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Methane was chosen for SpaceX Raptor because it has less coking problem that kerosene. But does it completely eliminate it? If not, how much less severe is it?
I googled 'thermal decomposition of methane", and got this
In the thermal decomposition of methane at temperatures from 880 to 1103 K, hydrogen and ethane are the only primary products.
This would suggest that it does not exist at all. Is that correct?
Methalox produces the least coking of any hydrocarbon rocket fuel.
Coke is a compound of almost pure aromatic carbon rings, whose chemical structure resembles chicken wire. Coke is formed by the amalgamation of carbon chains and aromatic rings. The heavier (longer chain) the reactants in the fuel, the more readily coke forms when the reactants are heated.
The easiest way to form coke is to start with coal, since its structure is so close to Coke:
Coke
https://www.ariyancorp.com/petroleum-coke-petcoke/
Coal
https://www.researchgate.net/figure/Model-of-bituminous-coal-structure-30_fig16_255748916
The average chain length of kerosene (RP1) is 12, so coke forms readily.
Methane is a single carbon (no chains of covalently linked carbon) so it is very resistant to coke formation.
That would suggest it doesn't occur in the range from 880 K to 1103 K, at whatever pressure they were testing at. Also, from a few sentences later:
Deposition of carbon on the surface was observed at a still later stage of the decomposition,and was quantitatively estimated by light absorption measurements
The advantage of methane is not that it never produces any coking at all under any conditions, just that the coking can be controlled or prevented by the right operating conditions.
