Encountering Pol without circularizing at Jool

[Reinhart Mk.1]

I did this same thing with Gilly and it seems WAY more efficient than even getting a gravity assist from Tylo to get into a Jool orbit... have i discovered something? Does anyone else do this? If not then why not?

[Aegolius13]

Do you mean to ultimately get into Pol orbit?  I've noticed similar things, at least on Laythe and Tylo.  If you encounter the moon with the trajectories tangent (i.e., while it's travelling in the same direction you are), you're taking full advantage of the moon's rotational velocity around the planet.  Kinda like how it's easier to catch a ball if you move your hand back as you catch it -- the difference in relative velocity is smaller.  A less-than-ideal gravity assist may give up as much of this benefit as it gains in apo/peri adjustment, especially once you factor in maneuvering burns.  And your approach certainly saves a lot of work over plotting an ideal assist.

I would think the outer moons would provide less of a benefit here than, say, Tylo or Laythe, since their orbital speed is lower (though of course YOUR orbital speed around Jool would be somewhat lower as well. And since their gravity is lower, the Oberth effect won't soak up as much of the delta-v needed to get into capture around Jool and ultimately the target moon.

I don't really follow with Gilly, though, as there's nothing else in Eve's SOI to get a gravity assist from, so no alternative to compare too.

 

 

[Reinhart Mk.1]

32 minutes ago, Aegolius13 said:

I don't really follow with Gilly, though, as there's nothing else in Eve's SOI to get a gravity assist from, so no alternative to compare too.

I had the video all edited but there was one clip that was bugged so i cant exactly show it now, but basically with both i get an encounter with the parent planet (if that's even a word but i.e. eve or jool) then do a course correction to where the trajectory enters the right trajectory for an encounter with the smaller planet (i.e. gilly or pol) then circularize. i did find it very hard to circularize and i didn't have enough fuel (for pol, i know gilly is notorious for that). then i checked the delta v map and noticed i needed 820 m/s and could've have made it even if i tried 

[Empiro]

You definitely shouldn't circularize at Jool or Eve if you're trying to get into orbit around around a far-away moon like Pol or Gilly, but I suspect that isn't what you meant either. I assume that you're actually talking about a course where (assuming you're in the right inclination already), you burn close to Jool just enough such that your AP intersects Pol's orbit. You'll eventually encounter Pol at your AP and can capture.

Whether or not that saves Delta-V compared to capturing directly at Pol depends on a lot of factors. The general intuition is that by capturing close to Jool, you're taking maximum advantage of the Oberth effect since you're moving so fast close to Jool. It's easy to burn off the excess orbital energy you had when entering Jool. However, the cost is that you then have to raise your PE to match Pol's. It's similar to the tradeoff between a Hohmann Transfer versus a Bi-Elliptic. I haven't done the math, but I suspect that Eve and Kerbin are close enough that there's probably some savings in capturing at Gilly directly. However, Jool is far enough (and massive enough) such that I think it's still probably better to burn close to Jool.

The optimum also can change if the moon is very massive, since you can take advantage of the Oberth effect there too, but both Pol and Gilly are small enough that I don't think it makes much of a difference here.