Interplanetary transfer math

[Windspren]

So I am planning to go to Duna, which will be my first ever interplanetary venture. I read some stuff about transfer windows and I think I get it, but I don't get the stuff about phase angles and stuff. I think you have to burn prograde at a certain part of your orbit, which gets you on an escape trajectory that takes you to Duna. Is this what you do? How do I know which part of the orbit to make the burn? Should I just play around with maneuver nodes? I need help with da maths

Edited May 30, 2015 by Laythe Dweller

[Empiro]

Phase angle means the angle between the target and yourself when looking at a the Kerbol system (http://i.imgur.com/dXT6r7s.png). You can also use one of the transfer window planners, which tell you exactly which date it's best to launch from.

Once the time is right, get into as circular of an orbit as you can. Then, create a maneuver node that's burns prograde the necessary amount. You should see that it leads to an escape trajectory. Then, drag the node back and forth so that your escape is about parallel to Kerbin's orbit in the prograde direction when you zoom out (if you're going to an inner planet like Eve, you want it to be retrograde).

Edited May 30, 2015 by Empiro

[MoonMan22]

The easiest way is to make sure that Duna is 45 degrees behind Kerbin along their orbits then preform your escape burn on the side of Kerbin opposite the sun. Check THIS SITE out for a little more detail but still easy to understand, also has a calculator that will help you find the exact time and place to burn along your orbit.

Hope that helps! Goodluck!

[Windspren]

Phase angle means the angle between the target and yourself when looking at a the Kerbol system (http://i.imgur.com/dXT6r7s.png). You can also use one of the transfer window planners, which tell you exactly which date it's best to launch from.

Once the time is right, get into as circular of an orbit as you can. Then, create a maneuver node that's burns prograde the necessary amount. You should see that it leads to an escape trajectory. Then, drag the node back and forth so that your escape is about parallel to Kerbin's orbit in the prograde direction when you zoom out (if you're going to an inner planet like Eve, you want it to be retrograde).

The easiest way is to make sure that Duna is 45 degrees behind Kerbin along their orbits then preform your escape burn on the side of Kerbin opposite the sun. Check THIS SITE out for a little more detail but still easy to understand, also has a calculator that will help you find the exact time and place to burn along your orbit.

Hope that helps! Goodluck!

Wow, thanks so much! So basically I just use KAC to see when the transfer window happens, then burn prograde parallel to Duna? It now seems much simpler!

[Red Iron Crown]

Your burn shouldn't be parallel to Duna, it should be at the "angle to prograde" or "ejection angle" specified by the transfer calculator. The site MoonMan22 linked has a good visualization of it.

[GoSlash27]

The simple version for ejection angle:

Assuming that you're in an eastbound equatorial orbit

If you're transferring to an inner planet, burn at sunrise.

If you're transferring to an outer planet, burn at sunset.

Best,

-Slashy

[Red Iron Crown]

You sure that's good advice, Slashy? For outer planets it holds up, but it seems like too soon for burns to an inner planet. I think of it like this, the red areas are the spots for good transfers (apologies for the crudeness of the diagram):

Also assuming an eastward equatorial orbit.

[GoSlash27]

You sure that's good advice, Slashy? For outer planets it holds up, but it seems like too soon for burns to an inner planet. I think of it like this, the red areas are the spots for good transfers (apologies for the crudeness of the diagram):

https://dl.dropboxusercontent.com/u/61004449/KSP/Misc/IPEjection.PNG

Also assuming an eastward equatorial orbit.

RIC,

Yeah, it's a crude rule of thumb and not optimal, but it's generally "good enough for government work".

Best,

-Slashy