Methane can be made from Martian carbon dioxide if hydrogen is brought. The Sabatier step is CO2 + 4 H2 -> CH4 + 2 H2O. Splitting that water returns hydrogen to the reactor and frees oxygen. The net, once the water's hydrogen is reused, is CO2 + 2 H2 -> CH4 + O2.
MOXIE showed that oxygen can be taken from the atmosphere by a different reaction, solid-oxide electrolysis, which does not make the methane. [1] This page is only the mass arithmetic that pairs methane with the oxygen the net reaction produces. The oxygen-only path is Oxygen from air.
Screen
Count with integer molar masses. Methane is 16. The hydrogen that stays imported is 4, which is two H2 molecules, not the four H2 molecules the Sabatier step consumes before the water is split. That imported hydrogen is 4/16 of the methane mass. Carbon dioxide is 44, which is 44/16 of the methane mass. Oxygen is 32, which is twice the methane mass. Type a methane mass. Hydrogen, carbon dioxide, and oxygen scale from it by those three factors. The mass ratio 2 is the chemistry, not an engine requirement.
Class A instrument
For methane mass m: carbon dioxide is m * 44/16, imported hydrogen is m * 4/16, oxygen is m * 32/16, and the water made on the way is m * 36/16. The oxygen-to-methane mass ratio is 2. That is the chemistry, not an engine mixture.
What the ratio is not
An engine may burn a different proportion of oxygen to methane than the reaction hands you. This page does not choose that proportion, and it does not size tanks, power, or a plant. Crew budget is the people-and-days oxygen line. It is not this reaction. Design envelope is the index of the screens.
See also
What links here
References
- Hoffman, Hecht, Rapp et al., Science Advances 8 (2022), doi 10.1126/sciadv.abp8636 https://www.science.org/doi/10.1126/sciadv.abp8636