Bit short of fuel - Gravity assist required but where?

[Speeding Mullet]

Hi everyone.  This isn't a first for me. I tend to sling mission together with little regard for tolerances, especially when I am headed somewhere for the first time.  So my situation is this:

Orbiting Dres (first time visiting Dres yay!) with about 1067m/s Delta V left in the tanks and a requirement to Kerbin from Dres of around 1.3 km/s.  I'm a little short as they say!  The mission has gone perfectly up to now with a successful landing, and planned canyon skydive being completed, as well as a rove.

I don't really want to get out and push as while possible, I did that the first time I went to Moho and want to try something different.  I'm thinking Gravity assist would be the best way.  I can get to Jool for around 700m/s by the looks of it so what do I do from there?  Do I swing round Tylo (I've heard you can get a free ride home from there under the right circumstances) or has someone got a better solution to get me home within the 1km/s budget?

Answers muchly muchly appreciated - I've never really got into Gravity assists so I'm a little out of my depth!

EDIT: @Zhetaan wow that's some superb information there so thanks for pretty much dropping an education on my sorry mission to Dres!  I will explore this in depth this weekend and report back for you. @PLAD thanks also for your input

 

SM

Edited June 30, 2016 by Speeding Mullet

[Zhetaan]

In his thread LKO to Jool for 1051 m/s, @PLAD goes into a bit of detail on how to do multiple gravity assists.

In this thread, it is further mentioned how to take advantage of the great disparity in orbit size to easily get the resonances that you will need for good encounters.

I'd also suggest going into your settings.cfg file and changing your patched_conics_limit to something high enough that you can plan far enough ahead to see what your assist is doing, but without going so far as to really hurt your computer's performance or introduce glitches.  Six seems to be the de facto standard choice.

To really put this together, I think that Flyby Finder is what you need.  I looked at it myself and found that it appears to be possible to get a Dres-Jool-Kerbin burn starting on day 2095.96 (Year 5, day 391, hour 5.7) for 964 m/s from a starting altitude of 75 km, that then flies by Jool at an altitude of 42770 km (about Vall's orbit) on day 3026 (Y8 D44), and finally encounters Kerbin on day 4146 (y10 D 312). Braking delta vee brings this trip to nearly 3100 m/s, but since you don't have that, slowing down when you get there is your problem.  I hope you have lots of ablator.  Bear in mind that I am no expert on multiple gravity assists--you may well need to figure out something else--but this is what the first-pass search has found.

Here's the detailed information on the burn:

Start Planet: Dres
Orbit Departure Time:
   45251136 seconds UT
   2095.96 days UT
   Y5   D391   H5.7
Start Orbit Inclination: -4.9 degrees
Start Boost from that incl.: 964 m/s
Start Equatorial Z velocity: -110 m/s
Start Equat. Prograde velocity: 960 m/s
Start Boost from Equat. Orbit: 966 m/s
V Infinity Leaving Start Planet: 1201 m/s

1st Encounter Planet: Jool
Time from Start to 1st Encounter: 930 days 0.2 hours
Vinf in: 1706 m/s
Brake to Orbit?: 2962 m/s
1st Encounter Periapsis:
   3026 days UT
   Y8   D44   H0
   42770 km altitude
Vinf out: 1706 m/s

2nd Encounter Planet: Kerbin
Time from 1st to 2nd Encounter: 1120 days 1.1 hours
Vinf in: 3021 m/s
Brake to Orbit?: 2131 m/s
2nd Encounter Periapsis:
   4146.2 days UT
   Y10   D312   H1.1

Total delta V expended: 3096 m/s
Total Travel Time: 2050 days

[PLAD]

Zhetaan did the work here, Dres-Jool-Kerbin is the best route I know of. It sounds like it's going to be dramatically close, I don't know what orbit you are in (altitude and inclination) so I don't know how close to the 964m/s you can go and still get on the right path. And you'll surely need some course corrections.  My first flyby advice is to time the Jool periapsis as close to the time FF recommends as possible.

  You have an intriguing idea- you could get from Dres to Jool for something in the low 700's, but then you wouldn't have enough energy to get thrown from Jool down to Kerbin. But if you flew by one of the big moons just right you could probably get the extra energy you'd need. Flyby Finder can't calculate that for you though.