It's just the casing becoming "red hot", i.e. radiating in the visible spectrum.

The heat flux is not uniform, so the temperature is not uniform, so the color is not uniform.

Metal is opaque to visible light.

It can't. What is visible are places where the heat has increased the surface temperature to incandescence and there is some contrast with areas that are less heated. What you cannot see is why the heating is uneven. There is a similar effect using a FLIR brand (or other) thermal camera and looking at a house. It's often possible to deduce where the wall studs are because of the uneven flow of heat, but that's not the same as seeing the studs themselves.

I think this premise is possible... under certain circumstances.

In Richard Bach's early book 'Stanger to the Ground'... there is a recollection where he is in a formation of F-84s... then the lead calls for a formation position shift. I think the wingmen exchange positions.

In the midst of changing positions, one of the wingmen's jet pauses directly behind the lead... they all 'understand silently', that he is looking directly into the turbine of the lead's engine and seeing a unique, and almost godly-view, into the core of the engine. I wish I could find that passage. Bach wrote it in his deep-inside-his-soul way... making the incident unforgettable.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

I thought that book was great.

him of the famed seagull ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

Yep...

Richard Bach was an ANG Pilot in the 1960s. Stranger to the Ground was written in relationship to a courier-mission flown in Europe in an F-84. His writing style is totally unique... puts you in the cockpit with him... while his mind is both 'focused' and 'drifting'. After reading this book in the early 1970s... I fell in love with flying... I knew what I was experiencing on a small scale... what he was writing-about on a broader scale... and helped me understand my 'USAF-pilot-dad'. Flight is wonderous!

NOTE1. I think Richard Bach's ANG unit was tasked to fly combat in Vietnam early in the war... but HE refused 'to go' and was removed from service [BCD?].

Here is a favorite excerpt from STRANGER TO THE GROUND – Richard Bach

Tonight I, who love my airplane with all its moods and hardships and joys, am looking upon the stars. And tonight, 20-minutes to the east, there is another pilot, another man who loves his airplane, looking out at these same stars. These symbols.

My airplane is painted with a white star, his with a red star. It is dark, and paint is hard to see. In his cockpit is the same family of flight instruments and engine instruments and radio control panels that is in my cockpit. In his airplane as in mine, when the stick is pressed to the left, the airplane banks to the left.

I know unquestioningly, that I would like the man in that cockpit. We could talk through the long night of the airplanes that we have known and the times we were afraid and the places that we have been. We would laugh over the half-witted things that we did when we were new in the air. We have shared many things, he and I, too many things to be ordered into our airplanes to kill each other.

I went through flying training at a base near Dallas, he went through it at a base near Stalingrad. My flight instructor shouted at me in English, his at him in Russian. But the blue fire trickles once in a while across his windscreen as it does mine and ice builds and breaks over his wing as it does mine. And somewhere in his cockpit is a control panel or circuit breaker panel or a single switch that he has almost to stands on his head to reach.

Perhaps at this moment his daughter is considering whether, or not, to accept a pair of Siamese kittens. Look out for Your curtains, friend. I wish I could warn him about the kittens.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

I was heat treating some fancy Waspaloy about a decade ago. I opened the furnace and I'm fairly certain it was transparent. The edges of the parts were cooler than the core, and I remember thinking I could see the opposing cool edges through the core.

My dad's hunting partner made a wood burning camp stove out of Inconel 625. When the fire was cranked up on a near-winter evening in the mountains, you could see where the fire was by where the heat was radiating, and know when you needed to add more wood. The metal was only 1/16" thick. Not really transparent...just really good at conducting and radiating heat through the wall.