The final flight of the unmanned Boeing X-51A Waverider accomplished a breakthrough in the development of flight reaching Mach 5.1 over the Pacific Ocean. The US Air Force (USAF) has stated that the cruiser travelled more than 230 nautical miles in just about six minutes over the Point Mugu Naval Air Warfare Center Sea Range, California. It was the longest of the four X-51A test flights and the longest airbreathing hypersonic flight ever.

The X-51A took off from the Air Force Test Center at Edwards Air Force Base, California, under the wing of a B-52H Stratofortress. It was released at approximately 50,000 feet and accelerated to Mach 4.8 in about 26 seconds, powered by a solid rocket booster. After separating from the booster, the cruiser’s supersonic combustion ramjet or scramjet engine lit and accelerated the aircraft to Mach 5.1 at 60,000 feet.

After exhausting its 240-second fuel supply, the vehicle continued to send back telemetry data until it splashed down into the ocean and was destroyed as designed. This was the last of four test vehicles originally conceived when the $300 million technology demonstration programme began in 2004 and the programmes objective was to prove the viability of air-breathing, high-speed scramjet propulsion.

The X-51A is distinct primarily due to its use of a hydrocarbon fuel in its scramjet engine. Without any moving parts, hydrocarbon fuel is injected into the scramjet’s combustion chamber where it mixes with the air rushing through the chamber and is ignited in a process likened to lighting a match in a hurricane.