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New Shepard is described as using liquid oxygen and liquid hydrogen for its propellants. When hydrogen and oxygen burn, as in the Space Shuttle main engines, there is a short conical blue flame, beyond which the exhaust is clear (until the water vapor condenses). The New Shepard exhaust appears to have a long, distinctly red/orange/yellow flame, which would suggest that there is something else burning besides hydrogen and oxygen. What's going on?

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  • $\begingroup$ A good question, especially considering the example of SpaceX's Raptor's hydrocarbon-rich but very clear blue flame (when it's not an early prototype engine slowly consuming itself). Sodium in dust can strongly color the exhaust when near the ground, but this clears up with altitude. The RS-68 exhaust is colored due to its ablative nozzle, but I haven't heard any indications that BE-3 uses such a thing. Though it might explain why BE-3U is basically a new engine... $\endgroup$ – Christopher James Huff Jul 21 at 0:03
  • $\begingroup$ Have we ever actually seen the flame of an SSME at similar altitude in similar atmospheric conditions filmed with similar exposure settings, without any other bright source of light nearby? In the SLS Green Run Hot Fire test, you can clearly see an orange flame, for example. $\endgroup$ – Jörg W Mittag Jul 21 at 2:32
  • $\begingroup$ @JörgWMittag the closest match I can think of is the DC-X, and never mind exposure settings, the imagery was done with a radically different technology. The exhaust was visible at times, but the film used might respond to wavelengths common camera sensors would miss, and the manipulations in printing and scanning might exaggerate its visibility, especially if done by someone who's expecting the exhaust to be visible. $\endgroup$ – Christopher James Huff Jul 21 at 2:45
  • $\begingroup$ @JörgWMittag note that in the Green Run test, the exhaust is hardly in open, clean air. Even tiny amounts of sodium contamination from dust/concrete/etc will turn things bright orange/yellow. $\endgroup$ – Christopher James Huff Jul 21 at 2:52
  • $\begingroup$ Pic of DC-X (regeneratively cooled RL10 engines) showing pinkish-red plume: americaspace.com/2013/08/20/… $\endgroup$ – Russell Borogove Jul 21 at 6:30
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The first few feet of exhaust from the New Shepard is clear, and the reddish plume fades with altitude, so we can rule out an ablative liner like the Delta IV's RS-68 engine uses -- that would produce a more consistent yellow plume.

Hydrogen's emission spectrum includes lines in ultraviolet, violet, blue, green, and red. Very hot hydrogen combustion, like that occurring in the engine's combustion chamber, gives more of the violet and blue emission, but the exhaust is fuel-rich, and cooler than the combustion chamber due to expansion in the nozzle, so when the exhaust hydrogen combusts with atmospheric oxygen at lower temperatures, you see more red.

The space shuttle's RS-25 main engines also produce a little pink light in the exhaust plume, but it's faint compared to the very bright plume of the solid rocket boosters, so not so noticeable in photos. The exact exhaust chemistry may be different as well; I don't know how the propellant mixture ratios compare.

The DC-X also used regeneratively cooled, hydrogen-oxygen RL10 engines with no solid rocket boosters, and shows similar pinkish-red plumes:

enter image description here

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