Fuel

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In the context of chemical processes, such as chemical rockets, fuel refers to a reducing agent which can be chemically combined with an oxidizer to release energy. In other contexts, such as nuclear energy, it normally refers to the substance which is used as an energy source. In chemical propulsion, Fuel plus the oxidizer are propellants. In electric and most forms of nuclear propulsion, there are only propellants.

On Mars, fuels could be hydrogen, methane, carbon monoxide and solids, such as aluminum or iron. Oxygen is the most likely oxidizer. The abundance of water on Mars favors hydrogen and methane. Methane could be produces in situ using Water electrolysis and the Sabatier process.

List of fuel + oxidizer combinations that could be produced on Mars
Combination Specific impulse (s) Exhaust velocity (m/s) Stoichiometric ratio

fuel:oxidizer

mass ratio
Hydrogen + oxygen 2H2+O2=2H2O 388 - 454 3816 - 4462 2:1 (stoichiometric)
Methane + oxygen CH4+2O2=CO2+2H2O 309 - 368 3034 - 3615 1:2 1:3.6
Carbon monoxide + oxygen 2CO+O2=2CO2 260 - 300 ~2770 (est.) 2:1
Aluminium + oxygen 4Al+3O2=2Al2O3 4:3

Actual ISP and exhaust velocities depend on engine design and external pressure. Exhaust velocities are ideal ones for one atmosphere and near vacuum. Table gives stoichiometric fuel ratios, not volumetric or mass based ones.

Notes:

  • Hydrogen / oxygen gives a high power, but hydrogen is hard to store at Martian conditions. The best engines burn a bit hydrogen rich (to cool the flame & reduce the molecular mass of the exhaust which gives more thrust).
  • Methane / oxygen can be created from carbon dioxide (CO2) and water, both from the Martian atmosphere. Also, the two liquids have a similar boiling point which makes storing them inside the rocket's tankage simpler. Methane created on Mars is likely to have a small amount of carbon monoxide mixed in it; this should not materially effect the rocket's performance.
  • It is fairly hard to ignite carbon monoxide (CO) and oxygen. Such engines have never been flown, and the exhaust velocity above is an estimate.
  • Aluminum + oxygen produces a solid exhaust product which might clog reaction chambers. Such fuel might be able to be used as part of an expendable solid rocket booster. If ht clogging problem is resolved aluminum oxygen might be an interesting combination for in-situ fuel production on the Moon.
Cost of fuel and oxidizer production (2016)
Propellant Density

(kg/m3)

Cost

($/kg)

References $/m3 $/tonne
Oxygen 1140 0,16 NASA paid 67 cents per gallon for the shuttle, so about $0.16 per kg 182,4 160
Hydrogen 70 3 Hydrogen Production Cost Using Low-Cost Natural Gas, DOE projection, 2012 210 3000
Kerosene 800 0,4 Commodity price for jet fuel, july 2016 320 400
Methane 420 0,2 0,1$/kg natural gas + delivery, refining and liquefaction 84 200