velocity changes in docking

[davidpsummers]

I'm doing a docking and have everything lined up. I'm slowing approach (say <10m/s), the ship points right at the target, the relative velocity indicator on the ball is also centered on the target. So I have no lateral velocity, right? But as I approach, the relative velocity indicator drifts off to one side. I keep having to expend RCS fuel to keep it centered. What is that happening?

Edited June 2, 2014 by davidpsummers

[Taki117]

I have a feeling your RCS thrusters are not balanced so when you thrust forwards (Or in any other direction) your also applying a rotational force to your craft. I suggest this mod to help with that.

[Claw]

Additionally, depending on how close you get the marker will drift slightly.

There are also some instances where KSP misplaces the target indicator. So you target a given docking port, but for some reason the game displays the marker off axis. As you get closer, the marker will stray even though you are pointed directly at the targeted port.

[cantab]

How far are you approaching from? If it's from several hundred metres, then remember you and the target are both in curved orbits, so the target prograde vector will drift. Just use small nudges of lateral RCS to keep it in line.

[davidpsummers]

I have a feeling your RCS thrusters are not balanced so when you thrust forwards (Or in any other direction) your also applying a rotational force to your craft. I suggest this mod to help with that.

Well, I think that may well be true. But this happens when I'm not apply thrust...

[davidpsummers]

How far are you approaching from? If it's from several hundred metres, then remember you and the target are both in curved orbits, so the target prograde vector will drift. Just use small nudges of lateral RCS to keep it in line.

Well, last time I cam in from 8 km (my rendezvous burns were nothing to brag about).

The orbit would cause subjective rotation. But would it impart translational separation?

Last time I had a big Rockomax Jumbo 64 tank (I was refueling my station) and it took a lot of hard thrusting, though maybe thats just because it was so big.

Do use nudge it to keep it in line, but

[benzman]

It could be that your target is slowly rotating. If you can control it, switch to it and orient your target docking port so that it is 90 degrees from the direction of the orbit, for instance, if you are in an equatorial orbit turn the craft until the docking port is pointing north/south.

[Claw]

Well, last time I cam in from 8 km (my rendezvous burns were nothing to brag about).

The orbit would cause subjective rotation. But would it impart translational separation?

Last time I had a big Rockomax Jumbo 64 tank (I was refueling my station) and it took a lot of hard thrusting, though maybe thats just because it was so big.

Do use nudge it to keep it in line, but

Yeah, I would say if you are coming in from 8km away at 10 m/s then you're definitely going to get some drift. Astronauts learned early the hard way that, because of orbital mechanics, you cannot simply point at your target and burn. Once you get within a few hundred meters, it becomes a lot more stable. Although still subject to the things mentioned previously.

[davidpsummers]

Yeah, I would say if you are coming in from 8km away at 10 m/s then you're definitely going to get some drift. Astronauts learned early the hard way that, because of orbital mechanics, you cannot simply point at your target and burn. Once you get within a few hundred meters, it becomes a lot more stable. Although still subject to the things mentioned previously.

So I guess I burn RCS or come in faster...

[Claw]

Perhaps give this a try. This gentleman makes quite a few videos and they are pretty good.

You don't want to necessarily "come in faster." It sometimes takes a bit of practice at adjusting maneuver nodes to get the closest approach just right. You can also grab the white part of your maneuver node (the white ring part) and drag it around your orbit. That can help fine tune the closest approach. Hover your mouse over the intercept tags and they will show you how close you'll get.

As you get close to the rendezvous, burn a little like you have been to get the markers lined up. However, don't burn off all the speed right away. Let the rendezvous happen until you get a few km apart. As I approach, I usually try to leave my speed up at around 50 m/s at 1km. That should allow enough time to slow down as you approach, but it will depend on your thrust to weight ratio. Always burn to keep the markers aligned, but make sure you don't run over the target. I'm not usually down to 10 m/s until I get around 100-200 m from the target, but it depends on how much practice you've had.

You can also drop another maneuver node right where the rendezvous will happen and pull on the prograde marker until the orbits are pretty closely aligned. That will give you a countdown to the closest approach and give you some idea of how much you'll have to burn. However, don't wait until you hit this node to start the burn or the target will zip past you.

Edited June 2, 2014 by Claw

[GoSlash27]

2 points:

#1 the target doesn't fly along a straight path and neither do you. you each fly a circle from apogee to descending node to perigee to ascending node and the circles add together. If you were flying a uniform distance from the target with no delta v, it would seem as if you were orbiting the target or vice- versa. Therefore you will always have a slow drift in azimuth and elevation that reduces as you approach the target.

#2 KSP seems to have a point at 200M where it "fixes" an exact point for objects. You can always expect an instant error to occur when you cross that distance. At 201M the game figures "the object is roughly there" but at 200 it suddenly says "this is precisely where it is" and the 2 positions are different. Just roll with it...

HTHs,

-Slashy