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:
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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:
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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:
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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:
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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:
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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:
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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:
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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.
