Docking in Kerbal Space Program

We will discuss two docking methods:

  • A slow, careful approach by repeatedly changing the orbit altitude to first catch up with, and later match the speed of the target. This is easier for beginners, as we gradually approach the target and its speed. It uses more fuel, in real life this method is not used.
  • Using a transfer orbit to quickly catch up with the target and slow down just before the intercept. This is a more dynamic process and requires more skills and attention.

The launch, orbit injection, and matching the inclination of the target are the same for both methods, we will start with these.

Key bindings

KeyAction
CClose the cockpit view and return to the normal view

Set the target

  • Click the target object and select Set as target

Launch the docking vehicle

  • Launch the docking vehicle before the target will pass the launch pad overhead as the rising vessel needs time to gain altitude and catch up with the target.
  • Set the Nav Ball to Target mode
  • Launch the docking vehicle
  • At 10 km altitude turn towards the target in a 45 degrees angle between the zenith and the target
  • Right-click the Apoapsis node to keep the values displayed
  • When the “T-” of the Apoapsis is over 1-minute, switch the Nav Ball to Orbit mode
  • Aim for the orbital velocity vector (prograde marker)
  • Continue to burn until the Apoapsis reaches the altitude of the target orbit
  • In the Apoapsis, create a maneuver node to move the Peiapsis to the same altitude to circularize the orbit, and reduce the inclination of the Ascending and Descending nodes. Adjust the Normal and Antinormal vectors to move the Ascending and Descending nodes 90 degrees to the Apoapsis and Periapsis.
  • Adjust the Prograde and Retrograde vectors to compensate for the Periapsis and Apoapsis altitude change caused by the Normal and Antinormal vector change.
  • Select Maneuver in SAS and start the burn. As we get close to the end of the burn duration, the velocity vector will deviate from the maneuver icon, so reduce the throttle and track the blue maneuver icon on the Nav Ball.

Match the inclination of the target

  • Right-click the Ascending or Descending node to display the inclination relative to the Target.
  • Create a maneuver in the closest Ascending or Descending node
  • Adjust the Normal vector to achieve “0” inclination relative to the Target on the Ascending or Descending node. Try moving the maneuver node itself a bit if it does not reach exactly zero.
  • Open the Maneuver Mode panel and the Graphical maneuver editor.
  • Select the Advanced orbital info tab
  • Select the Next Maneuver
  • Take note of the planned Inclination of the maneuver. We will stop the burn just before we reach that number on the ORBIT tab during the maneuver and fine-tune with a low burn.
  • Set the SAS to Maneuver and warp to the maneuver.
  • Start the burn, and watch the Inclination on the ORBIT tab and stop the burn a little earlier just before the inclination would reach the desired value
  • While the maneuver exists, the Ascending and Descending node on the map still displays the planned inclination. To display the actual value, delete the maneuver node and continue a low burn until the Ascending or Descending node indicator on the map turns to zero.

Catch up with the Target

There are multiple ways to catch up with the target.

Catch up slowly by gradually changing your orbit

Change your speed by altering the altitude of your orbit

  • Change the Nav Ball to Orbit mode
  • Change your speed to go faster to catch up or slow down to “wait” for the target to catch up with you.
    • If the target is ahead of you, lower your orbit to go faster
    • If the Target is behind you, raise your orbit to go slower.

If your target is in a 120 km orbit, depending on the altitude of your orbit each full orbit changes the distance:

– in a 110 in orbit: 70 km
– in a 130 km orbit: 80 km

  • Right-click the Target to display the altitude and distance information.

Bring the orbits closer

  • Warp ahead until the two spacecraft are at a 200 km distance.
  • Change the altitude difference to 5 km
    • Create a maneuver in the closest Periapsis or Apoapsis to change the altitude difference to 5 km
    • Set the SAS to Maneuver mode
    • Burn with the lowest throttle for better control
    • Circularize the orbit in the next Apoapsis or Periapsis.
  • Warp ahead until the two spacecraft are at a 50 km distance.
  • Change the altitude difference to 1 km, it will change the distance change to around 20 km per orbit.
    • Create a maneuver in the closest Periapsis or Apoapsis to change the altitude difference to 1 km
    • Set the SAS to Maneuver mode
    • Burn with the lowest throttle for better control
    • Circularize the orbit in the next Apoapsis or Periapsis.

Match the speed

The speed difference between the very close orbits should be below 400 m/s.

  • Warp ahead to reduce the distance
  • On the Approach Info tab of the Maneuver Mode panel, you can see information about the next two intersects.
  • When the distance value of Intersect 2 is greater than Intersect 1, the closest encounter will happen between them. It means Intersect 2 will happen after the two vessels already passed each other.
  • When the target indicator is 90 degrees from the velocity vector, the two spacecraft are above each other. Kill the speed difference by changing your orbit to match the target.

Align your vessel’s velocity vector to the target’s movement

IMPORTANT!!!

This is crucial, as for successful docking at the moment of contact the two vessels should have close to matching speed and direction. During the final approach, as you adjust the speed of your spacecraft, also continuously adjust your ship’s velocity vector, so in the final moments, the two will hover next to each other without much relative movement.

If arriving from below

If your orbit is below the target, burn prograde to raise your orbit. This will eventually slow you down.
Pull the Prograde symbol to the top of the Target symbol to move your ship towards the target.

If arriving from above

If your orbit is above the target, burn retrograde to lower your orbit. This will also speed you up.
Push the Retrograde symbol on top of the Anti-Target symbol.

Tip 1

When to push, and when to pull

When we run the engine, we always push the retrograde and pull the prograde symbol.
If the appropriate symbols are not visible at the same time, you can still pull or push the velocity vectors. In this example the target and the retrograde symbols are visible, and we want to speed up a little bit. We can still pull the prograde symbol toward the target by aiming between the retrograde and the target and pushing the retrograde symbol to the other side of the ball toward the anti-target symbol.

Tip 2

Curved path

As we are flying on a curved path, the target symbols and the velocity vector symbols (prograde and retrograde) continuously move relative to each other. As we approach the target, the retrograde vector tends to move toward the horizon. “Pushing” it back we can keep the velocity vectors of the ships aligned.

Approach the target

  • Make sure the Nav Ball is in Target mode and burn towards the target at 20 m/s speed
  • We continue this staggered approach to get closer to the target:
    • Catch up with the Target
      • If you are faster than the target in a lower orbit, burn retrograde
      • If you are slower than the target in a higher orbit, burn prograde
    • Kill the speed difference
      • When the target indicator is 90 degrees from the velocity vector, kill the speed difference by slowing down or speeding up to the speed of the target.
    • Move toward the target
      • Turn toward the target and start a burn
      • Cut the engine at 10 m/s speed difference
    • Turn retrograde and prepare to slow down
    • Slow down around 60 – 70 meters from the target

Catch up quickly via transfer orbit

  • Create a maneuver to get into the Transfer Orbit. Use the prograde or retrograde marker to reach the orbit of the Target, and move the maneuver to set the distance between the two orange markers below 5 km. If the pink markers appear, your trajectory will cross the target’s orbit and overshoot it. Reduce the burn duration until the pink markers disappear. You may need to select an orbit in the future to be able to burn at the correct time for a close intersect.
  • Make sure the new orbit stays above the 70 km thick atmosphere, otherwise the spacecraft can even burn up at the periapsis.
  • Set the SAS to Maneuver mode, and stop the burn before the estimated duration. Fine-tune the intersect with short low-throttle burns until the distance reaches the planned amount.

If the orbits are not parallel

If the orbits are just intersecting, we can create a maneuver in the intersecting point and adjust the prograde/retrogarade and radial in/ radial out vectors to bend our orbit to match the target

  • Create a a maneuver in the intersection point
  • If you Apoapsis or Periapsis is too extreme dial back the prograde or retrograde vector, and adjust the radial in / radial out vector to bend the curve
  • Adjust the prograde/retrograde and radial in/radial out vectors to match the trajectory of the target
  • If there is still a vertical separation between the orbits because of the different inclinations, use the normal/anti-normal vectors to bring the Ascending or Descending node close to the encounter.

Slow down

  • Click the speed indicator to switch to Target mode
  • Once in Target mode turn to Retrograde
  • Warp ahead even through multiple orbits until the two spacecraft reach the intersect point at the same time.
  • At 15 km distance from the Target burn until the relative velocity drops below 50 m/s. See Align your vessel’s velocity vector to the target’s movement

Align the docking ports

The easiest way to dock is if both vessels actively turn toward each other. We will set up active tracking on both ships. In this example, we will dock “Ship” to “Station”. When the vessels are close to each other you can use the “[” and “]” keys to switch between them.

  • Switch to Station
  • Right-click the docking port and select Control from Here
  • Select Ship as target
  • Turn on SAS and point Station to Target
  • Switch to Ship
  • Right-click the docking port and select Control from Here
  • Set Station as target
  • Turn on SAS and point Ship to Target

From now on the ports should continuously face each other.

Approach

  • Turn Retrograde to be able to use the engine to slow down if needed.
  • Turn on RCS with “R”
  • In Retrograde press the “H” RCS key to slow down, “N” to speed up until the Target speed drops to zero, or use the engine if more power is needed.
  • Turn Prograde, “H” is forward, and “N” is backward.
  • Use the RCS to approach it with no more then 10 m/s until it is in 100 distance.
  • Right-click the target to see its distance
  • Set the docking port as the target for a precise approach.

Final approach and docking

It is important to learn the basics of docking with a visual approach, to understand the steps necessary for success, but there is an excellent mod that provides a user interface to perfectly align the docking ports even in total darkness. Install the Docking Port Alignment Indicator mod (see Recommended mods for details)

Advantages

  • Adds port renaming functionality during construction and flight, so you can later select the target port by name at the bottom of the instrument.

Usage

  1. In the map view switch to the target vessel
  2. Right-click the receiving port, and select Control from Here, so the SAS of the approaching vessel can properly track it.
  3. Switch back to the approaching vessel, and make sure the target vessel is selected. If not, select it in the map view
  4. Right-click the port on your vessel and select Control from Here
  5. When you are 2.5 km or closer to the target, select the target port on the Docking Port Alignment Indicator UI.
  6. During the final approach, the only thing you need to do is to
    • set your SAS to track the target
    • with your lateral RCS controls (I J K L) keep the green prograde marker on the intersection of the green lines, and control the approach speed with H and N. If you successfully track the green lines, your vessel will slowly line up with the target port. The orange marker at the top of the circle shows the 0° of the port of your vessel. Use Q and E to rotate your craft.
  7. If the two lines are red, you are approaching the target port in the wrong direction. The orange arrow on the UI shows the direction you should move the prograde or retrograde icon for the correct alignment. If the yellow symbol changes from prograde to retrograde, you are getting farther away from the target.

Fine tune the docked module rotation angle

When a module has been docked, you can fine-tune the angle +- 15 degrees using the docking port menu. It looks like the module position is not saved, so when you reload the game all modules turn back to the original angle.

  • Right-click the docking port
  • Move the Alignment Angle slider to fine-tune the rotation of the docked module
  • The Invert Direction button changes the direction of the rotation.

Re-docking

When you just want to re-align a docked module the docking port needs to be reset. When you undock, the magnetic force which attracts the ports is turned off and the port is deactivated to let you leave the vicinity of the port. This prevents quick re-docking the same module. The port is reactivated when the module is moved at least 1.5 meters away, but if you just want to rotate the module sometimes it is not practical.

To quickly re-enable the docking port

  • Undock the module
  • Press F5 to “quick save” the game scenario
  • Hold F9 to reload the last quick save
  • Dock again

Transfer the crew

To transfer the crew between docked vessels

  • Click the hatch, or right-click the module, where the crew member is located and select Transfer Crew
  • Click the Transfer button of the crew member to transfer across the docking port
  • Click the target module to select it. In a few seconds, after a “tv static” video effect
  • the crew member will appear in the new environment

Docking tutorial videos

Teaching the transfer orbit method

Teaching catching up slowly by gradually changing your orbit

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