# How calculate ideal gravity assist

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Calculating gravity assist is rather hard for someone who is lousy in math and aren't Scott Manley

My biggest achievement it's probe to Jool, i want learn how create perfect gravity assist. I want send me Kerbals to Minmus using Mun as gravity assist, looking for advice.

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I too have been wondering how to do this. I've been wanting to send a probe on a polar mission to the sun and wanted to use Jool to help me change my inclination. I hopefully await the collective wisdom of the forums to answer the question.

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You want to set up a rendezvous just like you would if you were visiting the planet, except instead of closing your orbit you keep on going. The idea is to get as much inflection as possible.

On a perfect up-transfer (going to a higher orbit) you would set up a situation where you were directly prograde of the body, heading towards it. After the assist, you would then be flung 180 degrees backwards and thus picked up twice the body's orbital velocity.

If you are going to a lower orbit (such as a visit to moho by way of jool) you'd do this in reverse, setting up an inflection so that you leave jool out of its retrograde end (ha ha).

Practice on the Mun as it is very easy to get to and you don't need to wait for any tedious launch windows. Hope my 2 cents helped!

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Using Mun as a gravity assist to Minmus is indeed possible. I did do that on one mission saving fuel and matching orbital plane in the process. The two have to be properly aligned for the process to work. No math, just used the Add Maneuver control and noticed that Mun influenced the final intercept to Minmus.

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Calculating gravity assist is rather hard for someone who is lousy in math and aren't Scott Manley

My biggest achievement it's probe to Jool, i want learn how create perfect gravity assist. I want send me Kerbals to Minmus using Mun as gravity assist, looking for advice.

Theres more knowledgable people than just Scott Manley out there, you know.

anyway, heres a rough diagram of how you can do what you're trying to do.

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Calculation and general description are a far cry from one another.

The first thing I would do is calculate the deltaV for a direct burn and keep it in your back pocket. All the fancy gravity assists in the world are useless if they exceed this number. To get a net savings out of the maneuver your free deltaV must make up for the deltaV missed out by not burning in LKO. The baffling thing about the calculations is how the gravity assist factors into the entire mission plan. For example the max gravity assist comes from a pitiful entry into the Mun's SOI so while you get all of it, you don't come out with much more than that. Do you GA to eject in a nice Hohmann trajectory or do you get more dV out of the GA and let the trajectory hit the target at a sharp angle. Maybe you're heading for Duna so the dV to slow down at the end matters much less.

No maneuver except in the simplest of cases has a clear best. Each one's gains come at the expense of the other. The calculation's difficulty in seeing which gains outweigh other expenses to get the maximum economy out of the whole series of maneuvers. There is a lot of info to be gained by setting the number of SOIs the conics projection extends through to a high number so you can see well into the future.

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OP already said hes lousy at math. A general description would seem to be pretty much what hes asking for, even if it does say "calculate" in the title. Not a lot of people really pull out a calculator to play KSP. (I have before, though)

getting DV to compare these things wouldn't be too complicated. just make a maneuver node for with assist and later without. And the best way to do that would be to start with a general description, or better still - an illustration.

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OP already said hes lousy at math. A general description would seem to be pretty much what hes asking for, even if it does say "calculate" in the title.

But he's asking for a solution to a math problem that doesn't involve math. I'm conflicted. :I

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Theres more knowledgable people than just Scott Manley out there, you know.

anyway, heres a rough diagram of how you can do what you're trying to do.

Reverse direction, that means 180 degree???

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I want send me Kerbals to Minmus using Mun as gravity assist, looking for advice.

Getting to Minmas's SOI already takes barely more delta-V than getting to the Mun's. It's almost certainly not going to be worth it to use a Mun assist for this maneuver.

To get a net savings out of the maneuver your free deltaV must make up for the deltaV missed out by not burning in LKO.

Kerbin to Eve Hohmann transfers are more efficient if you start from the Mun's altitude than from LKO. People get stuck on the Oberth effect, but forget that there is a gravity well you have to overcome when you're near Kerbin, i.e. at the Mun's altitude it only takes 225 m/s to escape Kerbin's SOI whereas down at 100km, it takes 930 m/s. The two effects compete and for some transfers, the most efficient altitude for transfer ends up being a higher one.

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I'm terrible at math, but the game has this great little interface that does it for me. I don't re-arrange my missions in search of gravity assists, but if one presents itself I use the navigation thing in the game to work out how I can best maximize the opportunity.

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Reverse direction, that means 180 degree???

Yeah, though now I recheck your OP I for some reason didn't notice you were specifically after going to Minmus. For lower planet orbits yeah you do this around 180 degrees. For going to Minmus, you'll have to find the right angles between the Mun and Minmus, or adjust your burn time to alter the angle of deflection you get off the Mun (which makes it all somewhat tweakable) But indeed for optimum, thats quite some trouble. I can see the math being a bit troublesome if done that way. (Though I'm pretty tempted to look at the math now)

So in other words, ironically - Its easier to swingby to a planet outside of Kerbins local orbit than it is to one inside. (Ie Kerbin-Mun-Duna assist easier than Kerbin-Mun-Minmus)

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Yeah, though now I recheck your OP I for some reason didn't notice you were specifically after going to Minmus. For lower planet orbits yeah you do this around 180 degrees. For going to Minmus, you'll have to find the right angles between the Mun and Minmus, or adjust your burn time to alter the angle of deflection you get off the Mun (which makes it all somewhat tweakable) But indeed for optimum, thats quite some trouble. I can see the math being a bit troublesome if done that way. (Though I'm pretty tempted to look at the math now)

So in other words, ironically - Its easier to swingby to a planet outside of Kerbins local orbit than it is to one inside. (Ie Kerbin-Mun-Duna assist easier than Kerbin-Mun-Minmus)

I have not had much time to play the KSP, which I regret. I wonder does it any MOD that help with calculating gravity assist. It would be good for people that aren't good at math like myself

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I want send me Kerbals to Minmus using Mun as gravity assist, looking for advice.

If you're Polish speaker, you might want to check this video, in which I'm sending probe to Minmus with gravity assist of Mun

. Maneuver planning from 9:10 onwards.

Basically I've waited with launch for optimal alligment of Mun and Minmus for Mun->Minmus transfer using this: http://ksp.olex.biz/.

It would be much more difficult with inter-planetary assists if you want to hit target in the 'first shot', since Kerbin, planet A and planet B would have to be in a very specific alligment. If you don't mind waiting, orbiting few times, then it is possible. I've made Kerbin->Moho transfer with gravity assist from Eve that way, that saved me some fule as opposed to direct transfer. (as can be seen here http://forum.kerbalspaceprogram.com/showthread.php/38196-Using-gravity-of-Eve-to-get-to-Moho-easier?p=486916&viewfull=1#post486916)

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• 2 weeks later...

Unfortunately Scott Manley's video on this is rather scattered (not a very good tutorial) and he just changes targets for convenience rather than explaining how to use a gravity assist to get to a specific target. The one important thing to remember is that if you want to speed up, you pass behind the object, if you want to slow down or reverse, you pass in front of the object (in this case the Mun). I'm right about at this point and have been playing with Protractor to start flinging probes all over the solar system.

This Scott Manley video is better at showing that Gravity Assists off Mun aren't worth it:

Edited by Oddible
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This Scott Manley video is better at showing that Gravity Assists off Mun aren't worth it:

For interplanetary missions. Using Mun for Minmus transfer, or Joolian moons within Joolian subsystem is perfectly reasonable.

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Kerbin to Eve Hohmann transfers are more efficient if you start from the Mun's altitude than from LKO. People get stuck on the Oberth effect, but forget that there is a gravity well you have to overcome when you're near Kerbin, i.e. at the Mun's altitude it only takes 225 m/s to escape Kerbin's SOI whereas down at 100km, it takes 930 m/s. The two effects compete and for some transfers, the most efficient altitude for transfer ends up being a higher one.

Burns will always* be more efficient in lower orbits and shorter in higher orbits. The question is, which do you want?

*May not be accurate for all instances of always.

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Burns will always* be more efficient in lower orbits and shorter in higher orbits. The question is, which do you want?

*May not be accurate for all instances of always.

Well, the only way I can see this being the case is if you don't do your burn all in one shot, IE you rendezvous on the Mun, refuel, and take off from there. Otherwise a Launch fro LKO is most efficient I still think.

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Kerbin to Eve Hohmann transfers are more efficient if you start from the Mun's altitude than from LKO. People get stuck on the Oberth effect, but forget that there is a gravity well you have to overcome when you're near Kerbin, i.e. at the Mun's altitude it only takes 225 m/s to escape Kerbin's SOI whereas down at 100km, it takes 930 m/s. The two effects compete and for some transfers, the most efficient altitude for transfer ends up being a higher one.

As others are suggesting, this is true if you are already at Mun's orbit. From LKO, it's better to burn straight for Eve than to raise your orbit to Mun and then go straight for Eve. In terms of dV cost, {Mun -> Eve} < {Kerbin -> Eve} < {Kerbin -> Mun -> Eve}. An LKO/Keostationary transfer is significantly lower in altitude than Mun but costs roughly as much as an LKO/Eve transfer.

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Well, the only way I can see this being the case is if you don't do your burn all in one shot, IE you rendezvous on the Mun, refuel, and take off from there. Otherwise a Launch fro LKO is most efficient I still think.

We are in agreement.

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Yeah, Mr. Shifty: If it takes 225m/s from an orbital plane of munÃ‚Â´s altitude, and 930m/s from LKO, how much does it take from LKO to munÃ‚Â´s altitude? Some 800m/s if i am not mistaken. On the same premise, the trick to Gravity Assist is to only spent those 800 and get the 225 (+/-) for free, pretty much.

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We are in agreement.

Just re-read your Original comment and my comment; realized I just reposted what you said XD. Sorry about that.

This makes me think that those really long burns can be accomplished in less time if you do them from Munar Orbit. If you can set up a Kethane refinery on Mun and just refill your interplanetary ship with it whilst transportig Kerbals fron Kerbin using some sort of shuttle craft I can see this actually being a cool strategy. Otherwise I would just prefer to transfer straight from LKO

Edited by AmpsterMan
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Never actually understood what a gravity assist is until today. Turns out one of these managed to save my first ever Mun lander by sheer luck. Leaving the Mun left me with no fuel and in a highly eccentric orbit of Kerbin until the Mun shot past us half an hour later and lowered the periapsis just enough to get us into the atmosphere. Didn't even realize how lucky that was until today. I'm guessing using the Mun could also assist with launching me to Duna as could Duna's satellites be used to lower my orbital speed upon arrival and vice versa?

If so that's going to really change my plans for interplanetary travel.

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• 3 months later...
Calculating gravity assist is rather hard for someone who is lousy in math and aren't Scott Manley

My biggest achievement it's probe to Jool, i want learn how create perfect gravity assist. I want send me Kerbals to Minmus using Mun as gravity assist, looking for advice.

hmm..... I'm researching for the mechanism of the Gravity assist by using mathematical and physical informations of oval orbit and universal gravitition so I'll give you an mathematical sentence to you if I success.

(You can just type the information of mass and orbit speed to the letters on it when I complete to create the sentence

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Never actually understood what a gravity assist is until today. Turns out one of these managed to save my first ever Mun lander by sheer luck. Leaving the Mun left me with no fuel and in a highly eccentric orbit of Kerbin until the Mun shot past us half an hour later and lowered the periapsis just enough to get us into the atmosphere. Didn't even realize how lucky that was until today. I'm guessing using the Mun could also assist with launching me to Duna as could Duna's satellites be used to lower my orbital speed upon arrival and vice versa?

If so that's going to really change my plans for interplanetary travel.

Of course it can,

If a body has gravity, no matter how minuscule, will be able to edit your orbit

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