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Space shuttle nozzle flame

Why is there some space between the flame and the nozzle on the space shuttle? (see above picture)

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The flame is inside the combustion chamber, up at the top of the bell. What you see is very hot supersonic steam, which is clear. The visible light below is, I think, the very first shock diamond, but maybe somebody else knows for sure. – Mike Dunlavey Sep 15 '13 at 17:42
Isn't the nozzle just so hot, that the blackbody radiation emitted by the hot gas molecules has slightly peeked into the UV region (which our eyes can't see)? – Waffle's Crazy Peanut Sep 15 '13 at 17:51
up vote 12 down vote accepted

According to Wikipedia,

Pure hydrogen-oxygen flames emit ultraviolet light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the Space Shuttle Main Engine

(The picture they provide is the same or very similar to that in the question).
So, maybe Crazy Buddy is right.

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You can often see the same phenomenon close too the wick of a candle. – jdj081 Sep 16 '13 at 4:47

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