Question about docking

[Fez]

So I've been trying to tackle docking recently, and I'm starting to get the hang of it, but I thought of a question that interests me.  At some point, you obviously have to be going at a different velocity than the ship you're trying to dock with.  But this difference in speed would cause the trajectory, and therefore the orbit, of your ship to be slightly different than that of the ship you're trying to dock with.  But it seems to still work nevertheless.  So what's going on? 

[Bill Phil]

You're on a very similar trajectory, and when docking happens, the orbit is the same. Usually the differences in velocity is centimeters per second.

[Gaarst]

The orbit is indeed different. And this is a good thing: if you had exactly the same orbit as your target, then you would just hang around it while never reaching it.

The difference in orbit is what allows you to get closer to your target, as the orbits meet at one point. The orbits are identical only when relative speed is 0 and when the two ships are really close to each other, but that basically means you are one RCS puff away from docking.

[Fez]

But i'm talking about when you're right behind the ship you're docking with, and you increase speed to get closer, wouldn't that put you in a slightly higher and more elliptical orbit, since your trajectory now slightly changes?

[andrewas]

Yes, but its very slight and you can correct for it with RCS.

[magnemoe]

45 minutes ago, Fez said:

But i'm talking about when you're right behind the ship you're docking with, and you increase speed to get closer, wouldn't that put you in a slightly higher and more elliptical orbit, since your trajectory now slightly changes?

Yes and this can bee seen the effect is however low then you are close. It increases with higher orbital speed and gravity gradient, its also larger on huge ships since the center of mass is far from each other. 
 

 

[Kaos]

This effect is especially low, if you are fast relative to the distance.

[Fez]

So it doesnt cause that big of a difference, because it's only a slightly different velocity?

 

[Bill Phil]

1 hour ago, Fez said:

But i'm talking about when you're right behind the ship you're docking with, and you increase speed to get closer, wouldn't that put you in a slightly higher and more elliptical orbit, since your trajectory now slightly changes?

Orbits are really weird.

In this case, what matters more is your position and velocity relative to the target. Increasing velocity towards your target changes your orbit, and you then get an intersection. 

[Fez]

Thank you guys

[magnemoe]

10 hours ago, Bill Phil said:

Orbits are really weird.

In this case, what matters more is your position and velocity relative to the target. Increasing velocity towards your target changes your orbit, and you then get an intersection. 

Changing the speed has two effects.
You are behind target in the same orbit, increasing orbital speed so you ar 1 m/s faster and you are 60 meter behind and you will reach him after 60. Yes it also make your Ap higher but this does not matter. 
If you are 10 km behind and accelerate with 10 m/s you will use 1000 seconds or 16 minutes, here the higer Ap is important you will pass to high and target will move ahead of you. 
You can see this effect very clearly if you try to dock two ship around Gilly as the orbital speed is so stupid low. So low that you can ignore orbits totally if you want, just remember that target moves in an circle around Gilly.

[rudi1291]

Both vessels, that are about to be docked together, may have different velocites and therefore different orbits, but after they docked, that doesn´t matter. In KSP, the CoM just merges, and that is used to calculate the resulting orbit. In real life, its a bit more complicated: Both 'parts' still want to use their own orbit, but they are forced into position by a solid connection (docking adapter) and can´t do their own orbit anymore.

[K^2]

49 minutes ago, rudi1291 said:

Both vessels, that are about to be docked together, may have different velocites and therefore different orbits

Two ships coming together from different orbits is called high velocity collision, not docking. You have to match velocities almost perfectly to successfully dock, and that means that orbits of two objects also match right before docking.

[WestAir]

I'm sure it's possoble to have vessels in opposing orbits dock if the orbital speed is ridiculously low.

[rudi1291]

Isnt every docking some kind of collision? 

[quyxkh]

Yes, it does happen, exactly the way you think it does.  You can even see it.  Get yourself into as close to a perfectly matched orbit as you can, say ten seconds ahead of it. Burn prograde, directly away from your target, long enough to lengthen your orbit by ten seconds.  Then watch your target for one orbit.

[sevenperforce]

Related question about docking...

In real life, how long does the actual process of maneuvering into position and making the docking connection take? Assuming, for the sake of argument, that you're already on a "collision course" to align orbits and positions to an arbitrary distance (e.g., crossing within <10 m at relative speeds of <5 m/s), how much time does it take to RCS into alignment and go through the docking process? Two or three minutes? Ten or fifteen minutes?

[Nibb31]

The final approach takes several hours.

https://www.youtube.com/watch?v=MNe577ttvfw

Edited February 1, 2016 by Nibb31