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Advanced techniques: the gravitational slingshot
- Thread starter Altaïr
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Altaïr
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Ah, I used to do that when I was playing with 1.35 too. I even designed a technique that implied two slingshots with the Moon, and the second one would throw you on a transfer trajectory with only a small boost needed.
It still works now, but because the Sun is heavier under 1.4, a transfer to Venus is more demanding, and that technique doesn't provide enough speed anymore. Of course you can boost your second slingshot to compensate, but you practically nullify all the benefits then.
That's why I don't use it anymore, and to be honest, I would be surprised that a single slingshot with the Moon could significantly save fuel...
While this looks cool and attractive, sometimes this is not as efficient as you could think, you shouldn't overestimate that technique when used with light bodies. A single injection burn from LEO makes you benefit from the Oberth effect from Earth. Sometimes going simple can also be very efficient...
It still works now, but because the Sun is heavier under 1.4, a transfer to Venus is more demanding, and that technique doesn't provide enough speed anymore. Of course you can boost your second slingshot to compensate, but you practically nullify all the benefits then.
That's why I don't use it anymore, and to be honest, I would be surprised that a single slingshot with the Moon could significantly save fuel...
While this looks cool and attractive, sometimes this is not as efficient as you could think, you shouldn't overestimate that technique when used with light bodies. A single injection burn from LEO makes you benefit from the Oberth effect from Earth. Sometimes going simple can also be very efficient...
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Hello, first post here.
Excellent tutorial but I have a few questions.
1. Does it matter where Venus is in relation to earth for the first burn to Venus? I ask this because it seems to make a difference as far as the angle that the ship approaches Venus which also affects the escape point dramatically. In some cases I cannot get the same orbit that I achieve in other instances.
2. Your tutorial talks about distance in MM which I’m guessing is mega meters? And version 1.5 now uses kilometers. Has something changed in version 1.5 because the numbers don’t seem to line up.
3. After successful entry into Venus’ influence you mention to adjust the trajectory to capture earths entry the first time. Is it important to have that done before you cross Venus? Again with the new version it’s a bit tricky as far as the transfer point and in counterpoint are displayed.
funny enough, after I thought I did this right my little rocket burned down to 44% fuel. So I was excited to do a direct burn from earth to run out of gas and was projected to be a Jupiter with 44% fuel so I’m obviously doing something wrong, haha.
Thanks in advance!
Excellent tutorial but I have a few questions.
1. Does it matter where Venus is in relation to earth for the first burn to Venus? I ask this because it seems to make a difference as far as the angle that the ship approaches Venus which also affects the escape point dramatically. In some cases I cannot get the same orbit that I achieve in other instances.
2. Your tutorial talks about distance in MM which I’m guessing is mega meters? And version 1.5 now uses kilometers. Has something changed in version 1.5 because the numbers don’t seem to line up.
3. After successful entry into Venus’ influence you mention to adjust the trajectory to capture earths entry the first time. Is it important to have that done before you cross Venus? Again with the new version it’s a bit tricky as far as the transfer point and in counterpoint are displayed.
funny enough, after I thought I did this right my little rocket burned down to 44% fuel. So I was excited to do a direct burn from earth to run out of gas and was projected to be a Jupiter with 44% fuel so I’m obviously doing something wrong, haha.
Thanks in advance!
Horus Lupercal
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Yes, this tutorial works great in 1.4.
However.
1.5, the solar system has been re-scaled, so the distances Altair uses are no longer valid anymore and the nav system works totally differently also. Hence why the distances and 'units' are messed up
Also yes with respects to the position of Venus, it does matter. If you read through again, you'll see that you're supposed to burn a bit later than the standard transfer window position. The idea being is that it generates a much better angle for the ensuing slingshot and the better the angle, the more kick Venus gives you.
With the first Earth capture burn, you don't have to havethat sorted before you leave Venus SOI, but doing the burn either as you pass Venus or at the solar periapsis means the burn is more efficient than doing it somewhere random along the orbit.
However.
1.5, the solar system has been re-scaled, so the distances Altair uses are no longer valid anymore and the nav system works totally differently also. Hence why the distances and 'units' are messed up
Also yes with respects to the position of Venus, it does matter. If you read through again, you'll see that you're supposed to burn a bit later than the standard transfer window position. The idea being is that it generates a much better angle for the ensuing slingshot and the better the angle, the more kick Venus gives you.
With the first Earth capture burn, you don't have to havethat sorted before you leave Venus SOI, but doing the burn either as you pass Venus or at the solar periapsis means the burn is more efficient than doing it somewhere random along the orbit.
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Hi and welcome to the forum Walkthesky 
Horus explained it well, just to add a few precisions, the biggest problem is that the Solar system's scale has changed in 1.5, and the Sun has been made heavier too. All distances have been multiplied by 2.5, and the Sun mass has been multiplied by 3 approximately. All in all it doesn't make the game significantly harder, but the gravity assist procedures are no longer valid, I have to recalculate them.
For your questions:
1- Yes, Venus position is important. The VEEGA path implies to burn slightly past the optimal transfer, which was shown by the transfer window before, but this has changed too. Now your transfer is showed in a dotted line with the new navigation feature. To have a favorable approach angle, your periapsis should go below Venus'orbit. It would be interesting that I calculate that periapsis
Now you'll probably have a different result on 2 successive attempts, but that's what the corrections are for.
2- MM is mega meters indeed, it's how it used to be displayed in 1.4. And yes, distances have changed, I already answered that.
3- You can't really do the corrections before entering Venus's SOI, because the purpose is to get an encounter with Earth. You can only visualize the trajectory once you've entered Venus SOI, so it's best to make them at that moment.
Also, something that will be hard to manage is the second Earth encounter. The first gravity assist with Earth sends you onto a resonant orbit, which in short means that the ship will naturally encounter Earth again after a few turns. Because the distances changed, you would have to multiply your great axis (periapsis + apoapsis) by 2.5 for this to work.
I'll redo this when I have some time, feel free to ask if you have more questions
Horus explained it well, just to add a few precisions, the biggest problem is that the Solar system's scale has changed in 1.5, and the Sun has been made heavier too. All distances have been multiplied by 2.5, and the Sun mass has been multiplied by 3 approximately. All in all it doesn't make the game significantly harder, but the gravity assist procedures are no longer valid, I have to recalculate them.
For your questions:
1- Yes, Venus position is important. The VEEGA path implies to burn slightly past the optimal transfer, which was shown by the transfer window before, but this has changed too. Now your transfer is showed in a dotted line with the new navigation feature. To have a favorable approach angle, your periapsis should go below Venus'orbit. It would be interesting that I calculate that periapsis
Now you'll probably have a different result on 2 successive attempts, but that's what the corrections are for.
2- MM is mega meters indeed, it's how it used to be displayed in 1.4. And yes, distances have changed, I already answered that.
3- You can't really do the corrections before entering Venus's SOI, because the purpose is to get an encounter with Earth. You can only visualize the trajectory once you've entered Venus SOI, so it's best to make them at that moment.
Also, something that will be hard to manage is the second Earth encounter. The first gravity assist with Earth sends you onto a resonant orbit, which in short means that the ship will naturally encounter Earth again after a few turns. Because the distances changed, you would have to multiply your great axis (periapsis + apoapsis) by 2.5 for this to work.
I'll redo this when I have some time, feel free to ask if you have more questions
Thanks guys, I’m gonna keep trying this out. I essentially burn 32% fuel to get the venus flyby setup. Then only 2-4% for the one earth sometimes 2 earth flybys. After that, I either under or overshoot Jupiter’s orbit. Again, with the newer software, its hard to see what its trying to predict with transfers and encounters in mind. It’s almost erratic.
Altaïr
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I made a quick calculation of the most interesting resonant ratio. I didn't test them in practice, but this should allow you to plan the second Earth encounter. Here they are:
2:1 : great-axis = 23677.89 Mm
3:1 : great-axis = 31048.42 Mm
5:2 : great-axis = 27486.92 Mm
For example, if you choose the resonance ratio 2:1, it means the great-axis (periapsis + apoapsis) of your solar orbit should be 23677.89 Mm. Then you'll encounter Earth after one turn, the Earth will rotate twice around the Sun during this time.
The ratio 2:1 is the safest. It's not the one that will allow you to go the furthest, but for Jupiter it should be enough.
3:1 would be the most efficient, but it's also the one that requires to go the highest. Not sure you can use it without using a powered slingshot.
5:2 is in-between. It most certainly works, but the drawback is that you have to wait for 2 turns before encountering Earth again.
2:1 : great-axis = 23677.89 Mm
3:1 : great-axis = 31048.42 Mm
5:2 : great-axis = 27486.92 Mm
For example, if you choose the resonance ratio 2:1, it means the great-axis (periapsis + apoapsis) of your solar orbit should be 23677.89 Mm. Then you'll encounter Earth after one turn, the Earth will rotate twice around the Sun during this time.
The ratio 2:1 is the safest. It's not the one that will allow you to go the furthest, but for Jupiter it should be enough.
3:1 would be the most efficient, but it's also the one that requires to go the highest. Not sure you can use it without using a powered slingshot.
5:2 is in-between. It most certainly works, but the drawback is that you have to wait for 2 turns before encountering Earth again.
So this time I let the ship encounter and escape earth the first time, and time warped slowly until the new speed window showed up and it was close, -12mps. So I burned barely any to zero it out then time warped til I had an encounter, and here it is...
what’s your suggestion from here to encounter Jupiter? My great axis is 11,051,209 km, correct?
(what is that new speed window on transfer? Delta V)
And thanks, you guys are great!
what’s your suggestion from here to encounter Jupiter? My great axis is 11,051,209 km, correct?
(what is that new speed window on transfer? Delta V)
And thanks, you guys are great!
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Altaïr
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Indeed, on the second slingshot you should have tried to go higher than Jupiter, or at least at the same level. As is, your apoapsis is high enough to let you enter in Jupiter's SOI, but not enough to get an encounter with it. Don't be afraid to go higher, it will give you more room for your next maneuvers after that.
More generally speaking, getting the encounter with Jupiter is the tedious part. You can time-warp like crazy until you get a lucky encounter, or there's a smarter method, that also uses resonance: put your ship in solar orbit so that it's close to a resonance ratio (not exactly equal!)
In this case, you case use the resonance ratio 1:2 (great-axis = 48979.09Mm). In the time your ship performs 2 orbits, Jupiter will orbit once.
If you are close to this ratio, Jupiter's position will only slightly vary every 2 turns, which lets you anticipate a cheap encounter.
Of course, don't aim for the exact ratio, as your orbit would be synchronized with Jupiter and you would never encounter it.
When you entered Jupiter's SOI (finally!), you can use a slingshot with Ganymede to help you to slow down indeed. But if you don't manage to encounter it, don't make it a priority. After all, now you can aerobrake, and burning at the perijove is very efficient too.
More generally speaking, getting the encounter with Jupiter is the tedious part. You can time-warp like crazy until you get a lucky encounter, or there's a smarter method, that also uses resonance: put your ship in solar orbit so that it's close to a resonance ratio (not exactly equal!)
In this case, you case use the resonance ratio 1:2 (great-axis = 48979.09Mm). In the time your ship performs 2 orbits, Jupiter will orbit once.
If you are close to this ratio, Jupiter's position will only slightly vary every 2 turns, which lets you anticipate a cheap encounter.
Of course, don't aim for the exact ratio, as your orbit would be synchronized with Jupiter and you would never encounter it.
When you entered Jupiter's SOI (finally!), you can use a slingshot with Ganymede to help you to slow down indeed. But if you don't manage to encounter it, don't make it a priority. After all, now you can aerobrake, and burning at the perijove is very efficient too.
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That's the problem, you can't anticipate Ganymede's position. You can try slowing down when you enter Jupiter's SOI. Your trajectory won't change much, but because your speed is different you'll cross Ganymede's orbit at a different moment. At some point you will manage to get an encounter with Ganymede. I've already managed to do it, it's even possible to tweak your trajectory before you encounter Jupiter. The problem is that it shouldn't be too expensive...
All being said, how much fuel do you save direct injection vs veega? I’m seeing 15% ish.
Maybe I’m missing something but, I start in both cases LEO 100% fuel. 44% left to get Jupiter encounter with a direct injection vs 60-65% VEEGA. I ask this because in your demonstration on page 1, you run out of fuel before getting to Jupiter with your direct injection example. Maybe it’s all because of the fuel tank and engines you used...
Maybe I’m missing something but, I start in both cases LEO 100% fuel. 44% left to get Jupiter encounter with a direct injection vs 60-65% VEEGA. I ask this because in your demonstration on page 1, you run out of fuel before getting to Jupiter with your direct injection example. Maybe it’s all because of the fuel tank and engines you used...
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