In yesterday's post on bypassing the moon, a commenter writes:

As long as you're going somewhere where there are no in-situ resources to produce fuel with, you've got no, repeat NO advantage in terms of the amount of mass you have to put into space to get something somewhere.

Orbital refueling is the same whether it takes place next to a space-station or in the middle of nowhere. You still have to launch all your fuel from Earth. Constructing a fuel-factory base on the moon, on the other hand, means that you only have to get the payload in an agreeable orbit for the booster rockets/tanks to be launched to it from the moon.

This isn't necessarily the case. Not all payloads are created equal. It's conceivable that propellants could be launched more cheaply than other things (for instance, with catapults, or relatively unreliable but cheap boosters). So fueling in LEO would make sense under those conditions. In addition, you might be able to deliver propellants to GEO or EML1 much more cheaply than other payloads (e.g., by sending them on a slow tanker with a high Isp, with trip times that wouldn't be tolerable to humans, particularly through the Van Allen belts). So there is potentially a lot of benefit to orbital fueling even in the absence of ISRU.

[Early afternoon update]

I should note that it's also not true that "you've got no, repeat NO advantage in terms of the amount of mass you have to put into space to get something somewhere."

If you can deliver propellant to a staging point (like EML1) for your return more cheaply than conventional means, you can in fact reduce the total amount of propellant required for the mission, and that must thus be delivered to space. That's because it takes propellant to move propellant. If you deliver your return propellant as part of the total lunar insertion payload, it costs just as much, in terms of injection propellant requirements, as a pound of anything else. But if you can get it out there using low-thrust systems or (as Jon Goff suggests in comments) by Weak Stability Boundary trajectories, you can get the propellant there with a lot less propellant. There are really huge payoffs to the ability to store and transfer propellants on orbit, regardless of the cost of launch from earth.

Rand,
I was looking at Weak Stability Boundary transfers (at least for delivering propellants to the moon or to L1/L2), and it looks rather interesting. With a three month cycle where the full tanker goes out via WSB, and then comes back empty using a direct Hohmann transfer, you end up getting payload/IMLEO ratios not much worse than with SEP (better than the payload/IMLEO ratio of the fastest 5-month round trip designs Dennis was talking about). And that's with far less exotic and expensive propulsion systems, and you get 4 turns per year instead of 1-2 on your expensive piece of hardware...

Just a thought.

~Jon

This seems like an ideal application to supply re-useable LSAMs. Add lunar LOX and the incremental cost of a lunar mission falls through the floor.

"Not all payloads are created equal."
Launching LOX and LH2 is probably not subject to ITAR either, so using cheapest launch available on global market becomes an option.

But if you can get it out there using low-thrust systems or (as Jon Goff suggests in comments) by Weak Stability Boundary trajectories, you can get the propellant there with a lot less propellant. There are really huge payoffs to the ability to store and transfer propellants on orbit, regardless of the cost of launch from earth.

True.

But do we want NASA to deploy and operate these fuel depots?

NASA (and the potential propellant operators) faces a chicken and egg issue here -- NASA cannot make their architecture dependent upon a fuel depot until they can be assured it will actually get built while the private sector cannot finance a fuel depot without assurances NASA will buy fuel.

Dr. Doug Stanley (in the now frozen nasaspaceflight thread) explicitly stated that if a fuel depot were built and fuel delivered by NewSpace, EELV, railgun or even by teleportation and if buying fuel from that depot was cheaper NASA lifting it for itself, NASA would absolutely buy that fuel on orbit from the private sector.

= = =

Once again, to deploy a re-useable single stage LSAM assures the private sector that a market for an EML-1 and/or EML-2 fuel depot will exist.

But do we want NASA to deploy and operate these fuel depots?

No. But better that NASA do it than no one. Ideally, NASA would put out a contract to someone for depot services.

If I understand correctly, another advantage of fuel depots is that you can use cheap launchers to save money - that is, launchers that would be far cheaper because they're built to lower reliability standards.

You wouldn't launch humans or satellites with them - even on unmanned flights, the payload's value usually dominates the launch costs. A payload of fuel on the other hand is cheap and easily replaced. You don't have the insurance costs. If you lose a launcher, you launch the next one.

"But do we want NASA to deploy and operate these fuel depots?"

It'd be nice if they did a technology demonstrator and maturation program. Same goes for SPS-related technologies and lots of others.
Oh, wait, thats not what NASA does, its a state-sponsored space trucking institution

A dedicated tanker can be cheaper and deliver higher payload than a launcher that carries a propellant payload. The tanker can carry its payload in the same tanks as its upper stage. There need be no payload fairings and interfaces. Just a delivery pipe.