The advantage of sulphur was that is a solid over lunar temperature so it does not need refrigeration to prevent boil off. This could simplify both the depot and the rocket.

Unfortunately the Moon (unlike Mars) is short of the normal fuels like carbon, hydrogen and nitrogen. For use as a fuel or oxidiser lunar sulphur may be too rare without major mining. Oxygen is plentiful but only exists in compounds. Common lunar elements are silicon, iron, calcium, aluminium and magnesium – all of which burn when powdered.

Using solar power to extract both oxygen and metal from luna regolith then burning the pair as a propellant is possible, particularly if the mixture is lox rich.

Are you referring to the “brimstone rocket” proposed by Vaniman, Pettit, and Heiken? It still uses LOX, the sulphur is the fuel, not the oxidiser. The intriguing advantage is that sulphur is apparently relatively plentiful on the moon, so you could produce both fuel and oxidiser on the moon, instead of importing it all the way from earth. Combined with reusable landers, vasimr cargo transport and orbital infrastructure this could lead to considerable savings in fuel or ,uch more downloaded cargo.

From what I understand ESA, unlike NASA, wants to focus a lot of its attention and lunar activities on building the sort of orbital structures inside the Earth moon system that many people on this blog are interested in…

The value of depots (notice the plural) is not necessarily in the cost of the launch, but will find its greatest utility by being an “enabler”. There are only a few nations capable of launching rockets large enough to actually participate in a meaningful way in space exploration. But there are many more with decent launch capability to LEO that could participate if they were able to dedicate their lift capacity to the spacecraft, and not to the mission propellant. Remember that in some cases up to 80% of the spacecraft mass is propellant. Being able to “gas up” once in orbit before setting off on the mission would enable many more players from around the globe to participate and would enable vastly superior probes from all nations, even the big boys, to head into the solar system. A 20 ton spacecraft that is actually 20 tons of spacecraft and not 4 tons of spacecraft and 16 tons of propellant at launch is a far more useful probe.
-Chuck

For the purposes of demonstrating the key technologies required for propellant depots, only small amounts of hydrogen, oxygen, and methane need to be set aside for testing the handling properties of cryogenic fluids in micro-gravity. If the tests are successful, then perhaps a pilot plant could be established. In addition to providing fuel for reboosting the station, the depot might also be able to support a handful of ISS-servicing tugs.

If you do not want to use cryogenic LOX then sulphur could be used as the oxidiser. You would have to design new rocket engines.

Does anybody have any idea what it would cost to pitch this idea to NASA?

~Jon