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By Daniel Lametti

There’s nothing worse than a long flight. Perhaps that’s why the Internet was filled this morning with reports that the United States Air Force was testing an experimental plane that could, in theory, fly from New York to London in about an hour. (According to the Washington Post, the test flight did indeed take place Tuesday, but there’s been no word yet on the results.)

The X-51A Waverider uses a special scramjet engine to propel it to five times the speed of sound—that’s hypersonic speed, or more than 3500 miles per hour. Scramjet stands for supersonic combustion ramjet. Simply put, rapidly moving air is funneled into an engine, mixed with fuel, and ignited to produce thrust. (A conventional jet engine needs the help of heavy compressors and turbines to achieve the same result.) In a ramjet, the air inside the engine is typical moving at subsonic speeds when it’s ignited; in a scramjet, the air inside the engine is lit at supersonic speeds. This makes scramjets more efficient while increasing thrust.

There are several downsides to scramjet-powered planes. For one thing, they can’t start from a standstill. Since forward motion is what pushes air into the engine, the plane must be brought up to speed before ignition. This also means scramjet engines are very difficult to start. “Producing thrust with a scramjet,” reads one military report on the plane, “has been compared to lighting a match in a hurricane and keeping it burning.”

The X-51A is unmanned. The plan for today’s test flight was to drop it from the wing of a B-52 bomber, 50,000 feet over the Pacific. If this flight proceeded in the same fashion as the X-51A’s first test run in 2010, an army tactile missile would then propel it to about four times the speed of sound, at which point the conditions would be right for its scramjet engine to fire up. The plane was expected to fly for about five minutes before it, presumably, crashed into the ocean. (A NASA fact sheet on the aircraft simply says that it’s “not designed for recovery.”)

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