Off the coast of southern California on 16 November 2004, a NASA B-52B released a rocket with a strange, flat, surfboard-shaped object bolted to its nose. The Pegasus booster lit, climbed to about 110,000 feet and let go. For roughly ten seconds, the 12-foot X-43A breathed air, burned hydrogen and flew at Mach 9.6, close to 7,000 mph according to NASA.

No aircraft powered by an engine that breathes air had ever flown that fast. More than twenty years later, none has officially flown faster. As this week’s first HACM flight test shows, the technology the X-43A proved is only now turning into weapons.

How Do You Test an Engine That Only Works at Mach 5?

A scramjet, or supersonic combustion ramjet, has no compressor and no turbine. It relies on the aircraft’s own speed to ram air into the engine, where fuel is burned while the airflow is still moving faster than sound. That makes it elegantly simple and, below about Mach 5, completely useless. It cannot start from a standstill.

NASA’s answer was brute force. The X-43A rode on the nose of a modified Pegasus rocket, which itself hung under the wing of the agency’s veteran B-52B. The bomber carried the stack to around 40,000 feet, the Pegasus boosted it to test speed and altitude, and then the X-43A separated and fired its engine for about ten seconds before gliding into the Pacific. None of the three vehicles was ever meant to be recovered.

NASA’s Langley Research Center in Virginia led the programme, with flight testing from Dryden Flight Research Center, now Armstrong, at Edwards Air Force Base. ATK GASL built the vehicles and engines, Boeing Phantom Works supplied the thermal protection and onboard systems, and Orbital Sciences modified the Pegasus boosters.

A Failure, Then Two Records

The first attempt, in June 2001, ended seconds after launch when the Pegasus booster lost control and had to be destroyed. It took almost three years to fix the problems. On 27 March 2004, the second X-43A reached Mach 6.8 at about 95,000 feet, the first time an air-breathing hypersonic engine had powered an aircraft in free flight.

The third and final flight on 16 November 2004 was the hardest. At nearly Mach 10, the heating on the leading edges was far more severe, and the margins on everything from inlet geometry to fuel timing were razor thin. It worked. The engine ran for about ten seconds, and the vehicle then glided for close to ten minutes, sending back data before it hit the ocean.

Ten Seconds Worth Years of Data

For the engineers, ten seconds of engine time was an enormous haul. Ground facilities can only sustain Mach 10 airflow for fractions of a second, so the X-43A’s flight produced more real scramjet data at that speed than all previous tests combined.

The flight was also a farewell. The B-52B that carried the X-43A had launched X-15s in the 1960s and was retired shortly after the final Hyper-X mission. The commander of the Air Force Flight Test Center, Brig. Gen. Curtis Bedke, said the bomber could claim to have seen and taken part in more aviation history than any other single aircraft.

From Research Toy to Weapon

The X-43A never carried a payload or a pilot, and it was never meant to. Its job was to prove that a scramjet could be started and controlled in real flight at hypersonic speed. The Air Force’s X-51A Waverider took the next step a few years later with a hydrocarbon-fuelled engine that ran for minutes rather than seconds. Today, missiles such as HACM are built on what those programmes learned.

NASA’s footage of the X-43A launch from the B-52B:

Sources: NASA, US Air Force (af.mil, November 2004), Wikipedia