The British engine designed to fly from a runway directly into Earth's orbit

Scott Manley explains the ambitious SABRE engine, a hybrid design intended to function as an air-breathing jet up to Mach 5 before transitioning into a pure rocket cycle for orbital flight.

The SABRE engine was the centerpiece of an effort to create the 'Skyline' aircraft, a vehicle capable of reaching orbit without the need for jettisoning stages. The core innovation behind this concept was a specialized pre-cooler, a component engineered to rapidly chill incoming air to temperatures suitable for compression.

Once cooled, this air would be used to burn hydrogen within the rocket engine. While the ultimate objective was the development of the Skyline aircraft, the company, Reaction Engines, prioritized the technical demonstration of the engine's cycle and the viability of its critical pre-cooling technology.

Source: How Britain's Air Breathing Rocket Engine Would Have Worked... If The Builder Hadn't Gone Bankrupt.