So I read a Reddit comment explaining how to do multiple gravity assists (something like a kekkj assist) so I went to try it out myself but realized that I had no idea how to get the first eve assist. The comment just said to get an eve intercept and then your trajectory should pass kerbins orbit but after I got an intercept I was never able to get my apoapsis above kerbin’s. I’ve tried tweaking it but no matter what direction on the maneuver node I pull, I can’t raise it higher. I used astrogater to plan the intercept and I’m not sure if it matters what type of transfer you use to get to eve. What am I doing wrong?
#help with gravity assist
vagrant gust
tall knot
Ideal gravity assist has your exit vector in the same direction as the celestials prograde vector is.
You likely passed eve on the wrong side, exit vector aligned with eve retrograde.
Closer you pass the more energy youbget from the assist.
https://wiki.kerbalspaceprogram.com/images/thumb/f/f8/Gravity_Assist.svg/600px-Gravity_Assist.svg.png shows the idea pretty good.
The hard part is chaining assists in a reasonable timeframe
vagrant gust
is there a way to find out which way the planet is gonna be facing when i get there? In this picture it looks like im going to be launching directly prograde but when i eventually reach eve i actually end up losing energy
what i tried doing was creating a node around 80 days before the encounter and just pulling on the prograde or retrograde markers until the apoapsis got to the peak and started to fall back down but this barely changed my orbit and when i zoomed into eve my trajectory barely even touched its soi
but moving my trajectory closer to eve would result in my apoapsis lowering
i also made sure to test my trajectory on both sides of the planet but both of them kept lowering my apoapsis
actually wait i just realized i can check which way ill be ejecting
tall knot
is there a way to find out which way the planet is gonna be facing when i get th...
I either eyeball it based on the soi exit point, or change conics mode
tall knot
but moving my trajectory closer to eve would result in my apoapsis lowering
Yup, wrong side of the planet
vagrant gust
in this case going to the right or left (a 90 degree curve) would just add radial energy
and i would need my trajectory to go straight up to eject prograde but to keep my trajectory straight i would need to not have my trajectory curve but then that's not even a gravity assist
this maneuver node would make me eject prograde from eve
but look at my apoapsis with the assist
and look at it without the assist. barely anything changes
tall knot
What if you pass on the other side of the planet?
vagrant gust
this would eject me prograde on the other side of the planet
and im actaully losing energy here
tall knot
Only thing I can think of is to miss the transfer window a bit. That way you'll come at a different angle. Don't know why that isn't getting you more energy.
frank sphinx
and look at it without the assist. barely anything changes
To gain energy from an assist, you must leave the sphere of influence of the body you assisted off of pointing closer to prograde than you started.
Here, your orbit is already perfectly tangent to Eve's orbit at your interception, so you will arrive from perfectly retrograde and leave perfectly prograde. You can't point "more prograde" than already pointing prograde so you can't gain any energy from this assist.
Thus, to be able to get energy gain from the assist, your starting orbit must dip below the level of Eve's orbit, so that it crosses Eve's orbit at an angle.