Looking for advice on using gravity assisted capture

[DBowman]

I'm trying to minimize the dV required for a return from Eeloo - not sure if this query should be somewhere else, but the principal is real world orbital science.

I can easily setup a cheap Eeloo - Jool - Kerbin Pe trajectory. Then I'd been using Kerbin to gravity assist drop the Ap & Pe, and tuning it to give a resonant orbit with another Kerbin encounter in the future. The braking encounter also cranks the orbit outward (radial out dV?), with the Pe drop this makes the next encounter angle a little more radial out. I could drop my Kerbin capture burn from 2200 to 1600 m/s - but it feels like 'I'm doing it wrong' / 'I want more'.

Just from the luck of my Jool-Kerbin's orbit with Pe below Kerbin Pe my first Kerbin encounter was post Pe and so already not pre-retro aligned, and I just went with it. Seems like I could:

• take care to make Jool-Kerbin hit Kerbin Pe (and rely on timing to make this a Kerbin encounter / tune one from the Pe)
• gravity assist brake into a resonant orbit with lower Ap and lower Pe and radial out cranked
• do some deep space maneuver to raise Pe back to Kerbin Pe and thus encounter Kerbin earlier in it's orbit where the radial twist will cancel out (might have to setup the braking encounter and dsm nodes together (but GUI might not show me two encounters with Kerbin at once? mode / focus?)) 
• repeat

Is that plan misguided?

Is it practical to carry out?

Anyone have any idea what magnitude of final Kerbin capture burn I could achieve?

Maybe there is a bit thread somewhere talking about this? (I searched around a bit and couldn't find anything that goes into it deep enough)

Thanks in advance for any advice, ideas, or information you have.

[K^2]

Look up the profile for Rosetta Mission and do that in reverse. Their first boost was a self-assist from Earth using only a slightly eccentric orbit. That gives you very little delta-E, but significant delta-L, which gives you an orbit with Pe bellow Earth and Ap above, giving you opportunity for boost with Venus or Mars. I don't recall which one the mission actually went with, but both are viable.

Running this in reverse in KSP, you could use a gravity assist from Duna to bring you to a highly elliptical orbit that crosses Kerbin's path. You are looking for something with semi-major axis roughly that of Kerbin's own orbit. Use assist at Kerbin to bring you to a nearly circular orbit that intersects with Kerbin again almost exactly a year later. You can bring your dV down to a few hundred m/s.

Planning this is not easy, though. I don't know if you can do that with tools that KSP provides you. I'd put simulation of the Kerbin system into something a bit more sophisticated, and crunch out the numbers in advance.

Edited July 24, 2016 by K^2

[DBowman]

I could reduce a typical alex moon Eeloo-Kerbin from 3500 m/s to about 1370 m/s - so you can save a lot going 'the long way round'. 

I tried my plan in the OP. The initial Jool encounter (spent 440 m/s @PLAD fly-by-finder says should be able to do for 260) could get a capture for around 2200 m/s. I could get 2 Kerbin passes for 250m/s resulting in a 1380 m/s capture - I din't take that one. Then I spent 340 to set up a 340 m/s capture - I didn't take that one. Spent 320 to get the next with Ap at Kerbin and Pe below, 200 m/s to tweak up an encounter with 158 m/s capture. Something is a little off in my note taking because by mechjeb I spent 1000 m/s from Jool encounter to Kerbin highly eccentric, but if I add up the numbers I wrote down it's 1200-ish - maybe I double counted something. Also if I'd taken the 340 capture it would have cost 930 m/s all up

I made a cheat-sheet of resonant orbit periods and then eyeballed the +/- Kerbin days due to the radial cranking by the encounter. I had to tune/tweak after the Kerbin encounter cause I couldn't get it to show two encounters on Kerbin. Also it's easy to let the encounters add +/- normal without you noticing... It would be nice (for this task) if the Ap/Pe rollover showed you the period also.