Understanding Maneuver nodes for non orbit maneuvers

[helgeras]

Hi,

I understand how to use manoeuvre nodes to alter an orbit. But have no idea what is going on when I try to use one to fly to another body such as Minmus.

For instance, when I pull the prograde hotspot, I see a yellow ellipsis expand. That one I understand, as this is the orbit I will get into.

However, if I keep adding more prograde, suddenly the yellow ellipsis is cut off, a white piece of curve is added and then a new green ellipsis.

There is a circle marked Mun encounter, and further out another circle marked Mun escape.

If I move the node so that Mun does not interfere with the node, then yellow ellipsis will expand past the Minmus orbit but then suddenly just be cut off with a circle marked Kerbin escape, and another line, this time violet, shows up.

What does all these lines mean? It certainly cannot be my projected path.

I have tried to search the wiki and see several Youtube tutorials, but they explain the orbit change stuff in detail, then lightly skip over this more complex part.

Can you point me to a tutorial explaining this part of the maneuver node system?

[Captain Sierra]

Every time you get a new line color, you are in the sphere of influence of a new celestial body. The encounter and escape markers mark SoI changes for arrival and departure respectively. Minmus is inclined so how you get to it is you drag out prograde to minmus orbit distance. Unlike the Mun, you have to lead minmus with about a 120* phase angle while the Mun only needs 45* lead angle. Minmus is inclined so set it as target and you will get closest approach markers. You want to get these as on top of each other as you can. Then change the view so you can see if you are above or below and adjust using the normal and antinormal (purple node icons) to change inclination up and down respectively. SOmetimes this inclination change is best done mid-transfer.

Once you do that, you should get a blue line (current orbit) and a yellow line indicating your trajectory relative to Kerbin while in the Minmus SoI. You will also see a third color (green or violet) that indicates either an altered Kerbin orbit or a Kerbin escape into solar orbit. Once you reach the yellow, it will change to a blue line indicating your trajectory relative to Minmus. Burn at periapsis to circularize and complete the orbit around Minmus.

Congratulations, if you follow this correctly, you will have just completed your first Minmus transfer.

If this doesn't help you, go to youtube and find Scott Manley. He has a bunch of tutorials that are great for KSP beginners and non-beginners alike who need help with something.

[Specialist290]

First off, welcome to the forums!

Here's what's happening: Your orbit as projected by the maneuver nodes is leaving the Sphere of Influence of Kerbin (basically the zone where Kerbin's gravity has an influence on your ship) and is entering the Sphere of Influence of the Mun. The point at which that happens is the spot marked "Mun encounter."

Once you're within the Mun's SoI, instead of Kerbin, the Mun becomes the body exerting gravity on your spacecraft,* bending the shape of your orbit slightly. If you were to burn retrograde to cancel out some of your orbital velocity at this point, you would eventually end up with an orbit around the Mun. However, because your spacecraft would be moving so quickly, if you didn't perform any maneuvers and just let the craft coast, you'd eventually leave the Mun's SoI again at the point marked "Kerbin escape" (unless you're on a crash course aimed straight at the Mun -- if there's no "Mun periapsis" marker, that's probably what's going to happen instead). The path after that point is your craft's new orbit around Kerbin, because your craft's original orbit was bent slightly by passing close to the Mun.

In short, you've just plotted either a Munar flyby and return to Kerbin, or a Munar impact.

Likewise, if you accelerate even faster, you'll eventually exceed Kerbin's escape velocity and end up in an orbit around the Sun. If you zoom out on the map further, you should see that that violet curve you mentioned forms an ellipse around the Sun itself.

For a slightly more in-depth explanation of what's going on, and how you can apply it practically, I'd highly recommend reading Vanamonde's "Trip to the Moon" tutorial.

* In the real world, of course, every body is acting on every other body at the same time. KSP uses a simplified model called "patched conic approximation," in which only one body (the Sun, Kerbin, the Mun, or another planet or body) acts on your spacecraft at any given time, basically to allow you to play the game on something other than a Cray supercomputer. The planets and moons themselves are "on rails," following predetermined orbits coded into the system, for the same reason.

Edited August 24, 2013 by Specialist290
Adding a link to Vanamonde's tutorial.