The need for speed

Last month an experimental U.S. Army hypersonic missile was intentionally destroyed in midflight during a test in Alaska. A few weeks earlier, the Pentagon confirmed a second flight test by the Chinese military of a similar hypersonic weapon. Russia and India also reportedly have hypersonic aircraft programs. Are we witnessing a new type of arms race? What is hypersonic technology, and why the recent surge in interest in hypersonic aircraft?

The prefix hyper means “way beyond,” as in hyperactive. A hypersonic aircraft is one that flies between five and 10 times the speed of sound, or about 3,800 miles per hour to 7,700 miles per hour.

At those speeds, “all of our understanding in aerodynamics falls over,” says Paul Bruce, an expert in high speed aerodynamics at Imperial College London, in an interview with the Daily Telegraph. “Every equation that we use to describe subsonic and supersonic flow and flight … doesn’t work when the air starts changing at hypersonic speeds.” Which explains why many hypersonic aircraft tests last only minutes or are aborted.

The world’s militaries are interested in hypersonic weapons because of their ability to reach anywhere in the world in a matter of minutes and outrun virtually any defensive missile launched against them. These hypersonic glide vehicles, or HGVs, are launched from atop an ICBM. Once it’s very high, the missile pitches over, releasing the glide vehicle, which then dives to hypersonic speed and glides to the target.

But beyond the military applications there’s a huge commercial interest in hypersonic technology: getting satellites and other payloads into orbit cheaply. The cost of using traditional launch vehicles such as rockets or the Space Shuttle has been around $10,000 per pound of payload. Scientists and engineers believe they can lower this cost by a factor of 10 or more with reusable hypersonic aircraft.

The biggest challenge in hypersonic technology is developing a jet engine that can power an aircraft to hypersonic velocities. The earliest hypersonic aircraft such as the X-15, which flew from 1959 to 1970, used rockets to propel them. But since a rocket carries both its fuel and the oxygen to burn it in liquid form, it’s very inefficient. A jet engine, on the other hand, is “air-breathing”: It takes in air at the front and then burns it with the fuel, pushing it out the back at a much faster speed. But when you’re already traveling at 2.3 miles per second, it’s pretty difficult to push air back faster than that!

In addition, the engine has to slow down the air that’s entering it at Mach 10 to something close to the speed of sound so combustion can take place. “It’s a lot like trying to keep a match alight in a hurricane,” says Bruce.

A combination of international government and private investment is funding current hypersonic development. In the United States, the Defense Advanced Research Projects Agency, or DARPA, has awarded initial design contracts to Northrop Grumman, Boeing, and Masten Space Systems for what is being called the XS-1, or Experimental Spaceplane. The XS-1 operational requirements call for it to be able to fly 10 times in a 10-day period and launch 3,000 to 5,000 pounds of payload for less than $5 million per flight. The first orbital test flights are expected in 2018.

The X-15 still holds the world record for the highest speed ever reached by a manned, powered aircraft. Air Force test pilot Pete Knight achieved a maximum velocity of 4,520 mph on Oct. 3, 1967.