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During the first crewed flight of a Falcon 9 today, I watched the nozzle go from dark grey to glowing red hot, confirming that it was burning. Yet there were no visible flames. I understand this stage uses LOX / RP-1 for propellant, which is usually quite a show.

Why were no flames visible? I've attached a picture that I believe was from while the engine was about mid-burn. I can't verify that the picture is of the right time, but it looked like this through most of the burn today.

Falcon 9 Second Stage

Thanks in advance.

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The Merlin engine used by the Falcon 9 burns a fuel-rich RP-1/LOX mixture, like most other rocket engines. That means the exhaust coming from the engine contains unburnt fuel, mostly in the form of soot. You can see that in your picture: There is a dark exhaust.

At sea level the excess fuel/soot burns off with atmospheric oxygen, leaving a flame trail behind the rocket, which you can nicely see after liftoff. But the higher the rocket flies, the less oxygen there is, the less visible the flame gets.

Now, the second stage operates in (near) vacuum, where no oxygen is, so the soot cannot burn at all, thus there is no flame.

If you would see a flame in the vacuum of space, that would mean that not all fuel/oxygen is burnt inside the engine, implying that the engine is not as efficient as it could be, since fuel burning behind the rocket does not provide any thrust.

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    $\begingroup$ How to reconcile these statements " That means the exhaust coming from the engine contains unburnt fuel," and "If you would see a flame in the vacuum of space, that would mean that not all fuel/oxygen is burnt inside the engine" $\endgroup$
    – Organic Marble
    May 31, 2020 at 12:15
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    $\begingroup$ By fuel I mean RP-1 only, without oxygen. In the latter statement I mean RP-1 and/with oxygen. Could try to phrase that better. $\endgroup$
    – Floern
    May 31, 2020 at 12:40
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    $\begingroup$ I did not know it burns fuel-rich. Why does it burn fuel-rich? $\endgroup$
    – Greg
    May 31, 2020 at 13:22
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    $\begingroup$ Almost all rocket engines run fuel rich. It makes for simpler molecules in the exhaust (e.g. CO instead of CO2), which don’t “hide” as much kinetic energy in internal vibrations; more of the kinetic energy goes toward thrust. (Many sources will say it’s lighter molecules that are desired but I’m told the math doesn’t work out for that explanation.) $\endgroup$
    – Russell Borogove
    May 31, 2020 at 13:42
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    $\begingroup$ Fuel-rich combustion is also lower-temperature, so the combustion chamber cooling can be less extensive. Summary of pros and cons of fuel-rich here: space.stackexchange.com/a/26439/195 $\endgroup$
    – Russell Borogove
    May 31, 2020 at 13:51
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Too long to post as a comment.

See Modeling of kerosene combustion under fuel-rich conditions in Researchgate or here or in Advances in Mechanical Engineering 9(7) · July 2017

Also, from Thermophysics Characterization of Kerosene Combustion by Ten-See Wang l NASA Marshall Space Flight Center, Huntsville:

Soot Formation:

Under fuel rich conditions, kerosene/RP-1 forms soot readily. This is because Naphthene and aromatic hydrocarbons form soot rapidly (condenstaion-polymerization) by directly condensing themselves into polycyclic aromatic hydrocarbons (PAH). On the other hand, paraffins form soot slowly. This is because paraffins have to be break up into smaller fragments first, from which fusing of the fragments occurs to form naphthenes and aromatics, and PAH's form eventually and indirectly (fragmentation-polymerization). 8,9 These PAH's, also known as soot precursors, are then undergone a series of physical processes to form coagulated soot panicles. Frenklach et al.20 developed a comprehensive soot formation mechanism in which 180 species and 619 elementary reactions are used in an attempt to describe the aforementioned soot formation processes. However, at the present moment, it is far too expensive to be incorporated into a CFD code while the oxidation of those 180 species was not even considered...

8Lawver, B. R., "Testing of Fuel/Oxidizer-Rich High-Pressure Preburners,Final Report, NASA CR-16544, NASA-Lewis Research Center, May 1982.

9Nickerson, G. R., and Johnson, C. W.,"A Soot Prediction Model for the TDK Computer Program," AIAA Paper 92-3391 ,July, 1992

20Frenklach, M., Clary, D. W., and Ramachandra, M. K., "Shock Tubes Study of the Fuel Structure Effects on the Chemical Kinetic Mechanisms Responsible for Soot Formation, Part2," NASA CR174880,May 1985.

soot formation chemistry in rocket exhaust as a function of fuel/oxygen ratio

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