Gravity Assists

[Stochasty]

This is a cross post from /r/KerbalAcademy.

At the request of a commenter in a previous post, I have decided to make a tutorial on how to plot and fly gravity assist maneuvers.

Introduction

A gravity assist is a maneuver in which you use a flyby of a secondary celestial body (such as a planet or moon) to alter your orbit about the primary (typically the sun, but occasionally a planet: for instance, when navigating the Joolian moon system). Gravity assists are useful because they allow you to gain or lose orbital energy or make expensive maneuvers such as plane changes for free; however, they are difficult to set up and require careful planning and lots of patience. That said, once you master them, you can manage feats you wouldn't have thought possible.

In orbit around Jool and low on fuel? A little help from Tylo can get you home for free. Heading to the Mun? A bi-elliptic transfer saves gas, but you can save even more using a Munar assist to boost your orbit first. And with the magic of Eve, you can get anywhere.

For the purposes of this tutorial, I am going to assume that everyone has a basic understanding of orbital mechanics: that you understand what prograde, retrograde, radial, anti-radial, normal, and anti-normal are, know what a plane change maneuver is and why it's one of the most expensive things you can do, understand the basics of how to get to other planets (ie., Hohmann trajectories, launch windows, etc.), and know what a sphere of influence (SoI) is.

I'm going to use the following conventions when referring to orbits: all directions are given in terms of the compass directions on the surface of the primary (so, the directions with respect to your navball when in deep space). When I say "above" I mean further out from the primary; "below" is closer in. "In front" or "ahead" and "behind" are in terms of an object's orbit about the primary and correspond to east and west on the surface of the primary; north and south are with respect to the poles of the primary. "Slower" and "faster" refer to orbital speed about the primary. "Closing speed" means the relative speed between your spacecraft and the secondary (planet or moon) at the moment at which you transition into the secondary's SoI. A "tangential (transfer) orbit" is an orbit which touches that of the secondary at exactly one point and which is heading in exactly the same direction at the point of contact; a "crossing orbit" is one which touches the orbit of the secondary at either one or two points, but is not heading in the same direction at the point of contact.

Okay, so now that the technical definitions are out of the way, on with the tutorial! In the following four parts, I'm going to try to answer the following questions: 1) How does a gravity assist work? 2) When is it appropriate to use one? 3) How do you set one up? 4) What kinds of gravity assists are there and what can be done with them? Because these questions are difficult to answer using just words, I'm going to intermittently link screen shots from in-game examples. So, on to business!

Part One How do gravity assists work?

The first thing to understand about a gravity assist is that orbits about a single body are fixed; the only way to get an orbit to change is to either do something (expend fuel) or run into something (aerobrake, lithobrake, or change SoIs). This last option is the one we care about: any time we change SoIs, we are performing a gravity assist maneuver. Furthermore, changing SoIs is the only way to perform such a maneuver.

As everyone who has experienced missions beyond LKO will recognize, when you encounter another object your post-encounter trajectory about your primary will be different than your pre-encounter trajectory (sometimes markedly so!). The reason for this is that while you are within the SoI of the secondary you are subject to its gravity rather than that of the primary. However, in order to be able to make use of this difference, we have to understand how this effect changes our post-encounter trajectory.

This first thing to understand is that, because we are approaching the secondary from outside it's SoI, after we enter the secondary's SoI we will be on an escape trajectory (typically a hyperbolic one, but not always so). Rule One is "what goes down must come up" (unless we slow down by burning or hitting something); we have too much velocity to remain within the secondary's SoI.

The second thing to understand is that gravity is a conservative force; our kinetic energy at any point in our orbit is equal to the kinetic energy we had when we entered the SoI plus the energy we have gained by falling (which is linear with respect to our altitude). While it's not really necessary to understand the physics here, the up shot is that because the SoI is a sphere centered on the secondary, our altitude at the point where we leave the SoI is exactly the same as the altitude with which we entered, so our kinetic energy (and hence our speed) will be the same. Rule Two is "you leave an SoI with the same speed with which you entered."

If this is the case - if our speed is the same before and after - how has the encounter changed our orbit about the primary? Rule One and Rule Two apply to our trajectory while we're within the SoI of the secondary, and thus are given from the point of view of the secondary body. While all of this is occurring within the SoI of the secondary, that SoI itself is moving with respect to the primary; thus, our velocity with respect to the primary is the sum of our closing speed with respect to the secondary and the secondary's own velocity about the primary. Furthermore, while our speed with respect to the secondary doesn't change as a result of the encounter, our direction of travel does.

Suppose we enter the SoI of the secondary from directly ahead, and then we use the gravity of that body to alter the direction of our trajectory by 180 degrees. This means that we will be leaving from the front of the SoI at exactly the same speed (with respect to the secondary) with which we entered. However, from the point of view of the primary, we started off traveling more slowly than the secondary by an amount equal to the closing speed, and we ended the encounter traveling more quickly by that same amount. Thus, we have added exactly twice our original closing speed from our velocity about the primary, and in the process turned what used to be the apoapsis of our orbit into the periapsis. This change, and it's reverse, mark the most basic (and often the most useful) type of gravity assist.

In the Mun example above, we were traveling at a (Kerbin relative) speed of 251 m/s prior to the encounter and a speed of 608 m/s after. Not quite a gain of double our closing speed, but the Mun's not heavy enough to bend our trajectory the full 180 degrees.

Part Two When is it appropriate to use a gravity assist?

Now that we understand the mechanism underlying gravity assists - using the gravity of the secondary to deflect our orbit - we need to understand when they should and should not be used. The answer to this question depends on two factors: closing speed, and the mass of the secondary.

As we have seen above, the most extreme change which can be made is to add or subtract twice our closing speed from our velocity with respect to the primary. This lets us change a periapsis into an apoapsis, an apoapsis into a periapsis, and reach a variety of orbits in between - but it is contingent on a number of factors. If the secondary body already marks the apoapsis of our orbit, we cannot go any lower than that: to slow down we need to exit the secondary's SoI from the rear, but our pre-encounter trajectory is already crossing the secondary's SoI from front to back; any deflection will move us off of this trajectory, increasing our speed relative to the primary and widening our orbit. Likewise, if the secondary marks our periapsis we cannot go any higher. (There are some caveats to this that have to do with inclination which I'll get into in Part Four.)

Thus, the primary use for a gravity assist is to use an encounter with an object intermediate between us and our target to help boost us to where we want to go. For instance, if we wanted to get to Minmus, we could plot a course for the Mun and use the Mun encounter to boost us into a higher orbit. Likewise, if we wanted to get to Moho, we could use an encounter with Eve to lower our orbit. However, we could not use an encounter with Duna to get to Moho if we were starting from Kerbin; the reason for this is that Duna marks the apoapsis of our Hohmann transfer orbit so we're already as low as we can reach from that encounter.

The second caveat has to do with closing speed. Since we are limited by a change of twice our closing velocity, if our closing speed is too slow the gravity assist won't help us much. This is why the Mun is typically a bad choice of gravity assist; a minimum energy Hohmann trajectory to the Mun gives you a closing speed of around 250 m/s, meaning a Munar gravity assist can theoretically provide a maximum of 500 m/s delta-v - however, this isn't enough to actually get anywhere (except Minmus). Post encounter, you'll wind up in deep space with no place you can reach except Kerbin, and if you try to burn in deep space you'll lose more getting anywhere than you gained with the assist (due to the loss of the Oberth effect). A possible way around this is to hit the Mun faster than a Hohmann trajectory would allow, but this means that you're no longer entering the Mun's SoI from dead in front, so you aren't getting the full effect of the assist. With careful timing and planning you'll still save a little, but only a couple of hundred m/s at best.

The third caveat has to do with the size of the body you are encountering. You are using the gravity of the secondary to deflect your orbit, but you want large closing speeds to maximize your gains. This means that you need a massive secondary; if the secondary is too light it won't be able to deflect your trajectory enough to accomplish your goal. This is another reason why the Mun makes a bad assist target except in certain situations. It also means that you typically shouldn't use Gilly, Minmus, Ike, Dres, Pol, Bop, or Eeloo. Moho is not optimal, but there's nothing else near it so if you are that far down in the Sun's gravity well you have no other choice. Duna is marginal; I've used it with success when getting to Dres, but that's about it. The optimal targets are (of course) Eve and Jool. Within the Joolian system Tylo, Laythe, and (to a lesser extent) Vall make good candidates.

Part Three How do you set up a gravity assist?

This is the tricky part; it involves much patience and planning.

The first thing you have to realize is that there's no hard and fast rule for how close you should approach the planet or in which direction you should pass. These depend on where you are coming from, how fast you are going, where you are trying to go, and the mass of the planet. The only good way to do this is to plot a maneuver node some distance away from the planet as you approach (typically well before you enter the SoI; the further away the cheaper any course correction will be) and play with it. Watch how your post-encounter trajectory changes depending on how you make your flyby, and try to get as close as possible to your intended post-encounter orbit. With practice you should get an intuitive feel for how to set things up.

Second, when you are trying to get somewhere, you need to do two things: you've got to put yourself into a transfer orbit, and you've got to time your transfer orbit so that the target you are trying to reach is in the right location when you get there. This is where understanding phase angles and launch windows come into play. If you don't understand how to transfer to and from different planets, you'll need to practice that until it's intuitive and then come back; trying to make sense of gravity assists before then is wasted effort.

Assuming that you do have a good grasp on transfers, what you are trying to do with gravity assists is to string multiple transfers together. You want to leave planet A on a trajectory which intersects planet B, and then use a gravity assist around planet B to put yourself into a trajectory which encounters planet C. This is a lot harder than it sounds.

The key to managing this is to start at the end and work your way backwards. Suppose you are trying to get to Eeloo by way of Jool. The first thing you do is look for a transfer window between Jool and Eeloo (I recommend heavy use of alexmoon's Launch Window Planner for this step). You'll want to find one a bit in the future, since you'll need time to get to Jool.

Once you've found that window, mark down the date of departure. At the same time, make note of the required ejection delta-v (starting from an orbit of a given altitude; say 500 km). That's the date on which you will need to arrive at Jool in order to make the assist. So, your second step is to search for a trajectory which will arrive at Jool on that date. This will typically not be during an optimal launch window, but you want to try to get as close as possible (try to minimize your ejection delta-v for the outbound leg to Jool). If there are no good trajectories, then you'll have to wait until the next Jool-to-Eeloo window and hope for better luck.

A third thing to keep in mind is the listed insertion delta-v when you arrive at Jool (make sure the altitude is the same as the altitude you used when you recorded the ejection delta-v above). While you will not actually be inserting into Joolian orbit, this does give you a measure of how fast you will be going when you arrive. Remember that you cannot change speeds during a gravity assist, so you want to try to get the listed insertion delta-v as close to the ejection delta-v as possible. If they are too far off you'll have to make an additional burn during the encounter in order to put yourself into the right trajectory, and this could potentially wipe out the savings from the gravity assist.

The fourth thing to keep in mind is whether or not your intermediate target has enough gravity to actually make the required course change using one encounter. For Jool, this typically isn't a problem, but for other planets it can be.

Lastly, although planning makes everything easier, it is possible to some extent to wing it if you are willing to circle the sun a bunch of times - and if you are trying to accomplish something really complicated you'll probably have no choice. The idea here is to try to put yourself into a crossing orbit as cheaply as possible (without regard for the timing), and then wait for a close approach. Use small deep space maneuvers to alter the period of your orbit so that you get an encounter, then use the gravity assist to send you towards your next target.

For the trip to Eeloo in the examples above, I started off by finding windows from Eeloo to Jool that minimized the insertion delta-v at Eeloo. Unfortunately, most of the early windows were not useful; the required departure date could not be reached cheaply from Kerbin. However, after a bit of searching, I found a window trajectory that left Jool on Year 3, Day 265 and arrived at Eeloo on Year 4, Day 281. Likewise, there was a trajectory from Kerbin timed to arrive during this window with an ejection delta-v of 2100 m/s - not optimal, but not too inefficient. Unfortunately, I arrived with slightly too much speed, and had to kill 213 m/s while passing Jool. I should also note that it took me more than an hour to set up this burn correctly: in order to do this right, there's a very narrow keyhole that you have to hit (in this case, only a few hundred km wide), and the only way to get it right is through trial and error. However, patience (and practice) makes perfect, and I arrived at Eeloo needing only a 352 m/s burn to insert into orbit. Grand total cost for the trip? 2100 m/s getting to Jool, 213 m/s correction at Jool, 352 m/s inserting into Eeloo orbit, and about 50 m/s for course corrections en route - so roughly 2700 m/s from low Kerbin orbit to low Eeloo orbit; nearly 1,000 m/s delta-v better than trying to fly to Eeloo directly. Not bad.

Part Four What kind of gravity assists are possible?

For this part, rather than trying to give a theoretical explanation of gravity assists, I'm going to give a variety of examples to help you get an intuitive feel for what is possible and what is not. I'm going to start a new game, so the MET time listed in each screenshot is also the date that shot was taken. Feel free to use your favorite interplanetary ship to try to replicate these maneuvers as you follow along.

Before you start, however, I strongly recommend that you edit your settings.cfg file and set "CONIC_PATCH_DRAW_MODE = 0" and "CONIC_PATCH_LIMIT = 5." This will make it much easier to see the effects of various flybys on your trajectory.

1) From below to above.

For the first example, I'm going to start simple and show you how to use a Munar encounter to boost yourself into interplanetary space. Note that in general using the Mun this way is a bad idea; I do it here solely for demonstration purposes. The general procedure for boosting your orbit is the same no matter which moon/planet you use do it.

After takeoff and orbital insertion, start off by plotting a transfer orbit from LKO to the Mun.

For a typical transfer orbit, you place yourself on a trajectory which is tangential to the Mun's; this means that you encounter the Mun at the slowest point in your orbit, and that the Mun will be catching up from behind. However, this is not terribly useful when planning a gravity assist; we won't have much speed to work with when we arrive at the Mun. Instead, I've burned into a slightly aggressive transfer, so I'm still carrying quite a bit of speed at the SoI change. While this would be bad if I were intending to land, it gives me a bit of extra momentum to work with here.

We want to boost our orbit, so we want a trajectory that leaves the Mun heading prograde with respect to Kerbin. With a little bit of fine tuning, we find the right trajectory, and voila! We're headed off into interplanetary space.

2) From above to below.

We're in deep space, but now we're in trouble. Our apoapsis is only about 2/3rds of the way to Duna, and if we try to burn the rest of the way we'll waste a ton of fuel. The only place we can reach is back to Kerbin. (This is why using the Mun is a bad idea.)

Fortunately, I anticipated this issue, and deliberately placed us into an orbit with a 4:3 resonance with Kerbin (meaning Kerbin orbits 4 times for every 3 of our orbits). Let's see if we can use Kerbin to reach Eve.

With some early course corrections (the earlier the better) I set up the encounter. This time, we're catching up to Kerbin from behind, so we want to swing completely around Kerbin and leave Kerbin's SoI heading retrograde - or as close to it as we can get (with a little serendipitous help from the Mun).

With a little bit of fine tuning we put ourselves on an Eve crossing orbit. (It's almost like I planned this or something.)

3) The magic of Eve.

After only one orbit we get an Eve encounter, and fortunately for us Eve is a near perfect gravity assist target. It's heavy, meaning we can make large course corrections, and it's inclined relative to Kerbin.

We're going to use Eve to try to get to Duna. But wait, you say (at least if you've been paying attention so far) - Duna is above Eve, and we're coming from above, so an Eve assist won't help, will it? That's where the magic of Eve's inclination comes into play - even though we're coming from above, we still have room to go higher because we're not coming in tangentially. This means that there's a normal component to our Eve-relative velocity, which we can turn into a prograde component, boosting our orbit.

The end result is that, even though we're coming from Kerbin, we can get almost to Duna with a single assist.

4) Walking our way up the ladder.

But this is the same problem we had before! We may be almost to Duna, but almost doesn't cut it. How can we get the rest of the way without resorting to an expensive deep space burn? Fortunately, there's one more card left we can play.

I've intentionally put us into an orbit close to a 3:2 resonance with Eve, so after circling around once we find that we need only a small burn at apoapsis to set up another encounter. The important thing about this burn is that it is retrograde, dropping our periapsis to within Eve's orbit.

As a result, when we arrive at Eve our Eve-relative velocity now has a small radial component which we turn into a prograde component to further boost our orbit. More importantly, when we cross into Eve's SoI we find that we are traveling over 700 m/s faster than we were during our last encounter.

So, for the small cost of a 113 m/s burn at apoapsis, we have gained over 700 m/s velocity at periapsis thanks to Eve. This lets us boost all the way into a Duna-crossing orbit nearly for free.

5) The end.

Now we're at Duna; where can we go next?

Unfortunately for us, Duna makes a really lousy gravity assist target and we're approaching at speed. Therefore, there is very little we can do now; even though we literally scrape the edge of Duna's atmosphere, we can only manage a small change of direction - not enough to do anything useful.

There is one more technique you can use - by putting yourself into orbits with integer or fractional multiples of Duna's period you can use multiple flybys to slowly alter course. Emphasis on the slowly - after this particular flyby I wound up in a 7:10 resonant orbit, meaning I'd need to wait another 1400 days before the next encounter (and that's just one of the several encounters I'd need to change orbit enough to get up to Jool). I don't have the patience for that, so for now we're stuck. Welcome to Duna, Jeb!

6) Plane changes

There's one last thing that I only covered in passing; using gravity assists to make plane changes. The trick here is no different than using assists to boost or lower your orbit, except that rather than passing in front or behind the planet you pass over one of the poles. I made a small plane change when I used Eve's inclination to boost my orbit, and a larger one in the example in part three where I went from Jool to Eeloo. The technique is easy; play around with maneuver nodes and it should come naturally.

So that's it for the tutorial. Gravity assists are tricky to set up and take a lot of patience, but they aren't all that difficult once you get some practice. Go out and give it a try! Jeb will thank you for it!

Edited October 20, 2013 by Stochasty

[Specialist290]

Impressive work!

It's funny that you're posting this here now, actually, because last night I added the original Kerbal Academy link to the Drawing Board...

[Stochasty]

Thanks.

I should probably mention that I did some additional editting and revision when I copied over from KerbalAcademy, so could I ask you to replace the drawing board link with this post?

[Specialist290]

Will do

EDIT: And done.

[DMagic]

Very nice and thorough. You might have mentioned it, but I find that maneuver node mods can really help here (MJ, maneuver node improvement, or anything else that gives you fine control over nodes). Because such small changes in your approach to the secondary body can have a huge effect on your resulting orbit it helps to be able to make very fine adjustments when setting up your maneuver nodes.

I never had any success when trying to use Eve to get to Moho, but I've recently used Jool twice when getting to Eeloo. With a little patience you can easily shave off 500m/s or so, and as you said, if you really get it right you can save over 1000m/s.

[Stochasty]

The trick to using Eve to get to Moho is that you have to plot two encounters with Eve; the course correction is too extreme to make in a single flyby, but you can put yourself into a 1:1 resonant crossing orbit with the first flyby and then make the rest of the assist during the second. You'll also typically need a small burn at periapsis during the second flyby to push your orbit low enough to intercept Moho; you can't quite gain enough energy from the gravity assist alone to get there (unless you make a non-minimum energy transfer from Kerbin to Eve).

[SRV Ron]

Some examples of gravity assist or slingshots;

1. Major orbital plane change using very little fuel.

2. Even more difficult, a near orbital capture.

3. Speed reduction to reach inner moons of Jool, or land a probe there.

Do plan to use thrusters to fine tune the encounters to get the desired results. These are difficult maneuvers to do correctly and it is easy to mess them up when burn times are critical to fractions of a second.

Retrograde encounters will reduce your orbital speed while prograde encounter will add speed.

[SanderB]

if your going to skip more than 100s of days to to get to your resonance encounter you can just quicksave, alt tab go to the quick save (f5) file in the ksp folder and add Days * 86400 to the universal time (UT = #)of your save file (at about line 750), save and then quick load (f9) to save minutes maybe even hours of your time.

Also I suggest always including 2 to 4 prograde pointed RCSs thrusters or small radial engines (on hotkey) so you can carefully adjust your orbit to get exactly the right encounter even though you might be several years away from it.

Edited October 22, 2013 by SanderB

[Stochasty]

Also I suggest always including 2 to 4 prograde pointed RCSs thrusters or small radial engines (on hotkey) so you can carefully adjust your orbit to get exactly the right encounter even though you might be several years away from it.

Very little need for this in my experience. Just make multiple correction burns; one far away to get close to your intended trajectory, then fine tune when you get closer. The extra delta-v required is miniscule (on the order of 1 m/s).

With the advent of the new reaction wheels, I never include RCS on any of my craft. RCS is heavier and less effective.

[SRV Ron]

With the advent of the new reaction wheels, I never include RCS on any of my craft. RCS is heavier and less effective.

RCS is best used for fine tuned burns for interplanetary maneuvers and landing small probes on the moons around the various planets. Landing gear optional. Otherwise, the reaction wheels provide more then enough torque for orientating ships prior to a maneuver burn.

BTY, I believe it is called Yang Ming San Park, is a beautiful place I visited in 1967 while in Taiwan on R&R. Real egg drop soup served at a local restaurant was very good. I was even able to use chopsticks for the noodles.

[rottielover]

This is really good stuff, I'm not sure I've digested it all. I'm one of those visual Lerner types and the screenshots helped a lot. Any chance someone can do or has done a video on this?

[CaptainBullet]

Thanx for the detailed tutorial. I knew how it works in principle, but it was still helpfull to read through this in order to really understand how to set those slingshots up. I mostly did them when I got an encounter by chance, but I'm now aiming for those assists. Next project: replicate Juno's maneuver to get to Jool. I'm not sure whether one can actually gain a significant amount of fuel with a Kerbin fly by, but its worth trying. Also: Nasa did it, so irl it works.

[Stochasty]

This is really good stuff, I'm not sure I've digested it all. I'm one of those visual Lerner types and the screenshots helped a lot. Any chance someone can do or has done a video on this?

Scott Manley did a video on using multiple Eve encounters to boost your orbit; it's not really a tutorial but it gives examples of several things I talked about. Video is

.

[Docterchese]

Although it's not the ideal body, I like to use Ike to get an escape from Duna after missions there. It can save a few 100m/s extra and it'd be rude not to since Ike orbits so close to Duna and is very easy to get an encounter with.

[SRV Ron]

Duplicate Post caused by browser.

[SRV Ron]

Although it's not the ideal body, I like to use Ike to get an escape from Duna after missions there. It can save a few 100m/s extra and it'd be rude not to since Ike orbits so close to Duna and is very easy to get an encounter with.

Not only easy to encounter, it is large enough with no atmosphere that you can get close enough to make major changes in orbital plane and speed using very little delta V. The burn times and setting up the maneuver mode is very critical. The use of thrusters will greatly aid in fine tuning the critical burns.

See post 7 for the screen shots examples.

[Stochasty]

Ike is very similar to the Mun in terms of gravity assists. It's good for using to manuever around Duna, but it's not good for interplanetary manuevers.

You can save about 100 m/s using Ike if you time things right, so if you are running low on fuel trying to get home it's worth a shot; however, just like with the Mun, if you do things the wrong way an Ike assist can cost you more than you save due to loss of the Oberth effect.

[Iron4venger]

Hi dudes.

I must say I find this topic incredible useful. I'm a probe guy, and I am totally nuts about gravity sheganigans this post really helped me a lot. I'm at the process of duplicating NASA missions, and that involves a lot of gravity stuff.

In return, let me share some wisdom, and maybe you want to add this at the first post, so we can create a nifty guide here.

Maybe this is interesting. In the case of making to Eeloo, I found a consecutive encounter from Duna to Eeloo through Jool,

This particular window is at day 260. I think is worth mentioning because theres only a window between Jool and Eeloo every 12 years, this window uses the fact that the aligment with Eeloo is at the descending node, so this windows is also incredible efficent.

Secondly, I've "found" that a retrograde orbit in Kerbin going to mun, leads to very interesting orbital effects. In fact, you can do a two consecutive encounters with mun first then minmus and back, in an already escape velocity of Kerbin in a single burn.

Some proof: .

I've not tested this in depth, but the main idea here is to create the best perigee point at Kerbin for maximum oberth effect when interplanetary. But at the moment I don't know if this setup worth the Av. :/

Also I have this question: ¿Does the Conic mode have any effect ingame depending on the mode number? My default is number 3, but I see lots of people using mode 1. ¿I'm missing something?

Also, I want to know how to plot orbits with my ships that are 2:1, 4:1 periods of the main body. :/

Edited October 27, 2013 by Iron4venger

[Stochasty]

Going into a retrograde Kerbin orbit and then plotting a Mun encounter is going to give you substantially more closing velocity at SoI change than you would be able to manage under normal circumstances: you should be hitting the Mun with somewhere around 750 m/s relative velocity. Unfortunately, this is where the Mun's lack of gravity comes into play; if you could bend your orbit fully around the Mun so that you were leaving along the prograde vector, you'd be exitting the Mun's SoI with a Kerbin-relative speed of around 1,250 m/s - certainly enough to get to somewhere interesting. Sadly, as your picture demonstrates, you can't bend your trajectory nearly enough, so you wind up on an orbit that only barely exits Kerbin's SoI; in fact, were Kerbin's SoI not finite in size, your post encounter orbit would be bound. So, this is a creative use for a Mun encounter, but once again demonstrates that it's hard to use the Mun to get anywhere.

Regarding Duna-to-Jool-to-Eeloo: it's interesting that there's a cheap trajectory for Duna that will arrive at Jool during that first window. Good to know; I'll need to check that for future missions to Eeloo. I'm a little concerned about efficiency; though. You can't use a direct gravity assist from Duna to reach Jool if you are starting at Kerbin. A Hohmann trajectory will not have enough speed to get all the way to Jool, and Duna doesn't have sufficient gravity in any case to alter your trajectory enough. About the only way I can think of to make that transfer is to aerocapture into Duna orbit and then burn for trans-Jool injection, but that's costly. In the long run it might still be cheaper than a direct transfer from Kerbin to Jool (there are tricks you can use to prevent the need for a complete capture), so it could be useful. I'll need to run some numbers to be sure.

[SRV Ron]

I was able to get that Eeloo intercept without a major orbital plane change when sending several probes to Jool. As mentioned, that window is once every 10 Kerbal years.

[Stochasty]

Also I have this question: ¿Does the Conic mode have any effect ingame depending on the mode number? My default is number 3, but I see lots of people using mode 1. ¿I'm missing something?

Also, I want to know how to plot orbits with my ships that are 2:1, 4:1 periods of the main body. :/

Missed this when I made my earlier reply. Conic mode changes how the trajectories are drawn in map mode. I use 0, a lot of people prefer 1. 3 is, in my opinion, terrible (it's probably good for beginners, but it's not useful for the fine tuning required for gravity assist manuevers).

As for getting 2:1, 4:! etc. orbits: look up the period of the planet's orbit in the wiki; then, when you are plotting your flyby manuever, place a second manuever node on your post-flyby trajectory as close to one period later as you can manage. The period of your post flyby orbit is then the difference between the time until periapsis of your flyby and the time until the post flyby node. Adjust until this is a multiple of planet's orbit.

[Iron4venger]

I've solved the conics mode question before. U need to change inbetween conics, so you can zoom in into the target planet and fine tune your orbit with the node editor. That's the reason of conics mode 0.

More info here. (Picture 13, 14 and 15)

http://forum.kerbalspaceprogram.com/threads/27236-Tutorial-Step-by-step-Interplanetary-Hohmann-transfer-guide-and-tips

Edited November 11, 2013 by Iron4venger

[THX1138]

On 20/10/2013 at 11:01 PM, Stochasty said:

Heading to the Mun? A bi-elliptic transfer saves gas, but you can save even more using a Munar assist to boost your orbit first.

That should probably read "Heading to Minmus?"

[specialopsdave]

Not counting plane-change assists, I see this as so complex that I'd just forget all other gravity assists and get the extra deltaV using the infamous "moar boosters" method (Or a 3.75m lifter to lift my 2.5m lifter to be used as an interplanetary drive... Liftception )

 

I know, I'm lazy.