Tutorial:Orbital Rendezvous

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In version 0.18 docking was introduced. This allows two independently launched components to be intrinsically linked in orbit and allows them to function as a single structure. Functions of such a structure can range from a stationary way point, such as a space station for refuelling to a large scale interplanetary platform that would be impossible to launch in one piece.

The way this is accomplished is by linking (essentially touching) two docking ports to each other, after you have moved both components into an Orbital Rendezvous. This tutorial will explain how to achieve this. It can be a very complicated procedure but once you understand it, you'll learn it's not as hard as it seems. Just, remember to quicksave often. F5 to save and F9 to load last quicksave called "quicksave", or alt-F5 to save under a different name such "quicksave #1" (F9 still loads "quicksave"), and alt-F9 to choose which save to reload.

Specifications

  • Length: 30–40 minutes
  • Difficulty: Medium
  • For version: 0.18 and higher

Preparation

Payload Creation

The first step is to create the payload, this tutorial assumes that you will already have this design started, however keep the following in mind:

  • A docking port is required on at least one point of your payload.
  • Another port can also be attached so that another section can be daisy chained to it a later stage.
  • Make sure you use the same size ports where you plan to dock.
  • The sections should be kept at a reasonable size. The only one worth pushing the limits on is the first section, as it will not have to move once in place.
  • Usual requirements such as solar panels and batteries
  • Add some lights in case you have to dock in the dark. Illuminate both the direction your port is facing, as well as yourself (so you can see it from the target)
  • Add RCS thrusters for fine control once you are close to your target. The mod RCS Build Aid helps in placing balanced thrusters that won't torque you when you are trying to push a direction.
  • Pods come with a small amount of monopropellant, used for RCS thrusters. Recommend adding a small tank or two, just in case.

Tug Creation

Next you will need a small orbital tug to push your payload into position. This will mainly be liquid fuel and a small engine controlled by a command module.

A small amount of liquid fuel and an adequate sized engine should be used for the payload. Only a tiny amount is required as it will only have to cover a small distance to the target.

More importantly monopropellant and RCS thrusters are necessary to fine tune the position and allow the docking ports to touch.

After constructing the craft, the center of mass indicator should be used to fine tune the RCS thrusters' positions. The thrusters should be placed symmetrically around the center of mass of the completed craft to allow linear movement without rotation while in space.

Orbital Alignment

There are five distinct stages to an orbital rendezvous:

  1. Achieving stable orbit
  2. Fine-tuning orbit sync and inclination to match target
  3. Matching target orbit
  4. Dock positioning and locking
  5. Closing range

Achieving stable orbit

This is covered in other tutorials and I refer you to them. Initially set your orbit smaller than the target orbit if possible.

Fine tuning orbit

  • Go to Map mode and click your target, and select "Set as target". Switch to the target vehicle and mark your inbound vehicle as its target. Switch back to the inbound vehicle.
  • You will see Ascending Node and Descending Node markers appear on your orbit. Whichever node is coming up first in your orbit, add a maneuver node there. Use the normal/anti-normal adjustments to lower your final AN/DN number until it is as close to zero as possible (NaN is ideal). Precise Node really helps - it can move your node to the exact AN/DN, and fine tune your burn.
  • Execute the burn
  • Do prograde/retrograde burns aligned with your target's periapsis and apoapsis to get your periapsis-apoapsis set aligned with the target's set. This makes the encounter much easier to manage. Don't necessarily match altitudes, but both periapses and apoapses should be in a line with Kerbin's center of mass. Aim for an altitude ~10% lower than the target's to give you freedom to time the encounter.
  • Create a maneuver node at Periapsis and raise your Apoapsis until it matches the target. Encounter indicators will now appear. Ideally you want a single pair. If you see two pairs, instead of just barely touching, your orbit is going beyond the target orbit and back.
  • The up and down trapezoids indicate where you are/where your target will be (need to verify direction) during the close approach.

Matching target orbit

  • There are multiple ways to get the indicators closer together. The easiest way is to delay executing your encounter until they are close together. You can also resize your orbit if you need a sooner encounter, but it is more difficult to plan with the current tools. This covers the first method.
  • Open the maneuver node. Right-click on the circle in the middle of the maneuver node to switch to delay mode. Use the icon on the bottom right to delay the maneuver node time to future orbits. As you do so, you will see the closest encounter change. If your orbits are close in size, the distance will change by small amounts.
  • Once you are close as possible by delaying the encounter, re-adjust the prograde/retrograde burn until the encounter distance is <2.5km ideally. You might need to work the normal/anti-normal direction as well. To get the most precise encounter, it helps to Focus on Kerbin, and rotate the view to where the encounter occurs.
  • Set your SAS to point to the maneuver node. Often the node is low delta-V, and will wiggle. In that case, set SAS to Stability Assistance and manually align with the blue maneuver node direction.
  • Change to Stability Assistance to keep your ship from flopping around at the end of the burn as the maneuver node direction shifts.
  • Execute your burn. To help with accurate burning right-click your engines, and lower your thrust limiter. Using Kerbal Engineer Redux or MechJeb to get thrust numbers, aim for ~10% of your required thrust for the encounter. This gives you a nice slow burn that you can fine tune. Otherwise, quicksave, do a test burn to get a feel for the acceleration, and quickload.
  • Once you are < 1m/s, delete your node and switch to map mode. The encounter indicators will now show actual instead of potential future encounters.
  • Using prograde and retrograde SAS, set minimum thrust and adjust your orbit until your encounter is at minimum distance.
  • Add a node a few minutes before your encounter, and accelerate time until there. Delete the node.
  • Once you are less than 2.5 km, you can switch between your inbound and target vehicles using [ and ].
  • At this point, your navball will switch to target mode, and display relative velocity with your target.

Dock positioning

  • Switch to your target, and right-click on his Docking Port, and select Control from Here.
  • Select SAS setting Target, and the docking port should align with your inbound vehicle. If it points away, use Anti-Target.
  • Switch back to your inbound vehicle. You will want to switch back and forth every few minutes to let the target SAS align the docking port. If your target is a station, you will not want to adjust its SAS. Instead set it to Stability Assist.
  • Stay in Staging mode, and change Camera mode to Chase using V. This will align your camera with your RCS translation vectors. Enable RCS thrusters with R. QWEASD will still rotate your vehicle, but now HNIJKL will translate, ie push it. If your RCS thrusters are not well balanced, a translation will apply torque and start to rotate your vehicle. If you have SAS enabled, the SAS will cancel the rotation, but it will burn excess monoprop. Do a translation burn, turn off thrusters to let the SAS stabilize using reaction wheels, then turn the thrusters back on to save fuel.
  • If you are transferring Kerbals, you should be close enough for a simple EVA transfer now.

Closing Range with Stock tools

Once you are within a few km, use the navball in target mode (click the speed indicator box until it says target). If you are moving relative to the target, your speed will be shown there.

  • Aim for the retrograde symbol on the navball. Burn until your relative speed is zero.
    • It is better to get as close as possible to the target before doing this, but any distance will work.
  • Aim for the target prograde - the purple curvy one - on your navball, and burn until your relative velocity is ~1-2 m/s (or faster if you're brave / far away).
  • As you get closer, turn to face the retrograde symbol once again, and burn to a stop once again.
  • Repeat the above until you are within 50-500m, and use RCS thrusters from there.

Closing Range with DPAI

  • Docking Port Alignment Indicator is a great tool that makes the final few kilometers easy and painless.
  • Once you are within a few KM, activate DPAI. A window will come up with the indicators.
  • There are three key indicators
    • Orange indicates which direction the target docking port is pointing. Ideally it should already be in the middle if the target SAS is on Hold Target mode. If not, or if you are docking with a station, it will be offset.
    • Yellow indicator is your relative velocity vector. Prograde symbol means you are approaching the target, retrograde means departing the target. It's location on the screen is how much your are moving in the horizontal and vertical directions.
    • Green lines indicate the direction to the target docking port center line.
  • Additional information on the docking screen.
    • Bottom left number DST is absolute distance to target
    • Bottom right CDST is distance from the target using the center line of the docking port.
    • Bottom right CVEL is the center line velocity towards or away from the target.
    • Think of two dots on two sheets of paper, separated by a book. CDST is the shortest distance from one dot to the other sheet of paper. CVEL is how fast those sheets of paper are coming together. DST is the distance from dot to dot.
  • Set your inbound vehicle SAS to target. Use the H key to get your acceleration ~5m/s until < 500m, then slow down using N to 2m/s.
  • Ideally the CDST and DST numbers should already be close if you and your target docking ports are aligned with SAS. If you are docking with a station, you want to rotate using WASD until the orange indicator is in the center of your window. That gets the docking ports parallel to each other and pointing the right direction. They will still be offset.
  • Use IJKL to push the yellow target in the same direction as the green bars. Aim for pushing it twice as far from the center as where the green bars cross. As the green bars come closer to the center, move the yellow target closer to center too.
  • As you close range, try to keep the green bars in the center with the yellow target. Reduce forward speed to 1 m/s within 50 m, and .5 m/s within 10m. Experiment with how much delta-V you get for each RCS burst.
  • If there are any protrusions to your station or vehicle, rotate with QE so you can cleanly dock. You can use the Dock Align angle indicator at the top of the DPAI screen to help.
  • Sometimes if you are approaching too fast, you will bounce off. Take a deep breath, and redo your approach.
  • Breathe!