For takeoff, landing and subsonic flight, the A2's Scimitar engine sends intake air through a bypass duct  and into a turbine , like a standard jet engine. After reaching supersonic speed, though, the engine redirects air from the bypass duct through the engine core  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  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  using gaseous helium.
Five amazing, clean technologies that will set us free, in this month's energy-focused issue. Also: how to build a better bomb detector, the robotic toys that are raising your children, a human catapult, the world's smallest arcade, and much more.