So... I had a thought in the shower last night but I won't get chance to test it until tomorrow. If you launched precisely 2 years ahead of your transfer window to Jool/Eeloo/modded-outer-planet, and put yourself on a solar orbit with exactly 2 years period, you should re-encounter Kerbin again at the time you should be launching. Moreover, you will be tangential to Kerbin's orbit, just like you would be when launching out of LKO. Does this actually produce a gravity slingshot, or do y