How to do a gravity Assist Maneuver?

[Gooru]

How to do a gravity Assist Maneuver?

I know the transfer window planers, so i can find a window to jool or eve and then an maneuver from there to elo for example.

The 2nd maneuver of course has to be timed with the arrival at the first planet, but how do i match the other information i get from the TW-planers.

As far as i know i can alter the PE by adjusting my approach and that changes ejection speed and angle, but how do i aim for a specific planet?

[tomf]

There is a stand alone tool called the flyby finder that will help you to plan what is possible with gravity assists. Finding it is going to be a pain on the new forums though.

Otherwise it is a pretty much a matter of trial and error. If you remeber that passing in front of a planet will loose you energy and passing behind it will gain energy.

In general though I would question why you want to do gravity assists. If you are trying to create efficient missions than I don't believe it is worth it. The size and layout of the kerbal system means that the benefits are consideraly less than in the real solar system and the burns you have to do to adjust between encounters normally eat up any gains you might make.

[mhoram]

Have a look here for the mentioned tool: http://forum.kerbalspaceprogram.com/index.php?/topic/73174-win-flyby-finder-v082-ksp104/

[Taki117]

In all honesty, in KSP gravity assists are only marginally beneficial.  Just like in Real Life, you can sacrifice more mission time for less dV usage (and vice versa).  In KSP however, because the distances between planets are relatively close, if you wait for the proper transfer window the dV expenditure is minimal, for a very short mission time (relatively speaking).  For example, the Rosetta mission took 10 years not because it would have taken that long to get to the Comet, but because there was a serious dV restriction on the probe.  Had NASA had an unlimited dV budget (as we do in KSP) the mission time would have been much shorter.

[Geschosskopf]

2 hours ago, Gooru said:

How to do a gravity Assist Maneuver?

......

As far as i know i can alter the PE by adjusting my approach and that changes ejection speed and angle, but how do i aim for a specific planet?

Given that the net result of any gravity assist maneuver is to have the ship leave the planet on the desired trajectory, then technically speaking adjusting the Pe at that planet is all you as a player do, because this is what affects how you leave that planet.  Learning where to put your Pe to get the desired results is the trick.

There are 2 types of gravity assists:  those that speed  your ship up and those that slow it down.  To speed up, your Pe needs to be on outboard side of the planet, so as seen from the ship you'd be going to the right of the planet.  Conversely, if you want to slow down pass the planet on the inboard side (to the left side as your ship sees it).  And you can change your inclination by moving your Pe up or down in the normal direction.

So, how do you use this?  Well, it depends on what you're trying to accomplish, simple or complex.  Simple stuff, usually involving only 1 gravity-assist encounter en route and not requiring high precision, is fairly easy.  The most common use is to capture into the Jool system without having to aerocapture or use a lot of fuel.  Here, you set up a "left-side" encounter with Laythe or Tylo, adjusting your Pe so that you end up in a closed orbit around Jool without having to burn a drop of fuel.  You can also do the same trick with Mun when returning there from other planets, and use Eve to slow down with on the way to Moho to reduce the plane change and capture burns there.

NOTE:  Even with simple stuff, it's a good idea to change your (root KSP folder)\settings.cfg file to make the patched conics in the map view more useful.  The following edits are necessary:

CONIC_PATCH_DRAW_MODE = 3     <--- This shows the part of your path within the SOI of another body so is really helpful for tweaking your Pe from far away.

CONIC_PATCH_LIMIT = X  <---- Where 3 < X <= 8, however much you need.  This defines how many different colored segments of your future path you see into the future.  Just beware that more than going much more than 3 becomes a botheration for just normal flying because, for simple orbits, you have all these redundant paths stacked on top of each other making it very difficult to place a node on the one you want to.  But if you're trying to do a Voyager-style multi-gravity assist, you'll need a lot here.

Which brings us to the complex stuff, like a Voyager-style multi-cushion billairds shot through all the outer planets.  The problem with complex stuff like this is that the planets all have to be perfectly aligned for it to work like Voyager did, going from 1 planet directly to the next, and because the outer planets move very slowly, such alignments are quite rare.. IOW, Voyager launched when it did because that was a once-in-a-lifetime chance.  So you have to know when the desired alignments will happen.  That's what you use those tools for, that others linked above.  They're used to determine when to leave Kerbin so that Planet A will sling you into an encounter with Planet B, then to Planet C.  Then you just have to warp ahead perhaps a few decades of gametime to reach that point.

In the alternative, you don't have to go direct from planet to planet, so you don't need the timing tools.  This makes the flight a series of simple problems.  Use Planet A to sling your into an orbit that crosses that of Planet B, then just wait however many orbits it takes until Planet B and your ship end up in the same place at the same time.  Unless you've just got some horrible synchronous orbit that will never meet the target, you'll eventually get an encounter, which you use to sling yout to Planet C.  Only thing is, you might have spend a few years making several solar orbits before you meet the next planet.

Which brings up the key point in all of interplanetary flight:  fuel = time saved.  If your ship lacks the fuel to make the trip under its own power, so that you NEED to use gravity assists, then you pay for that in elapsed gametime.  You either do this up front by waiting for several planets to line up just right for a direct trip, or you leave on the next window for the 1st planet, then do your waiting en route between the various planets.  Or you can just take enough fuel to go where you want when you want.