Can you use Ganymede to capture at Jupiter like how you can use Tylo to capture at Jool in ksp?

[ILikeSoup]

Because I’m wanting to do a Jupiter mission but don’t want to waste my Delta V capturing

 

[Altaïr]

It was very efficient before, but Ganymede's gravity has been reduced, so it's not as easy now. But you can still use it.

First, to get captured, the safest way is to aim for Jupiter's periapsis:


Then burn retrograde at the periapsis. Just enough to be captured:

Stop once you're on an elliptical orbit. This burn is very cheap, so you don't have to worry about it.

Then time-warp until you reach the apoapsis:

And burn prograde until your periapsis is slightly below Ganymede's orbit:

This maneuver is a bi-elliptic transfer. It's very efficient. In my case it only costed something like 60 m/s.

Now time-warp until you reach the periapsis, and try getting an encounter with Ganymede by burning slightly prograde or retrograde. This is a pain because of how the navigation system is buggy, but you can normally get an encounter for not much:

If possible, try to get the encounter on the ascending node, this will be more efficient. Make sure you also perform a retrograde fly-by. We are going to perform a gravity assist.

Then time-warp, and after your gravity assist, your trajectoy should be something like this:


Now adjust your apoapsis so that it's around 380000 km, and you should be able to get a new encounter by tweaking your orbit:


That new gravity assist allows you to reduce even further your orbit:


After that, it all depends on where you want to go. If your destination is Ganymede, you can directly insert yourself in orbit. If you aim for another moon, then you may try to chain other gravity assists, or time-warp to the apoasis and adjust your trajectory from there:

This is if my destination is Io for example.

 

[ILikeSoup]

Ok, thank you for helping! I will use this information in my next Jupiter mission

 

[ILikeSoup]

Thanks mate

 

[Altaïr]

Thanks mate

You're welcome, I'm glad it helped

 

[Pink]

If possible, try to get the encounter on the ascending node, this will be more efficient.

Small question, I'm wondering why encountering on the ascending node is more efficient. I'm thinking it's because an encounter on the descending node would uselessly affect your periapsis a bit more?

 

[Altaïr]

Small question, I'm wondering why encountering on the ascending node is more efficient. I'm thinking it's because an encounter on the descending node would uselessly affect your periapsis a bit more?

Oh, it's for the gravity assist itself. The purpose of the gravity assist is to deflect your velocity vector relative to Ganymede's velocity. But during the process, Ganymede itself rotates around Jupiter, and its velocity vector turns a little too.

If you encountered Ganymede on the descending node, you would have to perform a prograde fly-by, and your velocity vector would be deflected by a few degrees in the prograde direction. But Ganymede's velocity vector would also move in that direction because of its rotation, so the total deviation would be less, and the maneuver would lose efficiency. What matters is the deviation of your velocity vector with respect to Ganymede's velocity vector.

With an encounter on the other node, the opposite happens: your velocity vector is deflected in the retrograde sense, while Ganymede's velocity vector evolves in prograde sense. Now the 2 effects add up instead of substracting from eachother. That's what makes it more interesting.

You can have a more detailed explanation in this chapter of my gravity assist thread about the velocity vectors.

In short, the most efficient method is:
- to aim for the ascending node if you want to lower your orbit
- or to aim for the descending node if you want to raise your orbit.

The fly-by should be retrograde in both cases.

 

[Pink]

Oh, thank you very much! I understand that aspect much clearer now.

 

[Altaïr]

Oh, thank you very much! I understand that aspect much clearer now.

You're welcome