For takeoff, landing and subsonic flight, the A2's Scimitar engine sends intake air through a bypass duct [1] and into a turbine [2], like a standard jet engine. After reaching supersonic speed, though, the engine redirects air from the bypass duct through the engine core [3] for flight up to Mach 5. Like a ramjet, the Scimitar works by taking in air from the atmosphere at high speeds and funneling (or "ramming") it until it's intensely compressed. At this point, the extremely hot air mixes with fuel and causes ignition. But the Scimitar one-ups traditional ramjet design by adding a turbine [4] that compresses the incoming air even further. Ramjets usually can't use turbines because the incoming air is so hot that it will melt the turbine blades. The Scimitar solves that problem by first cooling the air in a heat exchanger [5] using gaseous helium.
The A2 runs on liquid hydrogen, which produces water vapor and a small amount of nitrous oxide as exhaust and is energy-dense enough to fly this plane all the way from Brussels to Sydney. To be truly green, however, the A2 must use hydrogen produced in a carbon-neutral process such as electrolysis powered by a nuclear, solar or biofueled power plant.
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