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In this cool video of a presentation about SpaceX's GPU-based computational fluid dynamics, there is a slide early on about making rocket fuel on Mars - specifically using water from the ground and carbon dioxide from the atmosphere to make oxygen and methane:

2H2O + CO2 -> CH4 + 2O2

Going in that direction requires energy, and I'm guessing solar power in some form or other would certainly be one way.

But why not keep it simple ("because it isn't" is a possible answer) and just use solar photovoltaics and electrolysis to make LOX and LH2 from the same water used above:

2H2O  -> 2H2 + O2

What are the salient tradeoffs here? If the slide shows methane, and the acompanying discussion talks about the problems of trying to make a methane engine, why is LOX + LH2 not discussed?

SpaceX Methane Fuel on Mars

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Liquid Hydrogen is difficult to deal with. The temperature must be 33 K or lower. Liquid Oxygen requires 90K, and Liquid Methane is similar. The temperature requirements are far less as such. The surface of Mars varies between 140K to 300K. The values for storing Methane/ Oxygen are much closer.

Methane also requires less hydrogen than the LH2/LOX rocket. It has been assumed that hydrogen is relatively difficult to find on Mars.

Lastly, Liquid Hydrogen is very difficult to deal with, as is mentioned in this NASA article. The long term storage of the substance is something that hasn't yet been achieved.

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  • $\begingroup$ I'm pretty sure using light weight insulation and low pressure, keeping liquid gases cold on Mars is a heck of a lot easier than it is on Earth, so Earth-bound arguments can't be applied directly. Has any one looked at how hard Hydrogen is to store on Mars specifically? At night for example, you have a giant cold sink above your head. It's not dangerous or explosive (is it?) Hmm... Does Hydrogen react with CO2? $\endgroup$ – uhoh Jul 28 '16 at 10:08
  • $\begingroup$ It is hard on the Moon or in space, I don't see why on Mars use much different. $\endgroup$ – PearsonArtPhoto Jul 28 '16 at 10:22
  • $\begingroup$ Because "using light weight insulation and low pressure, keeping liquid gases cold on Mars is a heck of a lot easier than it is on Earth..." Thermal loading is far lower, vacuum pumps don't need to work nearly as hard. Hydrogen embrittlement is still a stickler though, and there is that video out there when Elon Musk says hydrogen is stupid. No he didn't say that, I'm paraphrasing. $\endgroup$ – uhoh Jul 28 '16 at 10:26
  • $\begingroup$ Here's some handy Hydrogen Fundamentals. The ortho/para issue seems troubling. $\endgroup$ – uhoh Jul 28 '16 at 10:44
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As briefly mentioned in the previous answer, H2 is very tricky to deal with. The temperature is one thing, but what he didn't mention was its extremely low density. If I recall correctly the LH2 tanks on the shuttle require around 4-5 times the space of its LOX tanks, if not more. The volume and mass of the tank itself make them very difficult to deal with, and adding cooling system to prevent the fuel from boiling off compounds that problem even further.

Methane, on the other hand, is very dense. What it lacks in performance it more than makes up for by requiring a comparably tiny tank volume. What your rocket lacks in ISP it will more than make up for in having more propellant and less dead weight from the tanks.

Edit: I forgot to mention that electrolyzing water into H2/O2 requires very large amounts of electrical power. The Sabatier reaction (2H2O + CO2 => CH4 2O2) is vastly more energy efficient.

So, in addition to my points about the low density and high tank/support system mass involved with LH2, you'd need significant additional powerplant mass, too.

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  • $\begingroup$ Thanks! Can you help me understand the "vastly more efficient" part? Are you saying that getting LH2 from H2O using sunlight can only be done with electrolysis, and that electrolysis of water is an inefficient process? But splitting the same water to make CH4 is more efficient? $\endgroup$ – uhoh Jul 28 '16 at 22:53
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    $\begingroup$ Electrolyzing water into H2 and O2 is the art of dumping electrical current through it to break its hydrogen bonds. Lots of current. The Sabatier reaction, on the other hand, involves passing carbon dioxide and water over a heated (A few hundred degrees C) catalyst. Since we're reacting molecules instead of just ripping them apart, the forces of chemistry do most of the heavy lifting for us. I just got off work and I'm really tired, so I apologize for the lack of figures. I'm just trying to share what I can remember. $\endgroup$ – UIDAlexD Jul 29 '16 at 0:21
  • $\begingroup$ OK is the Sabatier reaction CO2 + 4 H2 → CH4 + 2 H2O + energy ? If so, where does all the H2 come from, if not the use of energy to split water? If you watch the five minute section of Feynman's "The Pleasure of Finding Things Out" video, or read the book (I don't have a copy right now to verify) between 07:00 and 12:00, he says it better than I can. $\endgroup$ – uhoh Jul 29 '16 at 2:07
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    $\begingroup$ That's using the reaction for power generation. I'm talking about putting energy into the reaction to turn H2O and CO2 into methane oxygen. Confusingly enough, it's called the sabatier reaction regardless of what direction it's in. $\endgroup$ – UIDAlexD Jul 29 '16 at 2:18
  • $\begingroup$ In this context it seems to be called the Sabatier process when you get your hydrogen from a bottle, or the Sabatier/Electrolysis (SE) process if you use electrolysis to obtain hydrogen instead of ordering it and having someone deliver it to your laboratory. See: Zubrin, R., Muscatello, A., and Berggren, M. (2013). "Integrated Mars In Situ Propellant Production System." J. Aerosp. Eng., 10.1061/(ASCE)AS.1943-5525.0000201, 43-56. http://ascelibrary.org/doi/10.1061/%28ASCE%29AS.1943-5525.0000201. $\endgroup$ – uhoh Jul 29 '16 at 4:39

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