Duna transfer: from Kerbin, Mun or Minmus?

[DeepSpaceDutch]

Thanks again everyone! Tonite I'm going to send my first probe to Duna, will post pictures if I succeed

[georgTF]

That depends on what you mean by slingshot. If you are talking about a passive gravity assist, where you simply fly close to the planet, then no; how far away your starting point is doesn't matter. If you are talking about a powered maneuver, where you wait until you are as close as possible to the planet (and going as fast as possible), then igniting your engines, then yes, you can; this is what people are talking about when they refer to the Oberth effect.

Flying out to Minmus first won't help by itself, but if you do what you are planning, refueling in Minmus orbit, then it can definitely be useful. You will lose about 150 m/s getting out of Minmus orbit, but after that it will only take another 100-200 m/s to get to Duna.

That's not quite my question. What i wanted to know was wheter or not i could use Kerbin itself for a gravity assist by setting up a trajectory that starts almost at the edge of Kerbin's SOI, has a low Kerbin Periapsis (how low depends on the target trajectory but ideally lower is better), and end up flinging me on a favorable course.

Unpowered this is impossible because the orbit will of course stay the same. I'm looking for a powered maneouver that would be performed after Kerbin has pulled me in (giving me a free ride) and built up speed. Then after the burn Kerbin will drag me back down less than it did pulling me in, so i should end up with more velocity. Some of this extra velocity will come from the burn, but i wonder if there's anything to get for "free" from Kerbin's gravity (gain more coming in, lose less going out).

Let's disregard the fuel required to go up to the edge of Kerbin's SOI (or Minmus) and assume the craft starts the trip down to Pe with full tanks or at least as much as it's possible to save. Or that a similar thing could be done better using Mun for a gravity assist.

Perhaps setting up the node long before Pe is also a good idea. I don't know, i have not really done it much. As i said only once, and with a 5 orange tank station that i wanted full. My guess would be that the difference (if any) would be too small to offset the complications. I just wonder if it's theoretically impossible in the first place. Maybe i'm thinking wrong.

[DMagic]

How far away you are from Kerbin doesn't really matter as far as a gravity assist is concerned. The only way that 'free' energy (it's not free because, while the planet pulls on the spacecraft, the spacecraft pulls on the planet) can be transferred from the planet to the craft is when you are measuring velocity from another point of view (your velocity relative to Kerbin after an encounter with the Mun for instance, or your velocity relative to the sun after an encounter with Duna).

Kerbin's gravity increases your velocity as you get closer to the planet. The advantage of this is that you can perform your burn (and you want to do this as close to periapsis as possible) when you are going much faster than you would be in a circular LKO. Or, more specifically, that you cover a greater distance in a smaller amount of time. Meaning that you can burn your engines for a shorter amount of time than you would be able to if you were going slower.

The other advantage to starting with an elliptical orbit from Minmus is, obviously, that you are already close to being able to escape Kerbin orbit. It only takes another 50-100 m/s of delta-v to escape Kerbin's SOI at this point, if you burn at your periapsis. You could perform the burn at some other time, but the point when you are at maximum velocity is the ideal time to do it.

So the short answer is, no you never get any free assist from Kerbin, not unless you start and end your encounter from an orbit around the sun.

[The Ideal Gas Lawyer]

using Eve to get to Moho (passing over/under Eve when near its descending/ascending node to Moho can save you all the fuel you need to get the orbital inclination like that of Moho).

I gotta try that. I was going nuts trying to get to Moho to explore the polar craters and had to resort to using kind of broken hybrid engines (LF+Oxy+xenon+electricity) to get a ship with 9ish km/s of dV so I could get there.