Gravitational assist

[seaces]

Hi,

Could anybody point me in the art of gravitational assist? I have tried to find some nice tutorials on YT but could not find some tutorial that shows actually how to execute that in KSP all of the tutorials that I have found are usually about theory but they do not demonstrate how to do it actually in KSP. What I need to learn is how to plan in some mission to use gravitational assist? I guess there is some launch window? I cannot just go there whenever I want and hope that I will get where I want using assist? Which planet to use or how do I decided which to use to get to my destination? And finally practically how you do that in KSP? I guess by the help of maneuver nodes somehow? I play in unmoded KSP.

[MatterBeam]

Hi,

Could anybody point me in the art of gravitational assist? I have tried to find some nice tutorials on YT but could not find some tutorial that shows actually how to execute that in KSP all of the tutorials that I have found are usually about theory but they do not demonstrate how to do it actually in KSP. What I need to learn is how to plan in some mission to use gravitational assist? I guess there is some launch window? I cannot just go there whenever I want and hope that I will get where I want using assist? Which planet to use or how do I decided which to use to get to my destination? And finally practically how you do that in KSP? I guess by the help of maneuver nodes somehow? I play in unmoded KSP.

In the majority of cases, the correction burns and suboptimal windows make gravitational assists go from useless to a waste of delta V.

In the stock game, you don't have tools to plan a decent Hohmann transfer between two planets, much less the sligshot around a third planet in between.

the science behind it is that, starting from Kerbin, heading to Jool, passing through the sphere of influence of a planet with a faster, smaller orbit such as Duna adds the velocity of that planet to your spaceship's velocity. Thos can boost you on your way to Jool.

[seaces]

So if I have understood correctly if I want to hop from planet to planet I should use Hohmann and leave alone the gravitational assist?

[Blu_C]

For the most part, yes. You can actually plan out gravitational assists if you know a few tricks, but it is much slower then going by a standard transfer. In most cases it is not worth the effort.

The problem is that when you get a gravity assist you need to be VERY careful about the angle it is going to eject you on. If you aren't then ultimately you end up burning more deltaV correcting than you got by using the assist. For most everyone I suggest playing with standard transfers until you are really good at doing them (basically when you have a feel for what is a good ejection angle and start to learn some of the tricks involving maneuver nodes.

If you want to see, in action, how a gravity assist can easily jack up your transfer wait until the Mun and Minimus are more or less in phase with eachother (roughly the same direction realitive to kerbin) then try to get a minimus transfer. You will find really quickly that although it is easy to get a munar encounter that boosts your orbit up higher it is significantly more difficult to actually make that USEFUL. More often than not the Munar encounter ends up making it so you have to burn more fuel to get the minimus encounter than if you just went there direct unless you get lucky.

[Supernovy]

It's possible, I've done it before, and it does save delta-v.

Simplest gravity assist is a Kerbin assist. Eject out of Kerbin's SoI in the Kerbin Radial or Kerbin Antiradial direction. In half a sun orbit, you should re-encounter Kerbin. Do small burns as far out as possible to get the new Kerbin periapsis as low as possible. By going normal or antinormal you can change the inclination change the Kerbin assist will give you, and going radial/antiradial changes the post-assist periapsis and apoapsis.

The bits after this take a long time to do. Depending on whether your Kerbin assist put you in an Eve-crossing or Duna-crossing orbit, you can use those planets to assist again. Target the relevant planet, then put down a 0 m/s manoeuvre node. Use the "+1 orbit" button on the node to see closest approaches for later orbits. Once you get a fairly close approach, you can use that manoeuvre node (or one on your current orbit, though you'll have to constantly update the 0 m/s node to keep seeing the changes) to tweak your close approach to whatever you need it to be (for landing, orbit, or making another assist). Eve is very good for assists as it travels fast with respect to the sun, is high gravity, and is inclined. You can shave a lot of braking and inclination change delta-v from a Moho mission by going there via Eve.

Basically, gravity assists let you change orbits via patience instead of propellent.

[seaces]

Thank you on your inputs guys. Could somebody paste here bunch of good links with resources that could be helpful with this? To the worst part is that I was not able to find this done on YT step by step. I have watched one from Scott but he is using mod and the clip is going really fast and jumping around so it is really hard to follow it and on top of that on the map view is really cluttered. So the more resources you paste the better so that ke and also others can get more familiar with it.

[Boonestorm]

Try the wiki:

http://wiki.kerbalspaceprogram.com/wiki/Tutorial:_Gravity_Assist

Cheers,

Boone.

[quantumpion]

Check out allmhuran's YouTube channel, particularly the gravity assist, grand tour, and "over 9000" videos.

[Gaarst]

You can do very well without gravity assists in KSP but if they are well done they can spare up to 2000 m/s delta-v for interplanetary trips.

Best and easiest ones are:

- using Mun to exit / reenter Kerbin system;

- using Eve to get to Moho;

- using Ike to exit / enter Duna system (though Ike often interferes with you at the wrong time);

- using Jool to dive towards Kerbol;

- or for doing anything within Jool system, Laythe, Tylo and Vall will happily help you crash into / reach your destination.

[seaces]

Good to know that Gaarst.

[Deutherius]

Look up some missions done by PLAD to see just how much you can save if you really know how to exploit gravity slings. He even made a tool that lets you calculate gravity assists.

LKO -> Jool for 1051 m/s and 1011 m/s

Kerbal-X to Duna surface and back

I am not really all that good at gravity assists, but following his example I was able to get from LKO to Jool for ~1250 m/s, which was one of the most satisfying experiences I ever had in KSP.