Docking In Orbit

Some time ago, I built a space station that I could orbit around Kerbin and use to practice docking.

It’s a simple design, but I’m proud of it. I put a detachable cupola module (with parachutes so it could return its astronaut safely home) on top of a Mk1 Lander-can. Then I attached a fuel tank and an engine. I put a docking port on one side of the Lander-can. On the opposite side, I attached a single solar panel. (The idea was that the solar panel would provide a long-range pointer to the location of the docking port.) I put a light next to the docking port, and ladders down and around the space station.

I launched my space station into orbit, with a lonely astronaut aboard, to sit out there until the day I learned how to dock two ships together.

I built a simple docking ship – just a single-astronaut command pod with a docking port on its nose, some RCS thrusters, some parachutes, a fuel tank, and an engine.

Docking two ships in orbit is difficult – very difficult (very, very difficult). I’d say it’s twice as difficult as a controlled landing. (At least, for me.) I couldn’t even get the ship and the space station within visual range of each other.

Then I discovered MechJeb.

I brought the space station curator home, attached MechJeb to both the station and the docking ship, and then launched the new versions.

But, even with MechJeb, docking seemed impossible. I used the Rendezvous Guidance, but it put my docking ship in a seemingly endless series of chase, maneuver, chase, and maneuver – never getting close to the space station.

(I don’t apologize for using MechJeb. As someone in the forums pointed out, if using an autopilot is cheating, then NASA is cheating all the time. Besides, as I’ve learned from my experiences with flying to Minmus, and with orbital docking, even with MechJeb you still need to know what you’re doing.)

I set the docking experiment aside. I landed a crew of kerbals on Minmus (using half MechJeb and half my skill). I landed a second crew on Mun, at a beautiful spot inside the polar crater. I designed some rovers, along with landing platforms for them – and landed one on Mun and one on Minmus. I put a satellite in orbit around Kerbin, ready for a rendezvous with either Eve or Duna – whenever the alignment is right.

Somewhere in my experiments, I noticed that MechJeb’s Ascent Guidance module has an option to time a launch for a rendezvous with a target. That was my answer – the thing I’d been overlooking. I had been forgetting to launch my docking ship at the correct alignment with the space station. I time-launched my docking ship last night.

Then, once it achieved a stable orbit, I set MechJeb for a 5,000 meter rendezvous with the space station.

At first, it seemed as if it was going to become another endless chase-and-maneuver. But, eventually, things lined up, and the docking ship was orbiting 5,000 meters, or so, from the space station.

I knew that in order for the docking to work, I had to right-click on the ship’s docking port, select “Control from here”, and right-click on the station’s docking port and select “Set as target”.

But it didn’t work.

When I’d right-click on the ship’s docking port, the only option it gave me was “Control from here”. Then I’d switch to the station, and the only option it gave me was also “Control from here”.

(It turned out there was a large piece of space debris orbiting nearby, so when I’d switch from ship to station and back, I’d have to stop at the debris along the way.)

Maybe the ships were too far away for the “Set as target” to work. I had MechJeb move the ship to 500 meters from the station. That didn’t work. It didn’t work at 100 meters, either.

Maybe there are two types of docking ports – like a plug and a jack. I shut down Kerbal Space Program and turned to the Wiki. No, there is only one type of docking port.

I returned to my orbiting ships. But I was stumped.

Suddenly, I had a burst of inspiration. Without switching to the space station, I right-clicked on its docking port. (That light next to the port was a great idea.) There was the “Set as target” option. It’s only there when you’re setting the target from the other ship. That seems obvious – now.

I let MechJeb take over the docking procedure, while I sat back and watched how it was done. It was a beautiful scene, with the sunlit surface of Kerbin 100 kilometers below.

At a little past midnight last night, my docking ship docked with my space station at last. Except… they didn’t dock.

I had built my space station and my docking ship with different sized docking ports.

I shut the game down and I went to bed.

At least now I know that it can be done.

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