Is there any way to plan gravity assists?

[jasperhb]

Hello, KSP forums.

I'm launching my first Joolian probe, but I don't think I can get a reasonable amount of delta-V without going overboard. So of course, I chose using a gravity assist to propel my probe to Jool. However, I probably won't be able to 'wing' the gravity assists without the transit time becoming impractically huge. Is there a program that can plot out gravity assist maneuvers and perhaps plug them into MechJeb? (I don't trust myself to execute the maneuver properly)

[Wadusher0]

Might not have everything you're looking for, but it'll definitely come in handy.

[Aegolius13]

I have not used, but heard good things about this:

Side note: I really like gravity assists, and they can be fun just to play around with, but getting one TO Jool you may find yourself underwhelmed.  For example, I did a Kerbin-Kerbin-Jool assist on one mission, and it only saved me like 300-400 m/s, in exchange for 2 years of extra mission time. 

Gravity assists within the Jool system for braking, on the other hand, can be a lot more useful.  And they add much less to the mission time.  

[Red Iron Crown]

On 4/8/2017 at 0:22 AM, Aegolius13 said:

I have not used, but heard good things about this:

Side note: I really like gravity assists, and they can be fun just to play around with, but getting one TO Jool you may find yourself underwhelmed.  For example, I did a Kerbin-Kerbin-Jool assist on one mission, and it only saved me like 300-400 m/s, in exchange for 2 years of extra mission time. 

Gravity assists within the Jool system for braking, on the other hand, can be a lot more useful.  And they add much less to the mission time.  

A Kerbin-Eve-Kerbin-Kerbin-Jool path can bring the ejection burn down to about 1000m/s, about half a conventional Jool ejection.

[Gaarst]

On 08/04/2017 at 6:22 AM, Aegolius13 said:

Side note: I really like gravity assists, and they can be fun just to play around with, but getting one TO Jool you may find yourself underwhelmed.  For example, I did a Kerbin-Kerbin-Jool assist on one mission, and it only saved me like 300-400 m/s, in exchange for 2 years of extra mission time. 

Gravity assists within the Jool system for braking, on the other hand, can be a lot more useful.  And they add much less to the mission time.  

DV requirements for transfers being absurdly low in stock KSP I have never felt like it was necessary. The few m/s of dV you can gain are often compensated by the increased mission time, and you can obtain the same result by adding a fuel tank to your ship anyway.

On RSS on the other side, gravity assists are a real treat. Because you need so much dV to go anywhere and because transfers times are stupidly high (50 years for Hohmann to Pluto, no thanks), you find that gravity assists can save both dV and time for some missions. I'm still fiddling around (still have trouble planning 2 or more assists for a mission) but it's extremely rewarding.

It's unfortunate that assists in the stock system are often just a cool trick. The Jool system is an exception: I love swinging around the moons there.

12 minutes ago, Red Iron Crown said:

A Kerbin-Eve-Kerbin-Kerbin-Jool path can bring the ejection burn down to about 1000m/s, about half a conventional Jool ejection.

Any tricks for complex gravity assists like this one? I'm already using the planner and have some experience with assists but I never succeeded in pulling dead accurate assists requiring several bodies (for rendez-vous with an asteroid/comet for example).

Edited April 9, 2017 by Gaarst

[Red Iron Crown]

2 hours ago, Gaarst said:

Any tricks for complex gravity assists like this one? I'm already using the planner and have some experience with assists but I never succeeded in pulling dead accurate assists requiring several bodies (for rendez-vous with an asteroid/comet for example).

In all honesty I could never make such things work, either. Perhaps the gravity assist maestro @PLAD can shed some light.

[PLAD]

My Flyby Finder can find some flyby paths to get you from Kerbin to Jool for less than the standard Hohmann requirement of about 2000 m/s (from a 75x75-km orbit). Going Kerbin-Duna-Jool can be done for about 1900 m/s, not much savings there because Duna is so small and can't throw your ship around much. Going Kerbin-Eve-Kerbin-Jool can be done for about 1400-1500m/s, here is an example of me doing that that goes into a lot of detail in how to set it up and execute it. You will probably need a different window than the one I used, but if you study the values and search periods and max v at SOI in the first screenie you can search forward to the time you want. As an aside if you have never used FF before you should really study the primer on using it because there is a bit of complication to planning flybys.

You can also go Kerbin-Eve-Kerbin-Kerbin-Jool for about 1100m/s, I find this significant for big ships that have low acceleration even in the small stock system. Unfortunately FF does not plan paths that have a double flyby of a planet like this, I have to use the Lambert spreadsheet linked in the FF OP to find these. Here is an example of going K-E-K-K-J for 1011m/s, though a flyby of Mun is also used to save about 80m/s. Using Mun is very tricky, you have to time things to a couple of minutes. I don't recommend starting with this.

   I find that the timing of the flybys is the most important thing in setting these up, in other words if FF says your flyby periapsis of  Eve should be on Year 1 Day 180, 4:32, then you should try to make the flyby be within a few hours of that time even if your flyby periapsis or departure dV will be off.

    There are several tools out there to make this easier. okder's Mechjeb addon is spectacular for launching into the right departure orbit inclination and LAN and it works with the latest version of Mechjeb. Slingshotter, which has just been revived by linuxgurugamer, is great for making sure you will encounter a planet at the right time- tell it to show you where the flyby planet and your ship will be at the required flyby time and it is easy to adjust a pre-flyby maneuver node to make the two meet at the right time.

    If you want more assistance then Arrowstar's TOT might be more what you want. It is big and complex but will decide what flyby to use and can put nodes directly into KSP while it is running. TOT can't do double flybys either though.

   If you are using the Outer Planets mod by CaptRobau or the Galileo Planet pack then flybys are even more useful for getting to those outer planets, they can be both cheaper and faster than Hohmann, and FF can handle those two systems.

[Gaarst]

45 minutes ago, PLAD said:

(snip)

And I used to be proud of my Earth-Jupiter-Saturn trajectories... I feel really stoopid now