It gets a bit 'hot' and makes the metal glow on the outside as well as the inside
I very much doubt the engine doors would still be there had they actually been that hot!
However, I must admit to a double take when I first saw the image, especially having worked Tonka for a number of years, thinking of an imminent burn through.
But, as others have said, the reflection is also on the ext tanks, and the glow goes far further forward than does the hot section of the 199.
Nevertheless, an interesting shot I thought.