Sabatier Reaction - Manufacturing Propellant On Mars

Manufacturing Propellant On Mars

The Sabatier reaction has been proposed as a key step in reducing the cost of manned exploration of Mars (Mars Direct) through In-Situ Resource Utilization. Hydrogen is combined with CO₂ from the atmosphere, with methane then becoming a storable fuel and the water side product yielding oxygen to be liquefied for the oxidizer and hydrogen to be recycled back into the reactor. The original amount of hydrogen could be transported from Earth or separated from martian sources of water.

The stoichiometric ratio of oxidizer and fuel is 3.5:1, for an oxygen:methane engine, however one pass through the Sabatier reactor produces a ratio of only 2:1. More oxygen may be produced by running the water gas shift reaction in reverse. When the water is split, the extra oxygen needed is obtained.

Another option is to make more methane than needed and pyrolyze the excess of it into carbon and hydrogen (see above section) where the hydrogen is recycled back into the reactor to produce further methane and water. In an automated system, the carbon deposit may be removed by blasting with hot Martian CO₂, oxidizing the carbon into carbon monoxide, which is vented.

A fourth solution to the stoichiometry problem would be to combine the Sabatier reaction with the reverse water gas-shift reaction in a single reactor as follows:

3CO₂ + 6H₂ → CH₄ + 2CO + 4H₂O

This reaction is slightly exothermic, and when the water is electrolyzed, an oxygen to methane ratio of 4:1 is obtained, resulting in a large backup supply of oxygen. With only the light hydrogen transported from Earth, and the heavy oxygen and carbon extracted locally, a mass leveraging of 18:1 is afforded with this scheme. This in-situ resource utilization would result in massive weight and cost savings to any proposed manned Mars or sample return missions.