Precision orbits.

[Vozlov]

Hi I'm having trouble with putting my space station into orbit properly.

I can't seem to get a stable orbit at a designated height. Every time I get it there it is always fluctuating on one side of the other, going down a meter a second or rising instead. Is there a better way to level these out that I've not figured yet, or do I simply need to be more precise with my trajectories?

[Specialist290]

Unfortunately, there's a limit to the degree of precision you can get in the game (without resorting to things like editing orbits in the save file) simply because of how the physics engine handles the calculations using floating-point decimal values. The closer you try to get to a perfect circular orbit, the more noticeable this gets (as you can see the Ap / Pe markers actually jerking around as the game keeps recalculating the orbital parameters).

The good news is, there's rarely any time when you really need that much precision. The really important things to pay attention to, if you want something that's geosynchronous or something like that, are the semi-major axis and the orbital period. A slightly elliptical orbit will still roughly maintain its position relative to the surface if its period correctly matches the planet's rotation period, with minimal need for adjustment.

[Vozlov]

Ok. I just don't want to go through all the effort of finishing my space station to find it'll be catching the atmosphere in a short time and breaking up. I'm guessing It'll be a while though if I get it pretty damn close to perfect. Or perhaps I would be best to get it so it's gaining altitude slowly?

[Claw]

Ok. I just don't want to go through all the effort of finishing my space station to find it'll be catching the atmosphere in a short time and breaking up. I'm guessing It'll be a while though if I get it pretty damn close to perfect. Or perhaps I would be best to get it so it's gaining altitude slowly?

The altitude may slowly go up or down even if your orbit is close to perfect. However, if you never dip into the atmosphere the orbit will not decay. In other words, if your craft's maximum altitude is say 100,500m and the lowest is 99,500m around Kerbin, it will stay that way unless you mess with it. The altitude will slowly drift between those two altitudes as the craft orbits.

Press M in-game and take a look at the map. It will show you the highest (apoapsis) and lowest points (periapsis) of your orbit. As long as they are > 70km around Kerbin, your craft won't come crashing back down..

[Specialist290]

As Claw said, the parameters aren't that unstable (thankfully), unless you're already dangerously close to the edge of the atmosphere anyway -- and once a craft is unloaded and put "on rails," the physics engine (including atmoshperic drag) doesn't affect it; it's basically a data point in a fixed orbit, though it'll get automatically deleted if it goes below a certain altitude. In other words, in theory you could have a station safely in orbit with a Pe just above 23km or so and it'll stay there forever as long as you never load it while it's in atmo, but this isn't really representative of how a space station in orbit would actually work, obviously Just a consequence of the way KSP is designed.