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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?

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    $\begingroup$ I believe Elon has tweeted that methane was chosen as a fuel because it is the easiest hydrocarbon to manufacture from the CO2 in Mars atmosphere. I can't site a source. $\endgroup$
    – Woody
    Apr 30, 2023 at 6:58
  • $\begingroup$ Related: space.stackexchange.com/a/22950/6944 $\endgroup$ May 2, 2023 at 0:12
  • $\begingroup$ As Ignition! teaches us, rocket engine combustion produces literally everything that can be produced out of the fuel and oxidizer. Carbon monoxide, soot, assorted hydrocarbons, hydrogen peroxide, monoatomic oxygen radicals, ozone... I'm sure coke is somewhere in there as well. It's all about proportions and conditions - the engines are engineered not to allow accumulation of the undesired compounds in amounts that would impact the performance. $\endgroup$
    – SF.
    May 29, 2023 at 12:46

2 Answers 2

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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 enter image description here

https://www.ariyancorp.com/petroleum-coke-petcoke/

https://www.researchgate.net/figure/Model-of-bituminous-coal-structure-30_fig16_255748916Coal

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. enter image description here

Methane is a single carbon (no chains of covalently linked carbon) so it is very resistant to coke formation.

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  • $\begingroup$ Coking is a problem in the fuel passages used for cooling, not in the combustion chamber. space.stackexchange.com/a/59099/6944 $\endgroup$ May 2, 2023 at 0:14
  • $\begingroup$ @OrganicMarble ... good point. The comment has been edited out. It was a factoid that came to mind because of my interest in the chemistry of 19th century coal gas and 20th century ICE engines (which are both rapidly becoming extinct technologies). Coking of combustion chambers in rockets is irrelevant because of short operational lifetimes. $\endgroup$
    – Woody
    May 2, 2023 at 1:25
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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.

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