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Kerbal Space Program - Orbital Rendezvous Guide (Tin Can Program)

By 1Si6   /   Aug 4, 2019     Guides
Kerbal Space Program - Orbital Rendezvous Guide (Tin Can Program)

Step-by-step guide for an orbital rendezvous using early techs - intended for use in Career mode after completing the basic Tin Can Program.

Introduction



Welcome to Orbital Rendezvous training! I hope that you are here because you completed the Tin Can Program and you need to do a rendezvous for your next main Contract. If not, well, you can still learn orbital rendezvous here but you may experience confusion.

Today we are going to fly an old friend, Tin Can 4, which you may remember was our basic Kerbin orbiter. The mission is to put two of these orbiters up above Kerbin and match their orbits and positions so that they are in visual range at a low relative speed. While this is easier said than done, you will develop a feel for it in no time.

Flight Checklist



  • Launch Tin Can 4 to Kerbin Orbit.
  1. Launch to 60m/s before steering.
  2. Pitch over with maximum steering.
  3. Steer to horizon with Upper Stage (First Liquid Engine).
  4. Cut throttle at 70K altitude, coast out to AP.
  5. Burn remaining upper stage fuel at AP on a Prograde heading.
  • Launch another Tin Can 4 to Kerbin Orbit (Same flight profile).
  • Target the other orbiter, check ascending/descending node alignment.
  • Correcting burns to match PE and AP distance and alignment.
  • Place maneuver node and find the sweet spot.
  • Quicksave and then fire maneuver burn.
  • Set Navball to Target mode.
  • Retrograde burn near target to match speed - Orbital Rendezvous Complete.
  • Deorbit both craft safely!

Orbital Alignment



Launch

We will fly Tin Can 4 a little differently today than we did for your first orbit. The Launch procedure is the same, however. Engage SAS and set maximum throttle before launch. Wait for your velocity to hit 60 m/s before you steer - maximum pitch. Just hold down W the whole time your SRBs are burning - immediately stage the little one after the big ones finish.

Release the steering for a moment to fire the Upper Stage engine - the first liquid-fuel rocket. This is to avoid tumbling the rocket by firing the engine while it rotates. Stage it on right away, though, and immediately steer down to the horizon. All of this is the same as before.

This time, cut the throttle once you enter space at 70K altitude. Open your maneuver tab and look at your remaining time to AP. Fast forward if you need to, until there's only about 60 seconds left. Now steer yourself back down the the horizon at your Prograde navpoint. The navpoint itself may be just above the horizon but keep yourself pointed at the line until your time to AP is 30 seconds or less. At that time, burn the rest of your fuel and drop your upper stage. This should put you in a regular orbit, clear of all atmosphere. If you need to burn a little of the orbiter's fuel to get there, that's okay. You have plenty, and extra.

If you really screw up, and use a bunch of the orbiter's fuel, that's probably fine, as long as you can still deorbit yourself. You can still try again with your second launch, and only one craft needs extra maneuvering fuel for this mission to succeed.

Now, with your orbit secure, return to the Space Center and do the whole procedure again with a second Tin Can 4 launch. Be sure one of these orbiters makes it up with most of their fuel - we need to do two burns to make this work, and a third to deorbit when we are done.

Okay, that's the grunt work out of the way. Now we can begin the real mission.

Rendezvous - Aligning Orbits

If one orbiter has more fuel than the other, fly that one. Most of this procedure is done in the Map view, so start by opening the map. The first thing is to set the other orbiter as your target - just click on it. Now you can see your ascending and descending nodes. Positive or negative, it should be 0 or 0.1°. If not, you have a problem. You can start over, obviously, or scroll down to the section marked Problem.

Assuming your angle is okay, your next task is to align your PE and AP with your target. To move one, you do a burn at the other. To raise it, burn Prograde; to lower it, burn Retrograde. Just bump the throttle gently with the Shift key, do not fire at maximum throttle, and be ready to hit X and cut throttle right away. Try to match the target's PE and AP by less than 1km. You have pretty fine control, and it's good practice.

While you are matching the PE/AP distance, you may notice that the actual PE/AP points are not located at the same place in your orbit at your target's. Once you get the distances pretty close, you will need to correct this alignment with a Radial burn. The best spot is in the space between the two PE or AP points. It can be hard to tell which Radial node will correct the alignment. Instead of a confusing explanation, I offer you this procedure: align on one of the Radial nodes. Zoom in nice and close on the Map view (you are still in Map view, right?) so you can see smaller changes in your orbit. Start to burn very gently on the Radial node you chose. If the gap between the two PE or AP points starts to get smaller, you are good to go. Line them up. If you see the gap start to get larger, cut the throttle and use the other Radial node.

By looking 'through' Kerbin, you can view target's PE and AP so that one of them is completely behind the other, sideways on, so that the orbital ring is just a straight line. From this perspective, you can watch your own PE and AP swing into alignment with the target's. Now, go back to a more conventional top-down view and finalize the alignment of your PE and AP distance. On the map, it should look like one orbit ring with two craft on it.

Problem

Did you come up into orbit, target the other orbiter, and found your ascending/descending nodes to be at an angle wider than 0.1°? If so, here is your solution. If not, you can skip down to the Maneuver Node section below.

You should have enough fuel to correct this problem. Whether the angle is positive or negative depends on which node you are approaching: one will be positive and the other negative. You need to find the purple, triangular navpoints. Not the Target navpoints, which are more like Crosshairs - these are two triangles. Now, if your angle is negative, you need the 'smoother' triangle (Normal navpoint.) If your angle is positive, you need the triangle with the lines poking outward from sides (Antinormal navpoint.)

Line yourself up on the navpoint you need, and then warp ahead to the ascending or descending node nearest to you. Unlike the other navpoints, your heading will stay right on top of the Normal/Antinormal node while you warp through orbit. Zoom the map in close so you can get right on top of your node and then do a gentle burn to bring that angle down to 0.1° or less. Now you can go back to aligning your PE and AP with the target's.

Maneuvering



Rendezvous - Maneuver Node

With your orbits aligned, you are ready to deploy a maneuver node. Just drop it anywhere, like we did for our Mün flyby. Add some Prograde velocity - not a specific amount, just enough so your new orbit after the burn is visibly bubbled with kind of a crescent moon shape between it and your current orbit. You should see a set of Orange markers showing your closest encounter with the target. This encounter marker is actually two markers, showing your target's position and your own at the moment of your closest approach. There may also be a second set of purple markers which show a different close encounter after the first one.

What we are trying to do is bring those two arrows right on top of each other. You need to move the node around to figure this out. One of two things will happen when you move the node. Your encounter markers might move around the orbits but never get closer together - in this case, adjust velocity. Or, if you got lucky when you put a random amount of velocity on this burn, you might see the markers start to come together as you move the maneuver node. If so, move them as close together as you can.

You need to adjust both velocity and the position of the maneuver node. There is a little bit of fiddling required. For example, when moving the node, you may only be able to get the markers so close before they 'snap' away from each other. In this case, place the node right before this 'snap' and go back to adjusting velocity.

Eventually, when you get the markers very close together, you will need to hover over them for a readout of the encounter distance in km. Continue tweaking to get all the way down below 1km if you can. Remember that you can also just type in burn values manually down in the bottom corner. You want to get the plotted encounter as close as possible: no matter what, your burn won't be perfect, so you want as perfect a target as possible to aim at.

Just as a note - this entire procedure also works with a retrograde burn, which is sometimes required if your target is 'behind' you in your matched orbit. For this training mission, this shouldn't be necessary, or even possible, because your orbits are too low. Regardless, for future applications of this procedure, it is important that you know.

Rendezvous - Encounter Burns

Make a quicksave now, or warp to near your maneuver and save there if you prefer. Either way, lock your SAS on Prograde and warp to 60 seconds or so before the maneuver. You want to use a little patience and drift the rest of the way in real-time so everything can be settled and lined up when it is time to burn. Just like the Mün flyby, don't leave your Prograde lock engaged. When you fire the engine, or just before, switch back to basic SAS mode.

For a precise burn, watch that delta-V readout instead of the remaining time. Cut the throttle with 3 or 4 m/s still in the gauge for best results. To fine tune your encounter, start by deleting your maneuver node. This will show your actual close approach markers instead of the theoretical ones we plotted. If it looks bad, you can always load that quicksave. Don't panic, though! Try fine tuning your encounter. Zoom in so you can see which marker is your position and which is your target's. You need to burn a little gentle Retrograde burn if you are ahead of the target, and a little gentle Prograde burn if you are behind. You need to get that encounter distance nice and close to complete the Contract: go for less than 5km.

Of course, distance is only half the story. Now that you are on course for a close encounter under 5km, take a look at the Relative Speed on your encounter marker. It's probably hundreds of m/s, and that's too high. So, if you're confident, quicksave again. If not, push on. You want to warp around most of your orbit until you are getting close to your close encounter. Not all the way right to it, though - you need some time.

After your warp, set your Navball into Target mode by clicking on your speed indicator until it says 'Target'. Finally, I will let you go back into the Flight view. I know the Map is not as much fun, but now you see why we need it. In Flight view, find your target. You should be close enough that there is a small yellow bracket around it - that is likely all you can see. Hover over the brackets so that they show the distance to your target. Hopefully you are still like 20km away or more, but don't panic if you are closer. Double-check you have the Navball on Target mode and engage the Retrograde lock.

All is now prepared to complete our rendezvous. Watch your target's distance. Fast forward carefully if you need to - get back to real-time at 5-10km. You need to start burning early so that you have some time to kill your speed, but not so early that you don't get close enough. It may take a few tries - reload if you need to, of course, but if you burned too early, you're low on speed and still too far away, cut the throttle and check the Map to see if you are still going to get close enough. Then just wait to get closer and burn off the rest of your speed.

Your basic 'Explore Kerbin' orbital rendezvous contract should complete as soon as your speed and distance are both low enough. With this technique, and a empty seat, you can start picking up additional Kerbals to expand your space program. This EVA rescue mission is the main practical application of this technique until you learn Docking (which requires its own tutorial) so with that in mind, we will conclude with a short summary of how to complete that final, terrifying EVA hop after your successful rendezvous.

EVA Transfer



Get your speed relative to your target as close to zero as you can. The closer you get it, the easier this is. Be sure you are within 2km - again, the closer you can get the easier this is. As a slightly more advanced technique, you can try using the Target navpoint on the navball to move yourself a little closer to the target, but only after killing almost all of your existing relative speed first with the Retrograde navpoint in Target mode.

In any event, once you are 'parked', switch over to the craft (or whatever sad derelict you met up with) and put that Kerbal out on EVA. Use the map to set your pickup craft as your target once you are on EVA, then go back to Flight view and look around the Navball for your Target navpoint. Press R to activate your spacesuit jetpack, then use W to accelerate toward the navpoint. Accelerate as much as you dare - you quicksaved, right?

Don't bother with the Retrograde navpoint. Use Prograde and just press S instead of W to slow down. Once your ride is really in view, and you are approaching at a safe speed, just use your standard-issue Mk.1 eyeball, zoom in nice and close (just like one of those Third-Person Action-Adventure Video Games I've heard about), and guide the Kerbal to the hatch. Slow and steady wins the race - the prize is a ride home!
Written by 1Si6.