How do you do gravity assists?

[zxv_3815]

How do you gravity assist? I know that you have to go into the SOI of a body and then your periapsis has to be behind the body's trajectory to get a boost and in front to brake. But every time I do it, my periapsis is besides the body's trajectory and does nothing. Please help, I don't wan't to go to jool the inefficient way. 

[maceemiller]

Hi there. Maybe this will help you. If not at least I tried

 

Edited May 31, 2017 by maceemiller

[HebaruSan]

4 hours ago, zxv_3815 said:

your periapsis has to be behind the body's trajectory to get a boost and in front to brake.

Focus more on the angle your craft takes to exit the SOI. If it's heading more towards the planet's prograde direction than it was before, you've raised your solar apoapsis or periapsis. If it's heading more toward the planet's retrograde direction than it was before, you've lowered your solar apoapsis or periapsis. This means you probably want your incoming path to have a significant solar-radial component (either positive or negative), as opposed to a normal transfer where you'd generally try to minimize the radial component at encounter, which in turn means you want to launch and arrive before or after you would for a normal Hohmann transfer.

[bewing]

You have to be rather lucky to be able to get everything you want out of a pure gravity assist. If the gravity assist isn't going to be strong enough, you can "help" it for very little cost by burning retrograde at your encounter Pe.

 

[Spricigo]

Hard to tell what you may be doing wrong without further details. But how much you can get from a gravity assist depends, among other factors, from how close to the celestial body you pass and the velocity you enter the SoI.

Also, there is some particular reason you don't want the 'inefficient way'? More often I find the time and effort it takes to go with gravity assist more problematic than the fuel of a regular transfer.

[zxv_3815]

On 5/31/2017 at 1:23 PM, Spricigo said:

Also, there is some particular reason you don't want the 'inefficient way'? More often I find the time and effort it takes to go with gravity assist more problematic than the fuel of a regular transfer.

Because it is impressive.

[Spricigo]

1 hour ago, llanthas said:

hmm...   Wouldn't it be more effective to burn towards the body you're orbiting at Pe, to get a speed boost?   Retrograde doesn't make much sense to me, if the idea is to increase exit speed...

Reducing the speed while in the body SoI means its gravity have more time to bend your trajectory. Also, burning towards (radial in) is more effective if done far from the Pe.

But you have a point, only the angle of scape is not enough to determine how effective the gravity assist is.

[Daveroski]

Often, while transferring to Jool, I use Tylo so that I can get a Jool orbit without burning at all.
I used to use Jool's atmosphere to slow down but when it changed so that touching the atmosphere caused your ship to die, I had to find another way.

Gravity assist works well for me.