[jfengel]

That's true, but there is extra energy cost associate with putting it in orbit and then breaking orbit. Orbital calculations are about velocity, and delta-velocity, more than they are about height-above-ground.

Getting above the atmosphere is a big win, and may well make it worth it to do the final fueling from orbit. But in the end, getting to orbit may be as little as 100 miles of a multi-billion-mile trip.

[jerf]

I find it weird how your first paragraph is correct, then your second paragraph kinda seems to ignore it?

The whole reason I'm interested is obviously not that lifting fuel a vanishing fraction percent of the distance is helpful. The whole reason is that with a full fuel tank that's already in orbit, a probe ought to be able to take off straight for a planet without having to wait for an energy-impoverished orbit to be available. Or take off on a much more energy-intensive orbit that involves more aggressive slingshot maneuvers, or whatever else; having more energy doesn't preclude using that energy cleverly as well.

I also find myself curious about what we could do with an ion drive... or set of ion drives... fed with lots of fuel. And maybe an RTG for power, or some other real power source other than solar (which isn't much of a source out there in the gas giants). Or fed by solar cells, but first it takes a swing around the Sun for the almost sole purpose of gathering solar energy and driving the ion drives with it. All sorts of interesting things become possible if we can just use multiple lauches.