Depart For Duna

[SteveD80]

I have built a gas station at Minmus and Ike.  I think I have also built a ferry capable of flying between them with enough delta-v to ignore transfer windows.  And so my question is: Should my ferry leave Minmus/Ike directly to interplanetary transit, or would it be better to get close to Kerbin/Duna before departing?

[Aegolius13]

It's cheaper to burn from the moon in most cases, since you've already spent the delta v to get there.  Minmus especially is very close to escaping Kerbin SOI.  The trouble is that you'll need to burn when the moon is in the right place in its orbit around the planet, so that your escape trajectory is going where you want.  This is more of a problem on Minmus than Ike, since the former has a much longer orbital period.

Yet another option is to drop your peri down to the planet, so you have a highly elliptical orbit, then burn at peri.  This might be the most efficient since it uses the planet's Oberth.  But it's also pretty tricky to get lined up right.  Note you want to do this when the moon is on the opposite side of its orbit than you would to eject directly from the moon.

[Mister Dilsby]

2 hours ago, SteveD80 said:

I have built a gas station at Minmus and Ike.  I think I have also built a ferry capable of flying between them with enough delta-v to ignore transfer windows.  And so my question is: Should my ferry leave Minmus/Ike directly to interplanetary transit, or would it be better to get close to Kerbin/Duna before departing?

If you have a fully fueled ferry at Minmus then yes, eject straight from there to interplanetary. While a burn from low Kerbin periapsis is more efficient, you would have to burn a good amount of fuel to reduce your periapsis from Minmus's orbit down to that point. 

All of my interplanetary missions leave from Minmus orbit, in fact for a while I maintained a fueling station there ("Minmus Outbound") that I kept filled by drone "ore piggies" from the surface. And when I returned from Duna and Eve, I did so by topping off the tanks at Ike and Gilly respectively. Will do the same from Pol on my way out of the Jool system.

[Nicias]

If you have free fuel, then the answer is "burn where the fuel is" if you have a fuel truck that can fill you up wherever you are, then the answer is this: