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Mars atmosphere as propellant

Earth's atmosphere contains oxygen, which can be used as a rocket propellant, burning with kerosene or hydrogen. Mars' atmosphere on the contrary contains only carbon dioxide (CO2), that will not burn with kerosene or hydrogen.

But, metals can burn with CO2, like magnesium does in this video:

So, a spaceship landing on Mars, could in principle fill a reservoir with liquid CO2 harvested from the atmosphere then use that CO2 to feed a hybrid rocket motor burning metal. Or use a second reservoir containing molten metal and feed a liquid rocket motor. Or freeze the CO2 and mix it with powdered metal. A hybrid motor could be the simplest way to constitute the first stage of a rocket to get back into Mars orbit.

The reaction products of magnesium and CO2 have a very high temperature and become a powder at lower temperatures, which is both destructive and useless in a rocket motor. To cope with this an excess of CO2 has to be used, to lower to temperature and maintain a flow in the nozzle.

Beryllium oxide and lithium oxide having about the same formation enthalpy as magnesium oxide, beryllium and lithium should burn too. I found no data to confirm this. Having a much lower atomic weight than magnesium they would be more appropriate for a rocket motor.

A solid rocket motor that uses frozen water and aluminum powder has been tested:

Another way to use CO2 as propellant is to rely on the ease to liquefy it and get it back to expand, like in CO2 airsoft guns. The idea here would for example be a drone that makes hops and short flights on Mars. It would use the energy from solar panels to slowly fill a reservoir with liquid CO2, either by pressurizing CO2 pumped from the atmosphere or by cooling the reservoir and let the CO2 from the atmosphere just enter the reservoir and liquefy. One way to help refrigerate could be a thermal panel that is directed towards the sky at night, to radiate heath away. Once the reservoir is filled, it can be heated up to increase the pressure. The CO2 can then be ducted towards nozzles to lift the drone. If the drone has a powerful battery, it can contribute to heat and expand the CO2 further inside the nozzles and control the flight. The flight would last only a few minutes but that would be enough to explore a site or travel some distance over rough terrain. Once the drone lands back, it blows the dust away from the solar panels and starts again replenishing its CO2 reservoir and loading its battery. Apart from a few valves, this system can be a solid state device; requiring no moving parts.

The specific impulse of a steam rocket is quite low but still enough to constitute a first stage for a rocket that can rise from the Martian surface towards orbit. If CO2 can replace water effectively, that first stage would have the big advantage of being many times reusable.

See also:

Eric Brasseur  -  November 16 2016