Trouble with unintended colonization of Duna

[Gabe]

After Mun and Minmus landings I've been trying my first trip to Duna and seem to be using all my fuel going there. I'm over the delta-V recommended from various aids. Soon there will be more Kerbals on Duna than the mother-world.

After watching many videos and reading the wiki I have a few questions that I've been having a problem with:

When planning the encounter with the maneuver node, what is the acceptable distance of a planetary encounter? Or is any encounter a good encounter? I've seen it vary from millions to hundreds of thousands. Does the acceptable distance vary on the planet you're going to?

After the burn to exit Kerbin and you notice the encounter isn't as close as you hoped or is now not an encounter at all, when is the best time to do correction burns? Right after the first burn, near the planet encounter or at the ascending/descending nodes?

Edited June 6, 2013 by Gabe

[Vanamonde]

The closer your periapsis is at the far end of the trip, the more efficient your braking will be. But on Duna, you can save even more fuel by aerobraking through the atmosphere. Which leads to your second question. The half-way mark of the transit is the most efficient time to do corrections, and if you can set your Duna periapsis below 47,000m, you will pass through its atmosphere and benefit from aerobraking. However, you have to go pretty low to get much of an effect out of Duna's thin air, so aim for a periapsis of about 13,000m for best results. It's difficult to be that precise, though, and I find that I usually need a second correction burn, just after entering the planet's SOI, to make sure I hit the aerobraking altitude I want. By the way, you can also save a great deal of fuel on the return leg of the trip, by aerobraking in Kerbin's atmo at about 32,000m.

[Uberick]

The initial "acceptable distance" is ANY encounter. Keep in mind though, that if you want to save fuel, you want to hit the planet when it will be about 180 degrees from your current position(opposite site of the sun). Thus, if you find yourself encountering the planet far earlier in its orbit, you are probably burning too much fuel. The amount of Delta V required for a minimum fuel transfer to Duna is about 1050 d/v. . . if you slingshot past the moon you can get there for like 850 d/v. If you find your maneuver nodes adding up to much more d/v than 1050, you are not aiming for the optimal intersect.

I recommend using http://ksp.olex.biz/ to plot your course. It will tell you what phase angle to aim for, and what ejection angle and burn you need.

in regards to course corrections. The halfway point(or an ascending or descending node), are usually the best places to make course adjustments. To do this, hover your mouse over the periapsis node for your intersect(so you can see the Pe distance). Then, fire your ship at the cardinal directions on your nav ball, 0, 90, 180, 270, and then burn towards the sun(dot on the middle fo the blue side) and away from the sun(dot on the brown side). Obviously, opposite directions(for instance, towards and away from the sun), will undo what firing in the opposite direction does.

To describe the process. Wait until halfway point of your journey around the sun. Then, fire north(0 degrees), while watching your Pe. If it starts decreasing, hold your burn until it stops decreasing. If you "miss", and burn too long, the distance will start going up again. As an example, say you have a 100km Pe; you burn north and it takes it down to 75km, and then starts increasing. You see the increase, but by the time you cut engines, it's already at 80km. to correct this, simply fire South(180), and it will undo that previous burn. hold your burn there until you hit 75 again. that is the closest you can get while firing in that direction.

continue this process firing North/South, East/West, and Towards/Away from the sun. when you are finished, you will have an intersect that SHOULD make you hit the planet directly. Note that if you are not at an ascending or descending node, firing to adjust your inclination(north/south), will not be perfect, and as such you will never get a perfect intercept)

After you enter the planet's SOI, you can further adjust your orbit. Place a maneuver node and use the blue circles to move your orbit closer or farther away from the planet. On Duna, this will allow you to set up a specific aerobraking altitude so that you don't have to waste any d/V decelerating.