Why are scramjet engines intended for aircraft flying faster than Mach 6?

Scramjet engines are designed for flight at speeds above Mach 6 because, at these extreme velocities, decelerating the incoming airstream to subsonic speeds—as required by traditional ramjets—becomes inefficient and results in significant pressure losses.1

To maintain performance at such high Mach numbers, scramjets (supersonic combustion ramjets) are engineered to burn fuel directly within the supersonic airstream. This design allows the engine to function effectively where conventional turbomachinery or subsonic combustion systems would fail due to excessive heat and pressure constraints. Furthermore, these engines often utilize cryogenically liquified fuels, such as hydrogen, which serve a dual purpose: they provide high energy density and act as a critical heat sink to cool the engine and aircraft structure, which face intense temperatures during high-speed flight.12