For years, engineers have dreamed of building an aircraft that could reach hypersonic speeds, greater than Mach 5, or five times the speed of sound. Air-breathing engines have several advantages over rockets. To produce the same thrust, air-breathing engines require less than one seventh the propellant that rockets do.
Missions can also be more flexible.
Hypersonic air-breathing propulsion is just now finally coming of age. But turbojet engines, such as those found on commercial and fighter aircraft, are limited to Mach 3 or 4, above which the turbine and blades that compress the air suffer damage from overheating. Such an engine has an air inlet that has been specially shaped to slow and compress the air when the vehicle is moving rapidly through the atmosphere. To obtain higher speeds, supersonic-combustion ramjets, or scramjets, reduce the compression of the airflow at the inlet so that it is not slowed nearly as much. Because the flow remains supersonic, its temperature does not increase as dramatically as it does in ramjets.