Mercury and Back

[Roger Jolly]

I confirm, there's even a strong possibility that you spend more than what you would have saved. From the delta-V map, the difference between an injection towards the Moon and Venus is about 76 m/s. That's really few, and you have to substract from this all maneuvers, corrections, and the boost at Moon level. You'd already be lucky that the benefit is 0

A Moon assist doesn’t take any maneuvers at all, unlike a mid flight target you can just sit in orbit until the moon is in the right phase and lined up well enough for whatever the next target is and then ‘we’re going that way anyway’

 

[Altaïr]

A Moon assist doesn’t take any maneuvers at all, unlike a mid flight target you can just sit in orbit until the moon is in the right phase and lined up well enough for whatever the next target is and then ‘we’re going that way anyway’

I was talking about maneuvers because when I used this technique long ago I used 2 slingshots from the Moon, and I had to adjust the second encounter. But that was viable under 1.35, under 1.4 it was not advantageous anymore. Under 1.5 this is even worse. Actually Venus is closer to the Earth, which helps, but the Moon is also higher too. As a result it orbits slower and gives even less kick. And not to mention that for the same reason the difference between an injection towards the Moon or Venus is even tinier than before, so there is less to save.

I made a calculation, by supposing you manage to put your ship on the best trajectory, and if you attempt this you'll have to power your slingshot by an additional 100 m/s, which is more than the 76 m/s you intend to save compared to a direct injection.

So even in a perfect world this is a losing bet now. That technique is cool and everything, but seriously, it's not efficient. Sometimes simplicity works better. Believe me, if it was working I would be the first to defend that technique

 

[Roger Jolly]

I was talking about maneuvers because when I used this technique long ago I used 2 slingshots from the Moon, and I had to adjust the second encounter. But that was viable under 1.35, under 1.4 it was not advantageous anymore. Under 1.5 this is even worse. Actually Venus is closer to the Earth, which helps, but the Moon is also higher too. As a result it orbits slower and gives even less kick. And not to mention that for the same reason the difference between an injection towards the Moon or Venus is even tinier than before, so there is less to save.

I made a calculation, by supposing you manage to put your ship on the best trajectory, and if you attempt this you'll have to power your slingshot by an additional 100 m/s, which is more than the 76 m/s you intend to save compared to a direct injection.

So even in a perfect world this is a losing bet now. That technique is cool and everything, but seriously, it's not efficient. Sometimes simplicity works better. Believe me, if it was working I would be the first to defend that technique

Well heck, did a test run with an unpowered assist off the ‘New Moon’ vs a direct burn and saved quite a bit of fuel going direct;
I cannot comprehend why, will require more painful thinking

 

[Altaïr]

Well heck, did a test run with an unpowered assist off the ‘New Moon’ vs a direct burn and saved quite a bit of fuel going direct;
I cannot comprehend why, will require more painful thinking

That's the magic of astrodynamics
That can be explained by the Oberth effect: accelerating while being close to a heavier body will have a greater effect. In this case, it means that spending 100 m/s from LEO will send you further than if you spend 100 m/s at Moon level.
With the direct injection burn, you rely fully on the Oberth effect from the Earth. When you boost a Moon gravity assist, you get the Oberth effect from the Moon, which is lower. That could be efficient if you only needed little to no boost (like if you just wanted to go to a very high Earth orbit), but Venus is too far in this case.

 

[Roger Jolly]

That's the magic of astrodynamics
That can be explained by the Oberth effect: accelerating while being close to a heavier body will have a greater effect. In this case, it means that spending 100 m/s from LEO will send you further than if you spend 100 m/s at Moon level.
With the direct injection burn, you rely fully on the Oberth effect from the Earth. When you boost a Moon gravity assist, you get the Oberth effect from the Moon, which is lower. That could be efficient if you only needed little to no boost (like if you just wanted to go to a very high Earth orbit), but Venus is too far in this case.

Ahhh...
Of course, and is why I did so terrible burning from Sun orbit to Venus after an unpowered Lunar flyby

 

[Altaïr]

Ahhh...
Of course, and is why I did so terrible burning from Sun orbit to Venus after an unpowered Lunar flyby

Exactly
Maneuvers in solar orbit are referred to as deep space maneuvers. They are the most inefficient for that reason. They are still useful to adjust an encounter of course, but large maneuvers are to avoid in Sun orbit.