[freebeard]

Consider the Skylon

Skylon is a series of concept designs for a reusable single-stage-to-orbit spaceplane by the British company Reaction Engines Limited (Reaction), using SABRE, a combined-cycle, air-breathing rocket propulsion system. The vehicle design is for a hydrogen-fuelled aircraft that would take off from a specially built reinforced runway, and accelerate to Mach 5.4 at 26 kilometres (85,000 ft) altitude (compared to typical airliner's 9–13 kilometres or 30,000–40,000 feet) using the atmosphere's oxygen before switching the engines to use the internal liquid oxygen (LOX) supply to take it into orbit.[1]
....
One of the most significant features of the Skylon's design is its powerplant, known as Synergetic Air-Breathing Rocket Engine (SABRE).[65] ... The engines are designed to operate much like a conventional jet engine to around Mach 5.5 (6,740 km/h; 4,190 mph),[63] 26 kilometres (85,302 ft) altitude, beyond which the air inlet closes and the engine operates as a highly efficient rocket to orbital speed.[63] The proposed SABRE engine is not a scramjet, but a jet engine running combined cycles of a precooled jet engine, rocket engine and ramjet.[13] Originally the key technology for this type of precooled jet engine did not exist, as it required a heat exchanger that was ten times lighter than the state of the art.[45] Research conducted since then has achieved the necessary performance.[21][66]

I forget why the jet needs to be pre-cooled but that heat exchanger is phenonemal, dropping the temp a thousand degrees in a fraction of a second or something. [cittion neeeded] of course.

edit: Here it is

https://www.space.com/32115-skylon-s...e-vehicle.html

This device precools the air entering the engine at speeds greater than four times the speed of sound (Mach 4). SABRE's precooler will cool such air from more than 1,832 degrees Fahrenheit (1,000 degrees Celsius) down to minus 238 F (minus 150 C) in one one-hundredth of a second. The oxygen in the chilled air will become liquid in the process.

I undersold it.