Does it make economical sense to use an electricaly powered fuel tug to supply a moon landing program with fuel?

I am imagining an architecture of launching a 25t payload of mostly fuel into LEO which then uses ion thrusters to move to the moon to refuel spacecraft used in moon exploration.

Does this architecture make sense and are there any studies into this?

  • $\begingroup$ If I were an investor in such things, I would wait and see how the revolution of cheap reusable giant launchers develops. The cost of launching fuel is crashing dramatically right now. Let's wait and see if Starship/Superheavy turns out to be a success before we do the calculation. $\endgroup$ – LocalFluff Aug 30 '20 at 9:40
  • $\begingroup$ Different but related: Going from LEO to lunar using only low-thrust ion propulsion - can it be done? $\endgroup$ – uhoh Aug 30 '20 at 13:59
  • $\begingroup$ One question: what kind of fuel? The most useful fuels to have available are cryonic (LOX, LH2, liquid methane); these are not known for being effectively storable over long durations, and ion thrusters are a very slow road. Not only would you have to address the issue of boil off during the journey, but I would wonder how the incidental thrust from that boil off would compare with the thrust of the ion engine. $\endgroup$ – Anthony X Aug 30 '20 at 14:36
  • $\begingroup$ If you have power for ion thrust, will you not have power for cryocooling? $\endgroup$ – ikrase Aug 30 '20 at 23:41

So the basic fact is that the delta-V from LEO to LLO (low lunar orbit) using a high-thrust system is about 4 km/s and using a low thrust system it's about 8. source

So, using something like a vacuum raptor engine ($I_{sp}$ 382s) you need a mass ratio of about 2:1. That is, for every ton you want to deliver to LLO you need 2 tons of methalox in LEO. Using an ion engine ($I_{sp}$ say 5000s though they vary a lot) the mass ratio is 0.17, so you need 170kg of xenon (economically it's probably better to use a krypton and accept a little less energy efficiency, but whatever).

An ion tug also needs big solar panels (using a fission reactor in LEO is generally frowned upon for a number of reasons) so it's "dry mass" may be higher.

So from a fuel perspective, you are comparing the cost (in LEO) of two tons of methalox, versus 170kg of krypton. Alternatively, including the cargo, you need 3 tons to deliver a ton of cargo using a chemical engine and 1.2 tons using the ion engine. A bigger differential if you want the tug back without aerobraking (depends on the "dry" mass of the tug).

On the other hand the ion tug will also be much slower. Months or years per round trip, most likely, versus a few days for chemical. This matters if the tug is expensive to build or launch. You need more of them to deliver the same number of tons/year to LLO. This could easily be the dominating factor, especially of the chemical tug was more or less off-the-shelf (eg a stripped down SpaceX starship).

A better solution in the longer term is almost certainly to make propellant on the moon, or on an asteroid. If you can find ice on the moon this is relatively easy. If not, you can make aluminium dust and liquid oxygen from rock if you have enough energy, or ship in water from Ceres.


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