Using Sphere of Influence for Gain

[eevenson]

Because KSP uses Sphere of Influence (SOI) in its trajectory modeling, there are some unique points in space. For example, you can hover 85,000 km ahead or behind Kerbin's orbit around Kerbol without orbiting Kerbin. You can also orbit Kerbol slightly faster or slightly slower than Kerbin without ever entering Kerbin's SOI.

Might these points provide some useful characteristics for trajectory planning? Characteristics similar to those of Lagrange points?

[strongest_2hu]

Nope.

You can try riding Kerbin's orbit around the sun just in front or behind but it's somewhat difficult to get the speeds and altitudes exactly right, and lots of timewarp across a long savegame will result in the satellite falling out of position without occasional stationkeeping.

You can orbit the sun slightly faster or slower than Kerbin (without ever getting grabbed by it) by being in an orbit which is fully inside or fully outside of Kerbin's orbit +/- its SOI, which is both obvious and not useful.

The best way to use the two-body system we've got for fun and profit is probably to time-warp across the SOI change which might alter an encounter to your advantage.

[eevenson]

If I travel to say, Duna, but stop just short of the SOI boundary, I don't have to spend any energy climbing out of Duna's gravity well. So I might only send my lander into Duna's SOI since it will have only a fraction of my transfer vehicle's mass. Thus, my mission mass is much less than it would be if KSP used an n-body system. Seems useful to me.....

[Cykyrios]

Stopping at the boundary of the SoI does mean you won't spend energy to get out of it, but you will spend more energy to both stop at the boundary, and to leave Duna afterwards, because your burn will be much less effective up there than it would have been close to the planet.

A better way would be to stay into a highly elliptical orbit, with the Pe very close to the atmosphere, and the Ap very close to the SoI boundary (you could manage that with aerobraking and a little delta v). This orbit would be rather efficient at getting out of the SoI, but docking the lander to the station in this orbit would prove difficult.