Kerbin-Kerbin Slingshot?

[Red Iron Crown]

@GoSlash27 This pic and its caption are illuminating:

"Making a small adjustment shortly after leaving Eve's SOI. I've rotated the view. The Kerbal encounter will be at about 8 O'clock, then I'll follow the red line in an orbit that returns to Kerbal at the same place 213 days later for the 2nd Kerbin flyby."

The red dashed line is the trajectory after the first Kerbin gravity assist, you can see that its apoapsis is short of Dres's orbit, let alone Jool's. Almost all the remaining required energy is added in the second Kerbin slingshot.

[GoSlash27]

RIC,

 Right... but all of the energy *could've* been added in the first pass with the same exact result, right? Except, of course, Jool would've been in the wrong place for an encounter.

So in that sense, the second pass didn't really "add" anything. It just restored what *could've* been added the first time.

 

 

[PLAD]

This thread is bringing up enough issues for about 5 threads, but here is my take on the initial question. @eddiew , if you leave Kerbin with a certain v-infinity (this is the speed your craft has relative to Kerbin just before it leaves Kerbin's SOI) and fly around the sun for a while and come back without making any maneuvers or flying by any other bodies, you will arrive back at Kerbin's SOI with exactly the same v-infinity as you had leaving (except of course you are coming in instead of going out). You can then flyby Kerbin and head back out of it's SOI moving in a different direction relative to the Sun then you had before, but you will once again leave Kerbin's SOI with the same v-inf as you had arriving. The big deal is that your craft's velocity is added to Kerbins velocity, and if, for instance, you leave Kerbin going 'backwards' relative to Kerbin's orbit around the Sun you will drop in towards the Sun, but if you leave Kerbin going 'forwards' your speed is added to Kerbins and you will go further away from the Sun. If you leave Kerbin at the right angle your orbit around the Sun might have the same period as Kerbin but be much more eccentric or inclined than Kerbin's, in which case you will encounter Kerbin exactly a K-year later. But always you will arrive back at Kerbin with the same v-inf you had (relative to Kerbin!) when you left it. An example of this is my Kerbin-Eve-Kerbin-Kerbin-Jool path, if you look at the craft's Kerbin departure angle relative to the direction of Kerbin's orbit around the sun you will see that although the v-inf is the same for both Kerbin passes the departure direction is different, and this is why the first pass only goes out to about Dres' orbit but after the second pass you get all the way to Jool. I took screen shots that point it out in the album. Indeed, the reason that two Kerbin passes are needed is that the ship is going too fast for Kerbin to turn it all the way to the right direction in just one pass.

  The excitement comes when something changes your path while you are outside Kerbin's SOI. In the example above that thing is the flyby of Eve. As a result of that, with no other changes, you come back to Kerbin with nearly 2km/s more of v-inf relative to Kerbin. Another thing you can do is make a small thrust while as far away from Kerbin as possible, this will cause you to come back with a much larger change in your v-inf than the thrust was. This is called a V-Infinity Leveraging Maneuver (VILM) and is the trick that the Juno probe to Jupiter is using. @Mad_Maelstrom came up with a way to plan this maneuver in the Kerbol system, check out his post here. This is also part of the trick that the Messenger probe used, after every flyby of Mercury it would do a VILM while opposite the Sun from its Mercury encounter, in this case it would greatly reduce the next flyby speed of Mercury (think of it as the Juno-maneuver being done backwards). I think what you are looking for is the VILM.

 To touch some other issues in the thread, Flyby Finder can also handle the Outer Planets Mod planets if that is the mod system you are using ( I haven't tried Kerbol Plus Remade so I don't know what that changes).  There is a complication to the v-inifinity rule if you are flying around the moons of Jool, and to a lesser extent Mun, in that the 2-body method that KSP uses causes some inaccuracies if you leave a moon's SOI at the planetward or anti-planetward edge. For instance, you might have noticed that if you are in an orbit that just barely leaves Laythe's SOI, say by only 1 m/s, you will fly outward past Tylo if you leave Laythe's orbit at the anti-Joolward side. This is because while you are climbing away from Laythe the game is not taking extra energy away  due to you also climbing away from Jool, because you are only in the SOI of Laythe. You can use this by approaching a moon on the inside and flying by so you leave on the outside and get some free energy, I use this in my Mun multi-flyby path that gets to Mun from LKO for only 845m/s instead of the normal 856 from a 75x75 orbit. But that is another story. (To me it is an entirely acceptable approximation and it compensates for not being able to do 3-body tricks like flying through L2 or L3 to leave a planet for free.) And then there's the trick that Vector found where you use a flyby of Mun during a flyby of Kerbin to change your Vinf, you can get to Duna or Eve for 873m/s from LKO with that one. 

 

[herbal space program]

OK, so I realize now that maybe I remembered it wrong and that what Plusck/Machinist said is true. Perfectly tangential encounters produce minimal changes to AP/PE but result in radial corrections instead. Off-tangent encounters produce higher or lower AP/Pe depending on which side you approach the SOI from and which way you go around the body. I will just go put that to the test now. I'm kind of embarrassed I spent so much time doing this stuff less than a year ago and I still don't remember exactly how it works...

[GoSlash27]

12 minutes ago, PLAD said:

This thread is bringing up enough issues for about 5 threads, but here is my take on the initial question. @eddiew , if you leave Kerbin with a certain v-infinity (this is the speed your craft has relative to Kerbin just before it leaves Kerbin's SOI) and fly around the sun for a while and come back without making any maneuvers or flying by any other bodies, you will arrive back at Kerbin's SOI with exactly the same v-infinity as you had leaving (except of course you are coming in instead of going out). You can then flyby Kerbin and head back out of it's SOI moving in a different direction relative to the Sun then you had before, but you will once again leave Kerbin's SOI with the same v-inf as you had arriving. The big deal is that your craft's velocity is added to Kerbins velocity, and if, for instance, you leave Kerbin going 'backwards' relative to Kerbin's orbit around the Sun you will drop in towards the Sun, but if you leave Kerbin going 'forwards' your speed is added to Kerbins and you will go further away from the Sun. If you leave Kerbin at the right angle your orbit around the Sun might have the same period as Kerbin but be much more eccentric or inclined than Kerbin's, in which case you will encounter Kerbin exactly a K-year later. But always you will arrive back at Kerbin with the same v-inf you had (relative to Kerbin!) when you left it. An example of this is my Kerbin-Eve-Kerbin-Kerbin-Jool path, if you look at the craft's Kerbin departure angle relative to the direction of Kerbin's orbit around the sun you will see that although the v-inf is the same for both Kerbin passes the departure direction is different, and this is why the first pass only goes out to about Dres' orbit but after the second pass you get all the way to Jool. I took screen shots that point it out in the album. Indeed, the reason that two Kerbin passes are needed is that the ship is going too fast for Kerbin to turn it all the way to the right direction in just one pass.

  The excitement comes when something changes your path while you are outside Kerbin's SOI. In the example above that thing is the flyby of Eve. As a result of that, with no other changes, you come back to Kerbin with nearly 2km/s more of v-inf relative to Kerbin. Another thing you can do is make a small thrust while as far away from Kerbin as possible, this will cause you to come back with a much larger change in your v-inf than the thrust was. This is called a V-Infinity Leveraging Maneuver (VILM) and is the trick that the Juno probe to Jupiter is using. @Mad_Maelstrom came up with a way to plan this maneuver in the Kerbol system, check out his post here. This is also part of the trick that the Messenger probe used, after every flyby of Mercury it would do a VILM while opposite the Sun from its Mercury encounter, in this case it would greatly reduce the next flyby speed of Mercury (think of it as the Juno-maneuver being done backwards). I think what you are looking for is the VILM.

 To touch some other issues in the thread, Flyby Finder can also handle the Outer Planets Mod planets if that is the mod system you are using ( I haven't tried Kerbol Plus Remade so I don't know what that changes).  There is a complication to the v-inifinity rule if you are flying around the moons of Jool, and to a lesser extent Mun, in that the 2-body method that KSP uses causes some inaccuracies if you leave a moon's SOI at the planetward or anti-planetward edge. For instance, you might have noticed that if you are in an orbit that just barely leaves Laythe's SOI, say by only 1 m/s, you will fly outward past Tylo if you leave Laythe's orbit at the anti-Joolward side. This is because while you are climbing away from Laythe the game is not taking extra energy away  due to you also climbing away from Jool, because you are only in the SOI of Laythe. You can use this by approaching a moon on the inside and flying by so you leave on the outside and get some free energy, I use this in my Mun multi-flyby path that gets to Mun from LKO for only 845m/s instead of the normal 856 from a 75x75 orbit. But that is another story. (To me it is an entirely acceptable approximation and it compensates for not being able to do 3-body tricks like flying through L2 or L3 to leave a planet for free.) And then there's the trick that Vector found where you use a flyby of Mun during a flyby of Kerbin to change your Vinf, you can get to Duna or Eve for 873m/s from LKO with that one. 

 

PLAD,

 Yeah, that's what I figured (other than the reason you did 2 Kerbin passes; too fast to do it all in one shot)

 So in the case of the OP's scenario, nothing can be gained from Kerbin->Kerbin->points beyond, right?

 I mean... he launches from Kerbin into a resonant orbit, leaving tangent prograde. When he arrives at Kerbin on the next pass, he will arrive tangent prograde. Therefore the most he can hope for from the encounter is to not *lose* any velocity, which isn't really possible.

 The only way to get another boost directly from Kerbin would be to alter his velocity (either with a boost to raise his Pe) or by using another body first.

 Is this all about correct?

Best,
-Slashy

*edit* Thanks for explaining the VILM maneuver. That's what I was missing!

Edited May 7, 2016 by GoSlash27

[OhioBob]

If it is any help, here is the section from my web page were I write about gravity assist:

http://www.braeunig.us/space/interpl.htm#gravassist

 

[Plusck]

 

 

1 hour ago, GoSlash27 said:

RIC,

 The way I see it, the second Kerbin encounter is actually correcting the inefficiency of the first Kerbin encounter. I understand why it would have to be done that way in order to to sync the encounters with the bodies (no sense in boosting for Jool when Jool isn't there yet), but he would've entered the same exact orbit had he left the first Kerbin flyby tangent... right? So in that sense, the second flyby didn't "gain" anything, it just restored what could've been gained in the first pass, but wasn't.

 Or am I missing something?

Best,

-Slashy

 

 

 

I've just been playing about with hyperedit and an approximately 2-year orbit with Pe near Kerbin.

I suppose in a way you coud say It's due to the inefficiency of the first gravity assist, but that's because no gravity assist can be wholly efficient - you'd need to be getting your assist from a miniature black hole at very low altitude, starting with a retrograde orbit, to maximise efficiency.

And I think that one reason why you're doubting this very real possibility of a gravity assist from the planet you started from is because you're imagining encountering that planet straight on. And indeed, if your Pe is exactly at the orbit of Kerbin, there is no way you can get a gravity assist. Even if you dove through a hole through the planet, you'd end up at the SOI edge with exactly the same energy as when you entered it.

However, as long as your orbit crosses Kerbin's orbit (i.e. Pe is slightly lower than Kerbin's orbit, for OP's example), you have two possible encounters where you enter the SOI at an angle, and you can use that to get a gravity assist from Kerbin dragging you around and ejecting you prograde.

And I think also that this is where the "passing inside" and "passing outside" myth comes from. It's not true as such, but it has an element of truth to it.

Indeed, when you cross the orbit, there are two points where you can get an encounter, as you "descend" towards the sun and your solar Pe, and when you start ascending again.

And the angle between your orbit and the orbit of Kerbin at those points is not the same, it is much more acute as you descend than when you ascend, and this does affect how much of an assist you can get*. The smaller the angle, the further from Kerbin you have to make your flyby to get the right ejection angle, and the less the potential benefit:

Spoiler

Getting an encounter while descending to Pe, passing behind and then on the "inside" of Kerbin: narrow angle of approach, orbit boosted from 27Gm to 31Gm.

another view of the same encounter:

Getting an encounter while ascending from sub-Kerbin Pe, passing behind then on the outside of Kerbin: wider angle of approach, orbit boosted from 25Gm to 37Gm.

So there you go: you can get multiple gravity assists from a planet but the most important point is that the last gravity assist, the one that has you making the final big push to your destination, has to have the biggest possible angle between your orbit and the planet's orbit when you get the assist.

 

 

edit: ah ok, I was a bit slow posting this so much of it has been rendered needless by @PLAD's informative post. And thanks @herbal space program for the nod.

Also I never realised that @OhioBob is the author of that excellent website.

 

*edit 2: regarding the difference between descending and ascending angles... I'm actually not sure that it makes a difference because it balances out between arrival and ejection angles. What does help, though, is the "SOI effect" where you get free energy from escaping the gravity of the major body as you rise up within the SOI of the minor body. So that difference between the best possible gravity assists in the pics could actually be just the result of the SOI effect.

Edited May 7, 2016 by Plusck

[herbal space program]

So having tested it now, it does seem that I can only lose energy if I re-approach Kerbin from a perfectly tangent orbit. I guess losing energy was what I was trying to do with my Moho multi-assist mission, so I never looked so much into trying to gain it! The KEKKJ route of course involved non-tangent Kerbin encounters, so I never had occasion to test it there. What does seem possible however would be to boost retrograde at a solar Ap a bit less than 2:1 resonant and drop your Pe so you re-encounter Kerbin earlier on its orbit and non-tangentially, you could actually get a net gain of energy from the second encounter. I suppose that's the Vinf leveraging PLAD was talking about. I'm gpoing to go mess about with this some more, maybe employing Mun as well, to address the more practical question of how possible is it to bootstrap yourself out of Kerbin with only the Kerbin system and DSMs.

 

[PLAD]

1 hour ago, GoSlash27 said:

PLAD,

 Yeah, that's what I figured (other than the reason you did 2 Kerbin passes; too fast to do it all in one shot)

 So in the case of the OP's scenario, nothing can be gained from Kerbin->Kerbin->points beyond, right?

 I mean... he launches from Kerbin into a resonant orbit, leaving tangent prograde. When he arrives at Kerbin on the next pass, he will arrive tangent prograde. Therefore the most he can hope for from the encounter is to not *lose* any velocity, which isn't really possible.

 The only way to get another boost directly from Kerbin would be to alter his velocity (either with a boost to raise his Pe) or by using another body first.

 Is this all about correct?

Best,
-Slashy

*edit* Thanks for explaining the VILM maneuver. That's what I was missing!

GoSlash27,

  Yup. There is no energy to be gained by going K-K-somewhere, as long as no maneuver or flyby changes the path while you're out of Kerbin's SOI. You can change the direction you leave Kerbin at, but why not just launch in that direction in the first place and save a year of time? Well, wait... there are some neat tricks that just a tiny VILM can do though, for instance your needed departure path from Earth might have a very high inclination to Earth's equator, such that you would have to launch into a 90-degree-inclined orbit. This costs a lot since you lose the 400+ m/s of your launch site by not launching due East. So instead launch due East and then depart Earth into a Solar orbit  that comes back to Earth a year later, but 90 degrees before you encounter Earth do a tiny normal thrust that gives you a 90-degree inclined flyby of Earth. Now you leave Earth in a highly inclined plane for just a couple of m/s more than a due-East launch would require! This competes with a bi-elliptic-style plane change, but is an example of why one might do a K-K flyby.

  Someone brought up the timing issue for the K-E-K-K-J flight, it is true that the mission is timed so that you cross Jool's orbit when it is there, and if you skipped the 2nd flyby you wouldn't encounter Jool even if you had the right direction after the first flyby. However the mission start is timed to take the 2 flybys into account, I use K-E-K-J flights sometimes if I'm in a hurry, but they are less efficient because the single flyby of Kerbin doesn't leave in an optimum direction so you have to start with more energy.

 

 

[herbal space program]

1 hour ago, PLAD said:

There is no energy to be gained by going K-K-somewhere, as long as no maneuver or flyby changes the path while you're out of Kerbin's SOI.... Well, wait... there are some neat tricks that just a tiny VILM can do though, for instance your needed departure path from Earth might have a very high inclination to Earth's equator, such that you would have to launch into a 90-degree-inclined orbit. ...

 

I definitely understand how you can get a big plane change from a small mid-coursecorrection, but what I'd like to know is if you eject from Kerbin (perhaps using a Munar assist)  in such a way that your Pe goes inside Kerbin's and your Ap outside, can you then set up a subsequent (non-tangential) Kerbin encounter at one of the resulting orbital intersection points,  from which you can ultimately harvest more energy back than you had to put in to get to that orbit in the first place?  It seems to me like the plane change maneuver doesn't really increase your total orbital potential energy as much as change your direction of motion, so it's not clear to me if you can get these sorts of effects keeping everything in the same plane...

[herbal space program]

...Well I've now convinced myself that no matter how I eject from Kerbin, I can't re-encounter it in a way that gives me more orbital energy than I would have had if I had just ejected prograde with the same amount of dV. I set up two ejection burns of `980m/s,  one of which swung low around Mun to eject prograde, and the other of which ejected about 60deg sunward of prograde. The prograde ejection got me to an Ap of ~21M Km, a bit higher than Duna's orbit, the anti-radial one to an Ap of only 18M km, with my Pe inside Kerbin's orbit such that there were about 80 days of Kerbin travel time from intersect to intersect. I timed the anti-radial ejection burn so that I would re-encounter Kerbin (non-tangentialy) after 4 orbits at the opposite intersect from where I initially ejected. When I tweaked that encounter to raise my Ap as far as possible post-ejection, lo and behold I could get to a tangential orbit with an Ap of ~21M km, just as if I had made the same burn ejecting prograde in the first place! There is a long and tedious lesson in conservation of energy for you. Next I will test what happens if I eject prograde with a smaller burn and then expend the difference boosting retrograde at my Ap to create a subsequent non-tangential encounter....

[cantab]

A general principle I've found:

You can't raise your apoapsis with a gravity assist at periapsis. You can't lower your periapsis with a gravity assist at apoapsis.

(Unless your orbit is inclined relative to the assisting body's orbit).

The consequence is that if you want to set up a Kerbin gravity assist to boost you out towards Jool, you need to make your orbit cut inside Kerbin's so they cross at an angle.

Similarly, if you want to use a Jool assist to go sundiving, it's not enough to get an apoapsis level with Jool, you need to push it out somewhat further.

[herbal space program]

4 hours ago, cantab said:

A general principle I've found:

You can't raise your apoapsis with a gravity assist at periapsis. You can't lower your periapsis with a gravity assist at apoapsis.

(Unless your orbit is inclined relative to the assisting body's orbit).

The consequence is that if you want to set up a Kerbin gravity assist to boost you out towards Jool, you need to make your orbit cut inside Kerbin's so they cross at an angle.

Similarly, if you want to use a Jool assist to go sundiving, it's not enough to get an apoapsis level with Jool, you need to push it out somewhat further.

As I have been thinking (and playing) this through, I have arrived at the formula that the maximum amount of orbital energy you can theoretically harvest from a given encounter is proportional to the relative velocity between the two bodies times the sine of the angle between their trajectories. I haven't actually looked this all up, but I'm pretty sure this is the right answer. IOW, if you are coming at the other body at zero angle, right along its orbital path, it can't accelerate you at all, regardless of how fast you are moving relative to it. But OTOH if you are falling straight at the other body towards the sun, exactly normal to its trajectory, it can add its whole orbital velocity to yours if its gravity is strong enough. If its gravity isn't strong enough, you can harvest the energy in successive encounters, like you do at the end of the K-E-K-K-J route. The end state is always an orbit that's perfectly tangent to that of the boosting body, from which you can't gain any more energy. What this boils down to in terms of bootstrapping yourself out of your local gravity well is that if you escape a given body, any maneuver that you can do to increase the orbital angle at which you encounter it the next time can have significant energy returns. For example, this evening I boosted my ship prograde out of a 100km Kerbin orbit for 1069 m/s, to reach a solar Ap of 21.01M Km. FWIW, if I had used the Mun, I could have done this for just 980 m/s. From this solar apoapsis, I boosted retrograde 165 m/s, dropping my Pe inside Kerbin's orbit and generating intersects with significant angles. I timed it (using a fairly simple Excel spreadsheet)  so that I re-encountered Kerbin on the inbound intersect 4 orbits later, and was able to boost my Ap to >35M km from that encounter, almost all the way to Dres. What this adds up to is that by losing 165 m/s of velocity I was able to gain it all back plus another 235 m/s, because dropping my PE from up there maximally increased the angle of re-encounter. Better yet, the higher your Ap, the cheaper it becomes to do this. My guess right now is that in this way, I'll be able to climb up resonant energy levels to to Jool's orbit for something like 1500 m/s total, which is obviously  significantly more than the PLAD route, but is still half what boosting straight there would cost and does have  the advantage of not depending on other bodies and thus being doable once every Kerbin year. I need a bit more time to work on it, but when I finish I'll post the whole thing with pictures. I must thank everyone who participated in this discussion. I didn't quite understand all this before and now I think I do!

Edited May 9, 2016 by herbal space program

[eddiew]

Gosh this turned into a long thread ^^;  Thank you for the detailed thoughts chaps  

The summary seems to be:

1. You can get more energy out of Kerbin-Kerbin.
2. But not at your solar PE/AP. You have to 'cross' the orbits to get an assist, meaning you cannot come in tangentially to Kerbin as I'd originally suggested.
3. The sharper the angle at which you can cross the streams orbits, the more of a boost you can gain.
4. You need to leave more tangentially than you arrive (thanks @GoSlash27 for this one) to get boosted.
5. Without some in-game tools, it will be a bugger to plot this sort of manoeuvre and actually get to your destination.
6. It might not save you enough to be worth doing anyway.

Have I got the right end of the stick?  

Edited May 9, 2016 by eddiew

[eddiew]

On 06/05/2016 at 3:40 PM, p1t1o said:

The Juno probe apparently achieved a 3.9km/s slingshot around Earth on its way to Jupiter after travelling for 2 years, seems to fits the OP's question pretty well. For a minute there I thought it was due to arrive tomorrow, but its that blasted american dating system...

This turned out to be an excellent example of the conclusion this thread came to. The second Earth encounter is about 30 degrees round from the original launch position due to those "deep space manoeuvres", and it's this different angle of interception that allows further harvesting of the planet's orbital velocity  

[GoSlash27]

1 minute ago, eddiew said:

Gosh this turned into a long thread ^^;  Thank you for the detailed thoughts chaps  

The summary seems to be:

1. You can get more energy out of Kerbin-Kerbin.
2. But not at your solar PE/AP. You have to 'cross' the orbits to get an assist, meaning you cannot come in tangentially to Kerbin as I'd originally suggested.
3. Without some in-game tools, it will be a bugger to plot this sort of manoeuvre.
4. It might not save you enough to be worth doing anyway.

Have I got the right end of the stick?  

Sounds about right to me. Except you *could* plot this sort of maneuver using a spreadsheet. And an addendum to 2: Not only must you not come in tangentially, you must also leave more tangentially than you entered.

It'd be much simpler to pinball your way to higher velocities by using other bodies, and of course the Mun is always there to help boost your initial burn. If you just lob it up to Munar altitude, it's like chucking a softball in front of a swinging bat.

Best,
-Slashy

[eddiew]

2 hours ago, GoSlash27 said:

It'd be much simpler to pinball your way to higher velocities by using other bodies, and of course the Mun is always there to help boost your initial burn. If you just lob it up to Munar altitude, it's like chucking a softball in front of a swinging bat.

I've played Jool pinball once before; scrape past Jool, capture by Tylo, swing by Laythe to get an encounter with Vall, then back to Laythe. Very satisfying to get quadruple flyby science and a low-speed entry into Laythe's atmo, but also totally unplanned and probably lucky. I don't fancy my chances of pulling this move off on the solar system scale... lot more empty space between SoI's. At Jool, there's a huge range of altitudes that will sweep you up eventually  

[herbal space program]

5 hours ago, eddiew said:

Gosh this turned into a long thread ^^;  Thank you for the detailed thoughts chaps  

The summary seems to be:

1. You can get more energy out of Kerbin-Kerbin.
2. But not at your solar PE/AP. You have to 'cross' the orbits to get an assist, meaning you cannot come in tangentially to Kerbin as I'd originally suggested.
3. The sharper the angle at which you can cross the streams orbits, the more of a boost you can gain.
4. You need to leave more tangentially than you arrive (thanks @GoSlash27 for this one) to get boosted.
5. Without some in-game tools, it will be a bugger to plot this sort of manoeuvre and actually get to your destination.
6. It might not save you enough to be worth doing anyway.

Have I got the right end of the stick?  

Items 1-4 are exactly correct. Item 5 is not really as hard as it might seem at the outset. You just need to construct a table of resonant orbits for Kerbin  (i.e. 2:1, 3:2, 5:7 etc., just multiplying Kerbin's period by these different ratios) and another table that lets you offset the return times to model encounters at spots other than where you ejected from. With those simple tools in Excel, it's fairly easy to tweak your orbital period up and down until you have your ship and Kerbin at the same spot at some future time. Remember you can always save your game state and run the clock forward until your ship and kerbin each in turn reach the desired encounter point, then use that information to go back and calculate all the return times to that spot for both Kerbin and your ship at the time when you saved. Again, I will illustrate all of this later when I have more time.  And Item 6 I would say depends on your personal temperament, but you can definitely save a significant amount of dV if you're willing to do the calculations. While figuring out exactly what to do might be a time-consuming exercise, once you know how to set it up you can save (in my current estimation) at least half the required energy to go to a body like Jool. It's true that using Eve may be more efficient in terms of energy, but those windows only come around once in a blue moon while if you do everything using Kerbin and DSMs, you'll get a window every 1.2 Kerbin years or so. Anyway, I think I'll try to use this method to actually get to Jool, and we'll see what it ends up looking like....

Edited May 9, 2016 by herbal space program

[Plusck]

4 hours ago, eddiew said:

This turned out to be an excellent example of the conclusion this thread came to. The second Earth encounter is about 30 degrees round from the original launch position due to those "deep space manoeuvres", and it's this different angle of interception that allows further harvesting of the planet's orbital velocity  

Agree completely.

I hadn't really understood the significance of those "deep space manoeuvres" before reading PLAD's post. Applying that knowledge to the various descriptions of the trips made by Juno, Rosetta and whatnot makes it all sound a lot more logical.

[eddiew]

31 minutes ago, herbal space program said:

Anyway, I think I'll try to use this method to actually get to Jool, and we'll see what it ends up looking like....

Hope you're going to post us a mission report when you're done, will make good reading 

[herbal space program]

5 minutes ago, eddiew said:

Hope you're going to post us a mission report when you're done, will make good reading 

I will definitely do that, but it's going to take a couple of days. I've already taken screens of the proof-of-principle mission, but I still need to figure out at least one more DSM to get all the way to Jool's orbit. Once I've done that, I'll just need to take note of the Jool-Kerbin phase angle at the time of my initial ejection burn and at the time of my arrival at Jool's orbit, and then I should be able to determine the correct Jool-Kerbin phase angle to start with so that the second time through Jool will actually be there when I arrive at its orbit. Stay tuned....

[eddiew]

1 minute ago, herbal space program said:

I will definitely do that...

Consider me subscribed to your newsletter, good sir.

[The_Rocketeer]

[REDACTED]

I am wearing my dunce hat this morning.

Edited May 13, 2016 by The_Rocketeer

[Interplanet Janet]

This may be belated. WAY belated. But I can actually give a simpler guide.

Burn fuel until your time to apoapsis is at least 15 days above a Kerbin year (426 days). Set up a maneuver node, pull the retrograde handle, and then wait until you get a Kerbin encounter. If you enter tangenial to the planet, you will not gain as much velocity if on the inbound or outbound leg of the orbit. If the closest approach is below or above the planet, pull either of the normal nodes.

If you are heading to Dres, Jool, or Sarnus, you need to pass behind the planet; you cannot get to Urlum, Neidon, or Plock with this because it will either a) pass within the atmosphere, or b) impact the planet. One to Duna will need you to pass in front of Kerbin to slow yourself down. Not that you'd want to do that.

Edited May 13, 2016 by Interplanet Janet

[Plusck]

I feel the need, for posterity's sake, to do a little summing up...

In essence, a kerbin-kerbin gravity assist quite simply cannot give you any boost whatsoever.  In that respect, GloSlash27's first post was abolutely correct.. but not quite for the right reasons.

And my posts - and the posts of a few other people - were absolutely wrong... but for the right reasons.

So rule No. 1: if you leave Kerbin going perfectly prograde (or retrograde) you quite simply cannot obtain a boost from encountering Kerbin again.

Rule No. 1a: if you leave Kerbin with an inefficient outward trajectory (not perfectly prograde), you can use that on your next encounter to obtain a boost, since you will cross Kerbin's orbit at an angle, but the net result of that boost will be as if you'd left Kerbin going perfectly pro- or retrograde the first time. You've just wasted a year or two to get to the same point.

Rule 2: once you're well away from Kerbin, you can make very minor adjustments to your orbit which, on re-encountering the planet, will change your approach significantly and allow you to gain a boost. Essentially, your are multiplying the energy expended in "deep space maneuvres" and using Kerbin to enforce that multiplication of energies, giving a boost where none could have existed beforehand.

Rule 3: knowing what changes to make in deep space is a dark art, knowledge of which is enshrined in the tesseract, and only obtainable by slaughtering chickens during a full moon while wearing the cloven hoofs of a goat. Or you ask PLAD about it.