I replayed that mission (
link) from a quicksave, when i only first time approaching Venus. If i change my trajectory near Venus just a little, i can catch Venus again over 4 laps around the sun in the same place and i can take one more deceleration from it, but that would be final, since my vector will be parallel to Venus after second rendezvous, and that means at any next lap i can't get more deceleration, i can only accelerate from it.
That second meeting conciderable reduced amount of fuel needed to burn to get to Mercury's perihelion. But it is still much cheaper to get to Mercury's apohelion - it requires almost no burn at all - if we just meet Venus from the other side of it's orbit (around the sun) in the first place.
And from Mercury's apohelion we can shift our orbit around the sun to Mercury's orbit around the sun by encounters with Mercury itself. Then we will have the same orbits and thus I cant find any difference at what point we approached Mercury first time.
So, i don't understand - how this benefit works? What do i get for extra fuel burned near Venus? It seems like just wasted fuel. Please tell correct strategy with it.
It's cheaper to aim for Mercury apohelion, but then you have to look at what it costs to get captured in Mercury orbit.
If you aim for Mercury's apohelion, you'll find yourself in this situation:
Your perihelion is at Mercury's apohelion, but your apohelion is at Venus level (Venus orbit is circular). Now you have to lower your apohelion to Mercury's perihelion, and that strategy maximized the difference between them, because the eccentricity doesn't play in your favor now.
On the contrary, if you aim for Mercury's perihelion:
That configuration costed more, but you're now in a more favorable situation: you have to reduce your apohelion from Venus level to Mercury's apohelion, the difference is much lower.
So all in all, you tend to lose on one side what you gain on the other side. So
why is it more efficient to aim for Mercury's perihelion then? In fact, mostly because Venus is more massive than Mercury. The fact that Venus is massive allows to perform some very efficient gravity assists. If you use this to your advantage, it'd better be for the most expensive part of the trip, since that's the part you will make free. On the contrary, Mercury is light, which means that chaining gravity assists with it is less efficient (but it still works), and you'll have less Oberth effect from Mercury. It means that your insertion burn would quickly be more expensive. So all in all, it's better to reach Mercury in the most favorable configuration since this is where you could spend a lot.
I see there that Altaїr didn't get a lot of slowing down from first rendezvous with Venus and instead used first rendezvous to gain even more difference in vectors with Venus, so at his next rendezvous he would get much more deceleration from Venus, than he could get with "normal" vector of first approach. And that would finally reach Mercury's perihelion without burning too much fuel.
That is very clever solution, it did not come to my mind. Thank you very much Mooncrasher!
That's exactly that. The first gravity assist is not that powerful, but the purpose is to chain a second gravity assist so I aim for a resonant orbit. Then the second one is performed at full power. All in all, that gives a better deflection than a single slingshot at full power.
In theory, it's possible to achieve the desired deflection in a single gravity assist, but that would imply to pass through the planet, and I'm afraid that the probe is not designed for that.