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Wow. 1,500C to -150C in 1/100th of a second sounds pretty remarkable. Any idea how that's achieved?

A helium cooling loop, cooled by a nitrogen boiler. More info and pictures: https://www.bbc.co.uk/news/science-environment-20510112

The Rolls engines designed for HOTOL were meant to do the same, by a different method. They were going to use the hydrogen fuel to operate the intake heat exchangers. The engine then burnt hot hydrogen. Any excess was used as reheat injected into the exhaust.

What do they ultimately do with the heat?

Or do they just boil of the nitrogen into the atmosphere and need to take a supply of it for the whole trip?

They will use fuel instead of LN2 in the real thing.

From what I heard liquid hydrogen is quite nasty. It will definitely require different materials than ln2

There was a movie showing their heat exchanger test going around. It is a gas to gas (helium.) And you can pump helium at very high flow rates. Speed of sound in helium is 1000m/s after all.

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