How do you know from where to get gravity assisted?

[seaces]

My dear KSP Veterans,

I have a question for you, agan.

What I do not understand when there is some delta v realted topic or how to get on X planet how do you pull from your head when and from which planet you get gravity assist? A typical answer goes like this on those topics: "OH or you could get just a assit from planet x and planet y and that will reduce your requred fuel to get to Z" I am like how do you know which to use when? Not to mention how do you know which to combine for multiple assists and for example once you get those transfer windows from that calculator how do you incorporate that in that one?

[HebaruSan]

Personally, I think of gravity assists as the most advanced part of KSP. I haven't used them, but they're on my list of things to learn, and I've done some reading and watching of tutorials to try to get started. They're complex enough to make it difficult to write good automation tools or even aids, though there are a few:

As I understand it, the most important thing is that the assisting body be massive enough to change your trajectory significantly. That's why in real life, it's very common to slingshot a probe past Jupiter. It's also why it's rarely worth it to use Mun, and pretty much never worth it to use Minmus. It also has to be easier to get to than your ultimate destination. Maybe that sounds obvious, but people use Eve a lot because it's nearby and its orbit isn't too crazy. And the exact angle and timing of encountering the assisting body is crucial to the effect it has; you can think of the shape that your orbit makes through the SOI as being like a maneuver performed at that point of your solar orbit, to some extent.

As for when to launch, you generally use a good launch window for the assisting body, but there's no guarantee that that will give you a good route to the destination body. That part is either luck or experimentation; some planetary alignments are very advantageous, and some aren't. As far as I can tell, even the experts can't plan assists as easily as you would plan a normal Hohmann transfer. Rather, they master the underlying mechanics of how assists work, then search and experiment until the optimal route appears. But there are some tricks you can use; for example, if you want multiple assists in a row from the same body, then your orbit after the first one needs to be in resonance with the assisting body, meaning that your orbital period is a multiple of its orbital period, or vice versa, or some fractional relationship like 2:3 (i.e., you do 2 orbits in the time it does 3), such that you end up at the same place at the same time again.

Again, my direct experience with this is no greater than yours, but those are the things that I'm pretty sure about. Maybe @PLAD will stop by to relate how he found the Kerbin-Eve-Kerbin-Kerbin-Jool route.

[mpk10]

1 hour ago, HebaruSan said:

you can think of the shape that your orbit makes through the SOI as being like a maneuver performed at that point of your solar orbit

This!

Depending on your direction of travel relative to a body's orbit, the "bend" that happens to your course as you pass through that body's sphere of influence can act as the equivalent of some combination of prograde/retrograde and radial/anti-radial.  (Or even normal/anti-normal if you want it, though I have rarely wanted that affect.)

It's taken me a long time to learn how to use planets or moons for gravity assists.  I now pretty much exclusively use EVE (for getting to Moho), TYLO (for arriving anywhere in the Jool system, and JOOL (for getting to Eeloo).  

One piece of advice I would add is that you should increase the "conic patch limit" in the game settings if you want to learn/use gravity assists.  I won't go into how to do it, it's easy enough to look up.  But it will allow you to see further into the future, through more S.O.I.s.

 

-mpk

[PLAD]

Flybys!

I know of two tools for finding flybys in KSP, my Flyby Finder and Arrowstar's TOT.  It is possible to leave Kerbin at a random time and find flyby opportunities by chance because the Kerbol system is small with high-gravity planets, but using the tools above makes it much easier to achieve a specific objective.

Here is what I have found after doing many, many flybys to and from all the stock planets. In general only Eve, Kerbin, and Jool should be used for flybys as the other bodies are too small to be worth the trouble. Here's how I get to them all...

To get to Moho from Kerbal you should fly by Eve. This is both the hardest one to execute and the one that gives the most benefit, little Moho is moving so fast that even a small mistake in timing the Kerbin departure or Eve flyby will make you miss Moho. However once your ship is in an orbit that crosses Moho's orbit it is usually easy to plan a Moho encounter several orbits out with only a small adjustment to your path. Even better is to go Kerbin-Eve-Eve-Moho but that requires a double flyby which neither FF nor TOT can plan for you. I use my Lambert spreadsheet to find double flybys, it can be found in the FF post. It requires trial-and-error to find a flyby, if you study this old challenge you will find us working out things like when the best times to flyby Eve are and how to set up the Eve double encounter.

To get back from Moho to Kerbin you should also use Eve, Moho-Eve-Kerbin is rather easy if you can wait a year or two at Moho for the next window.

No flyby needed to get to Eve, though you can save about 60m/s by using a Mun flyby, see the link in the Jool flyby below.

No flyby needed to get to Duna, but you can go K-Eve-Duna if you want to get to Duna cheaply outside the normal Hohmann windows from Kerbin. See here for an example of this.

Dres is the hardest. I have found that using a flyby of Duna to get to Dres is not useful, because Duna is too small to change your path much if you fly by it with enough energy to get to Dres, so the windows are very rare (~30 Kerbin years apart) and the gains are less than 50m/s. I even tried a K-D-D-D-Dres (triple flyby!) and find I spent more on mid-course corrections than I saved in using Duna. However, it is can be advantageous to use Jool to get to Dres! Although it takes about 200m/s more to get to Jool than Dres you can knock  more than 500m/s off your Dres arrival speed for a net savings. If you use Eve to get to Jool you can save even more.

To get to Jool cheaply you should go to Eve first. Using typical values (rather than best possible with perfect alignments) it costs about 2000m/s to get from low Kerbin orbit directly to Jool. It's about 1900m/s to go K-Duna-Jool. It's about 1300-1500m/s to go K-Eve-K-Jool. It's about 1100m/s to go K-E-K-K-J. Here is a thread that gives mission examples of KEKKJ. I found that superb window using Lambert and trial-and-error. Another intriguing option for getting to Jool is a Juno-style Kerbin flyby with a deep space manuever. Check out this post for how to get to Jool for about 1550m/s.

To get to Eeloo you should fly by Jool. Because they orbit so slowly there are extended periods with no fast and cheap window for this, but if you can get to Jool it can throw you to Eeloo wherever it is if you are patient. Note that you can use Eve to get to Jool and from there to Eeloo, check out this trip I made using KEKKJ-Eeloo, and then Eeloo-J-K to get back. Nowadays with the modern aero model requiring lower Kerbin re-entry speeds it is useful to flyby Eve on the way back from the outer system to reduce your Kerbin arrival speed.

Remember the concept of synodic periods, in the Flyby Finder primer I give a table showing the periods between which the relative positions of the planets repeat, if you find one good window it will often repeat after a set period.

NASA uses more tricks than FF or TOT can calculate, for instance deep space maneuvers and multiple orbits between encounters, I do not anticipate writing anything that handles those anytime soon. Does Rich Purnell play KSP?

And a final note on flybys- they are not too forgiving of errors in timing. The more flybys you do the more precise the early ones have to be or you will require bigger and bigger course corrections to stay on track, or you might miss the last planet altogether. Try to leave the start planet within an hour of the time that TOT or FF gives you, and remember when doing mid-course corrections that it is more important to have your flyby periapsis at the right time than at the right altitude or speed.

[Snark]

Taking the broad view:  there are two aspects of gravity assists:  the easy part and the hard part.

The easy part is:  speed me up or slow me down.  That is, it's really easy to set up a gravity assist that will add energy to your orbit (i.e. speed you up), or take away energy from your orbit (i.e. slow you down).

The hard part is:  send me to a specific destination.  That's really, really hard, for two reasons.  First, aiming is hard, because even a tiny error will make huge differences in where you end up.  Second, timing is hard, because if you're at planet A, and you want to use planet B to get to planet C, then you have to wait for a good alignment of all three planets, which is very tricky.

So, if all you want to do is the "easy" kind of gravity assist (where all you care about is changing your speed and you're not trying to aim at anything in particular), then that's pretty straightforward.  Probably the most common example of this sort of maneuver is using a reverse gravity assist from Tylo when you arrive at Jool.  You can use Tylo to slow you down so that you're captured to Jool, saving a lot of dV.  This is a very simple maneuver because you're not particularly aiming at any specific destination-- all you want is to capture to Jool, and you can fine-tune your orbit to arrive at a particular moon afterwards.

Another example of an "easy" type of maneuver is if you're using Eve to reduce your dV to Moho... but that is typically accompanied by a fair amount of multi-orbit navigation after the Eve encounter, unless you manage to get really lucky with Moho's placement.

On the other hand, if you want to do the "hard" kind of gravity assist (e.g. use planet A to get to planet B)... well, that's hard.  Personally, I never bother with this type of maneuver-- it's not worth the time or the hassle, and the amount of dV savings isn't all that much.  But then, I'm goal-oriented and am interested in getting where I'm going with the least amount of fuss.  If, on the other hand, you want to do it because it's hard (i.e. just for the sake of the challenge), then go for it! 

But bear in mind that that's really the tradeoff, here:  unless you're specifically in it for the challenge, I would say that this type of maneuver isn't worth the hassle in most cases.

[Blaarkies]

4 hours ago, Snark said:

...If, on the other hand, you want to do it because it's hard (i.e. just for the sake of the challenge), then go for it!  ...

because it's hard!
I have heard of this before...but he didn't have kerbal forums to help him along 

Edited April 7, 2016 by Blaarkies

[seaces]

45 minutes ago, Blaarkies said:

because it's hard!
I have heard of this before...but he didn't have kerbal forums to help him along 

Hhahaha @Blaarkies good one. That one made me send just another Kerbal to Mun.

14 hours ago, PLAD said:

Flybys!

I know of two tools for finding flybys in KSP, my Flyby Finder and Arrowstar's TOT.  It is possible to leave Kerbin at a random time and find flyby opportunities by chance because the Kerbol system is small with high-gravity planets, but using the tools above makes it much easier to achieve a specific objective.

Here is what I have found after doing many, many flybys to and from all the stock planets. In general only Eve, Kerbin, and Jool should be used for flybys as the other bodies are too small to be worth the trouble. Here's how I get to them all...

To get to Moho from Kerbal you should fly by Eve. This is both the hardest one to execute and the one that gives the most benefit, little Moho is moving so fast that even a small mistake in timing the Kerbin departure or Eve flyby will make you miss Moho. However once your ship is in an orbit that crosses Moho's orbit it is usually easy to plan a Moho encounter several orbits out with only a small adjustment to your path. Even better is to go Kerbin-Eve-Eve-Moho but that requires a double flyby which neither FF nor TOT can plan for you. I use my Lambert spreadsheet to find double flybys, it can be found in the FF post. It requires trial-and-error to find a flyby, if you study this old challenge you will find us working out things like when the best times to flyby Eve are and how to set up the Eve double encounter.

To get back from Moho to Kerbin you should also use Eve, Moho-Eve-Kerbin is rather easy if you can wait a year or two at Moho for the next window.

No flyby needed to get to Eve, though you can save about 60m/s by using a Mun flyby, see the link in the Jool flyby below.

No flyby needed to get to Duna, but you can go K-Eve-Duna if you want to get to Duna cheaply outside the normal Hohmann windows from Kerbin. See here for an example of this.

Dres is the hardest. I have found that using a flyby of Duna to get to Dres is not useful, because Duna is too small to change your path much if you fly by it with enough energy to get to Dres, so the windows are very rare (~30 Kerbin years apart) and the gains are less than 50m/s. I even tried a K-D-D-D-Dres (triple flyby!) and find I spent more on mid-course corrections than I saved in using Duna. However, it is can be advantageous to use Jool to get to Dres! Although it takes about 200m/s more to get to Jool than Dres you can knock  more than 500m/s off your Dres arrival speed for a net savings. If you use Eve to get to Jool you can save even more.

To get to Jool cheaply you should go to Eve first. Using typical values (rather than best possible with perfect alignments) it costs about 2000m/s to get from low Kerbin orbit directly to Jool. It's about 1900m/s to go K-Duna-Jool. It's about 1300-1500m/s to go K-Eve-K-Jool. It's about 1100m/s to go K-E-K-K-J. Here is a thread that gives mission examples of KEKKJ. I found that superb window using Lambert and trial-and-error. Another intriguing option for getting to Jool is a Juno-style Kerbin flyby with a deep space manuever. Check out this post for how to get to Jool for about 1550m/s.

To get to Eeloo you should fly by Jool. Because they orbit so slowly there are extended periods with no fast and cheap window for this, but if you can get to Jool it can throw you to Eeloo wherever it is if you are patient. Note that you can use Eve to get to Jool and from there to Eeloo, check out this trip I made using KEKKJ-Eeloo, and then Eeloo-J-K to get back. Nowadays with the modern aero model requiring lower Kerbin re-entry speeds it is useful to flyby Eve on the way back from the outer system to reduce your Kerbin arrival speed.

Remember the concept of synodic periods, in the Flyby Finder primer I give a table showing the periods between which the relative positions of the planets repeat, if you find one good window it will often repeat after a set period.

NASA uses more tricks than FF or TOT can calculate, for instance deep space maneuvers and multiple orbits between encounters, I do not anticipate writing anything that handles those anytime soon. Does Rich Purnell play KSP?

And a final note on flybys- they are not too forgiving of errors in timing. The more flybys you do the more precise the early ones have to be or you will require bigger and bigger course corrections to stay on track, or you might miss the last planet altogether. Try to leave the start planet within an hour of the time that TOT or FF gives you, and remember when doing mid-course corrections that it is more important to have your flyby periapsis at the right time than at the right altitude or speed.

Thnx @PLAD on the long answer

Edited April 7, 2016 by seaces

[tomf]

You might find this thread helpful.

It describes how to work out what is possible from a gravity assist