How do you correct a diagonal orbit?

[mulimulix]

I've had it a few times where my orbit will not be exactly horizontal. At one stage, I had an almost vertical orbit of the Mun and managed to somehow get it more horizontal get back to Kerbin, but I don't know what I did.

So, how is it done?

[Vanamonde]

There are complex ways to explain it, but here's a kind of sloppy simple method. Face your ship in the direction it's currently travelling (the prograde marker), rotate 90 degrees in the direction that you want to be orbiting, turn the engine on low and watch how your orbit changes on the map screen. The best times to do it are the two points where your current orbit is passing through the plane of the orbit you want. This will mess with your orbital speed too though, and you'll need to correct for that by slowing down later. It helps if you keep the nose of your ship aimed at the horizon the whole time, because then more of your thrust is changing your direction rather than your altitude. Play with that until you get an intuitive feel for what's going on, and then read some of the fancy and scientific methods that helpful people post here.

[Zephram Kerman]

If I understand correctly, it sounds like you're talking about a plane-change maneuver. There are a bunch of guides, including one I wrote in the KGSS Journal: http://kerbalspaceprogram.com/forum/showthread.php/12752-Writing-KGSS-Journal-Volume-I-Edition-1

Briefly, you can correct eccentricity by burning normal or antinormal. If the change is large, it can use a lot of fuel. So it's better to avoid the problem by

• closely following the correct heading during ascent;

• doing a mid-course correction in the middle of a transfer orbit, when speed is low but the distance is still large;

• in the case of returning to Kerbin, altering the ascent trajectory to compensate for latitude.

I hope that helps!

[TheDarkStar]

If you burn exactly between apoapsis and periapsis for the plane change, you don't change either one. When playing, it's not really possible to do this, but you can try to get close.