"Interest in precooled engines saw an emergence in the UK in 1982, when Alan Bond created a precooled air breathing rocket engine design he called SATAN. The idea was developed as part of the HOTOL SSTO spaceplane project, and became the Rolls-Royce RB545. In 1989, after the HOTOL project was discontinued, some of the RB545 engineers created a company, Reaction Engines Ltd, to develop the idea into the SABRE engine, and the associated Skylon spaceplane.So these guys have been working on this for THIRTY YEARS now!
You cannot have an air-air precooler because there is no air stream to which you can dump the heat. Therefore, all precooled jet engines use cryogenic liquid hydrogen fuel to cool the incoming air. Using liquid hydrogen means major changes to the entire airframe including making it a lot larger to store the fuel.
SABRE is a hypersonic hybrid engine. It draws in air to act like a conventional jet while accelerating to speeds of up to Mach 5 (3,704 mph, 5,961 km/h), then converts to a pure rocket engine burning hydrogen and liquid oxygen, making speeds of up to Mach 25 (17,521 mph, 29,808 km/h) possible.Key to this is a revolutionary heat exchanger that protects the engine as it approaches hypersonic velocity. At high supersonic speeds, the air is coming into the engine with the force 25 times that of a category 5 hurricane, generating temperatures that would melt any material that the engine might be made of. To prevent this, SABRE has a precooler that uses recirculated cryogenic hydrogen fuel to cool incoming air down from 1,000° C to -150° C in 1/100th of a second.
For comparison, the SR-71 engines were each gulping 200 lbm/second of air! that is a huge airflow to cool.
Below is the precooler they just tested in Colorado. They say this about their precooler:
Our pre-coolers are made from thousands of thin-walled tubes to provide high surface area to low weight. Each tube is joined to an inlet and outlet manifold, which allows coolant to be injected and removed for the cooling process. We're the only people in the world with the heat exchanger manufacturing experience to bond thousands of joints in a single operation, and achieve zero leakage. The joints in our pre-cooler modules are hermetically sealed, meaning that the gas which escapes can be measured by the molecule.That is quite an accomplishment (if true).
It will be interesting to see how they deal with all the shock issues and stay within size and weight constraints of a commercial aircraft. The story of the design of the SR-71 engine inlets is an amazing bit of engineering history. It was one of the toughest design challenges on the SR-71. Here's a good presentation on the SR-71 engine inlet design...