Tutorial: Docking Is Easy
Docking is very easy to do if you know how to use the navball. Most tutorials or discussions you see talk about getting into a similar but slightly smaller or slightly larger orbit to catch up to your target, then wait till you are within a kilometer or two. You do need to approach the target, but if you get within 200 or 300 kilometers you are plenty close. (Note however that the real question is the "angular" distance. That is, you have to be basically on the same side of the planet going basically in the same direction. You can't point your ship at the target when it is a quarter way around its orbit in front of you. So if you are in two very tight orbits, you will have to be closer.) (It will cost more fuel to match speed and trajectory if you are this far away, but it is not enough to matter much. If fuel consumption is a big concern, then you can get closer. [If it’s not a concern, you could start even farther away.] But you don’t need to sit and wait till you coast to within a couple kilometers.)
If you are not already familiar with how to rendezvous in orbit, read Tutorial: The Berry Maneuver. Start by matching orbital planes, then increase your apoapsis until it just touches your target’s orbit (or decrease your periapsis if your target is in a smaller orbit). Then move the navigation point back and forth until the intersection points are within a few hundred kilometers. (All this is covered in orbital rendezvous tutorials.) If your target’s orbit is highly elliptical, it will be helpful to line up the apoapsides and periapsides with radial in/out burns. Don’t worry about this too much, but you do want your ship and the target to be heading in generally the same direction when they encounter. Note also that their closest approach may not be where the “intersection” markers are.
Everything up to this point is covered in other tutorials. Once you are within a few hundred kilometers, it is time to close the distance. On the navball, click the little window at the top. It has three settings, “orbit,” “surface,” and “target.” It cycles through these each time you click on it. If you don’t see target, you forgot to set your target. Go to the orbital map, click on your target, and select “set as target.” This changes the markers on the navball. Now there are only four: prograde and retrograde, which are the same as always, and target and anti-target. Target is circular pink, anti-target is an upside-down Y, also pink.
To reach your target, you just need to make sure you are heading in the direction of your target. In other words, your prograde marker should be right on top of your target marker. Note that it doesn’t matter yet what direction your ship is pointing, all that matters is the direction it is traveling, which is the prograde marker. (The orange V with the dot, right in the middle of the navball, is the direction your ship is pointing.) Once you are heading directly toward your target, you can fire prograde to increase your speed relative to the target, say to 200 m/s. Watch your distance from the target, once you get closer, you should begin to slow down. Your only job now, as you approach, is to keep your prograde marker on the target marker. That is all, and you are guaranteed to hit your target. (You will literally hit it if you don’t slow down fast enough!) To slow down as you get closer, you must turn around and fire retrograde and watch your target speed decrease. Notice that when your prograde marker is over the target marker, your retrograde marker will be over the anti-target marker, on the directly opposite side of the navball.
The only thing that takes some getting used to is how to keep your prograde marker on top of the target. Whenever you fire your rockets, you pull the prograde marker toward the direction you are heading on the navball. So to position the prograde marker over the target, point your ship to the far side of the target directly opposite the prograde marker. As you fire you will see the prograde marker move across the target marker toward the direction marker. With a couple corrections you can quickly get your ship traveling directly toward the target. (If the prograde marker is drastically far from the target, just fire full blast for a few seconds toward the target. The prograde marker will quickly come into view on the navball. Then you can make more measured burns to adjust.)
Note that the retrograde marker goes the opposite direction. When you fire to one side of the retrograde marker, that marker moves away from your direction pointer. You push the retrograde marker rather than pull it. So you can “push” the retrograde marker onto the anti-target marker instead of prograde on target if it is more convenient, but you push it around the navball rather than pulling it.
Don’t worry about your orbit. As you are chasing the target, you will slowly align with its orbit automatically. (One caveat, if the target has a periapsis extremely close to its moon or planet, it’s possible you will crash! Just check on the orbital map to make sure that’s not going to happen, but once you get closer to the target, it won’t matter.)
When you are within 1 or 2 kilometers, it’s time to start using RCS and SAS. (These vital functions are also covered elsewhere.) Switch to the target ship and turn on SAS, and select the target marker. Now SAS on the target ship will keep the docking port pointed directly toward your ship. (Assuming of course your docking port is on top of your ship. If not, you can deal with that too, see below.)
Switch back to your ship and turn on SAS and select the target marker for it also. Now SAS on both ships will keep them pointed directly toward each other. Now keep prograde on top of target, and slow down as you get closer. Once you get to within 200 meters or so, and your speed is under about 20 m/s, it’s time to switch to the RCS translate controls (also covered elsewhere). SAS will keep your ship pointed the right direction, and you just need to use the translate controls to keep prograde on top of target. When you get to within about 10 meters, when the target ship is close enough that you can see detail, you need to make one last adjustment. By now you should have cut your speed to a crawl, about 0.2 m/s. Click on your own docking port and select “control from here.” Then click on your target’s docking port and select “set as target.”
That is all. Turn the camera sideways and watch the amazing spectacle of your two ships docking effortlessly. Note that when your speed is very slow, the prograde marker jumps all over the place, and sometimes disappears altogether. Don’t worry about that, just try to keep it so it looks like on average it’s pointed in the right direction. If you speed up a little, to 0.2 m/s or 0.3 m/s, it should calm down the prograde marker a little. But don’t worry, if they have been pointing in the same direction all this time, they are going to dock.
One final thing. Just before docking when you are about a meter away, disable SAS. The docking magnets will takeover. If you leave SAS enabled, it may "fight" the magnets and cause you to miss.
Note: SAS isn’t actually necessary for this. You can orient the ships manually using the navball. By clicking on "control from here on your dock, it will reorient the navbal prograde/retrograde to match the direction of the docking port. Warning: If your docking port is on the side and you forget to switch "control from here" back to the cockpit, your navball will be still off by 90 degrees after you undock!
Switch to the target ship and also click on the docking port and click on “control from here.” Then using the navball, orient the ship in a convenient direction. (Straight up, in the middle of the blue, for instance, is easy to understand, but anywhere will work fine as long as you can figure out what is the exact opposite point on the navball. For example, if you point the target ship at 270 degrees on the blue/brown horizon, then the exact opposite is 90 degrees.)
Then switch to your ship and orient it in the opposite direction. If you have the target pointing straight up, then point your ship straight down at the brown. Now the ships are oriented properly to meet. Everything else is the same. Your job is still to keep the prograde marker over the target marker, your speed at 0.2 m/s, and the ships will dock without any trouble.