At a test facility in China, a hypersonic engine that reshapes its internal airflow channel – much like a throat tightening and relaxing – has operated continuously from a relatively modest Mach 1.8 (nearly twice the speed of sound) all the way to Mach 6 without failing.And the material that kept superheated gases from escaping? Essentially the same black mineral found inside a pencil: graphite.For years, engines of this type could not ignite until the aircraft had already reached Mach 4 – meaning a separate rocket booster was needed to get up to speed first, adding cost and complexity.Now, researchers from Northwestern Polytechnical University and the Beijing Power Machinery Institute say they have solved a problem that has stumped engineers for decades.They ran the variable-geometry ramjet – a type of air-breathing jet engine with no moving compressor – at a ground-based facility that simulates high-speed flight conditions.The engine’s combustion chamber throat – a moving metal component that tightens and relaxes to manage airflow at different speeds – adjusted itself in one third of a second while inhaling gases at 1,650 degrees Celsius, according to a paper published in the Journal of Propulsion Technology on May 28.