The BBC News article Rolls-Royce and Boeing invest in UK space engine about Reaction Engines Limited (REL) and its Sabre engine states
REL is developing what it calls the Sabre engine. This power plant is designed to push a vehicle from a standing start all the way to orbit in a single step.
It would work like a conventional jet engine up to about Mach 5.5 (5.5 times the speed of sound) before then transitioning to a rocket mode for the rest of the ascent.
Key technologies include a compact pre-cooler heat-exchanger that can take an incoming airstream of over 1,000C and cool it to -150C in less than 1/100th of a second. This would permit Sabre to use oxygen direct from the atmosphere for combustion instead of carrying it in a tank with the weight penalty that implies. (emphasis added)
A temperature change of 1000C to -150C expressed in absolute terms is about 1273 K to 123 K, or more than a factor of 10 in absolute temperature, and in less than 10 milliseconds.
How does the pre-cooler work? How can it cool a continuous flow of air so quickly at such high volume, and still remain physically small and sufficiently light weight to be just one component of this amazing engine?
above: "Sabre would work like a jet engine in the lower atmosphere and like a rocket motor in the high atmosphere." From BBC. Credit: REL