[bfe]

One of the central costs of a round trip to Mars is the fuel for the return flight. With enough water at Mars we can manufacture all the methane/oxygen rocket fuel we need on site.

The Mars Direct mission architecture and the NASA Johnson Space Center's Design Reference Mission derived from Mars Direct both assume we send hydrogen from Earth to manufacture rocket fuel for the return trip at Mars, combining it with atmospheric carbon dioxide into methane and oxygen, and saving 95% of the mass of the fuel versus bringing it all from Earth. They are estimated at $20-30 billion and $50 billion respectively (spread over ten to twenty years), compared with the earlier $450 billion price tag of the Space Exploration Initiative announced by President Bush 1, which was based on bringing all our fuel for the round trip with us from Earth. Not even having to send hydrogen from Earth would cut the cost further.

Methane/oxygen rockets have been rare outside of Russia, but Pratt & Whitney demonstrated a working model of a modded RL10 rocket engine running on methane/oxygen.

More to the point in the context of Mars missions, SpaceX is switching to methane/oxygen for their new Raptor engine.

[kposehn]

Are there benefits to methane/oxygen outside what you referenced?

[bfe]

Liquid hydrogen has what Elon Musk calls the pain-in-the-ass factor. It requires much higher volume tank at much lower temperature, adding lots of mass to the rocket, and it's impossible to effectively seal. RP-1 kerosene has the inconvenience of requiring your planet to have been covered with life a hundred million years ago. So, methane is the liquid fuel of choice for future martians.

[jccooper]

Methane, while less dense (thus requiring bigger tanks) has a higher specific energy than kerosene, and thus has a higher ISP (rocket efficiency). Methane is also less sooty and should have better cooling as its a "mild cryogenic" like liquid oxygen.

Basically, it's between LH2 (hydrogen: high ISP, very low density, very very cold) and RP-1 (kerosene: lower ISP, high density, room temp) and may be a good compromise.

And its derivable on Mars. Downside is there's very little flight heritage for a methane engine, so most of this is theoretical.

EDIT: Paper: http://thehuwaldtfamily.org/jtrl/research/Propulsion/Rocket%...