Advanced techniques: the gravitational slingshot

Altaïr

Space Stig, Master of gravity
Staff member
Head Moderator
Team Kolibri
Modder
TEAM HAWK
Atlas
Deja Vu
Under Pressure
Forum Legend
#1
Today I'd like to show you a powerful space maneuver that will save you a LOT of fuel during your future missions: the gravitational slingshot.
This presentation is the result of a collaborative work:
- The original text and diagrams are from me
- @TtTOtW and @8bitCosmonaut translated my text from "frenglish" :p
- @Ana Imfinity and @8bitCosmonaut verified the content and added some precisions
- @Aiden @Danny Batten @Gecko Gekkota @Earlmilly1 @GuHP20 and @SupremeDorian read the presentation, tried to practice and gave feedback

Special thanks to all of them :cool:

First of all, I recommend that you familiarize with the Oberth effect first: https://jmnet.one/sfs/forum/index.php?threads/advanced-techniques-the-oberth-maneuver.1215/
The gravitational slingshot is a very powerful technique, but also a tricky one, whereas the Oberth maneuver is a more basic one that still allows some substantial benefits.

Done? Ok, let's go!

In this text I'll use the following abbreviations:
LEO: Low Earth Orbit
SOI: Sphere Of Influence

Regarding the lowest and highest points of an orbit, for more precision I'll use the following terms:
- periapse and apoapse when the ship is orbiting a planet or a moon
- perihelion and aphelion when the ship is orbiting the Sun

What's the gravitational slingshot?

Basically, the gravitational slingshot, also known as gravity assist, consists of making a fly-by of a planet (or a moon) and letting the gravity curve your trajectory so that it provides you a free speed gain.
Here is an easy example to reproduce. With a small rocket, first aim for the Moon as if you were going for a Moon mission:
01-lunar_injection.jpg
Then, once you entered the Moon's SOI, just let your ship continue on its trajectory until it leaves:
02-slingshot.jpg
Hey, your orbit is much higher than before now! What's interesting is that you didn't spend a drop of fuel to do that! That maneuver is called a gravitational slingshot.

Something important to notice is that your fly-by must be done over the rear side of the Moon. By "rear", I mean the direction the Moon comes from. If you make a fly-by over the front side, the effect will be reversed.

Now, generally speaking, maybe we could exploit this effect to our advantage... But to anticipate our future trajectory we will have to understand how it works first! What happens exactly? Isn't it weird that the ship magically gains speed by just flying over the Moon? And why does the result depends on the side you fly over?

To explain this I'll use an analogy. This will look completely disconnected from our problem at first, but this is an easier experience to understand, as the gravitational slingshot obeys a similar logic.

Let's make an experience!

Now, let's forget spaceflight for a while, and let's play ball for a moment. You see that truck parked in front of you? What will happen if you shoot your ball against it?
You don't expect anything special to happen right? Here is the experience:
03-immobile_truck.gif
Indeed, the ball just bounces against the truck, and that's all...

But hey, what would happen if we threw it against the truck while it's driving? Well, let's try:
04-truck-moving-forward.gif
Wow, now the ball bounces at a much faster speed! What happened is that because of the truck's speed, there was a higher relative speed between the ball and the truck. Thus the ball received a greater impact from the truck and bounced stronger. Seems logical right?

Now, let's try the same experience, but with the truck driving away. What do you think it would happen?
05-truck-moving-backward.gif
Correct, now the ball bounces at a reduced speed. In this case, the relative speed was reduced, and then the impact was lower.

Is that OK? So let's get back to our Spaceflight Simulator!

Oh, before that, just a warning: in case somebody would be tempted to reproduce that experience in real life, be warned this is a bad idea! You would expose yourself and other people to danger (you probably have no idea how much speed a ball thrown at a fast moving vehicle could acquire), so please, DON'T DO IT! This is really a bad idea.

OK, let's go on now.

So, How the gravitational slingshot works?

Let's think about it... What happens during that fly-by of the Moon we were talking about? First, the ship enters the SOI at low speed, and as it gets lower it accelerates right? OK, but once it passed the periapse, the altitude increases again, and the speed diminishes until you reach the SOI limit. Now, your speed is the same as when you entered the SOI, so we should not be able to gain speed like that...

Indeed, but... Hey wait, the Moon is moving right? Of course it does, when you look at it from outside it orbits around the Earth, so yes it's constantly moving!
Remember how the speed of the truck influenced the speed at which the ball bounced? This is exactly the same phenomenon here. Of course, the Moon doesn't impact your ship (Well, generally it doesn't... :rolleyes:) like the truck with the ball, but the Moon still attracts your ship and manages to deflect its trajectory, so the same reasoning applies.

Now we understood how it worked, let's see exactly how you should proceed to use this to your advantage. Let's suppose our ship is cruising in space, and is about to encounter the Earth. Let's use that opportunity to perform a slingshot!
06-forward_slingshot.gif
See how the Earth deflects the ship's trajectory and increases its speed?
I've also included a ghost ship in the animation. Unlike the real ship, it's unaffected by the Earth's gravity, and keeps going in a straight line. If you carefully observe the 2 ships, you will notice that the real ship is much faster in the end.
What is important here is flying over the side the Earth comes from, so that it deflects your ship in the direction the Earth is moving (towards the right here).

Now, what if on the contrary we want to reduce our speed? In this case you just have to fly over the other side:
07-backward_slingshot.gif
See? Now the ship's trajectory is deflected in the opposite direction. As a result, the ship is considerably slowed down.

Is it OK? Then you get it! I'll just summarize what you need to remember, and we'll see in practice.
  • As we saw it, what's important is the direction in which the ship's trajectory is deflected: in the direction towards which the planet moves? It will speed up! In the opposite direction? It will slow down!
  • the more you lower the periapse, the more your trajectory will be deflected. Thus you will generally want to fly over the planet as close as possible.
  • Your trajectory will be more deflected by a heavy planet than by a light one. Imagine the result with Jupiter!
  • something important to notice is that the speed gained doesn't depend on the mass of your ship. This means that the result will be the same with a small capsule or with your biggest space station. This makes that maneuver particularly valuable for heavy ships!

Now, let's practice!

To finish that tutorial, let's see an example of an easy and popular use of that maneuver. Let's suppose you want to go to Mercury. As Venus is quite massive and is approximately halfway on your path, this is a very good opportunity to use the gravity assist maneuver.
To do this, first get an encounter with Venus, as if you actually wanted to go there:
08-transfer_to_Venus.jpg
You may notice that I made my injection burn slightly after the transfer window, which is why my trajectory goes below Venus orbit before the encounter. The reason for this is because the slingshot from Venus will be more efficient after that. Otherwise I wouldn't reach Mercury directly. The counterpart is that I had to burn a longer time to get onto that transfer trajectory, but because of the Oberth effect the difference is actually pretty small.
Then, which side should you fly over? Mercury is closer to the Sun than Earth, so we want to decrease our speed. In this case you have to make a retrograde fly-by, so that your trajectory is deflected backwards:
09-Venus_fly-by.jpg
Adjust your periapse so that it is just above Venus' atmosphere, and let the magic happen:
10-slingshot.jpg
See? This makes a Mercury mission considerably easier!
To get it right, you can first practice with the Moon, as this is a close and easy target, and then, make some tries with more ambitious missions!
 
Last edited by a moderator:

Altaïr

Space Stig, Master of gravity
Staff member
Head Moderator
Team Kolibri
Modder
TEAM HAWK
Atlas
Deja Vu
Under Pressure
Forum Legend
#2
How to get the most from a gravitational slingshot?

Now that we understand the basics, let's see how to use this technique as efficiently as possible.

Firstly, let's debunk a current approximation about this: It's common to assume that a fly-by on the rear face of a planet will increase your speed, and one on the front face will decrease it.
This will generally give the expected result, but not always. In practice, you may experiment an effect much weaker than expected, or even a reversed one! :eek:

As we saw, what is important is the direction in which the ship is deflected. If the ship is deflected in the direction in which the planet moves, it will speed up. Otherwise, it will slow down. This is closer to reality, but still approximate. To be precise, you also have to take into account your ship's velocity when entering the SOI.

Let's take an example:
01-slingshot.png

The picture shows the ship's entry into the SOI on the left, and its exit on the right. Each time, the ship's velocity is showed with a blue arrow, the planet's one with a red arrow.
The planet's velocity has slightly moved between the entrance and the exit to simulate the planet rotation.
What you have to look at is the angle between the ship and the planet's velocities. It's represented below each situation.
In this case, the slingshot makes the ship speed up. Not only because the exiting ship's velocity is aligned with the planet's one, but also because those velocities were far from being aligned at first. Having made that alignment better is what actually sped up the ship.

On the contrary, degrading that alignment will make it slow down. Here is an example of this:
02-backward-slingshot.png

In this case, the ship's velocity form a relatively small angle when entering, but ends nearly in the opposite direction. This time the speed has been decreased.

Just a precaution to avoid confusion: to correctly evaluate the angle, be careful to match the arrow's origins as shown on the image:
03-correct-angle.png

Understood? OK, let's go on some exercises! Don't worry, it will be easy. I'll show you a gravitational slingshot, and given the velocities shown, you will have to predict the result. Ready? Go!

Let's practice!

Here is the first situation to examinate:
04-Exercise1.png
Will the ship's speed increase or decrease in this case? Check the answer to verify!
As you probably supposed it, the ship will increase its speed in this case. The angle between velocities is not that great at the entrance, but they end perfectly aligned, so the ship speeds up.
OK, let's continue with a similar situation:
05-Exercise2.png

You found the answer? Check below to see if it's correct.
This situation looks very similar to the previous one at first, so maybe you've been tempted to answer that the ship will speed up aswell. Actually that's the contrary, it will slow down in this case!
As I explained, what you have to look at is how the angle between the velocities evolves. In this case they were perfectly aligned at the entry, and the maneuver degraded the alignment. Of course, the velocities in the end are still far from being opposite to each other, but the fact that the alignment has been degraded is what counts.

Something interesting to notice is that in this case, as the alignment is the best possible at the beginning, you can only degrade it during the maneuver, so there is actually no way you can speed up in this configuration! Even making the fly-by on the other side will slow down your ship in the end.

On the other hand, if you actually want to slow down, that's an interesting configuration, but choosing the other side to make the fly-by is actually less interesting: as the planet's velocity rotates clockwise due to its rotation around the Sun, it's better to deflect your trajectory towards the other side (counterclockwise) to maximize the angle formed in the end.
Is that OK? Let's go on with that one!
06-Exercise3.png
Indeed, in this case the speed will increase. This is the opposite situation now: As the velocities are perfectly opposite to each other at the entrance, you can only reduce the angle between them, so the speed can only increase in the end.
The same as before, flying over both sides will increase the speed, but deflecting the ship's trajectory counterclockwise will maximize the result.
Let's see another situation now:
07-Exercise4.png
Take your time to think about it. Is that OK? Check the answer!
In this case, the angle has been reduced, so the speed will increase in the end. However, because the angle has not changed that much, the result may not be amazing.
What's particular in this situation is that the trajectory has been overfolded: if it hadn't been deflected that much, we could have had a better alignment, and thus a greater speed in the end. If this situation happens, it may be better to raise your periapse, or if you're willing to spend a little fuel, to perform a powered slingshot for an even greater effect.
We'll talk about that after this.
Let's try a last one! What will happen in this case:
08-Exercise5.png
Have an idea? Check the answer:
Again, the trajectory is overfolded in this situation. But what's particular here is that the angle at the entrance and at the exit is the same, so the speed will neither decrease, nor increase (or not significantly).
In this situation, your ship's speed hasn't changed, but its direction did, so you may end on a similar orbit as before, but your apoapse/periapse will be shifted compared to before. However, this is not something interesting to do in practice.
Is that OK? Now, let's see how to use this to your advantage!

How to adjust your trajectory?

Now you've understood the importance of the planet's and the ship's velocities, I'll show you 2 efficient ways to modify the trajectory in your favor:

  • Adjusting the periapse:
I already talked a bit about this in the first post: lowering your periapse will deflect your trajectory further, so that's a very simple way to adjust the direction your ship will have at the exit. Of course, you can anticipate as soon as you aim for the planet, before you even enters the SOI, but because this is imprecise, you may want to adjust your trajectory precisely when entering the SOI. You can do this by burning at very low throttle, so it will consume practically no fuel.

  • Performing a powered slingshot:
A powered slingshot consists in combining the gravity assist with an Oberth maneuver: When you reach the periapse, you can burn prograde to make the maneuver even more effective.
Burning at the periapse will combine 2 effects:
- you will benefit from the Oberth effect, which will drastically change your trajectory even with a small impulse
- this will modify the angle of your exiting trajectory

As a drawback, it will consume a small quantity of fuel. However, if using that maneuver modifies the angle between the ship's and the planet's velocities in your favor, the combination of the 2 effects will be extremely powerful.
For this reason, it can be interesting to put your ship on an overfolded trajectory and then to modify the exit angle in your favor with this maneuver. Remember the two last situations in the previous chapter? This is the perfect situation to use that maneuver.

Something to notice is that it can also be interesting to burn retrograde at the periapse, but this is pretty rare. This will only be advantageous if your trajectory is very opened (when your ship is very fast), and if it modifies the exit angle clearly in your favour.

Here is how to perform those two maneuvers:
09-adjustments.png
The diagram on the left shows how to adjust the periapse when entering the SOI, and the one on the right shows the Oberth maneuver and how it changes your trajectory.

Now, you have everything you need at your disposal, the rest is practice! Remember, to get the most of a gravitational slingshot, it's all about precision!
 

Altaïr

Space Stig, Master of gravity
Staff member
Head Moderator
Team Kolibri
Modder
TEAM HAWK
Atlas
Deja Vu
Under Pressure
Forum Legend
#3
Until now we have seen the theory, now let's see in practice!

In this post, I will teach you how to exploit the gravitational slingshot to make an easy trip to Mercury and back.

For this, I'll use the ship below:
01-ship.jpg

This ship is in Low Earth Orbit, and my aim is to reach low Mercury orbit, and then to come back to Earth.

1) Gravity assist with Venus

Level: EASY

Instead of burning directly for Mercury, we'll first aim for Venus to use its gravity to slow us down..
So I set Venus as the target, burn to put my ship on a Hohmann transfer trajectory to Venus:
02-transfer window.jpg 03-transfert to Venus.jpg

You may notice an important detail: I did not burn inside the transfer window, but after!

This is actually better as you will enter Venus' SOI at a more favorable angle. Otherwise Venus's gravity won't be able to bend your trajectory enough. If you burn in the transfer window, that's not a problem either, you'll just have to perform a powered slingshot around Venus to compensate.

Then, time-warp to enter Venus's SOI. Here is my trajectory:
04-slingshot Venus.jpg 05-Exit Venus.jpg

Now, I'm on a transfer trajectory to Mercury. Something important to do when you enter the SOI is to adjust your periapse: if your transfer trajectory to Mercury doesn't go low enough, you can correct this by lowering your periapse at Venus. If you can't lower it anymore, then you can burn prograde at the periapse (powered slingshot) to increase the slingshot effect.
However in this case, it may not be necessary though.

2) Gravity assist with Mercury

Level: TRICKY

Venus did a large part of the work, but now we will use Mercury to slow us down further and lower our orbit, making the insertion easier afterwards. However this may be more difficult, because unlike Venus, Mercury is smaller, so the Oberth effect will be less effective there. Moreover, its close distance from the Sun implies that the velocities involved are higher (which is why going to Mercury is hard in general), so modifying our trajectory requires greater effort.
It's still possible to save a significant amount of fuel, but this will still require us to perform 2 or multiple slingshots.

For this I'll indicate specific orbits to go for, as they are calculated so that you need very few corrections. It will also make your mission much easier, so I strongly advise to use them!

So back to our Venus slingshot:
05-Exit Venus.jpg

While you're in Venus, adjust your trajectory so that your perihelion will be at 1212.5 Mm (Mercury is at 1215.5 Mm).
Then exit Venus, and when you reach the perihelion, burn retrograde to adjust your aphelion at 1980 Mm:
06-transfert Mercury.jpg

Now. Set Mercury as a target, and time-warp until you get a encounter.
Your distance to Mercury will get slightly closer every 2 turns, so it's relatively easy to predict an encounter.

When you get it, adjust your trajectory to cross Mercury to perform a first slingshot:
07-slingshot Mercury.jpg

Now you have to adjust your trajectory so that your apoapse is at 1745.8 Mm.
For this, you can lower your periapse, and if that's not enough, you can burn at the periapse to boost your trajectory. This may not be necessary, but if you need it, you'll need to burn retrograde.

Now let your ship exit Mercury, and when you reach the aphelion, burn prograde to raise your perihelion at 1212.5 Mm:
08-exit Mercury.jpg

Now you just have to wait the next encounter. What's interesting with that orbit is that you will naturally encounter Mercury again after 3 turns. It will avoid the need to time-warp until you get an encounter. See why this trajectory is interesting?
You will probably have to make a small adjustment on your last turn.
Then, again, slingshot with Mercury:
09-exit Mercury2.jpg

This time, adjust your apohelion at 1486.3 Mm, and when you reach the apohelion, raise the perihelion to 1215.0 Mm, as shown on the image.

This time, you will have to wait 6 turns to get another encounter.
When you get it, you can get into low mercury orbit easily.
10-Mercury orbit.jpg

Something important to be aware about is that the 2 slingshots from Mercury allow to save slightly more than 200 m/s. It's not huge, and every unnecessary maneuver will reduce that gain.
That's why I consider it's better to aim for orbits that will be naturally synchronized.

Now, you may think that doing this to save "only" 200 m/s is not worth it, you could simply add a bit more fuel to your ship after all. In this case you may be right, but for a heavier ship this means adding more than a little fuel, and as your ship gets heavier, you may need more engines, which will make the ship heavier again etc... Not even speaking about the rocket that will have to propel that extra mass onto LEO first.
In such cases, that technique could prove very valuable! And especially as we didn't even take into account the return.

To be continued...
 

Altaïr

Space Stig, Master of gravity
Staff member
Head Moderator
Team Kolibri
Modder
TEAM HAWK
Atlas
Deja Vu
Under Pressure
Forum Legend
#4
Now we shall perform a return trip.

3) The return path

Level: HARD

We will use Venus's gravity again, but its still possible to use Mercury to your advantage for greater efficiency.

My technique still relies on synchronized orbits for practicality.

So the first step is to burn and exit Mercury so that your perihelion above the Sun is at 1060 Mm:
11-exit Mercury.jpg

Therefore will will fly below Mercury's trajectory.
When you reach the perihelion, burn prograde until you encounter Mercury again:
12-trajectory adjusted.jpg

Now we are ready for the first slingshot:
13-slingshot Mercury.jpg 14-exit Mercury.jpg

In this case I end up on an orbit which perihelion and aphelion are 1141.0 and 1559.9 Mm respectively. When reproducing this, you may have some slightly different values. In this case, what is important is that "perihelion + aphelion = 2700.9 Mm".
This is what guarantees that you'll encounter Mercury again after 6 turns, so try to adjust your trajectory so that you get as close as possible to this value. If there's a slight difference, you may just have to make a small correction on the last turn.

Then after 6 turns you can perform a second slingshot and aim for that trajectory:
15-exit Mercury2.jpg

Same as before, but this time, your "perihelion + aphelion" should be 2958.3 Mm, and you'll meet Mercury after 3 more turns. In this case, my perihelion and aphelion are 1181.0 and 1777.3 Mm respectively.

Finally, the third slingshot from Mercury:
16-exit Mercury3.jpg

Now we go for Venus. Try to adjust the aphelion at 2050.0 Mm. Personally, I had to graze Mercury on that last slingshot, and boost it a little by burning prograde at the periapse.
Then, when you reach the aphelion, burn prograde to raise your perihelion to 1250.0 Mm:
17-transfert to Venus.jpg

On this orbit, you're safe from encountering Mercury again. Now you have to get an encounter with Venus. On this orbit, your distance may slightly vary every 4 turns.
18-transfert To Earth.jpg

After the last slingshot from Venus, you'll be on a transfer trajectory to Earth. All you have to do is to cross Earth trajectory, encounter with Earth, enter Earth's atmosphere, and welcome your craft back home.
19-end.jpg

Now it's your turn.
For this, I advise you to try first with a similar ship with more fuel (at least a 20 tons fuel tank) and without the electric equipment. First limit yourself to the first few parts, then when you've got the hang on it, try the later few parts, by then you should have already understood and mastered the "Lower Goldilocks Region" gravity assist.
 

Altaïr

Space Stig, Master of gravity
Staff member
Head Moderator
Team Kolibri
Modder
TEAM HAWK
Atlas
Deja Vu
Under Pressure
Forum Legend
#5
In this last chapter, I'll show you a gravity assist combination that allows to reach Jupiter more efficiently than via a direct transfer. This combination is known as the VEEGA path ("Venus Earth Earth Gravity Assist"), and had been used in real life by the Galileo probe.

For this, I'll use this ship below, parked in LEO at the beginning of the mission:
01-Probe.jpg
It looks quite small but bulky for such a mission right? If you try to reach Jupiter with a direct transfer burn, this is what you will end up with:
02-direct_transfer.jpg
The mission will fail.

We will now try this technique out:

The VEEGA path

Level: TRICKY

So for this technique, our first target is Venus:
03-injection_burn.jpg 04-injection_burn2.jpg

As for the previous trip to Mercury, I didn't burn in the transfer window, but after it. As you can see I burnt after the transfer window. This is very important, otherwise you will not be able to raise your orbit later on.

This time, aim for a prograde fly-by, with your periapse close to the atmosphere line.
Then when you enter Venus' SOI, set the Earth as a target, and adjust your trajectory so that you encounter Earth again:
05-slingshot Venus.jpg
You may be able to do this by only adjusting the periapse.

Ideally, your periapse above Earth should be around 250 km. Then time-warp until you enter the Earth's SOI:
06-slingshot Earth.jpg
Now, you must adjust your perihelion so that your "perihelion + aphelion" value is equal to 10959.6 Mm. This generally happens when your aphelion is around 8335 Mm, so you can first aim for that, and then make the necessary adjustments.
If you do not match the exact value, you may have to perform a small correction later. This is affordable, but obviously, it's better to be as close as possible.

This value is important because it ensures that you'll meet Earth again after 2 laps. The closest approach line mechanism won't anticipate that, so it's better to rely on a pre-calculated value.

After 2 turns you encounter Earth again:
07-slingshot Earth 2.jpg
Ideally, before entering the Earth's SOI, you should adjust your periapse so that it's around 600 km.
Now, the objective is to set the aphelion of your exiting trajectory at 16200 Mm.

OK, now we are on a transfer trajectory to Jupiter. When you reach the aphelion, burn prograde to raise your perihelion to 3000 Mm. This will ensure you won't meet Earth again (you still have to watch out for Mars though), and you'll get slightly closer to Jupiter every 2 turns, until you get an encounter:
08-Approaching Jupiter.jpg 09-Approaching Jupiter 2.jpg 10- Jupiter encounter.jpg
Now compare this with the fuel consumed on a direct transfer burn, you'll immediately notice the impressive amounts of fuel saved.

In case you get an encounter with Mars in the process, you may just slightly modify your trajectory to avoid it (if you do it early enough this will cost practically nothing), and come back to your initial trajectory after one turn, once the "danger" has passed.

This trajectory will save around 400-500 m/s, which is quite valuable with a heavy ship.
 

BANDWITH

Embodiment of Made In Abyss spoilers
Professor
Swingin' on a Star
Man on the Moon
Registered
#8
Neat!
I'm sure I will find this very useful sometime in the near future!
 
#9
This presentation is very important. Your work will help many people, not only SFS players, but also people using other space simulator games and sci-fi writers. Thank you for the permission to share this with Terraforming Wiki. The techniques described here can be used also for flying between the moons of gas giants and not only.
 

Altaïr

Space Stig, Master of gravity
Staff member
Head Moderator
Team Kolibri
Modder
TEAM HAWK
Atlas
Deja Vu
Under Pressure
Forum Legend
#11
VERY VERY NICE JOB!!!!!!!!!!!!!!
I tried to Quote it all but it said,
"no more than 1000 characters
Wow, it seems that I wrote really a lot!
Thanks everyone for the positive return :cool:
 
#17
This post is masterfully crafted, and it really helped me understand some of the details of how the maneuvers work. Thanks for posting such content!
 

Altaïr

Space Stig, Master of gravity
Staff member
Head Moderator
Team Kolibri
Modder
TEAM HAWK
Atlas
Deja Vu
Under Pressure
Forum Legend
#19
This post is masterfully crafted, and it really helped me understand some of the details of how the maneuvers work. Thanks for posting such content!
Thanks for your return, I'm glad you appreciate it :)
 

Blazer Ayanami

Space Shuttle enthusiast // Retired Admin
Registered
Forum Legend
#20
@Altaïr could you make a guide about how to perform gravity assistances with Jupiter's moons to help achieve an Europa Orbit with less fuel??
Also If its possible, what's the most efficient way to get in and about of the Jovian System?
 

Horus Lupercal

Primarch - Warmaster
Professor
Swingin' on a Star
Deja Vu
Biker Mice from Mars
ET phone home
Floater
Copycat
Registered
#22
@Altaïr could you make a guide about how to perform gravity assistances with Jupiter's moons to help achieve an Europa Orbit with less fuel??
Also If its possible, what's the most efficient way to get in and about of the Jovian System?
VEEGA then Ganymede. Hit it right from outside and it'll cut your speed down easily enough to keep you inside Jupiter SOI. Once you're in, keep bouncing off moons until you're roughly where you want to be.

Exiting is easy. Hit a moon hard enough and it'll throw you right out of the SOI with change to spare. I normally do another Jupiter slowdown assist after that to bring my orbit as close to Earth as possible (or get it really wrong and end up past Uranus...) and then dive into Earths atmosphere at about 22km which usually slows me down enough to achieve Earth Orbit. And if I do end up out by Uranus, then it's easier than it sounds to bring your periapsis to the same as Earths and throw yourself into the atmosphere for some aero braking
 

Blazer Ayanami

Space Shuttle enthusiast // Retired Admin
Registered
Forum Legend
#23
VEEGA then Ganymede. Hit it right from outside and it'll cut your speed down easily enough to keep you inside Jupiter SOI. Once you're in, keep bouncing off moons until you're roughly where you want to be.

Exiting is easy. Hit a moon hard enough and it'll throw you right out of the SOI with change to spare. I normally do another Jupiter slowdown assist after that to bring my orbit as close to Earth as possible (or get it really wrong and end up past Uranus...) and then dive into Earths atmosphere at about 22km which usually slows me down enough to achieve Earth Orbit. And if I do end up out by Uranus, then it's easier than it sounds to bring your periapsis to the same as Earths and throw yourself into the atmosphere for some aero braking
THANKS @Horus Lupercal:) See, where i'm having trouble is precisely that "bouncing between moons" part. I know most of the rest...
 

Horus Lupercal

Primarch - Warmaster
Professor
Swingin' on a Star
Deja Vu
Biker Mice from Mars
ET phone home
Floater
Copycat
Registered
#24
THANKS @Horus Lupercal:) See, where i'm having trouble is precisely that "bouncing between moons" part. I know most of the rest...
Yeah, my path looks more like a game of Jupiter moons pinball than an organised flight plan. I've not done Europa yet either, Io and Ganymede mostly so mine will be different to yours and I still had to do a furious amount of burning to make an orbit once I'd done all I could with assists. And landing shouldn't be so bad, Europa ground speed is half that of Ganymede.
 

Lt. Snakestrike

The Kronian Serpent; Engineering Student
Head Professor
TEAM HAWK
Swingin' on a Star
Atlas
Under Pressure
Registered
#25
Yeah, my path looks more like a game of Jupiter moons pinball than an organised flight plan. I've not done Europa yet either, Io and Ganymede mostly so mine will be different to yours and I still had to do a furious amount of burning to make an orbit once I'd done all I could with assists. And landing shouldn't be so bad, Europa ground speed is half that of Ganymede.
I do the same thing with Saturn. Moons don't give as much of an assist though...