Yesterday morning, about 70,000 feet over the Pacific Ocean, the Boeing-designed X-51A Waverider “scramjet” set a new record. Reaching Mach 5 (almost 4,000 miles per hour), it wasn’t the fastest jet flight, but by burning for over 200 seconds it became the longest flight of its kind. The previous scramjet record, held by the NASA X-43, was 12 seconds.
A scramjet is similar to a simpler engine called a ramjet, but faster. The engines on most commercial jets have turbines to push air inside, but a ramjet is basically a tapered tube. As air flows through it, the shape of the tube compresses the air and, once the engine mixes this air with fuel, it ignites.
Unlike well-understood ordinary ramjets, which slow the air passing through them to subsonic speeds, the X-51 is intended to maintain combustion in a supersonic internal airflow — hence the name scramjet, for supersonic combustion ramjet — a feat often likened to “striking a match in a hurricane”. [The Register]