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Hey DT,

Wow!! That's amazing! What sort of relative humidity would there be
in the area. I would have thought that as the exterior surfaces of
the flex line began to cool, that it would form a heavier and heavier
layer of "ordinary" frost from the ambient room air. Is the LOX taken
from that frost? Or some other action? I guess if the frost built to
the point that it fell off, or the flex "flexed" and knocked it off,
then it would expose the super-cold surface directly.


You are correct, as normal ice builds up, it insulates the LH2 line from the
air and the liquification of the air stops. This was in a high performance
rocket engine test stand (I work for NASA) and the final fuel lines to the
engine were just plain flex lines. Each time the engine fired it tended to
shake off the frost and re-expose the bare metal line to some extent.

As an interesting note, we ran some low pressure LOX transfer lines uninsulated
for dozens of feet at a time, -297 F just isn't cold enough to worry about at
times. But the final LOX lines out of the high pressure tank ran in open LN2
troughs to supercool the LOX. In fact the entire high pressure LOX tank was
submerged in LN2. We always watched the LN2 to see if it turned blue,
indicating a LOX leak into the LN2 bath.

Further note: The LOX and LH2 systems were running at 3000-4000 psi!!

DT