This article from earlier this week revealed something that I had no clue was possible – that the abundance of CO2 in the Cytherean atmosphere could actually be used as an oxidizer!
Combustion is the process of chain-reaction oxidation. Most combustion (for example, with organic materials) takes oxygen from the air and uses it to oxidize carbon atoms, producing CO2 (and H2O in organic compounds). However, with the correct substrate, it is possible to “burn” carbon dioxide by using it as the source of oxygen for oxidation (this reduces the carbon, and black carbon residue is a product of the combustion) through a process called decomposition.
This is frequently done in chemistry demonstrations where magnesium is ignited and surrounded with dry ice (solid state CO2). This produces Magnesium Oxide (MgO) and carbon (C). MgO has slightly less free energy than CO2, making the reaction exothermic (and hence combustion possible), but it has a very high activation energy, mostly involved in the dissociation of CO2. Burning magnesium is quite hot and provides the necessary activation energy and will “burn” the CO2 if there is no O2 available (this is done by piling the dry ice on top of the burning magnesium, or a similar method).
So now the question becomes which fuel to use? Magnesium isn’t the only fuel that will decompose CO2 (and liquids are simpler to use for propulsion than powders):
- B2H6 Diborane cryo liquid at -165C
- B5H9 Pentaborane liquid at room temperature -46C to 60C
- SiH4 Silane or Silicon Hydride cryo liquid at -113C
- LiBH4 Lithium Borohydride solid at room temperature melts at 268C, ignite at 180C!
- Al(BH4)3 Aluminium Borohydride liquid at room temperature -64C to 44C
- H2 Hydrogen cryo liquid hydrogen -253C WTF that sounds expensive
- Mg Magnesium solid melts at 650C, auto-ignition at 473C?! WTF that won’t work
- NH3 Ammonia cryo liquid at -34C
- N2H4 Hydrazine liquid at room temperature 2C to 114C
- C2H2 Acetylene cryo liquid at -84C (not included in original research but found from a second source)
Chemicals that are higher density are more efficient to store, but need to result in a high specific impulse with CO2 as the oxidizer. Not sure where to get those numbers as well as mixing ratio. Are any of these hypergolic? Or do we definitely need an ignition source now?