Docking again - hope I'm not driving ya'll crazy.

[GungaDin]

OK so maneuvering 'in orbit' is radically different than it would be in deep space. In orbit I feel weightless but that's because if my engines are off I'm in equilibrium with the nearby planetoid.

I get that - but it brings up a few practical questions:

1 - In several posts - friendly Kerbanauts have suggested that I get close to my docking partner and 'zero out' my approach speed. I think that - in order to zero out my approach speed and keep it zeroed out I would have to be in the EXACT same orbit as my target. Instead, since I will always have a slight variation in either inclination or shape, there will always be a 'drift' on way or another. I get to a safe distance, bring my approach speed to zero but then find that I start to gain or lose relative speed. Is that other folks experience and am I right in thinking that it's because of imperfect orbital match?

2 - Forgetting the fine point of question 1, if I do manage to get on the '6' of my target and I'm ready to make my approach, I actually need to 'retrograde' slightly in order to lower my orbit and gain speed. Counter-intuitive but am I right? If I'm right than even though I start to gain orbital speed, won't I then drift down and have to adjust 'up' on the altitude plane? If I were in deep space and not in orbit this would be simpler.

3 - It drives me crazy that the 'relative velocity' readout does not go negative and instead I have to keep an eye on the distance readout to verify if I am closing in or losing ground. Is there any workaround for this?

4 - It's also frustrating to be only able t control one ship at a time and the only way to switch controls is to go out to orbital view - switch ships and then pop back into the other cockpit. I inevitably get close to my target only to find it askew and I have to jump over there to line it back up again.

A big thanks to anyone who can understand what I'm asking and help....

[thereaverofdarkness]

Once you have matched orbits, you have to slow down if you need to go faster, or speed up if you want to go slower. Sorry for the short reply, I need coffee. I'm not sure what your problem is but if you follow this general rule, approaching gradually shouldn't be too difficult.

[Ascension Islander]

1 and 2 - Yes these are right, but I think you're overthinking it. If you're within a few kilometers, just zero your relative velocity, point at the target, and thrust. Repeat until you're close enough to dock. If you're farther away, I find it easier to mess with maneuver nodes to get a close approach.

3 - The navball's prograde and retrograde markers during target mode will indicate whether you're moving towards or away from the target. For example, if you're pointing at the target with a relative velocity of 10m/s, and the prograde marker is visible, you're more-or-less moving towards the target

4 - If you're within physics range (2.3km) you can hit [ or ] to switch vessels. Otherwise, yes, you'll have to use the map or tracking station.

[Kryxal]

Once you're close (that is, under 1 km), the forces acting on both craft, in the semi-short term, can be considered near-identical. You shouldn't have a problem closing up on the target at several m/s, then docking in the .2-.5 m/s range once you're lined up and close in (under 100m, slowing some more within 50m).

Your second point applies better to orbital maneuvers than to docking operations, just don't take 20 minutes about things in close proximity and you should be good.

I sort of wonder if you aren't giving yourself too MUCH of a safety margin and having gravity give the two craft differing accelerations...

[Specialist290]

OK so maneuvering 'in orbit' is radically different than it would be in deep space. In orbit I feel weightless but that's because if my engines are off I'm in equilibrium with the nearby planetoid.

I get that - but it brings up a few practical questions:

1 - In several posts - friendly Kerbanauts have suggested that I get close to my docking partner and 'zero out' my approach speed. I think that - in order to zero out my approach speed and keep it zeroed out I would have to be in the EXACT same orbit as my target. Instead, since I will always have a slight variation in either inclination or shape, there will always be a 'drift' on way or another. I get to a safe distance, bring my approach speed to zero but then find that I start to gain or lose relative speed. Is that other folks experience and am I right in thinking that it's because of imperfect orbital match?

You're correct in that you're more or less never going to get it "perfect" until you've actually docked the two craft together. However, the point to "zeroing out" once you're at the point of closest approach is basically to reduce your relative speed to the point that the differences between the two orbits are mostly negligible. Once the two craft are close enough in position and velocity, it becomes less a matter of matching orbits and more one of steering your craft within a frictionless 3D volume so that it can link up with another stationary point within that volume. In theory you could probably do this from the point at which the navball cycles over from "Orbit" mode to "Target" mode (since most orbits are so big that even a 60km difference is a fairly small slice of the total pie relative to the orbit's circumference), but I generally try to aim for the closest approach I can get. Somewhere in the range of 5-10km might make for a good upper limit on your plotted closest approach simply for the sake of time and sanity for someone new to docking, but with a bit of practice I've been able to finish my "parking" burn within 200 meters or so of my target before.

2 - Forgetting the fine point of question 1, if I do manage to get on the '6' of my target and I'm ready to make my approach, I actually need to 'retrograde' slightly in order to lower my orbit and gain speed. Counter-intuitive but am I right? If I'm right than even though I start to gain orbital speed, won't I then drift down and have to adjust 'up' on the altitude plane? If I were in deep space and not in orbit this would be simpler.

You're more or less correct here in general, but again, unless you're a pretty significant distance away from your target, it might be simpler to make a "straight shot" at the target as explained above. It really depends on the situation. Can you consistently get a fairly good close encounter on your rendezvous approach?

3 - It drives me crazy that the 'relative velocity' readout does not go negative and instead I have to keep an eye on the distance readout to verify if I am closing in or losing ground. Is there any workaround for this?

In stock, the navball is your friend here. If the yellow prograde vector marker (open circle with wings) is right on top of the pink target heading marker (dot surrounded by broken circle), then that means you're heading straight at your target. With a little practice, you can learn to "steer" the prograde marker around on the navball using your RCS translation controls.

If you don't have any objections to using plugins, you may also find NavyFish's Docking Port Alignment Indicator to be helpful.

4 - It's also frustrating to be only able t control one ship at a time and the only way to switch controls is to go out to orbital view - switch ships and then pop back into the other cockpit. I inevitably get close to my target only to find it askew and I have to jump over there to line it back up again.

If you're within loading-distance range (~2.3km or so), you can pop back and forth between craft using the ] and [ keys. Extremely handy for these sorts of situations.

A big thanks to anyone who can understand what I'm asking and help....

Hope this helps. Don't be afraid to come to us if you still have questions

EDIT: Well, those replies certainly weren't there before Maybe I just take too long to type these things up...

Edited February 17, 2014 by Specialist290
Ninja'd

[technion]

OK so maneuvering 'in orbit' is radically different than it would be in deep space. In orbit I feel weightless but that's because if my engines are off I'm in equilibrium with the nearby planetoid.

I get that - but it brings up a few practical questions:

1 - In several posts -.

You'll notice this is exactly what happens. In some less than efficient manoeuvres, I've had one unit sitting in a low, circular orbit, and another at a highly elliptical orbit (usually a rescue ship and a lander). Create an intercept such that they come screaming towards each other at 1km/s. Then, when they are near each other, zero out the speed as discussed. At the end, you will find the orbits nearly identical.

[Starwaster]

OK so maneuvering 'in orbit' is radically different than it would be in deep space. In orbit I feel weightless but that's because if my engines are off I'm in equilibrium with the nearby planetoid.

I get that - but it brings up a few practical questions:

1 - In several posts - friendly Kerbanauts have suggested that I get close to my docking partner and 'zero out' my approach speed. I think that - in order to zero out my approach speed and keep it zeroed out I would have to be in the EXACT same orbit as my target. Instead, since I will always have a slight variation in either inclination or shape, there will always be a 'drift' on way or another. I get to a safe distance, bring my approach speed to zero but then find that I start to gain or lose relative speed. Is that other folks experience and am I right in thinking that it's because of imperfect orbital match?

This is an overly generalized way of stating things but, yes, the further you are the more you'll drift. The real answer is more complex depending on variables, but all else being equal that statement is more or less true. There's not much you can do except station keeping corrections and continue with your docking.

2 - Forgetting the fine point of question 1, if I do manage to get on the '6' of my target and I'm ready to make my approach, I actually need to 'retrograde' slightly in order to lower my orbit and gain speed. Counter-intuitive but am I right? If I'm right than even though I start to gain orbital speed, won't I then drift down and have to adjust 'up' on the altitude plane? If I were in deep space and not in orbit this would be simpler.

that can work and in real life they'll do that maybe but depending on how close you are you might not want to bother. That sort of maneuver is something you might do in anticipation of results after an orbit? or two? And yes, it would be tricky because you would lose some altitude so you need to plan for that in advance. And use RCS for that. Watch your Map from time to time. You can see when you'll hit your closest approach and how close that will be by glancing at the map.

3 - It drives me crazy that the 'relative velocity' readout does not go negative and instead I have to keep an eye on the distance readout to verify if I am closing in or losing ground. Is there any workaround for this?

Not sure... don't think so. Not on stock and I'm not sure any mods do either..... (though MEchJeb has a screen that tells closest approach and time without going to map)

4 - It's also frustrating to be only able t control one ship at a time and the only way to switch controls is to go out to orbital view - switch ships and then pop back into the other cockpit. I inevitably get close to my target only to find it askew and I have to jump over there to line it back up again.

Not sure there's a question there...? I'm gathering you don't use MechJeb or you'd know it could keep the other ship stable. Usually. But what you can do with only stock is to time warp just long enough to put that ship on rails. That will zero out all rotation unless you're victim of phantom forces.

[Jouni]

In KSP, orbit is a function of position and velocity. Once you are close enough to the target (e.g. 2.25 km, in physics range) and your relative velocity is low enough (e.g. 10 m/s), you are for all intents and purposes in the same orbit. At that point, it's better to forget that you are in orbit, throw away the map, and start pretending that you are in deep space. The approximation is good enough, so it's better to use it, instead of getting confused by thinking in orbital terms.

Of course, you'll drift over time due to the differences between your orbit and the target's orbit, but you'll probably drift more because you can't pilot the ship that precisely anyway. Or at least I can't. You are in deep space now, so the only relevant velocity is the relative velocity to the target. You can fly in straight lines, and you can also safely fly in circles around the target, because you are moving so slowly.

[Taki117]

OK so maneuvering 'in orbit' is radically different than it would be in deep space. In orbit I feel weightless but that's because if my engines are off I'm in equilibrium with the nearby planetoid.

I get that - but it brings up a few practical questions:

1 - In several posts - friendly Kerbanauts have suggested that I get close to my docking partner and 'zero out' my approach speed. I think that - in order to zero out my approach speed and keep it zeroed out I would have to be in the EXACT same orbit as my target. Instead, since I will always have a slight variation in either inclination or shape, there will always be a 'drift' on way or another. I get to a safe distance, bring my approach speed to zero but then find that I start to gain or lose relative speed. Is that other folks experience and am I right in thinking that it's because of imperfect orbital match?

You are correct, but you don't want a perfect orbital match ever. You want to ensure you are moving towards your target at all times (If only very slowly) this is ensured by first zeroing your relative velocity and then using RCS to thrust towards your target (Or your main engine at very low thrust) as you close in keep an eye on your velocity vector marker, and it should stay about centered on the pink meatball. If it starts to drift counter with RCS thrust.

2 - Forgetting the fine point of question 1, if I do manage to get on the '6' of my target and I'm ready to make my approach, I actually need to 'retrograde' slightly in order to lower my orbit and gain speed. Counter-intuitive but am I right? If I'm right than even though I start to gain orbital speed, won't I then drift down and have to adjust 'up' on the altitude plane? If I were in deep space and not in orbit this would be simpler.

Not exactly. If you are on approach to your target already then all you need to do is thrust towards your target. Yes Thrusting retrograde would speed up your orbit and cause you to close with your target, but you will also lose altitude and will have to counter with RCS thrust. There is no up or down in space (Contrary to popular belief)

3 - It drives me crazy that the 'relative velocity' readout does not go negative and instead I have to keep an eye on the distance readout to verify if I am closing in or losing ground. Is there any workaround for this?

If you are moving away from your target then instead of the prograde vector at the Target+ marker it will be the retrograde marker. Generally speaking if you're at that point you have done something horribly wrong. If you keep your prograde marker on your Target+ marker you are moving towards your target (The pink meatball)

4 - It's also frustrating to be only able t control one ship at a time and the only way to switch controls is to go out to orbital view - switch ships and then pop back into the other cockpit. I inevitably get close to my target only to find it askew and I have to jump over there to line it back up again.

What I like to do in this situation is to get ship A lined up with my target and moving in that direction. Then set SAS to hold that attitude. Swap over to Ship B and point it towards Ship A (Specifically the docking port I want to dock with) and activate SAS for it to hold attitude. Switch back to ship A, and perform the docking from there.

[Kerba Fett]

I do lots of docking. After the first couple of times it's always seemed pretty easy.

First I put my station in a circular orbit at 200km. Next a launch the ship that I want to dock and make its AP 200km. I move the PE just out of the atmosphere and let both vessels orbit till the ship is almost caught up with the station. Then I raise the PE till my intersect at AP is as close as I can get for the next orbit. (usually about 1.5 to 2km)

Whole on my final orbit I place a node at the intersect to raise my PE to 200km and match orbits.

This also acts as a countdown for the rendezvous.

Once the burn is done to match orbits I line up with the relative velocity - indicator. This looks like the retrograde marker on the navball. In target mode the nav ball doesnt show prograde/retrograde, the yellow markers are Rvel + and Rvel-.

I burn Rvel- till the speed shown on the navball shows 0.

Now I turn towards the station and use RCS to start approaching. The Rvel+ marker should show up once you start moving toward it. Strafe up/down and side to side with RCS to keep it over the pink target marker. It will tend to drift a little as you approach since your orbits are a close match, but not a perfect match.

Once you get to about 20m stop all relative motion and you can start lining up your final approach.

I think the suggestion about rotating the station to make docking easier is a bad one. Once you start your station rotating you'll need to use SAS to stop it. If any ships are docked with the station there's a good chance that their SAS will be fighting with the station SAS. Normally this starts as a small oscillation that increases till the station tears itself apart.

It may not happen the first time you try it, but sooner or later it's likely to happen.

If your docking port is on the wrong side, drift past a bit then kill your relative velocity and approach from the other side.

If you're ok with using mods the NavyFish Docking Port Alignment plugin is handy as mentioned previously, but MechJeb is even better. I've never tried the docking autopilot so I can't comment on that, but the smartASS is very handy for holding a particular attitude and custom windows can add a lot of info you wouldn't normally see.

My docking info window contains: Distance to Target, Closest Approach to Target, Relative velocity at closest approach, and RCS dV.

The relative velocity displayed by Mechjeb and the NavyFish plugin are also much more accurate than what the navball shows.

[DonLorenzo]

It's been said already that -when you are close already- it's more or less safe to treat the remaining closing maneuvers as 'straight shots' as if you were in deep space. If you find you can't get close enough for that, or you still find you're drifting around too much and are interfered with by orbital mechanics one thing to try is to perform your docking maneuvers at higher orbits. The higher the orbit, the lesser the effect of orbital mechanics on your relative velocities and vectors.

I'll also leave this here, this is how I do it

[AletzN1]

TL;DR

apoapsis reaches the target's orbit

maneuver wiggle at the apoapsis until you have a -5KM separation encounter

at the closest distance, burn either in the direction of the target or if you are coming to fast opposite of it. so the velocity-difference-prograde points at the target.

rinse and repeat until 100M

kill velocity difference

Point both crafts' docking ports to each other.

RCS on, press "H" until you have 1-6 m/s.

Dock'd!