Interplanetary flight, the Oberth effect, and stopping off for gas and a KitKat

[Crater]

I have a question on interplanetary transfers, the Oberth effect, and refueling stations.

Conventional wisdom for an interplanetary burn is ALWAYS ALWAYS do it at low altitude so you're moving faster, so you get maximum benefit from the Oberth effect.

All things being equal, it will always be more efficient to start at e.g. 70 km altitude than it would be to burn up to, say, 1000 km altitude, circularize, and then burn from there.

But... what about if you are refueling at 1000 km. In that case, what happens?

So, using AlexMoon's excellent calculator, at http://alexmoon.github.io/ksp/ , I find that to get from Kerbin to Duna

Starting at 70 km requires about 1714 m/s of delta-V

Starting at 1000 km requires 1433 m/s of delta-V

So, surely, if I started off from 1000 km with full fuel tanks (due to refueling at a station there), then surely I would still have almost 300 more delta-V when I got to Duna than if I had started at 70 km and taken advantage of the Oberth effect?

I know that it is, overall, less fuel efficient, since it would take me more than 300 m/s of delta-V to circularize the 1000 km orbit from the 70 km one. But if I'm refueling, that doesn't matter. It only matters how much I have in the tanks at Duna, or how big a tank I need in order to get to Duna.

So, am I missing something, or is it actually the case that the stock answer of always start low is only applicable if you are not refueling on the way?

Edited August 13, 2013 by Crater

[Tw1]

From what I've heard, you're pretty much correct. There was someone who even calculated the ideal orbits to put tanks in, for trips to each planet.

[Johnno]

This has been discussed several times and the consencus I've read is that it'll take you more fuel in the long run since you have to restock your station's depot.

I generally find that it's far easier to design a ship capable of getting me where I want than design one that can get from a refuelling station to where I want and then keep the station running.

[Crater]

Thanks all. Good to know I wasn't missing anything obvious