Rendezvous drift

[quasarrgames]

Sometimes, when im rendezvousing with another craft, i'll just start drifting away from them for no reason. I'll be on the final apporach, within a couple kilometers of the other craft, and for no reason at all, the prograde and retrograde markers will just start moving on their own, and the target-relative speed will go slowly change. I'm not doing any thing while it happens, sometimes it happens while i'm in time warp, it all happens on its own! Could somebody tell me what's going on?

[Deutherius]

That "no reason at all" is orbital mechanics. The other craft is simply on a slightly different orbit, therefore going to end up somewhere else than you are.

The only place where you can be so that your relative velocity will stay 0 is directly ahead/behind a craft, on the same orbit.

[maltesh]

Without knowing the magnitude of the change, it's hard to say. The typical answer is "Unless two objects in orbit are either physically fixed together, or in /exactly/ the same /truly circular/ orbit, they cannot maintain zero relative velocity to one another without thrusting." in every other situation, normal orbital mechanics means that they'll pick up relative velocity to one another.

[Empiro]

Unless you're perfectly within the exact same orbit as your target, you're going to drift apart after a period of time. To correct this, you'll want to use your RCS to move the prograde marker so that it continues to overlap with your target so you keep closing in.

In addition, if you're even a little off target, then as you get closer your relative velocity is going to point further and further away from your target. Think about it this way: you're aiming a dart at a small circle 2 miles away. You're off by 10 inches to the right. When you're first closing in, it looks like you're headed straight for it, but as you get closer, you'll notice that your velocity isn't exactly on target. Then as you fly past it, your velocity will start pointing away from your target.

Edited March 19, 2015 by Empiro

[Kryxal]

As a side note, if your approach is along the radial/anti-radial axis, your difference in orbits is going to cause you to approach the docking target instead of drifting to the side.

[Xannari Ferrows]

The only way you will not drift away is if you are PRECISELY the same orbit. Since the orbital parameters are changing almost constantly, and the slightest shift of any applied forces will be enough to change your orbit, it might as well be impossible.

Gravity is a mean thing.

[helaeon]

If your final approach is from a few km away rather than a few hundred meters away, there's your problem. Usually I want to be matching velocity with the other ship between 100m and 500m on rendezvous, then dock from there. Then normal visual cues work pretty well. Paying attention to your relative velocity indicator on the navball vs the target is critical still though.

If you're not against mods I can't recommend NavyFish's docking indicator enough.

[Bev7787]

If you what to stop the drift you need to keep your velocity compared to target at 0.

[eddiew]

The only way you will not drift away is if you are PRECISELY the same orbit.

If your final approach is from a few km away rather than a few hundred meters away, there's your problem.

That and that You and your target ship are on different elliptical orbits around the planet, and over time one will seem to be pulled away from the other - and sometimes back again next loop, if your orbital periods are in sync.

Even matching velocities at 2km distance doesn't change this; you've matched velocities for the moment, but your orbits aren't the same because you're in different places. The closer you are to the other ship when you match, the less noticeable the difference will be, and at 100m it really shouldn't interfere much with the docking manoeuvre. Outside that range, forget docking, just keep approaching slowly and compensate for orbital drift

It's worth playing with manoeuvre nodes a quarter of an orbit before you reach closed approach; I often find that a couple of m/s normal or anti-normal will turn a 5km close approach into a few hundred metres, which is much easier to handle.

[superm18]

If you are using kerbal joint reinforcement, that is properly doing it. It is known to apply phantom forces to"big" vessels