Simple Munar Gravity Assist

[Laie]

The common illustrations of a gravity assist show the spacecraft doing a u-turn around the body they are stealing an assist from. Don't try to actually do that, it will get you nowhere. Actually useful assists have a much smaller angle. Because a picture says more than a thousand words:

The above shows two maneuvers leading to Duna, one of them doing a close flyby of the Mun (PE <20km). Note how the trajectories leading out of the Kerbin system are nearly parallel. That is no coincidence.

How to set it up:

1. use a probe or whatever to plot an ordinary maneuver straight for your destination.
2. on the vessel that actually goes, set the probe as target (so you see it's trajectory), then plot a close flyby of the Mun that takes you out of Kerbin on a parallel course.
3. right-click the Munar PE so it remains persistently readable
4. zoom out, select actual destination as target. You should already be pretty close.
5. if you can't see an interplanetary trajectory, place a maneuver on the last stretch you see (probably between Mun and leaving Kerbin). That maneuver doesn't need to do anything, it just has to be there so you see the trajectory.
6. using Precise Node or similar tools, make small adjustments to the departure maneuver in LKO: a few seconds sooner or later, small changes in delta-V. Keep an eye on munar PE and see to it that it remains positive. That's why you clicked it in the first place, this will save you a lot of zooming in and out.
7. If you need a plane change, you can get that for cheap by doing a mid-course maneuver about 1/3rd of the way to the Mun. Put that maneuver down before zooming out.

This is more worthwhile the slower you go. For ballpark figures, you can expect to steal 80m/s or more when going to Eve or Duna, but only about 50m/s when going to Jool.

Edited April 4, 2018 by Laie