Does there any mod that help me with planing gravity assists?

[Pawelk198604]

or does i can do this with mechjeb?

How i can use Duna to reach Jool?

[ansaman]

You can do this with Mechjeb by using the advanced transfer function and assuring a close right (counter-clockwise) flyby with the fine tune closest approach function. This will cause your craft to gain delta-v, but make sure you stay above the Duna atmosphere.  Of course, this does not guarantee a Jool encounter unless the relative positions of the planets are in exact alignment and I suspect that would only occur rarely; therefore, you would have to do a separate maneuver which may also take some time to construct and execute.

Calculating an exact launch window for the most efficient transfer and executing it is likely a lengthy process.  

[Guest]

Check out KSP TOT if you're on Windows, it can plan multiple slingshot maneuvers.

[FungusForge]

There's SlingShotter, which helps visualize the locations of planets at a selected point in time.

[ptr421]

Personally I'm using Flyby Finder with this plugin for MechJeb to figure out when/where to launch. Works both for stock and RSS. 

With Jool you might be better off launching to Eve and then going Eve-Kerbin-Kerbin. That one is a bit tricky with Flyby Finder, might be easier in TOT (not sure, never used it). 

[Vanamonde]

Modding question moved to the modding subforum. 

[Spricigo]

14 hours ago, Pawelk198604 said:

How i can use Duna to reach Jool?

Worth noticing: Eve can provide a stronger push.

[PLAD]

TOT and Flyby Finder, both mentioned above, are the two tools I know of for planning flybys in KSP. I've done Kerbin-Duna-Jool and as mentioned above Duna does not give you much of a push, it reduces the roughly 2000 m/s a direct flight to Jool would need to roughly 1900 m/s. On the other hand it is easier and much faster to execute than the more powerful Kerbin-Eve-Kerbin-Jool route so you might want to try K-D-J first.