering your ship around on SAS, unless you have very poorly placed RCS thrusters.  Hard to give specific advice without seeing a screenshot.

Can't speak to MechJeb issues, since I've never used it.  But actually, Minmus orbit is a great place to train yourself to do things by understanding the orbital mechanics involved, since everything happens at such low speeds that you have plenty of opportunity to correct errors, and mistakes are relatively cheap.  Again, hard to offer specific advice without seeing a screenshot of your mapview with the orbits involved.

Edited February 25, 2016 by Snark

[Loren Pechtel]

2 minutes ago, Snark said:

Minmus orbital intercepts are actually really easy to do-- low dV requirements, everything's moving in slow motion.  However, depending on what your ship design is and so forth, it's possible there could be some other issue getting in the way and screwing things up for you.

First, can you post some screenshots so we have some idea what you're talking about?  e.g. a screenshot of your orbiter, and a screenshot of your map view showing your orbit and the orbit of the kerbal you need to rescue.

As far as RCS is concerned:  It shouldn't be wildly jittering your ship around on SAS, unless you have very poorly placed RCS thrusters.  Hard to give specific advice without seeing a screenshot.

Can't speak to MechJeb issues, since I've never used it.  But actually, Minmus orbit is a great place to train yourself to do things by understanding the orbital mechanics involved, since everything happens at such low speeds that you have plenty of opportunity to correct errors, and mistakes are relatively cheap.

I don't see how pictures are relevant as I'm looking at motion.  I'd have to post a video to show what's going on.  I'm seeing things like the apoapsis and periapsis markers jumping back and forth by about 10 degrees.  dV requirements are generally measured in a single digit, sometimes even less than 1 m/s.

Mechjeb got me within physics range and I cancelled it's intercept because it messes up the close rendezvous--trying to maneuver it wrecks it's orbit, the same problem I have.  Now I'm trying to bring the Kerbel in EVA.  They're so maneuverable and the jetpacks are weak enough that I think I can pull it off.

[Loren Pechtel]

41 minutes ago, Temstar said:

First, turn on Cap Lock, this turns on precision control so you can use RCS more accurately, it also gives RCS thrusters thrust compensation depending on their distance from CoM, if you have more than one set of RCS that should eliminate unwanted rotation while translating.

Then, don't worry so much about getting a 0.0km close encounter. You just have to get within oh say 5km and that's good enough. Minmus orbital velocity is so low that once you get close you can just cancel out your relative velocity to target and then point and shoot straight at it without worrying about orbital mechanics.

Thanks about the caps lock--a big part of my problem has been with precision.  I'm not having a rotation problem, the jittering RCS was when I used the RCS to speed the turn to position the main engine for a burn.

 

The EVA worked, I got my Kerbel.  Now for the next one, farther out and thus an even greater pain.

[Loren Pechtel]

Followup--now the jiggle is even worse.  Before I go after the retrograde Kerbel I decided to go down and have a look at the object in a low orbit.  I'm watching the apoapsis and periapsis markers jump back and forth, sometimes as little as 10 degrees, sometimes as much as 120 degrees.  It's trying to do a plane change to match my target the aim vector is jiggling by about 90 degrees and it simply can't line up to that other than by a lot of luck.

[Snark]

On 2/24/2016 at 7:30 PM, Loren Pechtel said:

I don't see how pictures are relevant as I'm looking at motion.  I'd have to post a video to show what's going on.

Pictures are very relevant.

A picture of your ship will show whether there's something wonky with the ship design that makes rendezvous a problem, and also whether there's an issue with RCS thruster placement that's causing it to give you grief.

A picture of the orbits involved (yours, and your target's) will allow giving specific advice on what sorts of burns to make when, in order to get an intercept.

On 2/24/2016 at 7:30 PM, Loren Pechtel said:

I'm seeing things like the apoapsis and periapsis markers jumping back and forth by about 10 degrees.  dV requirements are generally measured in a single digit, sometimes even less than 1 m/s.

Oh, okay.  That makes a lot more sense.  There's no actual problem there, you're just seeing math. 

About periapsis and apoapsis markers flitting about:  That's natural and normal any time you have an amost perfectly circular orbit.  A perfectly circular orbit's Ap and Pe are undefined.  When something is really really close to Ap and Pe, just tiny fluctuations in velocity (such as caused by SAS keeping you stable) can make the markers jump around.  Ignore them, they're not relevant.

On 2/24/2016 at 7:30 PM, Loren Pechtel said:

Mechjeb got me within physics range and I cancelled it's intercept because it messes up the close rendezvous--trying to maneuver it wrecks it's orbit, the same problem I have.  Now I'm trying to bring the Kerbel in EVA.  They're so maneuverable and the jetpacks are weak enough that I think I can pull it off.

Okay, now I think I see what you're getting at.

Ignore the orbit.  Use the navball.

Any time you're intercepting a target in orbit, anywhere, and you get within physics range (i.e. 2.3 km), stop looking at the orbit display.  When your position and velocity come very close to matching the target's, lots of indicators (intersect markers, AN/DN markers, etc.) are going to jump around erratically, for the aforementioned math reasons.

So when you get within physics range, just ignore the orbit completely.  Turn off map view, it's not helping you.  Go to flight view, put the navball into "target" mode if it's not already, and steer entirely by the navball.  The two markers that you're particularly interested in are the target-retrograde marker and the target-relative-velocity-retrograde marker.  Keep those two lined up, and you'll do fine.

Here's an illustrated docking tutorial I put together which you may find useful.  It walks you through the steps from "I just got a close approach within physics distance" to "I'm docked." 

[Armisael]

Is the object in a perfectly circular orbit? I think KSP has trouble with markers on those (understandably, given that the concept of apses breaks down in that situation).

[Snark]

44 minutes ago, Loren Pechtel said:

Followup--now the jiggle is even worse.  Before I go after the retrograde Kerbel I decided to go down and have a look at the object in a low orbit.  I'm watching the apoapsis and periapsis markers jump back and forth, sometimes as little as 10 degrees, sometimes as much as 120 degrees.  It's trying to do a plane change to match my target the aim vector is jiggling by about 90 degrees and it simply can't line up to that other than by a lot of luck.

Again:  Post a screenshot.  There's an excellent chance that an experienced KSPer looking at a screenshot of what your current orbit is, and what your target's orbit is, can just tell you quickly and simply what to do.

The periapsis and apoapsis markers jumping around doesn't mean anything.  It just means you're in a nearly perfectly circular orbit, and there really isn't any such thing as a "periapsis" or "apoapsis" because it's circular.  Don't worry about it.

Trust me:  Minmus orbital intercepts are easy.  There is no such thing as a difficult intercept at Minmus.  If you're finding it difficult, then there's a good chance that there's something about the situation that is leading you astray (for example, the way the Pe and Ap markers jump around).  As soon as you've put your finger on what the specific confusing thing is, and how to deal with it, you'll find the going much easier.

[Loren Pechtel]

13 minutes ago, Snark said:

Pictures are very relevant.

A picture of your ship will show whether there's something wonky with the ship design that makes rendezvous a problem, and also whether there's an issue with RCS thruster placement that's causing it to give you grief.

A picture of the orbits involved (yours, and your target's) will allow giving specific advice on what sorts of burns to make when, in order to get an intercept.

Oh, okay.  That makes a lot more sense.  There's no actual problem there, you're just seeing math. 

About periapsis and apoapsis markers flitting about:  That's natural and normal any time you have an amost perfectly circular orbit.  A perfectly circular orbit's Ap and Pe are undefined.  When something is really really close to Ap and Pe, just tiny fluctuations in velocity (such as caused by SAS keeping you stable) can make the markers jump around.  Ignore them, they're not relevant.

Okay, now I think I see what you're getting at.

Ignore the orbit.  Use the navball.

Any time you're intercepting a target in orbit, anywhere, and you get within physics range (i.e. 2.3 km), stop looking at the orbit display.  When your position and velocity come very close to matching the target's, lots of indicators (intersect markers, AN/DN markers, etc.) are going to jump around erratically, for the aforementioned math reasons.

So when you get within physics range, just ignore the orbit completely.  Turn off map view, it's not helping you.  Go to flight view, put the navball into "target" mode if it's not already, and steer entirely by the navball.  The two markers that you're particularly interested in are the target-retrograde marker and the target-relative-velocity-retrograde marker.  Keep those two lined up, and you'll do fine.

Here's an illustrated docking tutorial I put together which you may find useful.  It walks you through the steps from "I just got a close approach within physics distance" to "I'm docked." 

If it were just the apoapsis and periapsis markers jumping around I wouldn't mind.  The problem is that the burn direction jumps around with them--causing Mechjeb to miss burns and miss intercepts.

Trying to do it manually I make worse errors than Mechjeb.  A flew one intercept around Kerbin manually after Mechjeb got me in the ballpark, I was able to do it.  Out here, though, I can't turn fast enough to keep my craft pointed where I need to.  Do I need to bring something small for wandering around up here?  I still have a fair amount of booster along--I've burned probably 800 m/s up here so far and I still have to circularize for the last Kerbel then go home--I would say a quarter of that was wasted.

I got him--but my orbit at that point went out twice as far as his orbit.

[Gojira1000]

Do you, by chance, have multiple reaction wheels on that ship? I just had an issue (at Minmus, oddly) where several were fighting each other and resulting in similar issues. Plus violent shaking just to be exciting. On turning a particular one off, it stopped. Mind you I had other issues - like not being able to target Minmus.

[Loren Pechtel]

11 minutes ago, Gojira1000 said:

Do you, by chance, have multiple reaction wheels on that ship? I just had an issue (at Minmus, oddly) where several were fighting each other and resulting in similar issues. Plus violent shaking just to be exciting. On turning a particular one off, it stopped. Mind you I had other issues - like not being able to target Minmus.

Just one, the 1.5m one.

[Gojira1000]

Hmm, well there goes that theory.

[Snark]

19 minutes ago, Loren Pechtel said:

If it were just the apoapsis and periapsis markers jumping around I wouldn't mind.  The problem is that the burn direction jumps around with them--causing Mechjeb to miss burns and miss intercepts.

Trying to do it manually I make worse errors than Mechjeb.  A flew one intercept around Kerbin manually after Mechjeb got me in the ballpark, I was able to do it.  Out here, though, I can't turn fast enough to keep my craft pointed where I need to.  Do I need to bring something small for wandering around up here?  I still have a fair amount of booster along--I've burned probably 800 m/s up here so far and I still have to circularize for the last Kerbel then go home--I would say a quarter of that was wasted.

I'm having a lot of trouble understanding how "burn direction" could possibly be "jumping around".  By "burn direction" do you mean the maneuver node on the navball?  That should never be jumping around.  The only way it would jump around would be if you were trying to set up a maneuver node that had a really tiny dV, like a small fraction of 1 m/s, and you should never need to do that.

Making a rendezvous between any two objects orbiting Minmus is very easy and straightforward if one understands the principles of orbital mechanics and rendezvous.  It's perfectly understandable if you're having trouble with that, because everybody's a newbie at KSP at some point.  There are lots of very helpful and friendly people here on the forums who would love to give you specific tips and pointers that will not only help you with your current problem, but will also help you understand the mechanics better so you'll have fewer problems in the future.  However, all these helpful friendly people can't give you specific advice unless they know exactly what your situation is, and the best way to do that is to post a screenshot so they can glance at it and say "Oh, there's your problem right there."

One piece of advice that I can offer:  It kinda sounds like you've been using MechJeb to get by.  MechJeb is a great tool for lots of people who have already mastered KSP and just use it to skip the parts that are tedious and routine for them.  Using MechJeb is fine if you're doing it because KSP is too easy for you.  But if you're using it because KSP is too hard-- that is, if you're using it to do things that you can't do easily for yourself when MJ is turned off-- then you're likely opening yourself up to problems like this one, where if MechJeb can't handle it you're left high and dry.  And in such cases, forum posters can't help you much, beyond saying "learn to do it without MechJeb."

 

[Gojira1000]

It's odd, @Snark I think it's what I was seeing, not three hours ago at Minmus - the maneuver node marker was tap-dancing on the navball. I figured it was connected with whatever glitch kept me unable to set Minmus as a target or my duelling reaction-wheels. But now I'm wondering.

[Loren Pechtel]

28 minutes ago, Snark said:

I'm having a lot of trouble understanding how "burn direction" could possibly be "jumping around".  By "burn direction" do you mean the maneuver node on the navball?  That should never be jumping around.  The only way it would jump around would be if you were trying to set up a maneuver node that had a really tiny dV, like a small fraction of 1 m/s, and you should never need to do that.

Making a rendezvous between any two objects orbiting Minmus is very easy and straightforward if one understands the principles of orbital mechanics and rendezvous.  It's perfectly understandable if you're having trouble with that, because everybody's a newbie at KSP at some point.  There are lots of very helpful and friendly people here on the forums who would love to give you specific tips and pointers that will not only help you with your current problem, but will also help you understand the mechanics better so you'll have fewer problems in the future.  However, all these helpful friendly people can't give you specific advice unless they know exactly what your situation is, and the best way to do that is to post a screenshot so they can glance at it and say "Oh, there's your problem right there."

One piece of advice that I can offer:  It kinda sounds like you've been using MechJeb to get by.  MechJeb is a great tool for lots of people who have already mastered KSP and just use it to skip the parts that are tedious and routine for them.  Using MechJeb is fine if you're doing it because KSP is too easy for you.  But if you're using it because KSP is too hard-- that is, if you're using it to do things that you can't do easily for yourself when MJ is turned off-- then you're likely opening yourself up to problems like this one, where if MechJeb can't handle it you're left high and dry.  And in such cases, forum posters can't help you much, beyond saying "learn to do it without MechJeb."

 

 

That could be part of it.  I've never liked doing micromanagement in any game and Mechjeb sure takes a lot of the micro out of the game, turning it more into a strategic game.  I understand how the maneuver nodes work but they can be a pain to get right at times--you simply don't have that fine control and the best I can do with another body is hit it's SOI.

[mikegarrison]

Here's the right way to do it with Mechjeb:

1) Get in an orbit that's reasonably similar to the target orbit except with a different radius. (bigger, smaller, either OK.) It's best if it's roughly circular.

2) Use the "Match planes with target" function.

3) Use the "Hohmann transfer to target" function.

4) Use the "match velocities at closest approach" function.

5) When that is done you will be close. Hit the SAS or the MechJeb SmartASS and kill rotation.

6) Switch to the target with the [ key. EVA the target and fly manually to the rescue ship. Do not use map mode. Do not worry about orbits. Do not worry about anything. Just point the Kerbal to the rescue ship and fly him/her there using the suit thrusters.

7) Go in a hatch. You're done.

Also:

If you want to do it "without Mechjeb" but you have trouble with the maneuver nodes, one thing you can do it to just manually get close with the maneuver nodes. Then use Mechjeb's maneuver node editor for arbitrarily fine precision at manually adjusting the maneuver node. (You can also then use Mechjeb to execute the burn for you if you wish. Or do it yourself.)

Edited February 25, 2016 by mikegarrison

[seaces]

Do not look for orbit and AP and Pe. Look for target markers on the map. Usually that jumping around I would see once I really nail the encounter like 0.1km on markers or really similar orbits between me and target.. So just follow the markers get the encounter as close as you can. Around 1.5km from the target start cutting the speed towards the target. The positive side is that you do not need to dock your craft only get near the stuck kerbal vessel and EVA the stuck kerbal to rescue craft. Really post the screen shot so we can look at it. I could give you an answer how to do it since I am good with docking stuff or just write you the procedure. Really it is not hard to do it I do these rescue missions without RCS.

[Nich]

If you have RCS on while trying to burn a very small burn the RCS will definitely make the burn marker jump around.  Especially if you using SAS to track the burn marker.  Also the depending on the size of the ship a small reaction wheel can be too large and can cause SAS to do very weird things and cause phantom forces that will mess up small burns.  I.E. a command chair an ant and a small fuel tank

[RocketPropelledGiraffe]

10 hours ago, Temstar said:

First, turn on Cap Lock, this turns on precision control so you can use RCS more accurately, it also gives RCS thrusters thrust compensation depending on their distance from CoM

Is this true, there is actually a compensation for RCS when in precision mode?

[ElWanderer]

Orbits fluctuate due to the way they're continually being recalculated (unless you're on rails e.g. in timewarp). As such, really tiny manoeuvres (~0.1m/s) will dance all over the place by themselves as they're below the noise level of these fluctuations.

As your vessel rotates, particularly if it is applying force via reaction wheels or RCS, the location of the centre of mass will typically change. That, again, will change the calculated orbit. This will cause small manoeuvres to jump around, but how much of an effect there will be depends how small they are compared to your orbital velocity (which is where Minmus is more trouble than other bodies), how far the centre of mass is from the root part and the ratio of SAS torque/RCS thrust to ship mass.

Doing really fine burns manually is probably easier than using automation - hit caps lock to enable fine control, enable RCS, turn off SAS and gently tap the translation keys until you're happy with the orbit. Don't try to rotate (except perhaps initially to be pointed roughly at the node)! The joys of RCS with fine control are that it shouldn't cause any rotation unless the RCS thrusters are badly placed. It's also easier than a main engine burn as you can change the orbit in any direction without rotating. If you overshoot, just tap the opposite direction.

I've been writing kOS scripts to perform various tasks. Until recently, my node execution function had a similar problem to what's being described here: for small manoeuvres (<0.5m/s, mostly), the node would dance around as my ship rotated, so it would never be pointed in the right direction long enough for the engines to fire (it'd loop forever... *sadface*). For small burns I now find out where the node is currently pointing then store that vector as the aim point. Because this is fixed, that allows it to consider itself pointing in the right direction and so fire the engines... but not for very long. The burn ends when the current manoeuvre node is deemed to be pointed away from the original node, which can happen very quickly if the node is dancing around. I've not worked out how to deal with that yet.

[Snark]

59 minutes ago, RocketPropelledGiraffe said:

Is this true, there is actually a compensation for RCS when in precision mode?

Yes.

It's a super cool feature, and one of KSP's best-kept secrets.

[Dafni]

43 minutes ago, Snark said:

Yes.

It's a super cool feature, and one of KSP's best-kept secrets.

It is very cool indeed for those cases where your RCS is not perfectly balanced.

Another thing worth pointing out is that a joystick gives you proportional input for the RCS thrusters all over your craft. It's very nice, lets you do the finest corrections and movements.

This is kinda off-topic and directed @Snark: I just went through your docking tutorial that you mentioned above, and I was wondering if you have any tips for direct intercepts from the surface, be it Kerbin or any planet/moon?

Thanks

Daf

Edited February 25, 2016 by Dafni

[Snark]

15 minutes ago, Dafni said:

This is kinda off-topic and directed @Snark: I just went through your docking tutorial that you mentioned above, and I was wondering if you have any tips for direct intercepts from the surface, be it Kerbin or any planet/moon?

Here's what I do (with the caveat that there are lots of approaches to this... this is just my way, I don't claim it to be the best, but it works well for me).  Since this is kinda off-topic, I'll stick the following lengthy explanation in a "spoiler" section to avoid visual clutter.

Spoiler

I rely on eyeball estimates and experience.  For a given situation (i.e. what planet or moon am I on, and how high an orbit is the target in), I just go with my "gut" for deciding when the target's in the right spot for me to launch.  I usually do pretty well.  Partly that's because I've done a lot of launches (which doesn't help you), and partly that's because I try to minimize the variables involved (which can help you) to make the visual estimation job easier.

For example:  Your ship's TWR has a big effect on the right position of the target when you launch.  I control for this by keeping TWR very, very consistent.  When launching from Kerbin, my launchpad TWR is always exactly 1.5.  Doesn't matter whether it's a behemoth Jool mission or a tiny probe, I always use a launchpad TWR of exactly 1.5.  That gives me a very consistent experience on launch.  It makes it easier for me to judge my gravity turn (I do all my flying manually, never use MechJeb or the like).  And it makes it easy to estimate launch time to direct-intercept an orbital target.

For taking off from the Mun or wherever, I have less control over that, because the TWR on takeoff will depend a lot on the specifics of ship design, how much fuel is remaining, etc.  However, in practice that rarely matters.  Reason:  For most places other than Kerbin (i.e. basically everywhere but Eve, Tylo, and Laythe), the local gravity is much lower than Kerbin's, and therefore the ship's local TWR (i.e. relative to local gravity) tends to be very high.  And for purposes of launch, gravity turn, orbital intercept, etc.:  in practical terms, any TWR higher than 4 or so is effective infinity.  So as long as my TWR is "high" there, it doesn't make much difference whether it's 5 or 8; it works out to be the same timing on the intercept.

It also helps if there's some consistency to the altitude of the target you're aiming for.  There are two scenarios where I want to do a direct-launch-to-intercept:  1. rescue a kerbal in LKO (I do this a lot, it's how I populate my space program).  2. Launch my ship to an orbiting station (e.g. space station on Kerbin, mothership in orbit around some moon).  Keeping those altitudes consistent helps, too:  the LKO kerbals are always about the same altitude (70-90 km), and I make a point of parking my space stations consistently at the same altitudes all the time (e.g. slightly over 120 km, for Kerbin).

So:  for any given planet or moon, I've controlled for TWR, and I've controlled for target altitude, which means all I need to do is remember one "setting" per world.  For example:  "Okay, this is Kerbin, and I'm targeting an LKO kerbal, so that means I launch when the target is riiiiiight.... there."  To make it visually obvious, I always switch to map view, rotate the camera so it's over the south pole and I'm looking at the orbit from the side.  In the case of LKO kerbals, my launch point is when the target is about 10 degrees above KSC's western horizon, for example.

The other thing to do is flight technique.  This is where I can correct any small mistakes I may have made in launch timing.  As my ship accelerates into its suborbital arc, eventually its Ap rises close enough to the target's orbit that the game starts showing intercept markers.  Ideally, I want the "my ship" marker to be located directly underneath the "target ship" marker (i.e. not ahead or behind) at the point where my Ap finally kisses the target orbit.  So, after the markers are displayed:  if I see that the "my ship" marker is leading or lagging the "target ship" marker, I can adjust my pitch to compensate.  If the "my ship" marker is lagging, I pitch down some, which makes my Ap marker move forward as it rises.  If the "my ship" marker is too far ahead, I pitch up some.  The former correction is preferable to the latter, for reasons of fuel efficiency, so when I launch, it's better if I err on the side of waiting too long than launching too early.

That's about it. 

[Loren Pechtel]

10 hours ago, mikegarrison said:

6) Switch to the target with the [ key. EVA the target and fly manually to the rescue ship. Do not use map mode. Do not worry about orbits. Do not worry about anything. Just point the Kerbal to the rescue ship and fly him/her there using the suit thrusters.

Don't worry about orbits?  The first time I tried for the Kerbel in a low orbit about Minmus I just barely escaped with my life--one tiny burn trying to match velocities and my periapsis went well below zero.  (My target was below 5km.)