Last part of docking tutorial

[Dan Moos]

I'm hitting a wall on the docking tutorial.

 

Basically, I get to the point where I'm within 200 or less meters, I get my relative velocity pretty low, and then the thing drifts away. I can always burn as to get back where I was, but can't seem to get much closer than maybe 100m. I just keep doing this dance where I creep up on it, but we aren't exactly in the same orbit or something.

I have the navball in Target mode, and set to track the target. On map mode, the orbits look dang close. Closer than I know how to improve upon, as in only see extreme zoom do you see to orbit tracks. 

As a related question, when in target mode, what do the prograde/retrograde bugs mean exactly? Does it refer to those vectors in reference to the target's orbit, such that in a perfectly shared orbit, those bugs would be identical in orbit and target mode? Because they are not in my case.

 

 

 

[bewing]

Welcome to the forums.

The retrograde telltale is vastly vastly important in target mode. It is the vector difference in velocities between you and the target. Go into retrograde mode and burn until your velocity is exactly 0. Now you are at rest vs. your target.

Now point directly at your target, and do a little burn. Get 99% of the way there, turn retrograde again, and burn until your velocity is 0. When you are within 30 meters, the coriolis forces are small enough that you can gently burn toward your target and you will hit it, rather than doing the nasty dos-y-dos dance around it.

 

Edited September 5, 2017 by bewing

[Human Person]

4 hours ago, Dan Moos said:

I have the navball in Target mode, and set to track the target.

That's a common mistake. in order to really zero out your relative velocity, you have to burn retrograde relative to the target, not towards the target.

[BjM]

If you burn constantly tracking towards target you could end with some speed (sorry if I mistake the correct term) horizontal to the target. 

In other words. Usually, I burn at 5km from target towards it (if remeber correct the purple circle?) to something like 50m/s and wait, when its close to the target my prograde is not aligned with it. The farther you are the greater will be any error done at the inicial burn.

If you try to correct "on the fly", by tracking the target you could end with some of this vector thats passing the target. You need to kill this horizontal vector or you will drift away. (I have intended to make some drawing to show this but I'm at work now, sry).

What I suggest, and this helped me a lot; In your case, if you are at 200m of the target (first if hadn't killed your speed relative to it, set navball to target, retrograde and burn) aim at target and go for 10-15m/s. Wait, when at maybe 100m go 0m/s again, target again an bo to 10-5m/s. Wait, at 30-20m go 0m/s again. With some slow and careful simple steps you will easily come close to the target and stop relative to it. At this ponit you start the doking.

In no time you will get it and you do without so much burn and brake. Will be more efficient too. It worked for me. 

The navball, if it's in target mode, retrograde is to zero ir vector relative to the target. If you burn anti-target you will target to go away from it, retrograde you get you to an orbit where you stop relative to your target (hope it helps).

[Reactordrone]

If you're 200m away with a low relative velocity, point your ship at the target marker and use the IJKL keys with the RCS to keep the prograde marker aligned with the target marker on the navball. As long as they're lined up you'll fly straight at the target.

Edited September 5, 2017 by Reactordrone

[agrasyuk]

10 hours ago, bewing said:

Welcome to the forums.

The retrograde telltale is vastly vastly important in target mode. It is the vector difference in velocities between you and the target. Go into retrograde mode and burn until your velocity is exactly 0. Now you are at rest vs. your target.

Now point directly at your target, and do a little burn. Get 99% of the way there, turn retrograde again, and burn until your velocity is 0. When you are within 30 meters, the coriolis forces are small enough that you can gently burn toward your target and you will hit it, rather than doing the nasty dos-y-dos dance around it.

 

Partially agree - excellent answer as far as retro marker but instead of turning retrograde again at "99% there" I suggest to fine-tune the course  with RCS (IJKL) so the green pro-grade mark and Pink direction mark coincide one in another, slowing down with "N" as needed in case approach from 200m  was impatiently fast. If docking ports were selected as target and as control point (this one is obvious I think) this means it will be a "kiss" between ports every time  .

ADD: "kiss" maybe at angle of course if one doesn't position his craft aligned with target docking port for the final approach at 200m. which brings me to related question (hope OP doesn't mind a piggy back): what Indicators on stock Navball can one use to make sure docking ports are on exactly parallel planes ? I'm aware of align indicator Mod, but somehow to lazy to install.

 

 

 

Edited September 5, 2017 by agrasyuk

[Dan Moos]

Success!

 

Two keys. Finally learning how to push around the prograde/retrograde vectors. Also, after my initial rendezvous burn, I quit speeding up time. took longer, but gave me time to experiment without botching things.

 

I'm still not completely certain what the prograde/retrograde bugs represent in target mode. My feeling is that they are the prograde/retrograde vectors of the TARGET's orbit. This seems correct because it jives with lining up the target with prograde for a perfect orbit match. Am I correct here?

 

Whats funny, is while the docking was a success, I managed to strand all three Kerbal's! I wasted deltaV getting my orbit coplaner with Kerbin's equator, and ended up 5 m/s shy of getting below 70,000 on my rentry burn! I fiddled with the maneuver node forever, but that's as close as I could get it. I burned all my monopropelant and got a little out of it, but not enough. I made the misytake of not waiting 'till apoasis to burn the monopropelant, so I imagine I could have done better.

 

My six year old son hates it when my Kerbal's don't make it!

[bewing]

49 minutes ago, Dan Moos said:

I'm still not completely certain what the prograde/retrograde bugs represent in target mode. My feeling is that they are the prograde/retrograde vectors of the TARGET's orbit. This seems correct because it jives with lining up the target with prograde for a perfect orbit match. Am I correct here?

No, as I said above -- in target mode, prograde and retrograde are calculated from the "difference vector" between your ship's velocity and the target ship's velocity. As in, you do a vector subtraction of one vector from the other. If both velocity vectors are identical (the craft are at rest with respect to each other), then the result is zero.

[Spricigo]

2 hours ago, Dan Moos said:

Whats funny, is while the docking was a success, I managed to strand all three Kerbal's! I wasted deltaV getting my orbit coplaner with Kerbin's equator, and ended up 5 m/s shy of getting below 70,000 on my rentry burn! I fiddled with the maneuver node forever, but that's as close as I could get it. I burned all my monopropelant and got a little out of it, but not enough. I made the misytake of not waiting 'till apoasis to burn the monopropelant, so I imagine I could have done better.

 

My six year old son hates it when my Kerbal's don't make it!

 

Well, if you are feeling brave enough, you can EVA a kerbal and push. Not only EVA backpack have a generous amount of fuel but also are replenished when you board the vessel again*.

Another possibiliity its to send a rescue ship, either to refuel or to  get the kerbals.

 

*except for external command seats.