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At about 7:53 (T+2:30) in this official video of the SpaceX Starship SN10 test flight, the engines are shown in the lower right, with clearly different colors in the plumes. The one on the right is much more yellow-orange and the one on the left more blue. This is also seen earlier during three-engine flight, but it's easier to see the difference during two-engine flight.

Aren't these engines designed to be identical and experiencing pretty much the same atmospheric conditions?

Why the difference in plume colors?

image of rocket plumes

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  • $\begingroup$ I had heard that it could be indicative of an engine-rich combustion but I don’t think that could have been sustained for that long with such little variation $\endgroup$
    – R. Hall
    Mar 8, 2021 at 0:27
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    $\begingroup$ Engine rich on a Raptor will be very green: cdn.arstechnica.net/wp-content/uploads/2020/12/… Orange is just fuel-rich $\endgroup$
    – Saiboogu
    Mar 8, 2021 at 1:00
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    $\begingroup$ fuel rich = soot particles = incandescence = yellow + orange Maybe different throttling of the two is the root cause? $\endgroup$
    – uhoh
    Mar 8, 2021 at 2:48
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    $\begingroup$ I also think its due to being fuel rich, the unburnt carbon will glow with black body radiation. So its yellow because a normal sooty flame/incandescent light is yellow. Engine throttling isn't anything to do with it. $\endgroup$
    – Uzer
    Mar 8, 2021 at 21:12

2 Answers 2

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SN10 like all recent Starship prototypes is very over powered. It has 3 Raptor engines each capable of producing in excess of 200 tonnes thrust and for safety reasons it also takes off with a fairly limited amount of propellants and burns through them at a rapid rate. Consequently it is necessary to throttle down and then power down the engines one by one. If they didn't do that, then as Elon Musk said "it would go crazy high".

Raptor engines are throttled by adjusting the amount of fuel they burn (a little bit like a Bunsen burner) When deeply throttled the burn with a more yellowish flame.

Some times engines are seen to burn with a greenish tinge. This is not usual a good sign as it can be indicative of some of the inner copper lining from the nozzle burning off.

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  • $\begingroup$ Can you try to research the "green" origin? Saiboogu's comment above claims a different cause than you state here. $\endgroup$
    – Carl Witthoft
    Mar 8, 2021 at 15:55
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    $\begingroup$ @CarlWitthoft I said the green is "engine rich combustion" which means parts of the engine interior are burning. That agrees with what Slartysaid as well. $\endgroup$
    – Saiboogu
    Mar 8, 2021 at 16:10
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    $\begingroup$ Do you have a source for the statement that Raptor throttles by reducing just the fuel flow (thereby altering the mixture ratio), as opposed to reducing both fuel and oxidizer flow and maintaining the same mixture ratio? $\endgroup$
    – Russell Borogove
    Mar 9, 2021 at 21:01
  • $\begingroup$ If Raptor throttled by reducing the fuel and oxidizer flow it would not flame out.(extinction of the flame in the combustion chamber). twitter.com/elonmusk/status/1272655479420776449 $\endgroup$
    – Slarty
    Mar 9, 2021 at 22:46
  • $\begingroup$ Does "200mt thrust" mean 200 megatons (equivalent) of thrust? That's about 2E+12 N? Two trillion newtons? Have I got something wrong? $\endgroup$
    – uhoh
    Mar 10, 2021 at 0:37
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The orange color indicates they're running fuel-rich. With the methane/oxygen ratio out of whack there would be more carbon in the exhaust which burns orange.

It's possible that engine was malfunctioning. SN10 aborted at T-0.1s because one engine was producing too much power. They decided to reset the systems and launch anyway. When transitioning into hover, the second engine shuts down but combustion continues which is not a great sign. SN10 lands with a higher velocity than they intended indicating the second engine may have shut down early, or the one remaining engine was underperforming.

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