Early in 2019, the precooler had worked at 420C (788F) in conditions that replicated flight speeds of Mach 3.3, or more than three times the speed of sound. But the engineers wanted to reach the magic number of Mach 5. That is more than 6,200 km per hour (3,800 mph). It is also more than twice as fast as the cruising speed of Concorde and over 50 percent faster than the SR-71 Blackbird aircraft – the world"s fastest jet-engine-powered aircraft. Mach 5 also happens to be the limit of today"s materials used in aircraft production.