In a windswept corner of a military site on Salisbury Plain a small aircraft jet engine is undergoing tests that could one day lead to huge changes within the aviation industry.
The engine itself is almost completely conventional. It is a Rolls-Royce AE-2100A gas turbine, a design used widely on regional aeroplanes around the world.
What is wholly unusual about it is the fuel being used. This is the first time a modern aircraft engine has ever been run on hydrogen.
Devoid of bodywork, with its intricate wiring and pipework exposed, it sits securely fastened to a sturdy test rig, while engineers cluster around an array of screens in the control room, a safe distance away.