How to burn for Duna?

[GamerMitch]

So, as you may see in my sig, I miraculously got a probe in orbit around Duna,Just running out of fuel as I got there. So I can do it perfectly every time, is there a rule of thumb for a Duna encounter?I already have a ship with enough delta V, But how can I get there easily without having to guess when to burn?

P.S. I don't want to have to orbit Kerbin first either.Just straight up.

[ManTrelk]

This might help

EDIT: fixed my link

[Hodo]

http://alexmoon.github.io/ksp/

And this.

[kerbomatic]

I launched a probe into the exact same orbit as Kerbin, just some distance ahead (or behind, doesn't matter at all). Then you can try a maneuver node on its orbit and estimate when to launch. No extras needed Technically a guess but accurate to within a day I'd say...

[Frederf]

Phase angles are the key word for interplanetary burns. They aren't magic. When you eject from Kerbin's gravity well to a certain destination there is an associated ejection trajectory angle relative to where you started the burn relative to prograde. For Duna the usual assumption is that you are doing a Hohmann transfer where your ejection is aligned with Kerbin's prograde motion (most efficient).

The extreme case of ejecting with infinite speed is the phase angle 90° case. You can imagine that when you are a quarter orbit from the planetary prograde position you are positioned so your orbital prograde is pointed in the same direction as Kerbin is moving about the Sun. Your path escaping Kerbin's well isn't going to be bent because of the extreme speed of your departure.

As your transfer speed decreases the effect of Kerbin's gravity on your craft as you are leaving curves it more and more. If you started from the 90° position (progrades aligned) you'll find that during your escape gravity will bend your escape path in a partial orbit and your escape vector will be bent inward toward the Sun. To counter this undesirable path bending one burns earlier (greater than 90) so that the bending during the escape pulls your path aligned with Kerbin's prograde as desired for an efficient transfer.

This is easy-ish to verify with maneuver nodes, adjusting the position of your escape burn such that the resulting escape orbit is tangential to Kerbin's. There are web site calculators for this but they assume simple impulses and are only valid at the parking orbit assumed. With the proper deltaV one achieves a Hohmann transfer where the AP touches the orbit of Duna. It is another matter to time your ejection so the time at orbit intersection is one where Duna is actually there (around day 50-60 IIRC).