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Cheif Operations Director

Gravity Assist

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Hello I want to do a gravity assist from Dres  to Laythe but I a. Don't know how to do a gravity assist in the first place and b. Is it efficent thanks

Edited by Cheif Operations Director

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So you are flying from Dres, or want to use it for gravity assist? Because the latter won't help you at all. Dres is too inclined and too small.

Basically, to do a gravity assist, you aim at another quite heavy body to slow you down or speed up, instead of direct transfer. For Laythe, set your maneuver, let's say, to Tylo, and do some fine-tuning so you won't end up on escape trajectory from Jool, but get a capture instead.Once in Jool orbit, you can do it multiple times so you won't swing past Laythe at 3km/s or so. I've done that few times, all manually.

About the efficiency: yes it is efficient, but, in much bigger scale, more time consuming. Look at long distance RL probes, Cassini, Voyagers etc. They all used gravity assists to get where they should get. More time, less fuel used. 

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2 hours ago, The Aziz said:

 Look at long distance RL probes, Cassini, Voyagers etc. They all used gravity assists to get where they should get. More time, less fuel used. 

The other way to look at this, for these projects, is that they couldn't have gotten where they were going at all, with available launchers, if not for the multi-year, multi-gravity-assist path they took.  Cassini was the size of a camper van, but there were no more Saturn launchers for it -- it had to go up on a Titan IV (as I recall), and there just wasn't enough delta-V available to go direct to Saturn.  Without the gravity assists (from, IIRC, Venus, Earth, and Jupiter), there would have been a choice: make the actual vehicle much, much smaller (probably little if any bigger than the Huygens sub-probe), so the Titan could throw it (and the fuel it would need for Saturn capture) direct, or cancel the mission.

If you can trade off mission time against delta-V, you can do stuff like Clementine did, using a bunch of very weak Moon assists to raise its orbit enough to get to the Moon even after the insertion stage failed (but no, you can't do this in KSP, unless you have Principia installed and a computer that can handle the load -- the SOI and patched conics model doesn't do perturbations and distant gravity effects).

Edited by Zeiss Ikon

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@Cheif Operations Director I recommend this tool to help you with all your flyby problems. If you compare the dV to get on that flyby trajectory with the dV to go directly to Laythe (which I am almost certain you can do with this mod), you'll have your answer.

 

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This question about how to play the game has been moved to Gameplay Questions. 

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22 hours ago, Cheif Operations Director said:

Is it efficent thanks

Honestly, I don't think the little extra kick you might from Dres is worth the large amount of time and effort this is going to take to plan and execute (and, if you care, the enormous amount of in-game time it will take).   Since Dres and Jool both have very long orbital periods, it could be a long, long time before you get the alignment you want.

However, it's absolutely worth it to a reverse-gravity assist around one of Jool's moons to capture into Jool orbit.  Decelaration assists are a lot easier to set up since you don't have to worry about phase angles or getting a very precise path after the assist.  For a Jool deceration, Tylo offers the most raw power, since it's the biggest and does not have an atmosphere (meaning you can get very close to it).  Laythe is slightly less effective, but if Laythe is also your final destination,  it might be easier to use Laythe for the gravity assist.  

As far as how to do it: there are some good guides in previous threads on these forms, and in Scott Manley's YouTube videos.  But one piece of advice: don't get too bogged down in the theory.  Set up an encounter with the body you want to get and assist from, and then just play with the maneuver node to see how different paths affect your post-assist orbit.  Tweak iteratively until you find something you like. 

One trick i like to use when fuel is really tight is to set up your first gravity assist so that your apoapsis around Jool is way out past the moons, near the edge of Jool's SOI.  When you're that far out, for just a few m/s of maneuvers you can set up a second gravity assist that sets you up for a good final encounter with the target body.  

Edited by Aegolius13

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18 minutes ago, Aegolius13 said:

Honestly, I don't think the little extra kick you might from Dres is worth the large amount of time and effort this is going to take to plan and execute (and, if you care, the enormous amount of in-game time it will take).   Since Dres and Jool both have very long orbital periods, it could be a long, long time before you get the alignment you want. .

However, it's absolutely worth it to a reverse-gravity assist around one of Jool's moons to capture into Jool orbit.  Decelaration assistsare a lot easier to set up since you don't have to worry about phase angles or getting a very precise path after the assist.  For a Jool deceration, Tylo offers the most raw power, since it's the biggest and does not have an atmosphere (meaning you can get very close to it).  Laythe is slightly less effective, but if Laythe is also your final destination,  it might be easier to use Laythe for the gravity assist.  

As far as how to do it: there are some good guides in previous threads on these forms, and in Scott Manley's YouTube videos.  But one piece of advice: don't get too bogged down in the theory.  Set up an encounter with the body you want to get and assist from, and then just play with the maneuver node to see how different paths affect your post-assist orbit.  Tweak iteratively until you find something you like. 

One trick i like to use when fuel is really tight is to set up your first gravity assist so that your apoapsis around Jool is way out past the moons, near the edge of Jool's SOI.  When you're that far out, for just a few m/s of maneuvers you can set up a second gravity assist that sets you up for a good final encounter with the target body.  

Thanks for the help

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While it's probably not worthwhile to set up a Dres -> Jool slingshot, there are two common slingshots that are used to get there.

1) As Aegolius described, use Tylo to brake yourself into Jool orbit. While your initial orbit afterwards will be highly elliptical, you can then set up further gravity assists to brake yourself down into a more circular orbit. If you want to practice this first, you might try using Ike to gravity brake into Duna orbit.

2) Inner-planet slingshots. Using a KEKKJ (Kerbin Eve Kerbin Kerbin Jool) window, you can get out to Jool for around 1100 m/sec, little more than the initial transfer to Eve. The simplest path that gives significant dV savings would be the KKJ path, costing around 1550 m/sec.

To execute a KKJ path: your initial ejection from Kerbin should have an orbital period of 2 Kerbin years, resulting in an apoapsis around 2.17x Kerbin's orbital radius. This means that, when your probe returns to periapsis 2 Kerbin years after the initial ejection, Kerbin is exactly where it needs to be to get a slingshot. It's not quite as efficient as something like a KEKKJ path, but has the advantage of being much simpler to set up.

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On 02/12/2017 at 4:38 AM, Cheif Operations Director said:

Hello I want to do a gravity assist from Dres  to Laythe but I a. Don't know how to do a gravity assist in the first place and b. Is it efficent thanks

I found this site a great help in getting my head around the whole idea:

http://www.planetary.org/blogs/guest-blogs/2013/20130926-gravity-assist.html

Especially the various GIFs:

20130925_Fig1-assist-example.gif

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On 12/2/2017 at 6:07 PM, Aegolius13 said:

However, it's absolutely worth it to a reverse-gravity assist around one of Jool's moons to capture into Jool orbit.

+1 to this!  You can save scads of dV on Jool arrival if you use Tylo or Laythe to help slow you down.

On 12/2/2017 at 6:07 PM, Aegolius13 said:

For a Jool deceration, Tylo offers the most raw power, since it's the biggest and does not have an atmosphere (meaning you can get very close to it).  Laythe is slightly less effective

In my experience, they both work pretty well.  Tylo is great for raw gravity power (since you can get really really close to it).  On the other hand, I find Laythe really effective precisely because it has an atmosphere-- because I can actually do aerobraking there.

A Laythe aerobrake is nice because I don't actually have to capture to Laythe.  It's basically just a flyby where I skim through enough of Laythe's atmosphere to slow me down relative to Jool.  I never slow below Laythe's escape velocity (unless Laythe itself is my destination.)

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I found two Dres-Jool transfers, but they're terrible; they require 5.5 km/s of dV, take about ten years to complete, and require flyby altitudes of three and four metres in order to get enough of an assist to make it happen.  It's possible that better ones exist, but I don't see the point in looking; the problem is that Dres is, as others have said, too small.  If Dres only has a few m/s to give to you, then you have to arrive at Dres with the rest, but when you add the timing and conjunction to the problem, the end result is that you spend years on an orbit that reaches out to almost Jool while waiting for an absolutely perfect encounter with Dres.

Contrariwise, if you start at Dres, there are a couple of good assists via Jool that get you home.  Somewhere in this forum is a post from a person who ended up at Dres with a fuel miscalculation; while the Rocket couldn't get to Kerbin, it could get to Jool, and that was enough.  Jool gravity and Kerbin atmosphere took care of the rest.

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