Trouble Matching Docking Velocities

[luchelibre]

*shrug* Usually I just point at the screen and shout, "THE POWER OF MANLEY COMPELS YOU!!" and I end up where I want to be.....

Don't know how anyone else does it.

Also, post edited. Sorry.

[HeadHunter67]

Thanks for the pictures. It helps to have a visual like that.

[Anton P. Nym]

Glad to see things turned out okay. Now you're on your way to building unreasonably-large space stations!

-- Steve

[Apollo13]

Another prime video example. Here, I start in retrograde firing position. I've selected the target (Station Base). At closest approach, I will be at 200m. After I time warp, I'm out of alignment, so I adjust.

At 2 minutes until I rendezvous, I fire the engines, and my closing speed falls dramatically. When the retrograde pointer slides out, I halt the burn and readjust. I repeat this process several times. Then continue to burn, trying to get closing velocity to 0.0 (BTW: I don't make it). When I stop this, my closest approach is now 10 km.

I tried starting the burn at 4 minutes. I tried burning at 1 minute. What am I doing wrong that everyone else is doing right?

http://www.rickdomain.com/ksp/ksp_rendevous2.mpg

[blizzy78]

What am I doing wrong that everyone else is doing right?

First off, you're burning way too soon. In the video, your first braking attempt is going down from 114 m/s relative velocity to nearly zero in 15 seconds. Try burning at 8 seconds until closest approach. Exactly AT closest approach does work to, the most you could overshoot is by 114 x 15 = 1710 m. (I'm pretty sure the maximum overshoot while braking can be calculated

Second, you should try to aim precisely at the "target" retrograde marker. In the video, you burned slightly to the side of it. That, combined with the early burn, increases your closest approach distance by a large amount.

Other than that, I think you've got it right so far.

Edited August 14, 2013 by blizzy78

[Somerled]

Apollo13,

First: You don't need to get to zero velocity all in one burn. If you burn early, consider only cutting your relative velocity in half. As you get closer to the target, cut it in half again. This lets you manage approach speed and direction without slowing down so much that your closest approach moves way off.

Second: Notice in that video, around 1:45, how the retrograde marker tilts off and away from the target marker? You don't want that to happen until you are very close to your target. A good way to ensure that doesn't happen is to not slow down too much (especially stop burning when it starts moving away). Also, the retrograde marker tilts away from the direction you're pointing on the nav-ball, with it tilting faster the farther away from the marker that you burn. Use this to your advantage: if its tilting away, burn on the other side of it to push it back towards the target marker and keep you moving in the right direction. If you end up moving too slow doing all these small burns, just flip around and burn towards the target to add a little speed.

Third: It may be more awkward doing these burns in map mode. You can visually see targets within 100km, so you can track your distance from them by sight. If you place a maneuver node at the intersection point (no need to actually set up the node, just place it), the "Node in T-" time gives you an idea of how much time you have until the closest approach. As you keep doing burns, this gets more and more inaccurate, so use it just for your first one or two burns.

Edited August 14, 2013 by Somerled

[blizzy78]

Also, the retrograde marker tilts away from the direction you're pointing on the nav-ball, with it tilting faster the farther away from the marker that you burn. Use this to your advantage: if its tilting away, burn on the other side of it to push it back towards the target marker and keep you moving in the right direction. If you end up moving too slow doing all these small burns, just flip around and burn towards the target to add a little speed.

This is perfect advice, but I think the whole "pushing/pulling the markers" is advanced stuff that beginners shouldn't bother with. To start out, it's easier to just slow down to zero relative velocity, then accelerate slowly directly towards the target marker again. Of course this will take more fuel, but not so much that it should be significant.

[Somerled]

True, but it's only as advanced as keeping the yellow x close to the purple three-pointed cross by burning on the opposite side of the yellow x every now and then. No skill or knowledge involved beyond that, really.

The stop, burn, stop, burn method is easier to perform in theory, but can be much more frustrating to first-timers due to the unintuitive orbital ballet that's happening at all times.

[edit: I only just realized that the retrograde, target retrograde, and maneuver markers are all designed to visually line up on the nav-ball during a perfect approach. Same for the two prograde markers. Neat!]

Edited August 14, 2013 by Somerled

[HeadHunter67]

Glad to see things turned out okay. Now you're on your way to building unreasonably-large space stations!

Thanks, Steve! That was kind of the whole point behind my Evil Plan. I've already constructed a modest station (two Jumbo fuel modules, a hab core and power/RCS) but that was done using MJ's rendezvous autopilot (and sometimes docking autopilot, though now I'm actually better at manually docking than MJ is anyhow).

Now I just need to come up with some better station designs for my new project - there's lots of inspiration here on the forums.

Thanks to all of you for being far more patient with me than I would have, with all my newbie questions and failure to comprehend. I think I got the basics.

Only problem I still have is due to the SAS changes in .21. I find that even when SAS is engaged, my marker sometimes slides away during orbital burn maneuvers. The smaller consequence is that it makes my orbits a little eccentric or off-declination if it happens when circularizing... but if it happens when trying to kill RVEL, it can spoil a rendezvous entirely.

I'm wondering what's ultimately changed - they touted these changes as a replacement for ASAS, allegedly now you don't need it because SAS should function the same, except you can maneuver without disabling it... but the reality is, that very difference can lead to the problem I have. So how do I get around it? Adding more reaction wheels? The problem I then face is that sufficient torque for early maneuvers (ascent, orbital insertion, circularization) means the smaller craft that's left at rendezvous has too much torque and the slightest feathering of controls throws me way off whack (not what I need when fine-tuning a rendezvous!). I suppose I could toggle off the unneeded torque at that point, but honestly, my hands are already pretty full and time is of the essence by that point.

How do you effectively keep the ship from drifting off that blue maneuver node target?

[blizzy78]

How do you effectively keep the ship from drifting off that blue maneuver node target?

Personally, I chose to ignore it. I'm either right on the spot, or I adjust my heading manually to get back onto the maneuver node marker once I think it's too much deviation. At the end of maneuver node burns, I also tend to switch off the engine, correct the heading, then burn the rest of the maneuver node. Those few seconds of not burning usually don't matter all that much.

[HeadHunter67]

Also, on a slightly unrelated note... after undocking, I tried to land them both at KSC. Without any mechanical assistance, I was ablee to get one craft to land in the ocean 43km East of the pad, and one in the mountains 55km West of the pad.

I underestimated on one and overestimated on the other... but refining my landing technique is a topic for another thread, I guess.

[Anton P. Nym]

How do you effectively keep the ship from drifting off that blue maneuver node target?

I, uh, don't. At least, not always.

The blue target only shows the direction; it doesn't show magnitude, which is the other big part of a vector. You'll drift off the blue target primarily due to minor imperfections in lining up the burn*, particularly on long burns because being a pixel or two off means more over the longer burn. If you get drift when the velocity is a couple of meters per second or more, it's usually fairly simple to correct as for any other burn. But if the magnitude is less than 1m/s you really don't have to chase the node anymore... if you really want to get it to 0 at that point you could use RCS translation thrust which is way more precise (and aimable!) than turn-and-burn maneuvering, but rendezvous can still go from orbital dynamics to "pool hall" physics at that point.

-- Steve

* Also a factor, the bigger the difference in orbits (particularly in lower orbits) the faster the vector differential will change during the orbit as both craft swing around the planet at different rates... so even perfect aim will end up with a little drift then. So long as it's only a little drift, though, it's easily fixed by RCS or minor correction burns after the maneuver's over.

[Apollo13]

blizzy and Somerled: thanks for the insight and patience. I'll give it another go

Sidebar: I had fun with vers. 0.20, because MJ autodocking worked. With vers. 0.21 and MJ2, MJ autodocking is not as effective (read: failure). If the MJ folks get autodocking working again, I may not manually dock again. Ever. KSP is about as much fun as root canal without lidocaine.

Edited August 14, 2013 by Apollo13

[blizzy78]

If the MJ folks get autodocking working again, I may not manually dock again. Ever. KSP is about as much fun as root canal without lidocaine.

Last I hear is that the MechJeb author(s) lost interest in maintaining the addon. Anyway, I think docking is fun once you really got the hang of it.

[luchelibre]

The blue marker will drift if you start your burn too late or early, or if you didn't do the burn centered exactly on it. Also, the new ASAS is much slower in its corrections, so you'll need to pay more attention to your positioning.

Don't start the node burn right when the counter reaches zero. As a rough and dirty measure, divide how long the burn will take by 3, double that number, and start your burn early by that amount. (Example: you're projected to have a 20 second burn. 20/3 is about 6, doubles to 12. Start your burn with 12 seconds left on the 'T-' timer.

[HeadHunter67]

I usually do burns half-and-half (so a 20 second burn at T-10s) but I'll try the 2/3 method and see if it helps.

I usually take great care to line myself up on the marker - at least as well as the finer level of control allows. But I'm not buying the devs' claim that the new SAS system is better in every way, any more than I buy their boasts that the new graphics improvements make the game run "smoother"...

[ArmchairGravy]

Thanks for the great info! I, too, have been getting frustrated with docking.

[Max Grant]

but there has got to be a more sensible and fuel-efficient way to cancel relative velocity and still end up close enough to the target to finish via RCS. I know where to point and what to do - the problem seems to be in the "when", and nothing I've seen anywhere has answered that question for me.

Ok, you want to get within a useful rendezvous distance, which is under 4 km, under 2 if you can manage. Have a tank of real fuel not RCS for achieving this. Your target needs to be in a relatively circular orbit. I try to set space stations to be targeting the Mun, and then I correct the plane difference using the little navigation ears in map mode. Then, any ship into orbit can align with the Mun and rendezvous is easier. Line up one edge of your approaching ship's orbit with the target, and then make one orbit bigger or smaller than the other, and cycle through to get a good rendezvous set up.

As that time approaches, turn to the retrograde marker on your nav-ball, which should say "target" which is the velocity difference between you two. First rendezvous will be a high velocity, so plan for a longer burn. We're talking 30 m/s and above. On that first rendezvous, try to get it as close to zero as you can. Once you do, immediately turn to the target marker and pick up some velocity, say 6-10 m/s. Looking in map mode, you should see your next rendezvous sliding around the now-nearly-matched orbits, to a closer rendezvous. It may overshoot and start to grow apart in distance again, and in that case stop burning and coast, while you adjust your appraoch. At that second rendezvous, again lose your velocity difference, and then burn to target. This time a little slower. Your target velocity adjustments at each rendezvous should be smaller as you get closer.

You may take 2-3 orbits to get this right even after you've mastered it.

it is a series of closer, and closer rendezvous(es?) until you are within about 500 meters. Then, you can use RCS. You should find that as you get closer to the target, you "lose" your zeroed-out velocity over a much longer period of time, until you are effectively in almost the same orbit. Then you can dock.

[Apollo13]

*shrug* Usually I just point at the screen and shout, "THE POWER OF MANLEY COMPELS YOU!!" and I end up where I want to be.....

Please send cash for new keyboard and monitor. With all the frustration I have with rendezvous, I hit your comment. Cola spewed all over the place. This is now in my signature.

[blizzy78]

With all the frustration I have with rendezvous, I hit your comment.

Perhaps you might want to check out my new in-game rendezvous tutorial? (see signature)

[luchelibre]

Please send cash for new keyboard and monitor. With all the frustration I have with rendezvous, I hit your comment. Cola spewed all over the place. This is now in my signature.

*bows*

It's always my pleasure to ruin your good stuff in the name of funny.

Edited August 15, 2013 by luchelibre

[HeadHunter67]

Perhaps you might want to check out my new in-game rendezvous tutorial? (see signature)

Do it, Apollo! Do it now. I had the same trouble as you and this thread and Blizzy's new tutorial mission sorted it out for me.

I'm not perfect, I can't always do it unassisted, but I understand the basics and I'm learning now.

[Apollo13]

Do it, Apollo! Do it now.

I wish I could. Alas, I cannot. I installed it in the GameData directory; I cannot see it in the Training section. My GameData structure looks like:

GameData->

--- blizzy->

------ DockingTutorial->

--------- DockingTutorial.dll

Edited August 16, 2013 by Apollo13

[blizzy78]

I wish I could. Alas, I cannot. I installed it in the GameData directory; I cannot see it in the Training section.

Did you download the rendezvous tutorial? If both are installed, your installation should look like this:

• GameData\
  
  • blizzy\
    
    • DockingTutorial\
      
      • DockingTutorial.dll
        
      • ... other stuff ...
        

      [*] RendezvousTutorial\

      • RendezvousTutorial.dll
        
      • ... other stuff ...
        

  [*] saves\

  • training\
    
    • E_Docking.sfs
      
    • E_Rendezvous.sfs
      

That is to say, the file under saves\training\ must be installed, too - it contains the save file that will be loaded up when you start the tutorial.

Edited August 16, 2013 by blizzy78

[Apollo13]

I had not downloaded the Rendezvous interactive tutorial; I saw only the Docking and though that included all. I just downloaded/installed the Rendezvous now.

Also, I thought everything went under GameData. I just moved the SFS files to "saves" directory.

I'll give it a whirl today.

thank you