I
That may be the one (partially) true claim, actually.

Hydrogen remains largely transparent as it burns, so it emits very
little light. Blackbody radiation is called that for a reason --
perfectly reflective or transmissive materials don't emit light when
they're hot; bright yellow flames get that way because soot is black,
which is part of the reason that hot soot glows. So if you looked at
the flame with an infrared pyrometer it would show up as not much warmer
than it's surroundings.



Even that claim is dubious. It's true that a hydrogen flame doesn't emit much
light, but I can assure you it emits plenty of IR radiation.

I worked many years at NASA's Rocket Engine Test Facility (RETF) at the (now)
Glenn Research Center in Cleveland. One of our continuing programs was liquid
hydrogen cooled nozzles. The hydrogen was run through the nozzle and skirt to
cool the throat and preheat the hydrogen before it was injected. Since this was
a test of the cooling and not the combustion, we just dumped the warm hydrogen.

It went up a stack about 120 ft high, where it exited a manifold and was
ignited by a pilot flame. The flame was huge (this was about a 20,000 lb thrust
motor), about 50 feet high and 20 feet across. Since the RETF was right next to
Cleveland Hopkins airport, we had to get clearance to allow this flame so close
to the runway. My friend Ken actually flew a small aircraft above the flame to
test for thermals.

You could barely see the flame on overcast days, it surely is invisible. But,
if you went outside, Hole mackeral, it was hot! It was just like a second sun
on the other side of your face, really a strange feeling since there didn't
appear to be anything there to cause it.

Among other neat things about this testing was the fact that the final run of
piping to the engine was not insulated (it was a SS flex line). When the
hydrogen came on at 2000 psi the lines stiffened up and liquid began 'raining'
around them. It is liquid air! The non insulated line is so cold the atmosphere
condenses around it. The oxygen rich mixure rained down on the test stand and
if any oil or dirt was present it poofed into flames.

More than once I called into the intercom, "Fire on the stand!" The test
conductor (a half mile away) would always reply with, "Hang on, we need 5 more
seconds!"

As the fuel operator I was only 30 feet from the stand, in a foot-thick,
hardened building ominously called the 'Termination Room" (it's where the
instrumentation terminated). I looked out through 40 panes of bullet-proof
glass (totaling 12" thick) with a a thick steel plate that held a mirror at a
45 degree angle. Above this was another mirror facing the stand, so it
amounted to a periscope.

Near my right hand was the big red abort button, but woe to the tech who pushed
it too early!

Dennis