Propellant

The formation of the CH₄/O₂ bipropellant for the ERV and surface mobility is an important aspect in the Mars Direct plan. Because hydrogen is only about 5% of the total propellant weight it can be brought from Earth in the form of liquid hydrogen. Carbon and oxygen are the two most plentiful elements in the Martian atmosphere as 95.3% is CO₂ and there are traces of water, oxygen, and carbon-monoxide. Pure CO₂ is obtained by compressing is to about 7 bar pressure, when it will condense into the liquid state because of the Martian temperature conditions. Nitrogen and argon that is also found in the atmosphere will remain gaseous and can be removed. CO₂ is vaporized and all dust or other such impurities are left behind. The following reaction will occur as it is exothermic and spontaneous in the presence of a catalyst such (as nickel).

This is also known as the Sabatier reaction, after the chemist who studied it extensively in the late 19^th century. The equilibrium constant is far to the right and production yields greater than 99% utilization. Because this process is so simple, the manned Mars Direct mission can acquire all of the 33 tonnes of CO₂ needed to react with the 6 tonnes of liquid hydrogen in just 26 hours (though it would be needless to overwork the reactors and pumps when the crew will be there another 1.5 years). The methane produced in (1) is liquefied and stored while the water is subjected to electrolysis:

Oxygen is then produced and can be cooled and stored while hydrogen is reused in the Sabatier reaction (1). This will yield an oxygen to methane mixture ratio of 2:1, but the optimum ration is about 3.5:1. Extra oxygen can be obtained from the direct reduction of CO₂ :

This can be done by heating CO2 to 1100 C where the gas will start to dissociate and the oxygen can be separated by applying a voltage.