The Illustrated Guide to Docking Redux

[AHHans]

Disclaimer

This is my interpretation of @Snark's Illustrated guide to docking. A lot of this will be just copy&paste from his guide, not only the text but I'll also "borrow" many of his illustrations. So you cannot really say that this is wholly my work. But I will give it some personal flavor (every one has a different way to rendezvous and dock) and will incorporate some of the new features, in particular the new orbit and approach information that was introduced in 1.7.

This is work in progress! And it's taking me longer than I thought. But I'll get there...
Even though I'll appreciate useful comments and suggestions and may include them into this guide.

Contents

I plan to group this guide into the following sections:

1. Setting up the Rendezvous
   • You are in orbit somewhere, your target is in orbit somewhere else, and you want to meet it.
   • This is the part where you use maneuver nodes to arrange for your ship and the target to be in the same place at the same time, or at least as close as possible.
   • Careful fiddling with the maneuver node will generally get you a rendezvous within a kilometer or so.
2. Fine-tuning and Finishing the Rendezvous
   • This involves making small burns as you approach the target, in the last few kilometers, so that your closest approach is zeroed in to a few dozen meters (from the kilometer-or-so you got in step 1).
   • This involves close observation of the navball and light touches on the throttle.
   • The final step is to kill your velocity relative to the target
3. "Lazy mode" Docking
   • You have full control of both vehicles, and can rotate both around so that their docking ports face each other.
   • Using your RCS thrusters you make sure that the two docking ports hit each other face on at a survivable velocity.
4. Standard Docking
   • You cannot (or don't want to) rotate around you target.
   • You still dock.... (Jaja, I'm not done here...)

1) Setting up the Rendezvous

Navigation tool: Maneuver nodes and map view
Control via: Main engine and throttle

The first thing you have to do is to send your ship on a course that will put it and the target in the same place at the same time -- or, at least as close as you can get with maneuver nodes. The distance how close you need to get depends on where you are (how curved the space where you want to dock is(*)). A 5km encounter in LKO may be too far away to successfully rendezvous, while a 5 km encounter in Keostationary orbit (2863.33 km above Kerbin) will pose no particular difficulty. In general getting an encounter that is closer than 1 km should be good enough.
(*) Yes, I'm a physicist, space is curved, deal with it. Well, except maybe around Gilly, you can't really call that curved.

I'm assuming here that you in fact have maneuver nodes (i.e. you've unlocked them by upgrading Mission Control and the tracking station), and also that you're familiar with how to use them.

Setting up a fairly good orbital rendezvous with maneuver nodes is actually a significant challenge, and if you're a new player just learning how to dock, there's a pretty good chance you may not have mastered "orbital rendezvous with maneuver nodes" as a skill yet. This guide is not really about setting up an orbital rendezvous! That is a whole complex topic in its own right, and this guide is about docking rather than general orbital navigation. Having said that, I'll still present one way how this can be done. I'll use this method a lot, and I thought it makes for a better story.

In this guide we follow the voyage of AHS Minmus Big Miner. She isn't the prettiest ship in space, she doesn't have a fancy name, but she is a big piece of industrial equipment and can reliably haul 100 tons of ore into Kerbin Orbit. And here she is, in low Minmus orbit after just having filled her ore tanks on Minmus.

She needs to deliver that ore to the refilling station in a 500 km orbit around Kerbin. On the way there she'll stop by a probe in orbit close to the station to practice docking.

As we can see below, just burning prograde at the right time would get her into a Kerbin orbit, but with a high inclination to the refilling station. By combining the prograde with a (anti-)normal burn she can reduce the relative inclination of the orbits by a great amount, without using any extra fuel:

By burning normal or anti-normal at the node (here a normal burn at the descending node) that she encounters on the way to the periapsis, she can correct the plane of her orbit around Kerbin to be in the same plane as the target station.

Now that she is in an elliptical orbit in the same plane as her target that intersects the target orbit. By burning retrograde at the periapsis between the two intersection points (if you are coming from a lower orbit, burn prograde at the apoapsis) the time when she arrives a the second intersection point (after going though most of the orbit) can be changed by a large amount. So this time can be chosen so that she encounters the target at the point.

Note: in order to reduce the relative speed at the intersect I first set the maneuver node to get to about the same orbit as the target, and then reduce the burn until I get that encounter.

Before performing that burn I suggest to select the "Approach Info" for the current orbit in the maneuver display. This updates in real time during the burn and allows one to fine tune the burn to get the best rendezvous.

 

2) Fine-tuning and Finishing the Rendezvous

Navigation tool: Navball and approach information
Control via: Main engine and throttle

Your eventual goal is to coast to a stop so that you're parked right next to the target (as in, just a couple of dozen meters apart).

Chances are, however, that unless you're either super lucky or an ace at maneuver-nodes, the rendezvous that you set up in Step 1 above will only get you to within a few hundred meters at best-- perhaps a kilometer or two. You'd like to be closer than that when you match velocities, since your final approach (using RCS) will be very slow, just centimeters per second, and you don't want to have to do that for a kilometer or more.

So the way to solve that is to make minor course adjustments with light touches on the throttle of your main engine, using the navball for guidance. Here's how:

First things first: Make sure navball is in "target relative" mode
Once you get close to the target, you don't care what your orbital velocity is. You only care about your velocity relative to the target. Chances are, the navball is probably in "Orbit" mode when you're in orbit (the game does that automatically, unless you've manually tinkered with it).

Normally, when you approach your target within 60 km, the navball will switch automatically to "Target" mode instead of "Orbit": this causes your prograde/retrograde markers to indicate your target-relative velocity rather than your orbital velocity, which is what you want. It looks like this (note the "Orbit" or "Target" indicator at the top of the navball):

...Typically, it will do this automatically for you and no action is required on your part. I only bring it up because it's possible this might not happen, and you need to make sure that the navball is in "Target" mode, or nothing after this point will make any sense. You can change the mode manually by clicking on the label. This will toggle the navball between Orbit, Surface, and Target modes.

Why this fine-tuning step is needed
As you get within several kilometers of your target, your navball will start to look something like this (again, make sure your navball is in target mode):

This looks pretty good: your target-relative-velocity retrograde marker (yellow circle) is pretty close to your target-direction retrograde marker (pink Y with dot in middle). The fact that these two are close together means that you are heading almost directly towards your target.

However, the key word here is "almost". If you just coast towards your target and do nothing, you'll see the target-direction retrograde marker slide farther and farther away from the velocity retrograde marker, like this:

That's because you're not heading directly at the target. Unless you got super lucky (or are an ace) with the original maneuver node, your closest approach is likely to be at least several hundred meters away from the target. This means that without any course corrections, you're going to go past it, and it'll slide off the side of your navball in the same way that someone standing beside the road slides off to the side of your windshield as you drive past them.

Therefore, you need to adjust your course when you get close by, so that you are sitting right next to your target (and not a kilometer away) when you come to a stop relative to it.

What to do
When you get reasonably close to your rendezvous (say, when it's 1 minute away, which you can see in the map view when you mouse over the intercept marker), start the process.

What you're trying to do is to move your velocity-retrograde marker so that it's precisely centered on your target-retrograde marker (i.e. so that you're heading directly at the target). To do this, start by lining up your navball so that your center-marker, velocity marker, and target marker are exactly lined up in a straight line, like this:

Note that the yellow velocity-retrograde marker is exactly on a straight line in between the navball crosshairs and the pink target-retrograde marker.

Why do we do this? Well, firing your engines "pushes" the velocity retrograde marker away from the center crosshairs on the navball. Since what we want to do is move the velocity marker onto the target marker, then by lining the navball up this way, it will "push" the velocity-retrograde marker in the direction we want it to go.

Once you've got it lined up, gently throttle up to move the marker. How much throttle you need will depend on your ship's TWR and how big your target-relative velocity is. So just keep a careful eye on the navball, gently throttle up, and be ready to kill the throttle immediately when things line up.

As your engine burns, you will see the velocity-retrograde marker drift towards the target-retrograde marker, like this:

When the two markers line up perfectly, cut throttle. (Don't worry if it's not pixel-perfect; all that matters is that it's better than it was. If you're off by a smidgeon, what will happen is that as you get closer to the target, the two markers will drift apart again, and you can do this correction maneuver again. Repeat as needed.)

Edited September 3, 2019 by AHHans