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Propellant from Mars

Rockets and landing · level 8, lesson 2 of 6

A question to start

A crew's ascent rocket needs about 100 t of methane and oxygen to reach orbit from Mars. Mars's air is 95% carbon dioxide, and there is ice under the ground in many places, including Arcadia Planitia. Can the propellant be made there, and what would it take?

The idea

In-situ resource utilisation (ISRU) means making what you need from what is there. For methane-oxygen propellant, two reactions are enough.

Electrolysis of water, with electricity:

\[2\,\text{H}_2\text{O} \rightarrow 2\,\text{H}_2 + \text{O}_2\]

The Sabatier reaction, over a nickel or ruthenium catalyst at about 300 to 400 °C, giving out heat:

\[\text{CO}_2 + 4\,\text{H}_2 \rightarrow \text{CH}_4 + 2\,\text{H}_2\text{O}\]

The water it makes goes back to electrolysis. Run both together and the net reaction is

\[\text{CO}_2 + 2\,\text{H}_2\text{O} \rightarrow \text{CH}_4 + 2\,\text{O}_2.\]

By mass: 44 kg of CO₂ and 36 kg of water give 16 kg of methane and 64 kg of oxygen. That is 4 kg of oxygen per kg of methane; engines burn them at about 3.6 to 1, so there is oxygen to spare for breathing.

For 100 t of propellant at 3.6 to 1: 21.7 t of methane and 78.3 t of oxygen. Making 21.7 t of methane takes 48.9 t of water and 59.8 t of CO₂, and yields 86.9 t of oxygen, more than enough.

The energy. Splitting water takes at least 286 kJ per mole. Each kg of methane (62.5 mol) needs 125 mol of water split: \(125 \times 286\) kJ \(= 35.8\) MJ \(\approx 10\) kWh, in theory. Real electrolysers, plus digging and melting ice, compressing CO₂ out of thin air, and liquefying the products at −162 °C and −183 °C, may need 2 to 3 times that.

At 2.5 times: \(21{,}700 \text{ kg} \times 10 \times 2.5 = 540\) MWh. A 100 kW power plant running all the time would take 5,400 hours, about 225 days, inside the roughly 450 days a crew waits at Mars for the window home, and the plant could start work on a cargo flight before the crew arrives.

Already done, a little. MOXIE, an experiment on Perseverance, made oxygen from Mars's CO₂ by a different route (solid oxide electrolysis, \(2\,\text{CO}_2 \rightarrow 2\,\text{CO} + \text{O}_2\)) on 16 runs between 2021 and 2023, 122 g in all, at up to about 12 g an hour.

Worked examples

1. Mass balance. 1 kg of methane: \(44/16 = 2.75\) kg of CO₂ and \(36/16 = 2.25\) kg of water in; \(64/16 = 4\) kg of oxygen out.

2. The saving. From lesson 1, every tonne carried to Mars's surface costs about 5 t in low Earth orbit. Making 100 t of propellant on Mars saves about 500 t in Earth orbit, for a plant of perhaps a few tens of tonnes plus its power.

3. Without water. Carrying hydrogen from Earth instead of mining ice: hydrogen is only 1/9 of water's mass, so for 21.7 t of methane, \(48.9/9 = 5.4\) t of hydrogen from Earth, the rest from Mars's air. Bulky, but about an 18-fold leverage on 100 t of propellant.

Try it

  1. Check that the net reaction balances in atoms and in mass.
  2. How much water and CO₂ are needed for 10 t of methane?
  3. At 25 kWh per kg of methane in practice, how long does a 40 kW plant take to make 5 t of methane?
  4. Why is methane, rather than hydrogen, the usual choice for Mars?
  5. Why does the Sabatier reaction produce more oxygen than the engines need?
Answers
  1. Left: 1 C, 4 H, 4 O; right: 1 C, 4 H, 4 O. Mass: \(44 + 36 = 80 = 16 + 64\).
  2. 22.5 t of water and 27.5 t of CO₂.
  3. \(5{,}000 \times 25 = 125{,}000\) kWh; \(125{,}000/40 = 3{,}125\) h, about 130 days.
  4. Methane can be made from Mars's air and ice, is denser, and stays liquid at a much warmer temperature, so it is far easier to store.
  5. The net reaction gives O₂ and CH₄ in a mass ratio of 4, while engines burn at about 3.6; the rest can be breathed.

Common mistake

Thinking the CO₂ is the hard part. CO₂ is all around, in the air. The water (or hydrogen) and the energy are the hard parts: finding and mining ice, and building a big enough power plant.

Where it leads