[Tutorial] Orbital rendezvous: You can do them!

[hubbazoot]

So, you want to do an orbital rendezvous? Well, let's get you started, then.

We'll start by assuming both you and the target craft are in orbit around the same celestial body, and are orbiting in the same direction. We'll also assume you have basic knowledge of how to perform basic orbital maneuvers, including Ap height change, Pe height change, and orbital plane changes. I HIGHLY recommend quicksaving before and after each maneuver, to make sure you like the results of it before you go on, so you can quickly go back and try again. Keep your first rendezvous craft light and small, as larger craft can be harder to maneuver.

Terms:

Ahead: You will pass through a point in space before your target does, i.e. you will pass over the command center before your target does.

Antivector: The direction pointing backwards along your velocity. If you were driving a car down the highway, the antivector would point out the back window.

Behind: Your target will pass through a point in space before your target does. See "Ahead"

An: Ascending node. The point at which your craft will be going from south to north of your target.

Ap: Apoapsis. The highest point of your orbit relative to a celestial body.

Dn: Descending node. Same as ascending node, but going north to south.

Orbital Period: The time it takes to complete one full orbit around a body. For Earth, one orbital period is one year long.

Pe: Periapsis. The lowest point of your orbit relative to a celestial body.

Plane: A two-dimensional slice of 3d space. when two things are in the same plane, they can be drawn on a sheet of paper and held up to represent their actual positions.

Vector: The direction pointing forwards along your velocity. If you were driving a car down the highway, your vector would point out the front window.

Note that these instructions will work for highly inclined orbits (polar), but the headings here may not be accurate to your orbit. Also, loose tolerances are given. As you get better at doing rendezvous, you'll be able to get these numbers to match up closer and closer. Missing coming within a certain tolerance isn't going to make your rendezvous fail, but it may cost you time/fuel.

First step: Setup.

Select the craft you want to rendezvous with, and choose "set as target." This should highlight the selected craft's orbit and display the An, Dn, and the closest encounter with the target craft. Ideally, your target craft should be in a round orbit. While a round orbit is not necessary, a rounder orbit will be easier to match orbits with.

Second step: Plane change.

Start getting the orbits to match up is to do a plane change. The goal with an orbital rendezvous is to get two objects in the same place, at the same time, going the same speed. We do this by slowly eliminating the differences between them, one at a time. As you can see in this picture, our target craft is in an orbit inclined relative to ours. In this case, when we pass through the ascending node, we will burn at a heading of 000 as soon as we pass through the An, and will burn until the relative inclination is within about 0.1 deg. The closer you can get it, the better. Ideally, you should get them close enough that the relative inclination marker will "jitter," meaning that you are close enough to the same orbit the calculator cannot pick a closest point (that's good).

Third step: Ap OR Pe matchup, rendezvous burn adjustment.

At this point in time, our orbits are in the same plane.

At this point, we want to wait to our Ap or Pe to match them together with our target craft. It isn't a requirement that you match with the target's Ap or Pe, but it will save on fuel. In our example, we will match the Pe to our target's Pe, even though our target is in a mostly round orbit. (Pro tip: You can make your current point in your orbit the Ap or Pe by cancelling off all your vertical velocity. Burn radially inwards or outwards, depending. This may have adverse effects on your orbit, but can save drastic amount of time.

Now, depending on if you are ahead or behind of your target, you will need to make your orbit's Ap higher or lower than the target, respectively. From the picture above, should we make our Ap higher or lower than our target?

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Got an answer yet?

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We should make our Ap lower than the target, because the target is ahead of us. This will make the time it takes to complete one orbit faster for us than our target, so we will catch up to it. If it helps, think of an extreme example: If you are in low Kerbin orbit, are you going to complete one orbit before or after Mun does? You will complete one before, so the lower our orbit is, the faster we will complete one orbital period. The closer the orbits match up, the slower our target will approach us, or it approach us.

Step Four: Wait for the "overshoot"

The overshoot is when the next closest encounter will be further away than your current encounter, and should be on the opposite side of you as it currently is. Think of it as one car having passed another. It will look like this.

Now, we slowly burn either prograde or retrograde to make the orbits match up until the closest approach distance cannot decrease any more. Ideally, this should get you within ~10km of the target, but closer is better.

It should look like this, now.

Step Five: Short approach.

Now, you're on your final approach. A good guideline for this is that for each Km you are away, it should take you a minute to reach your closest approach. I.e. if you're 10km away, you want to start 10 minutes out. For 2km away, start 2 minutes out.

This step takes some getting used to. Burn up, down, prograde, retrograde, radially inward, and radially outward as needed to decrease the distance. Start with just a little burn, then you can increase throttle. Do this step slowly, as overshooting will cost you as much as undershooting. If your closest approach distance increases, burn in the opposite direction. Try to get as close as possible. If you can get within 1km, you're doing good.

It should look like this, now.

Step six: Final approach.

Don't time warp yet, you should have just finished doing your short approach and quicksaved (you are saving, right???). Switch out of map view, and press "V" to cycle your camera around into "Chase Camera." Align the camera so it's behind your craft, as the next few steps are going to happen very quickly. While you're at it, make sure you toggle F4 to show the distance to the target craft (yellow text). Change your velocity to display "target" velocity, and align along your antivector.

Time warp until about 20~30 seconds before your closest approach, and then burn off all your relative velocity. Your target velocity should be less than 2m/s.

Point your craft towards your target, and switch to docking mode controls. Adjust your velocity so it is pointing towards the target marker on your navball. A good target speed curve to follow is to make your speed 1/100th of your distance. I.e. if you are 1km out, approach at 10m/s.

You should look something like this:

Step seven: Coming in to dock.

Now, you're slowly approaching. When you get close enough, you can right click on the docking port and select the specific one you want to dock with. It should look something like this:

Maneuver so you are in front of the docking port, and then cancel off all your relative velocity. Orient yourself so you are in the same orientation as the docking port, and then slowly come in for your final approach!

Final notes:

When your docking ports actually touch, slower is better, and the more aligned they are, the better. Toggle off RCS and SAS as soon as they start to "pull" inwards. If the craft starts to wobble, toggle SAS to help stablize the craft. Eventually the camera should "snap" to show the newly docked craft.

Congratulations! You've done it! As you improve in your docking ability, you'll be able to increase your payload sizes and you'll be able to blend these steps together so you'll be able to just "swoop" in. Congratulations, and happy flying!