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SpaceX's Merlin engine, which burns RP-1 and LOX, has soot visible after the first stage lands. Many articles on the web say SpaceX's Raptor engine, which burns methane and LOX, is clean. Refurbishment is easier as it doesn't produce soot.

The general formula for hydrocarbon complete combustion is:

$$\text{Hydrocarbon fuel} + \text{Oxidiser} → \text{CO}_2 + \text{H}_2\text{O}$$

Why do CH4 (Raptor) engines produce less soot than RP-1 (Merlin) engines?

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    $\begingroup$ I think it's because Merlin runs fuel-rich so not all of the RP-1 gets fully burned with the LOX during combustion, it has a mixture ratio of 2.34, compared to the 2.58-2.77 stoichiometric ratio, while the mixture ratio of the Raptor is 3.6, compared to the 4.0 stoichiometric ratio. Another idea might be that the extremely-fuel-rich gas generator exhaust is pumped directly overboard so soot and unburned fuel could accumulate on the engine. $\endgroup$
    – WarpPrime
    Commented Feb 3, 2022 at 13:50
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    $\begingroup$ @fasterthanlight agree about the GG exhaust, was coming here to mention that. $\endgroup$
    – Organic Marble
    Commented Feb 3, 2022 at 15:53
  • $\begingroup$ @fasterthanlight I had totally forgotten about mixture ratio, thanks for reminding. $\endgroup$
    – Ashvin
    Commented Feb 3, 2022 at 17:44
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    $\begingroup$ The world would be much, much simpler place if hydrocarbons always burned into only CO2 and H2O $\endgroup$
    – fraxinus
    Commented Feb 4, 2022 at 16:18
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    $\begingroup$ @fraxinus A lot of people would vote for C + H2O as the combustion products: far easier to capture and sequestrate. $\endgroup$
    – Mark Morgan Lloyd
    Commented Feb 5, 2022 at 12:38

3 Answers 3

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As GandalfDDI notes, you always run your engine fuel-rich so it doesn't start running engine-rich. This results in incomplete combustion of the fuel.

Methane is a single carbon atom bonded to four hydrogen atoms. Burn it with insufficient oxygen, and you get hydroxyl radicals and carbon monoxide. These eventually react with atmospheric oxygen to produce carbon dioxide and water vapor.

RP-1 is a mix of long-chain hydrocarbons, where a chain of carbon atoms are surrounded by hydrogen atoms. Burned with insufficient oxygen, it produces the same hydroxyl radicals and carbon monoxide as methane, but it also produces chain fragments of pure carbon. These stick to each other and to the rocket to produce the soot markings you see on re-used Falcon 9 boosters.

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    $\begingroup$ @Uwe - RP-1 has ~12 carbons, so incomplete combustion of just one molecule gives a nice soot nucleus which can scavenge other unburnt bits and build up with only a few inter-molecular collisions to something pretty big. $\endgroup$
    – Jon Custer
    Commented Feb 3, 2022 at 23:01
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    $\begingroup$ @Puffin, the RD-180 has an oxygen-rich preburner, but as far as I can tell, the combustion chamber/nozzle is fuel-rich. $\endgroup$
    – Mark
    Commented Feb 3, 2022 at 23:50
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    $\begingroup$ One can force methane into sooty burning even in a kitchen stove. It is just much harder than for longer-chain hydrocarbons (gasoline, diesel, kerosene, RP1) who, generally, prefer producing soot. $\endgroup$
    – fraxinus
    Commented Feb 4, 2022 at 11:20
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    $\begingroup$ As well as running fuel-rich during normal operation, you also want to turn off the engines without causing fires/explosions. As the turbo pumps spin down, the fuel / oxidizer mix will be poorly controlled. It makes sense to err on the side of stopping the oxidizer slightly quicker than the fuel: a fuel rich fire due to residual propellant can be suppressed by CO2 / N2 / Argon gas, starving the fire of oxygen. And oxidizer rich fire is harder to suppress. $\endgroup$
    – user1998586
    Commented Feb 4, 2022 at 20:13
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    $\begingroup$ @Pere Too much oxidizer and you will start burning your engine. $\endgroup$
    – Mys_721tx
    Commented Feb 5, 2022 at 20:03
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As explained to me (carefully and fully) by a Rocketdyne engineer you ALWAYS run rocket engines fuel rich. If you run lean the oxidizer looks at the metal parts of the engine (remember the engines are typically running VERY hot) and says "Oh ... Look, metal. A source of fuel" and bad things start to happen. Same with other parts that may come in direct contact with the oxidizer, more fuel.

RP-1 is basically a petroleum derivative very close to kerosene, so being fuel rich unburned output from the flame is partially / not fully burned and the oxygen in the atmosphere burns the fuel rich output, along the lines of diesel smoke being unburned carbon from a diesel engine: https://en.wikipedia.org/wiki/RP-1

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    $\begingroup$ "... Another source of fuel." Good for a chuckle $\endgroup$
    – fred_dot_u
    Commented Feb 3, 2022 at 17:55
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    $\begingroup$ You haven't addressed why RP-1 gives more soot. $\endgroup$
    – PM 2Ring
    Commented Feb 3, 2022 at 18:00
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    $\begingroup$ @GandalfDDI: You might want to edit your answer to include your previous comment which directly answers the question, when combined with what you posted. $\endgroup$
    – Peter Cordes
    Commented Feb 4, 2022 at 6:27
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    $\begingroup$ It's not "Another source of fuel". It is simply "Another fuel" instead. In at least one Starship or probably F9 series of unsuccessful landings one can see green flames - a good hint for copper alloy burning. $\endgroup$
    – fraxinus
    Commented Feb 4, 2022 at 11:14
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    $\begingroup$ Another important reason for running engines fuel rich is that incomplete combustion produces smaller molecules (CO instead of CO₂, some H2 left unburned), which produce more thrust compared to larger molecules, so fuel rich is just more efficient. Raptor and some russian staged combustion engines run oxygen rich preburners, so the metals burning problem is solvable, at least for the preburners. $\endgroup$
    – JanKanis
    Commented Feb 4, 2022 at 22:26
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CH₄ contains relatively much more H than C$_{12}$H$_{26}$ (a typical RP-1 molecule). In CH₄ the atomic ratio of C:H is 1:4, while in C$_{12}$H$_{26}$ it's close to 1:2. So there is just more C to go around and form soot in RP-1 engines.

Another reason is the reaction probabilities. Soot is bunches of carbon atoms stuck together. If all the carbon atoms start out separated, as in CH₄ where each molecule contains only one C, the chance of many C atoms coming together without being burned is much smaller than when you start off with a C$_{12}$H$_{26}$ molecule with 12 C atoms, so I would expect you end up with smaller soot particles.

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