Setting up a proper polar orbit.

[rythin]

Hi.

I have difficulties setting a polar orbit around the planet for my contract. I have already launched a ship with a 90 deg. inclination, so I'm close to the target plane. How do I set it correctly? The target orbit says Ascend node 178.8 deg. Is there a MechJeb feature I can use to make the ship fall into the target orbit plane perfectly? I don't mean apoapsis or periapsis - I can set those well enough by myself. Just the plane.

r.

Edited January 11, 2015 by rythin

[vexx32]

I found the best way to do it is to fiddle with the camera in Map view. On the 'target' orbit you should see two points marked AN and DN (Ascending and Descending Nodes) -- these are essentially the two points where your orbit and the target orbit intersect (ignoring altitude). What you'll want to do is orient the camera so that the target orbit looks like a flat line, and the AN and DN markers are pointing to more or less the same point on the screen. Then... it depends on whether or not you have manoeuvre nodes unlocked.

If you do have manoeuvre nodes unlocked, click that point and create a manoeuvre. Don't move the camera. Fiddle with the Normal/Anti-normal handles (pink triangles) until your predicted orbit matches that of the target orbit. You don't need to be super precise, but it's not overly difficult to manage a 0.0º or ±0.1º match in the orbits. Don't worry if the AN or DN appear to move around the orbit once you get close to matching them; this is normal, and a result of having quite a close match to your target orbit. Then perform the manoeuvre once your craft reaches the node.

If you don't have manoeuvre nodes unlocked yet, it's more tricky. First you need to figure out whether burning normal (pink triangle) or anti-normal (pink triangle with lines poking out of the middle of its sides) is what you need to do. Generally speaking, if you want to flatten your orbit out a bit, you'll want to burn anti-normal when you're at the AN, but normal when you're at the DN. You'll need to orient your craft towards these markers on the nav-ball as appropriate. Using the camera trick outlines before, you want to wait until your craft is just a little bit before either the AN or DN, depending on which one you've planned to burn at. Fire thrusters, watch the orbit change, but just take a little bit of care, especially if you're doing a plane change of more than 10º or so -- the normal/anti-normal markers on the navball will move as your orbit changes, so make sure to keep up with them as best you can to avoid wasting fuel and mucking up your Ap/Pe too greatly.

[Starhawk]

Are you going in the correct direction in the orbit? If your ascending node is almost 180, I think you're going the wrong way.

On my first satellite launch contract ever, I couldn't figure out why it wouldn't complete. I thought whoever set up this contract must be crazy if they wanted me to get closer than I was. I read a few forum posts and realized I hadn't looked at the inclination value. I was perfectly in the orbit going exactly the wrong way!

There is also a visual indication, the little beads of light that circle around the orbit, but I missed that at first.

[rythin]

vexx32,

That was great advice, thank you! Yeah, I didn't know what AN and DN are exactly, so it was difficult for me to navigate. When I followed your advice and aligned them, it was easy to match a target plane.

Starhawk,

No, I was going the right way. But you have a good point, I did miss a correct direction on another occasion. Thanks for your help!

[LethalDose]

A bit of advice for launching directly into a polar orbit: set your velocity reading to "orbit" mode from "surface" mode straight off the pad. That way, when you can get your prograde straight onto 0 degrees (or 180 degree) heading for a near perfect polar orbit.