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 tactical 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.”)
This isn’t the first time the military has taken the X-51A out for a spin. The Washington Post reported that the craft was conceived in 2004. The initial 2010 test flight lasted just more than three minutes. Before that the longest scramjet powered flight was about 12 seconds by NASA’s X-43.
So when, exactly, can we expect a scramjet engine to zip us from New York to London in about an hour? Given that a five-minute flight would be a big success, probably not any time soon. But even if sustained scramjet flight could be perfected, passenger planes that used the engine probably wouldn’t be economically viable. As Peter Robbie from the European aerospace company EADS told the BBC this morning, flying on a hypersonic passenger plane would be very expensive because of the energy required to get the plane up to speed.
The trouble associated with making really fast passenger planes a commercial success is well known. The Concorde traversed the distance between New York and London in about half the time of a normal passenger plane, but was shut down in 2003 largely because it couldn’t turn a profit.
In the case of the X-51A, the U.S. government seems mostly interested in developing hypersonic flight technology for use in weapons. “The Pentagon believes that hypersonic missiles are the best way to hit a target in an hour or less,” reported the Los Angeles Times. “The only vehicle that the military currently has in its inventory with that kind of capability is the massive, nuclear-tipped intercontinental ballistic missile.”
So even if the test flight was a success, it’s unlikely it will make your trips across the Atlantic any speedier. Missiles, though, could be in for a faster ride.
Update, Aug. 15: CNN reported this afternoon that Tuesday’s X-51A Waverider test flight ended with the plane breaking apart and falling into the Pacific Ocean—before it was supposed to break apart and fall into the Pacific ocean. "Officials said a problem with a tail fin caused the missile-like vehicle to fly out of control before the main engine could be ignited, leading researchers to destroy it early,” CNN’s Mike Mount explained.
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