A "neat set of reactions" that makes fuel on Mars

[sciencegem]

Hi,

I'm reading The Martian by Andy Weir (fantastic book). A character, describing how a Mars mission works, says of fueling their ascent vehicle:

"Through a neat set of chemical reactions with the Martian atmosphere, for every kilogram of hydrogen you bring to Mars, you can make thirteen kilograms of fuel. It's a slow process, though. It takes twenty-four months to fill the tank."

Usually I don't concern myself much with the scientific accuracy of particulars in SciFi, but this author took great pains to be accurate so I don't believe he pulled those numbers out of a hat. I'm assuming by "Martian atmosphere" he means CO2, the fact that they're using hydrogen makes me think they're creating hydrocarbons. Assuming they use product oxygen as fuel as well as the hydrocarbons (would that work?? I really know nothing of this stuff), I can't figure out what "neat set of chemical reactions" gets the Mars mission nearly 13kg of fuel for every 1kg of hydrogen. Does anyone have any ideas?

Thanks!

 

[DaveC426913]

I'm not sure but in Kim Stanley Robinson's Mars series, the fuel they used on Mars was Hydrogen Peroxide (H₂O₂). Maybe they're related.

 

[Borek]

Nothing I can think of produces 13 kg per 1 kg of hydrogen.

Sabatier reaction - producing CH₄ - yields either 2 or 4 kg of methane per 1 kg of hydrogen used (depends on whether you assume the byproduct to be oxygen, or water).

Hydrogen peroxide production yields almost 17 kg of the compound per 1 kg of hydrogen used (unless there are some byproducts in which hydrogen is lost, that could account for the final ratio o 1:13).

Production of hydrazine yields up to 8 kg per each kg of hydrogen.

 

[sciencegem]

Thanks guys! Never thought of hydrogen peroxide...I'm assuming the oxygen would come from the carbon dioxide in Mar's atmosphere? Where'd the carbon go?One reaction I found would produce carbon monoxide and oxygen, and if I've calculated things right there'd be more than 13kg of carbon monoxide and oxygen for every kg of hydrogen (CO2 + H2 -> CO +H2O). Problem is it appears this reaction would involve extreme temperatures (400C) and the electrolysis of the water to free the oxygen. Also there's only +13kg of fuel if carbon monoxide can be used as fuel. What's the feasibility of this?

 

[Borek]

Neither of the processes mentioned is "free" - they all require input of the energy. Where does the energy come from is another problem, but you can be sure you need an energy source in all possible cases. 400 °C is not that high for an industrial synthesis.

 

[Bystander]

great pains to be accurate so I don't believe he pulled those numbers out of a hat.

You don't really want to know from where they came.

 

[Bystander]

Actually, he's probably doing any of several syntheses of benzene.
https://www.google.com/search?q="be...X&ved=0CC8QsARqFQoTCIz3k5esosgCFVItiAod7kgDCg

 

[sciencegem]

Thanks for all the input guys! I'll look up benzene syntheses. Also for any future readers of this thread I found this very interesting: http://web.stanford.edu/~cantwell/Recent_publications/Boiron_AIAA_2013-3899.pdf