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bjerrang

Jool braking

Question

Interplanetary speed , think i saw 9000ms at jool burn

Im 14 days out of jools periaps. (can warp back but since i have many ships its a lot of extra work)

got 2000dV sience/relay sattelite with no heatshield. 

I manage to catch jools orbit via areobrake and burn. 

But i always end up with to little dV left to manuver. 

I see some talk about gravity assist to brake but i cant find the correct one. 

How do i brake in jools system without using up all dV

Does not matter if i end up on any other planet. 

Edited by bjerrang

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Generally, gravity assists near Jool use Tylo.  This is because Tylo has high gravity but without an atmosphere, so it has all of the mass needed to warp your trajectory but none of the re-entry heating.  This allows you to get down nearly to the surface, and if you need to supplement your assist with a burn, you get the most out of the burn there, as well.  Unfortunately, since you're only two days from Jool periapsis, you may be too close to get efficiently the assist you want--though you may be able to get an assist on the trip out; don't forget that.

In order to get a good assist, you need to approach Tylo from behind:  that is,  your approach to Tylo must be from Tylo's retrograde side.  To get the most from the assist, you want Tylo to warp your trajectory as close to 180 degrees as possible; however, while it is important, your angle of approach is less important than your angle of exit.  It is more important that your exit vector from Tylo's sphere of influence be exactly tangent and retrograde with respect to Tylo's orbit.  The reason for this is because getting an exit vector that is tangent to Tylo's orbit allows you to more easily get additional encounters for additional assists.

If you cannot use Tylo, you can get a gravity assist from Laythe, but Laythe has an atmosphere and a much smaller sphere of influence; it will not be ideal.  Vall provides a small amount of help but not enough, and the outer moons cannot help you at all.

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Ah!  This explains it.  There are a couple of things wrong with your approach:

  1. You're coming to Tylo from the side.  I can tell by the fact that the blue line of your orbit is coming from Jool's direction to the left of the picture:  for an ideal transfer, you'll need to approach from Tylo's retrograde, which would be the bottom of this picture.  That being said, you have other encounters that I cannot see; the line through Tylo's sphere of influence ought to be dull orange, not purple, if Tylo is your next encounter.  Since I don't have a larger picture, I cannot predict how your other encounters will affect this one, so I will refrain from saying more about this approach than to add that you may already be too committed by how far you are in the Jool system to do anything about it, so you may need to learn to live with it.
  2. You enter Tylo's sphere of influence from its prograde side.  You can see this from where the dotted purple line begins (in the upper middle of the picture:  the circle-with-an-arrow is the escape point and the blank end is the entry point).  You get a close encounter with Tylo (10 km isn't a bad altitude) but the problem is that the trajectory is almost straight through Tylo's sphere of influence.  This gives you little to no assist.  What you want is to keep the exit point where it is, but try to get an encounter that has your entry point in nearly the same place:  that is, you want Tylo's gravity to warp your trajectory back on itself.  That may not be possible with the orbital speed you have, so instead, it will probably take multiple assists, a burn, or a combination of burns and assists to shed your excess velocity.  You may also consider taking your Tylo periapsis even lower; that will warp your trajectory more.  In every case, it is okay to enter Tylo's sphere of influence from the side so long as you exit retrograde:  retrograde exits always assist a capture.

Edited to add:  69 days to apoapsis is not insane if you came into the Jool system insanely fast; the important part is to get captured into Jool orbit.  Refinements can come later, after you know that your ships aren't going to fly into interstellar space.  Also, an apoapsis that is 69 days away gives you a great place to make substantial adjustments to your periapsis (and subsequent Tylo encounters) for a minimum of your fuel reserve.

Edited again:  It is more important to leave Tylo's sphere of influence 'backwards' than to enter it 'backwards'.  This is because leaving Tylo correctly will always take away from the amount of delta-V you need to capture, but leaving Tylo incorrectly can add to the burn.

Edited by Zhetaan

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Do you have a photo of the manuver , then i might see it more clearly .

On the photo below i tired , but how do en entry from "behind"

Edited by bjerrang

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Can someone make a simple drawing of this , i still don understand how i should approach tylo or exit. 

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Think i understand some now. Here is a entry from behind.  Ill try to adjust that i will exit from behind now.  

5IXRtCo.jpg

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