Teaching myself to dock, what's wrong with my orbit?

[Commander Wilmot]

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I am in this orbit, but I can't arrange an intercept closer than 2.5-3.1 kms. Can anyone figure out if I am doing something wrong?

[astropapi1]

2.5 km should be more than enough for a visual approach. Did you try watching Scott Manley's video?

If you simply can't do it, be sure to PM me and I'll send you a written docking guide.

[Frederf]

If you're out of plane and have different orbital dimensions there is a minimum separation in most cases. If you reduce your inclinatation relative you'll find you can get much closer approaches. That said 2-3km is plenty close for a quick scoot to co-location.

[Apollo13]

This will help as well

http://kerbalspaceprogram.com/dock-align-indicator/

[leax256]

It takes time to learn how to get closer intercepts, I manages to get a coliding encounter from over 2/3 an orbit away

[blizzy78]

FWIW, I've made an in-game rendezvous tutorial mod, perhaps it's a bit of help. (see signature)

Also, what Frederf said: If you can't get the intercept any closer, it's most definitely the relative inclination that is off.

Edited August 25, 2013 by blizzy78

[magnemoe]

Distance depend a bit of both resolution of the node manager. Can you plot an closer approach? but also of how well you can fine tune an burn. A upper stage with an mainsail and a almost empty tank is very hard to do an 0.1 m/s accuracy burn with.

Overall the orbits look nice, now you make an new node at the bottom intercept who zero out the speed pretty well, do that burn and leave map mode view and burn to reduce speed relative to target down to 0-0.2 m/s (green marker with cross in center), then towards the target (pink marker with dot in center) at some speed say 15 m/s, you might want to use warp here after you turn your ship to burn to cancel speed again. Do this burn then the pink and green marker get to wide apart (60-90 degree), reduce speed to 0-0.1 again, this time you should be perhaps 3-500 meter from target so repeat the process again, I restrict speed to 1% of distance so 5 m/s at 500 meter. Do this again and if you are lucky you get inside of 50 meter, if not just repeat.

[Nibb31]

Your orbits seem to be on wildly different planes. You first have to align your planes (by burning normal or antinormal at the ascending or descending nodes). Once your planes are lined up, it's just a matter of timing your Hohmann Transfer.

[RoboRay]

Your orbit is fine. To rendezvous, you don't need matched orbits... you just need a close encounter. And you're getting plenty close enough.

Once you have an intercept within a few kilometers, close the map and quit even thinking about your orbits, because any differences there are now irrelevant.

When you get to the minimum distance, make sure your NavBall is in Target mode and match velocities by aiming at the yellow retrograde marker then burning until your relative velocity reaches zero. From there, you can simply point directly at your target and thrust toward it to begin the docking approach.

Edited August 26, 2013 by RoboRay

[Commander Wilmot]

Thanks for the replies. I am using the LV-909 I believe. I will try to do the plugin tutorial and re-watch and reread the Scot Manley and wiki materials on orbital docking maneuvers. I think this pretty good for a first attempt at setting up a docking. I would like to keep this thread open in case I have any more issues before I resolve this.

[HeadHunter67]

Indeed, 2.5km is plenty close for a rendezvous. Unless your relative speed is huge, you have plenty of time to match velocities and the target will be "visible" at that distance. I've actually gotten into the habit of starting at an even larger intercept distance.

[ArmchairGravy]

Good luck! Docking is the toughest bit of the game. I broke down and got MechJeb as there have been some times I was frustrated as anything trying to get some docks. Yeah, it's a cheat, but keep it handy to save your sanity, IMHO.

[Mr Shifty]

Good luck! Docking is the toughest bit of the game. I broke down and got MechJeb as there have been some times I was frustrated as anything trying to get some docks. Yeah, it's a cheat, but keep it handy to save your sanity, IMHO.

Docking before 0.21 was tough because you couldn't lock your orientation with ASAS; it would just use up your RCS fuel. Now that you can lock orientation and just use RCS for translation, the only difficult part of docking is camera management, i.e. rotating the camera scene around the two vessels in order to determine when your approach is lined up correctly. However, the Docking Port Alignment Indicator mod takes care of this for you without requiring any camera gyrations. Docking in 0.21 with the DPAI is pretty easy once you get the hang of it.

[lajoswinkler]

Manley's video was never clear to me, even during the time I was beggining to understand how docking is done.

I don't like rushing things. Kerbin is too small and if the orbits don't match a lot, you've got few minutes until you see the target moving away from you.

It's very hard to finish docking in few minutes. I've never done it.

When you burn towards the target, you're making a bulge on one side of your orbit and that's where the problem is created. The user spends much fuel just to see the target all of the sudden going away.

Therefore after you use the map for initial adjustment (5 km separation at most, try to get closer than 1 km; I adjust to 0.1-0.2), go into the live mode and turn on the camera chase mode. Burn towards the target until the speed is positive. Watch the target's numbers. 5 m/s approach is fine.

Then switch to map and correct your bulged orbit by burning retrograde (yellow marker).

When you reach 100m separation, go pink retrograde (navball, of course) and reduce the relative speed to 1 m/s. Again, fix your orbit in the map view.

When you approach the target, use pink retrograde and reduce the speed as much as possible. 0.0m/s if you can.

Turn on the docking mode and use RCS for positioning, rotating, translating.

[Mr Shifty]

Therefore after you use the map for initial adjustment (5 km separation at most, try to get closer than 1 km; I adjust to 0.1-0.2), go into the live mode and turn on the camera chase mode. Burn towards the target until the speed is positive. Watch the target's numbers. 5 m/s approach is fine.

It's important to realize that the target relative speed on the NavBall does not indicate whether you are approaching the target, moving away from it, moving at right angles to it, etc. It's not a closing speed; it's just relative velocity. You have to adjust the relative velocity so that it is going toward the target, i.e. so that your prograde marker is on top of the target marker on the Navball. Usually, this means that you shouldn't be burning directly toward the target. (The two exceptions are: when your relative velocity is 0.0 m/s and when you are already moving directly toward or away from the target.) I have created a very crude drawing that shows how this works:

Note that in the left image, burning directly toward the target gives you a final velocity vector that will get you closer to the target than the original trajectory, but will still miss it by a bit. In the right image, you can see that by burning to the right of the target, you can pull your velocity vector so that it points directly at the target. Hope this helps.

Edited August 26, 2013 by Mr Shifty

[blizzy78]

Note that in the left image, burning directly toward the target gives you a final velocity vector that will get you closer to the target than the original heading, but will still miss it by a bit. In the right image, you can see that by burning to the right of the target, you can pull your velocity vector so that it points directly at the target. Hope this helps.

You're essentially trying to hit a moving target, only in space.

[Mr Shifty]

You're essentially trying to hit a moving target, only in space.

Right, and the problem is actually complicated by the fact that both your ship's path and the target's path are curved in 3-space. But, if you're within a 10-20 km, the relative trajectories are sufficiently flat that this method works.

[lajoswinkler]

It's important to realize that the target relative speed on the NavBall does not indicate whether you are approaching the target, moving away from it, moving at right angles to it, etc. It's not a closing speed; it's just relative velocity. You have to adjust the relative velocity so that it is going toward the target, i.e. so that your prograde marker is on top of the target marker on the Navball. Usually, this means that you shouldn't be burning directly toward the target. (The two exceptions are: when your relative velocity is 0.0 m/s and when you are already moving directly toward or away from the target.) I have created a very crude drawing that shows how this works:

http://i.imgur.com/OLt1cbC.png

Note that in the left image, burning directly toward the target gives you a final velocity vector that will get you closer to the target than the original trajectory, but will still miss it by a bit. In the right image, you can see that by burning to the right of the target, you can pull your velocity vector so that it points directly at the target. Hope this helps.

True, it's the relative speed. It would be great if the navball traget speed would show "-" sign when the distance is increasing.

But if you:

1) arrange a close encounter

2) correct orbits to match

3) do series of target prograde burns and yellow retrograde corrections

you essentially have no problems. The third step actually increases the precision of the second step. It's fine tuning. The yellow and the pink marker line up.

Ever since I've learned how to rendezvous and dock, I keep making encounters at less than 1 km, even less than 100 m.

I understand the graphic you've made, but I rarely do such maneuvres of forcing the markers together.

When you plan a rendezvous, your orbit should be shorter and then just time warp and see what happens. After you get a nice future encounter, you correct your orbit to match the target's orbit as much as you can, so you don't drift away fast. Why bothering with drifting and trying to hurry up if it can be corrected and avoided?

I must admit, I always do docking with targets that have pretty circular orbits with almost 0 inclination. I've never tried to dock something in a molniya orbit, just to name one weird example.

Edited August 26, 2013 by lajoswinkler

[RoboRay]

Kerbin is too small and if the orbits don't match a lot, you've got few minutes until you see the target moving away from you.

It's very hard to finish docking in few minutes. I've never done it.

You don't need to "finish docking in a few minutes" and it certainly doesn't matter that you're only close to the target for a few minutes if your orbits aren't similar. "A few minutes" is more than enough time to match velocities as you get close to target. Once you match velocities, you've also matched orbits and have all the time in the world for docking.

When you burn towards the target, you're making a bulge on one side of your orbit and that's where the problem is created. The user spends much fuel just to see the target all of the sudden going away.

If two people are on opposite sides of a room and want to shake hands, one of them has to start walking.

"Making a bulge on one side of your orbit" is just part of the approach. It's not a problem to avoid... it's an essential part of the rendezvous. From what I've seen, the people who struggle the most with rendezvous and docking are the ones who stare at the map and try to maintain perfectly matched orbits. It's completely impossible to approach the target if you have nearly identical orbits. Your goal is to put your closest point of approach as near to the target as you can get it. The shape of your orbit is absolutely irrelevant.

Then switch to map and correct your bulged orbit by burning retrograde (yellow marker).

When you reach 100m separation, go pink retrograde (navball, of course) and reduce the relative speed to 1 m/s. Again, fix your orbit in the map view.

Once you have an approach within a few kilometers, close the Map and do not open it again. Every bit of effort you expend trying to make perfect orbits on the Map is completely wasted, because you have to turn right back around and undo that perfect orbit to start moving toward the target again.

Once you're within a few kilometers, bring your relative velocity to a comfortable value (depending on your distance, but slow enough that there's no danger of a collision) and keep your yellow prograde marker on the pink target marker. You can steer with lateral RCS or by turning the ship to the side and thrusting with the engines.

The NavBall shows the direction of the target, your direction of motion and the relative speed, while the on-screen target marker shows the distance. That's all you need. The Map has nothing useful to you at this point and trying to make perfect orbits on it is not only a distraction from what you actually need to do, it's counterproductive because it's often driving you to do the opposite of what you really need (close the distance).

I rarely do such maneuvres of forcing the markers together.

...

Why bothering with drifting and trying to hurry up if it can be corrected and avoided?

An approach where you steer while drifting in rather than coming to a stop numerous times during the approach to correct is not "hurrying up." It can be just as slow and patient as the stop-and-go method, and it's a good bit more fuel efficient. The closure rate can be as low as you want it to be, and steering by correcting your velocity vector rather than zeroing it and and then building it right back up again can add up to a significant amount of propellant.

Edited August 26, 2013 by RoboRay

[Mr Shifty]

2) correct orbits to match

3) do series of target prograde burns and yellow retrograde corrections

If this method works and you're comfortable with it, then that's great. But here's the analogy:

Imagine two cars travelling down the freeway in the same direction, both going 100 kph, with one car 1 km in front of the other. You're in the back car trying to catch up to the front car.

- In your method, you speed up to 110 kph, gain 50 meters on the other car, then slow down to 100 kph, re-evaluate and repeat until you catch up, 50 meters at a time.

- In my method, I speed up 110 kph and stay at that speed until I'm close to the other car, then slow down gradually to match speeds at the meeting point.

Your method has the advantage of being more careful and more deliberate. My method has the advantage of being much faster and more efficient. (Once I have an intercept that is less than about 20km from the target, it rarely takes me more than 5 minutes to get within 100 meters of the target.)

[Cartz]

I find the hardest part of learning to intercept/dock is sifting through the reams of misinformation out there, from 'experts' that have docked once.

Some folks in this thread have it right, others not so much. Heres an easy recipe for an intercept.

1. Launch target vehicle to 150km orbit, make it as circular as possible.

2. Launch docking vehicle to 75km orbit, again, circularizing makes it easy.

3. Make target vehicle your target, plot a maneouver at an ascending or descending node, burn anti-normal or normal (purple indicators) until ascending and descending nodes are 0.0

4. Plot transfers up to 150km, note the distance between vessels at closest approach. Your ship in the lower orbit will 'catch up' with the ship in higher orbit over time. so use time acceleration until that burn up to 150km has a < 500m approach. When the time comes, burn on up to 150km.

5. Switch your velocity indicator to 'target mode' and burn retrograde when you're at closest approach until relative velocity is 0.

6. You should now have a nearly identical orbit to your target, although you may need to fine tune a little bit, you should be set to dock.

Every intercept, no matter the orbit, is a two step process imo.

1. Match target orbital plane

2. Match target orbital altitude and velocity

As a few others have said, once you're within a few km of your target, through the map views and the shape of your orbit out the window. If you simply ensure that you're heading towards your target, when you get there and null your relative veolcity your orbits will nearly match.

To the OP, in your images, it looks like you have a craft that has a periapsis lower and an apoapsis higher than your target's orbit. That makes plotting an intercept a lot more difficult because your vehicles velocity changes as you orbit. Circularizing helps because it ensures constant velocity and a more predictable intercept timing.

[HeadHunter67]

Manley's video was never clear to me, even during the time I was beggining to understand how docking is done.

I'll be honest, that video was confusing to me. But then I watched Pebblegarden's Phoenix Project videos and she made the whole process crystal-clear. Once you learn to nudge the yellow orbital vector markers into synch with the pink (or purple if you wish) target vector markers, you're set. It's honestly not as complicated as it sounds there.

Blizzy78's in-game tutorial missions for docking and rendezvous make it simple enough for anyone to grasp.

[lajoswinkler]

Granted, what I do takes some time, but I dock at pretty low LEO so sometimes and if I don't match orbits after arranging a future encounter, I drift away too much and that can turn into a constant push-me-pull-you struggle, especially for people who are new to this.

If you do these things at 300, 400, 500 km orbits, can be pretty bold with your orbital maneuvers, but if you're right above the atmosphere, the curvature is too big and that creates the neccessity to correct stuff all the time and go small steps, or simply be very skilled and fast, something I'm not.

I prefer to go low because I've figured out I save much more fuel by using baby steps and more time than to climb high and doing it quickly.

I'm not saying it's perfect, but it works.

I'd like to see someone docking in molniya, that would be interesting. As a matter of fact, I might try it myself.

[RoboRay]

That raises a great point... if your orbit is so low you're skimming along right at the top of the atmosphere, you might find yourself dipping down into the goo during the approach if you deviate too far from a circular orbit. The drag won't be enough to pull you down, but it can be enough to throw off your intercept.

But that's just a complication that comes with extremely low orbits. The delta v requirements for a higher orbit are pretty miniscule, and the shallower gravitational gradient at higher altitudes makes docking marginally easier (the craft drift more slowly relative to each other) as well as eliminating the low-altitude issues.

The stop-and-go approach isn't wrong... there just aren't many situations where it's the best choice. But extremely low-altitude operations certainly can be one.

Edited August 26, 2013 by RoboRay