Is this the correct exit burn to get on track to a lower Sun orbit?

[Dave Kerbin]

I've drawn up a diagram of a ship in circular Kerbin orbit. I want to escape Kerbin's gravity and set myself on a course to a lower orbit of the Sun (like Eve and Moho). Technically I've already plotted a trip to Eve, but that was using maneuver nodes, I need to do this one with just the navball.

The way I see it I should make my prograde burn when the ship is at A so that it escapes Kerbin's orbit at C. Not because C is closer to the Sun, but because if I exit at C while going 'right', my orbital velocity around the sun will be less then the body I just left (Kerbin). If I escape at A, going 'left', I think I'd be increasing my orbital velocity from that of Kerbin.

First, am I correct in the assumption that I should be exiting to the 'right' if I want to reduce my orbital velocity around the Sun?

And should I burn at A to do that, or does the change in angle when I burn hard to break the orbital 'circle' mean that I actually need to burn at B, since after a burn at B the opposite end of the orbit (which is now an escape node) will be pointing straight to the 'right' instead of curving 'up' as it was previously?

[Sirrobert]

Escape at point D.

Scott Manley explains why here:

(that's for Minmus, but it works on all orbits I would guess)

[Vlk]

The easy answer is that just play around with the nav node, and find the spot where you need the lowest deltav to get to a close sun orbit.

But i try to explain the hard one, which is, if im correct, that you have to start the burn around B:

If there is no Kerbin, you have to burn retrograde, so against your current veloicty around the sun. (Kerbin's velocity)

If you want the same while orbiting kerbin, you have to plan an escape route that goes out against kerbin's velocity. To do that, you have to start burning prograde At around B.

Where i think you are mistaken: if you burn at A, your velocity at C will be pointing to the right, if you still stay just in orbit, thats okay. But if its a high orbit, it would be a very-very little magnitude vector. But you will not complete the orbit, you will escape. And it is much better if the escape is fast, and if the direction of this escape is a sun-retrograde direction.

[Evrion]

Yes, escape at C if you want to go in, A if you want to go out.

You will have to play with maneuver nodes to get the right place to do the burn so that your exit path matches the desired escape direction. If I were going to Eve, I would probably burn prograde about half-way between B and C.

[blizzy78]

First of all, if you want to go outwards, you will want to go in the prograde direction of Kerbin to profit from its orbital velocity. For going in, you will want to go in the retrograde direction.

Now how to figure out where that is? It's rather easy. Consider the following picture. This one is actually plotting a course to get from the Mun back to Kerbin, but the same principle applies.

Notice how the tangents of both the projected trajectory and the Mun's orbit are exactly parallel. That's the way to do it. At the exact point where you leave the sphere of influence, imagine a line going straight out. That line must be parallel with the tangent at the body's orbit at that point in time (ie. when you're exiting the SoI.) By doing so, you're going exactly into the body's orbit's retrograde direction.

Like I said, in the picture that's a course plotted for a Munar escape headed for Kerbin, which goes into the retrograde direction of the Mun. This is also what you will need to get closer to the Sun. To actually go prograde to go further out, just flip the maneuver 180 degrees around the orbit.

[Kasuha]

You need to start your burn between points B and C so that you exit perfectly retrograde to Sun somewhere between points C and D. You need that point to be your apoapsis of the Sun orbit. And where exactly you need to start the burn depends on your planned periapsis. The lower the periapsis, the earlier you need to start the burn.

Edited October 22, 2013 by Kasuha

[Kasuha]

Okay I was wrong. In fact the lower the periapsis the later the point where you place the maneuver. Where you burn depends on how long the burn is. Here are examples for setting trajectory with periapsis around Eve and around Moho.

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[The_Rocketeer]

This is real simple.

Just like orbiting Kerbin, when orbiting Kerbol faster things go higher, slower things lower.

If you accelerate prograde in the same direction that Kerbin is moving (at A), you will head out towards Duna and Jool. If you accelerate prograde in the opposite direction i.e. decellerate in Kerbolar orbit (at C) you will begin to fall in towards Eve and Moho.