The Conundrum of Rendezvous Manuevering

[MaxwellsDemon]

One thing that continually challenges me is rendezvous manuevering.

When you're close (within a few hundred meters), it's straightforward.   You essentially can act as if your target is at rest with respect to your vessel, and a thrust towards the target gets you closer to it, one away from it gets you farther away from it, and so on.

When you're really far away, it's still straightforward.  To speed up, slip into a lower orbit; to slow down, raise the orbit. To adjust inclination, thrust 'south' or 'north' as required.

But there's a middle zone, one that appears to extend from a kilometer to several kilometers from your target, where it's not at all obvious... a thrust toward your target does not get you closer to your target.  It's all because of the relative orbital motions, of course, and is perfectly explainable in mathematical terms, but it can be doggone frustrating in seat-of-the-pants flying.  I remember first encountering this phenomenon on my old Apple //e in a space shuttle simulator; matching orbits was (relatively) easy, docking was (relatively) easy, but the approach between the two was the hard part.

My brain might just not be wired right to fully "get" it.  I can understand what's going on in an intellectual sense, but it doesn't seem that I can fully absorb it for some reason...   Anyone else notice this, or is it my own special handicap?

[Randox]

I have the same problem. I've actually had to abort missions because I did something wrong or out of order, and ended up close to the target, but not close enough, and the lines all end up so close to each other that I can't clearly see what is going on. I can also never tell which of the intercept indicators belong to which ship, which is...frustrating. Mostly I just rely on the fact that once I am in my grove and following procedure properly, I can usually count on at least a 700m intercept if not better, and that's usually close enough to make a linear approach, then sort out the actual docking.

The worst part is I probably used to be better at this, in the days when I was preparing for docking ports to be added. Fewer tools to help me. That said, I was docking much lighter ships with a lot more control authority as well, so they were easier to chase from the 1-2km range. The stuff I am docking now just isn't up to that.

[Pecan]

Personally, I can't be bothered to set up the initial rendezvous, which may be several-to-many orbits ahead, so I let MJ do it.
Either way once you're (more or less) stationary relative to your target <10km or so there is only one tool you should be looking at, right up until the final few tens of metres:

Navball, Navball, Navball.
(So good they named it, er, three times)

Where is the target?  It's on the navball.
Which way am I going relative to the target?  It's on the navball.
Which way do I need to burn to get to the target?  It's on the navball (sort of - pull prograde, push retrograde*)
How fast am I going relative to the target?  It's on the navball.

[*Pull prograde, push retrograde:
The prograde marker on the navball - which way you are moving - is 'attracted' to the direction in which you burn.  If your current prograde is left of the target on the navball then you want to burn to the right of the target, 'pulling' prograde until it is ON the target - now you're going the right way.
The retrograde marker is, conversely, 'repelled' from the direction in which you burn.  If your current retrograde is left of the target on the navball then you want to burn even further to the left, 'pushing' retrograde until it is ON the target - hey, guess what!]

The only thing that isn't shown on the navball is your separation distance; for that you do need to look at the rest of the screen.

[PS: I am taking your question as 'helping yourself' and am therefore taking my turn.  Wally will be next].

ETA: @Kermanzooming below gives you pictures, and @GoSlash27 further below has dug out an old (ignore the launch bit!) video which is absolutely perfect (docking - it's a pain and a half to learn but is a beautiful 'space ballet' when you get the hang of it).

Edited August 10, 2016 by Pecan

[ZooNamedGames]

For me it's the same process from launch til docking.

This middle experience I have never experienced. Maybe my brain isn't geared in a break that this conundrum causes me to break? :/

[magnemoe]

It's two ways to solve the 5 km from target issue 

The cheapest is to make an node and adjust it to get closer next orbit. Or just use rcs an watch distance decrease. Then tweaking nodes it's often smart with an second 0.0 m/s node half an orbit later to prevent flickering or jumping.

Or just burn towards target, do adjustments so you move towards it. You can also stop relative an do an second burn

[Fearless Son]

I tend to avoid the middle distance by treating anything above a kilometer as a variation on the "long distance".  

Suppose I am at the closest point in the current orbits, and there is a five kilometer separation between the vehicles.  Okay, I go into the map mode, make sure the nav ball is up, then set another maneuver node.  I tweak the values until there is an intercept on the other side of the planet that has a separation of less than one kilometer.  If I cannot do that, then just try to get the separation as low as possible, and then do it again at that point.  Repeat as many times as necessary.  

One method that helps is to set your velocity indicator to show velocity relative to the target, then when you get close you thrust toward the retrograde marker (which at this point is showing the difference between you and the target) until the relative velocity is very close to zero.  That will give you an opportunity to plan your next maneuver to get even closer much more carefully.  

Oh, and when doing an approach from that middle distance, yes, thrusting toward the target will get you there, but your course will seem to "drift" away from it.  Use the lateral RCS systems to re-center the prograde marker over the target marker, and keep yourself on course.  

[RW-1]

You won't encounter the middle distance conundrum if when setting up your initial encounter you match up planes.

Match Orbit planes, then when you select it as a target and have a intercetion point, when at that point, make a burn prograde (or retro if you have the height) to bring your sep down to under 2Km, and then you won't have to deal with it.

From my experience and watching others, it's when you don't have the planes matched that you run into this, because they'll burn to get closer, but differing trajectories cancel it out.  Or you "drift" away.

 

[SpaceplaneAddict]

I do manual docking within 1km. MJ does rendezvous for me.

[suicidejunkie]

I believe it is not so much a 'middle distance' thing as a 'too long before arrival' thing.

If you close in faster, there is less time for your orbital motion to throw your approach off.

 

You can also compensate for the approach skew at midrange by burning downwards (antiradial) if you're approaching from behind, or upwards (radial) if approaching from ahead.

 

Also; if you've got a fast closing speed, you can burn about 30 degrees away from retrograde to keep pushing retrograde to anti-target as the skew shifts it.  Ideally, you'll have slowed down to a good approach speed at the same time as you reach short range.

[TheSaint]

34 minutes ago, RW-1 said:

You won't encounter the middle distance conundrum if when setting up your initial encounter you match up planes.

Match Orbit planes, then when you select it as a target and have a intercetion point, when at that point, make a burn prograde (or retro if you have the height) to bring your sep down to under 2Km, and then you won't have to deal with it.

From my experience and watching others, it's when you don't have the planes matched that you run into this, because they'll burn to get closer, but differing trajectories cancel it out.  Or you "drift" away.

This. Almost every time I screw up a rendezvous it is because I either failed to match planes first or something I did during the subsequent maneuvers screwed up the plane match. If you match planes really well (less than 0.01 relative inclination is my target), then the rest of the rendezvous should go really smooth. 

[Guest]

From a moderate distance as you describe you can simply set the navball to target mode (click on the velocity readout) and kill relative velocity by burning retrograde (not anti-target or anything — literally retrograde, but make sure it's in target mode), then aim at the target and burn. Since I learned how to rendezvous through this two-step method I've generally been pretty aggressive about it. I've even been meeting up with objects on parabolic trajectories (asteroids specifically, but this would also be a useful method of science recovery from returning ships without fitting them to re-enter Kerbin from the outset). If you want to be more precise (as in hitting the target instead of approaching to around 0.5km) you can translate with RCS to get and keep your prograde vector lined up with the target one. By the way, I'd recommend the Better Burn Time mod since it'll show the ETA until and distance at closest approach below where manoeuvre burn times appear.

Edited August 10, 2016 by Guest

[suicidejunkie]

13 minutes ago, String Witch said:

From a moderate distance as you describe you can simply set the navball to target mode (click on the velocity readout) and kill relative velocity by burning retrograde (not anti-target or anything — literally retrograde, but make sure it's in target mode), then aim at the target and burn. Since I learned how to rendezvous through this two-step method I've generally been pretty aggressive about it.

Now that you mention it, I think this is a good technique to start.  And once you've done it a few times to get the idea, you can optimize the maneuver by burning halfway between those two directions and gradually improve it into a smooth approach.

[Kermanzooming]

 

2 hours ago, MaxwellsDemon said:

But there's a middle zone, one that appears to extend from a kilometer to several kilometers from your target, where it's not at all obvious... a thrust toward your target does not get you closer to your target.

My rendezvous & docking experience improved greatly when I understood there is no need to cancel all relative velocity to the target until you are really close.

I always did the same; kill all your velocity, point towards your target, burn. Kill again velocity, point again, burn again. And it works, but only if you are reasonably close and on a similar orbit to your target. The problem is that it is wildly inefficient.

There is a better way; let me try to explain with a very crude graphic. The arrow marks what the velocity vector would do when burning.

• Burning pulls your "towards the target" velocity towards your attitude indicator
• Burning pushes "away from the target" velocity away from your attitude indicator

That means that by burning not directly towards or away your target, you can ensure you are always moving towards your target without the need to kill all your velocity, which makes it way more efficient and easier. 

 

[Guest]

7 minutes ago, suicidejunkie said:

Now that you mention it, I think this is a good technique to start.  And once you've done it a few times to get the idea, you can optimize the maneuver by burning halfway between those two directions and gradually improve it into a smooth approach.

Huh... I figured there would be an optimal single burn direction, but somehow never made the connection that it would simply be between the two.

[regex]

Navball in target mode plus Mk 1 eyeball.  In map mode you can also see the time to intercept and the relative speed which will help you to stop at the right time if you pay attention to how long your burns last.  Without informative aids your best option is always to pay attention to the craft so you can best intuit these sorts of things.  Even with informative aids you still need practice reading the information.

For instance, if MJ/KER says you have 1km/s delta-V with a 60 second burn time, you know that reducing relative speed from 100m/s to zero will take six seconds and can plan accordingly.  Without those aids you pay attention to maneuver node expenditures and how long they take during circularizations, that sort of thing.

[MaxwellsDemon]

Stuff to think about, and stuff to try.  Excellent!   Thank you.

As I said, it's not an insoluble problem-- just some concepts I need to fold the right way to stick into my brain-slot.  

[Enceos]

@MaxwellsDemon To avoid the middle zone one should start aligning the retrograde and anti-target vectors in Navball Target Mode at 6 km distance during rendezvous.

If that happened and your vehicle is drifting near the target at a distance of 4-10 km with low relative speed, you can:

- Check the orbital period of your target.

- If your active vessel is on a trailing orbit and needs to catch up - reduce the orbital period relative to your target

- Increase the orbital period if your vessel is ahead of your target on the orbit.

Make sure your current orbit still matches the orbital inclination of your target.

Usually 1-3 min difference in orbital periods will guarantee that you will drift past the target at a distance of around 400m in 2-5 orbits.

P.S. All numbers are crude approximations from my head, they differ depending on your orbital altitude, I followed this simple rule in my early KSP days and it works.

Edited August 10, 2016 by Enceos

[magnemoe]

4 hours ago, SpaceplaneAddict said:

I do manual docking within 1km. MJ does rendezvous for me.

Totally opposite of me, I always set up rendezvouses manually, this from launch or entering low orbit to dock where I don't circulate, well if target is in 80-90 km orbit I find it best to come up a bit early and then raise Ap up to 100-120 to get 1-3 km then use mechjeb to get distance down to 100-200 meter, match speed, then point target docking port against me, go to 1-3 m/s point docking port towards target, use docking autopilot then distance is 50-100 meter, unless its two smaller ship then its faster to dock manual. 

Then coming in to orbit from other bodies you either aerobrake into an elliptic orbit or burn first into en ellipse first and in this point its nice to select your target, do the matching burn who bring you close together on next orbit, you want the lowest posible AP, your next orbit should take less than twice the time of the target to orbit, also an good time to match planes on this orbit, next burn in the adjustment burn followed by matching speed and orbit. 

Now two objects in the same orbit require some judgement, how much dV do you want to spend on this, you tend to want to do the rendezvous with the lightest ship unless its very low on fuel, still you could transfer from target.
Raise Ap on active craft, use 40% of the dV budget you want to spend on this, now match planes closest to Ap, mark node just after intercept point, click click forward until closest approach of target jump to the other side, now do an burn to reduce Ap until you get close, you can set up both burns at once to get an time estimate. 
Rest is treated like an normal rendezvous.

[Snark]

I avoid the "middle distance" problem by setting up my long-distance maneuver node so that I get an intercept that's within a few hundred meters.

Then, as I'm approaching the target, still at high relative velocity, I do a bit of off-retrograde burning to shove my target-retrograde-velocity marker closer to the target-retrograde-position marker, as described in this illustrated docking tutorial I put together.  By the time I get to where I need to actually kill my velocity, my trajectory passes within a few dozen meters of the target and my main worry is making sure that I don't collide with it if I've lined it up a bit too precisely.

Works like a charm. 

 

[GoSlash27]

As the others have said, the trick is to keep your velocity vector overlapping the...

Nevermind. Let's just do this instead:

https://www.youtube.com/watch?v=5ciFCKgyx48

The important parts are "pushing/ pulling the marble" and the 10x rule for closure rate.

Best,
-Slashy

[Kyrt Malthorn]

Don't know if anyone's suggested this yet, but one thing I like doing is, after matching planes,  in map mode create a maneuver node right next to the closest spot in your orbit to that of the target's (I.e. the general region your intercepts and close-approaches will happen.)  Then hit TAB.  This changes your focus to the maneuver node and you can scroll in to see how close you're orbits really are.  Ignore the node itself and tweak you're orbit to get right on top of the target's.

[Xavven]

Don't worry... Real astronauts had trouble with this initially too. We didn't really understand orbital mechanics quite as well in the 1960's compared to today. If only they had KSP back then

[RW-1]

6 hours ago, Xavven said:

Don't worry... Real astronauts had trouble with this initially too. We didn't really understand orbital mechanics quite as well in the 1960's compared to today. If only they had KSP back then

Oh they kind of did. There's no way Apollo designs would have been man rated today, forgot the site that talked about this however ...

[Blaarkies]

Use the camera...no really, switch camera mode to "FREE". This will make the bottom of the screen stick to kerbin.
-Now focus on eliminating any Inclination difference(get that AN/DN down to 0.0').
-When you are close enough to have visual contact with the target(2.3km i guess), burn to cancel all relative velocity(end with 0m/s).
-Now watch the target(with Kerbin at the bottom of the your screen, you are using camera "FREE" mode, right?).
 +-Is the target at a higher altitude than you?

Thats a higher/slower orbit than you, so to reach it, you must burn a little bit backwards(retro orbital) and also somewhat upwards(towards target)...the navbal TARGET indicator will seem way off, but with some practice you will be able to perform a near miss every time(almost as good as the russians with the MIR "docking")

[Fearless Son]

I have been doing a lot of docking recently (building a big interplanetary ship in orbit,) and I have been running into the "middle distance" issue a lot.  This is how I deal with it: 

When you get a intercept that is "close enough" (within about five kilometers or so) you zero out your velocity relative to the target.  Then, point toward the target and do a gentle burn, fast enough that you are headed toward the target in a reasonable time but slow enough that you can comfortably break with RCS.  Continue to point your craft prograde (use piloting stability assist if you have to) and let yourself drift toward the target.  As you do so, your prograde marker will gradually slide off of the target marker, this is normal and expected.  As this happens, use lateral thrusts from your RCS system to get the prograde marker back on top of the target marker.  

If you keep doing corrections like this, you will eventually get to your target.  It just depends on how close you already are and how patient you are.