Should we use Mars as the main source of interplanetary fuel? [closed]

Question

Robert Zubrin makes a compelling case for using Mars as a site to make methane fuel for the return trip back to Earth. However, maybe there is an additional refinement to this model;

In addition to the landers and habitats to be shipped to Mars, I would like to suggest that a 'tanker' vehicle be left in Mars orbit. As each batch of Methane is completed on Martian surface, it is shuttled up and transferred to the 'tanker' vehicle. This tanker vehicle should be large enough to contain enough fuel not only for the return trip to Earth, but also for the return trip to Mars.

Earth can then restrict its activities to providing the Hydrogen feed stock for further Methane production (back on Mars) and for the supply of manufactured goods. Does all this make any sense?

Answer 1

Does all this make any sense?

Part of it does. The idea of leaving the tanker as an orbital fuel depot in Martian orbit is OK (if you can reuse a spent upper stage for that), but it would be better to leave propellants themselves on the surface of Mars until you actually need them in orbit, to prevent propellants boil-off and risk of explosion due to thermal cycling (thermal expansion during one complete orbit as the vehicle enters and exits Martian shadow, and rotation of the vehicle itself to cycle surface area exposure to the Sun and other sources of radiation).

Methane itself isn't such a big deal, it's a fairly big molecule compared to some other relatively good performance fuels so boil-off rate, assuming your orbital fuel depot doesn't spring a leak, would/should be fairly slow. It does have a rather big isobaric coefficient (expansion due to temperature and pressure), so it would have to be actively cooled. Bigger problem is the oxidizer, which would in this case be LOX (cryogenic liquid oxygen) that is more difficult to store long-term in orbit and it would require active cryogenic cooling which is a fair bit trickier to achieve since it's highly corrosive, doesn't react well in contact with most surfaces and with increased pressure (e.g. if your cryocoolers fail) becomes more metallic, i.e. any exposure to electric wiring or even vehicle's surface charging through solar weather and discharging through the insulation and tank walls would result in loss of vehicle. And if you depended on it, also loss of mission or worse.

Otherwise, and I'm not entirely sure if that's what you meant with your question, Mars isn't really suitable as an intermediary stop to interplanetary exploration of other targets than Mars, its two small moons Phobos and Deimos, and perhaps some asteroids in the Main Belt, because it's rarely suitably aligned with outer planets, if those would be your final destination. Refueling at Mars would also mean that you'd first have to inject into its orbit and rendezvous with the orbital fuel depot, instead of using it for gravity assist during flyby, so you're really losing a lot of delta-v, propellants, and time and with it exposure to radiation and so on. It would be more of a detour than a shortcut, unless you meant with interplanetary specifically to support / facilitate Earth-Mars transports.

And no, such setup wouldn't depend on delivery of hydrogen from Earth. Hydrogen is locked in Martian water and can be extracted from it fairly easily with electrolysis. It's unclear how much of that hydrogen is deuterium though (atmospheric escape is slightly faster for normal, lighter hydrogen isotopes than slightly heavier deuterium), so you'd perhaps also have to mass-separate it before use first. Besides, delivering hydrogen from Earth, with trip times of roughly 9 months (depends on orbital positions of Earth and Mars), would be even more difficult than already described long-time storage of oxygen. Typical LH2 (cryogenic liquid hydrogen) boil-off rates, depending on tank design, are too high to store it for so long in the vacuum of space, and by the time you'd reach Mars, it would be either all gone and lost to space, or you'd have to insulate and thicken tank walls so much that it becomes impractical from mass economy perspective.

Answer 2

The first thing to note, is that the fuel generation gear would be perfectly sized to create the amount of fuel required to bring the astronauts back to Earth. It is not like it has surplus capacity. So we really need to consider bringing fuel back from Mars, somewhat separately to the plan of bringing the astronauts back. It is going to require bringing a lot of extra equipment.

If the intention is to bring something to be used as fuel back to Earth, the logical thing to bring would actually be plain water either in liquid or ice form. Water can be turned into H2 and O2 in a perfect ratio for fuel using electrolysis. However, in the H2O form it is highly stable and easily transported, while in the H2/O2 form it is highly volatile - with both forms having exactly the same mass. Instead of bringing extra electrolysis gear, extra power generation and extra tanks all the way to Mars, it'd make more sense to put all that extra gear in Earth Orbit, bring water back from Mars, and perform the electrolysis in Earth Orbit, where the fuel would be used.

It would also be possible to bring CO2 back from Mars (probably as dry ice) and perform the Sabatier reaction and electrolysis in Earth Orbit, with Hydrogen brought only from Earth to Earth Orbit instead of all the way to Mars. CO2 has the advantage of being easily extracted from the Martian atmosphere, while water has to be mined from the ground or extracted from the atmosphere at considerably greater effort.

Note that these options result in just as much usable fuel in Earth Orbit, with much less mass needing to be sent to Mars. Of course, it would still be necessary to generate a lot more methane and lox to bring the H2O or CO2 back to Earth - the whole point of in situ generation is that carbon and oxygen are heavy elements (compared with hydrogen) and not desirable things to be delivering between planets, so the plan of bringing carbon or oxygen from Mars to Earth somewhat defeats the purpose of in situ generation - which is to save on mass. Yet it could still be cheaper to bring them from Mars to Earth Orbit, than from Earth, at least in terms of delta-V.

But there are potentially cheaper places to get water (and maybe carbon) from, it would actually be a lot cheaper in terms of delta-V to bring fuel from the moons of Mars than Mars itself. This comes down to gravity, Mars has a relatively deep gravity well while the moons have very shallow gravity wells. In fact, various factors conspire to making Phobos and Deimos even closer in terms of Delta-V than the Moon - but only for very restricted windows when the Earth and Mars are properly aligned. At the moment we don't really know what resources are available on Phobos and Deimos, it is only speculated that there are exploitable quantities of volatiles.

The other good candidate to bring water from is the Moon, even though Phobos and Deimos are occasionally slightly closer in terms of Delta-V, the Moon is far more accessible in every other way, most importantly the trip time is mere days or weeks instead of 8 months. The moon likely has water in the form of ice in permanently shaded craters, which could be extracted and processed into fuel in-situ, or delivered to Earth Orbit for processing.

Considering the much greater accessibility of the Moon, I believe it does not make sense at this time to bring extra equipment to Mars with the purpose of bringing fuel back to Earth Orbit. In situ generation of fuel on Mars only makes sense if that fuel is to be used on Mars.