AW&ST: Pentagon, NASA Look to Renew Push in Hypersonics

Pentagon, NASA Look to Renew Push in Hypersonics
Aviation Week & Space Technology 01/30/06
author: Ann Finkbeiner

The dream of hypersonic flight is lovely: an aircraft whose engine has almost no moving parts, flying maybe 10 times faster than the speed of sound. The principle is elegant and simple: Air moving at hypersonic speeds funnels into a chamber, mixes with fuel, burns and essentially explodes out the back. Short of gliders or paper airplanes, hypersonic vehicles are as close to pure physics as airplanes get.

Hypersonic speeds--generally above five times the speed of sound, or Mach 5 (3,800 mph.)--are often exceeded by rockets and space shuttles that reenter the atmosphere, but until recently, no aircraft has been capable of sustained hypersonic flight. After years of stalled efforts, the Pentagon, and possibly NASA, now look set to make another concerted push toward a hypersonic test vehicle. The Pentagon has a number of programs for hypersonic aircraft. One design under consideration is a waverider--an aircraft that rides its own shock waves. That's the idea behind the X-51, a single-engine demonstrator that's the latest example of joint work on a hypersonic vehicle by the U.S. Air Force and the Defense Advanced Research Projects Agency.

Mark Lewis, the USAF's chief scientist, describes the X-51 as the "jewel in the crown of hypersonics." The X-51 follows work done on the earlier X-43 series, which foundered after NASA pulled funding from the program. Lewis says the X-51 is well-funded and one of the best arguments for sustained support of basic research.

Getting to the first test flight, though, will take work. Hypersonic aircraft, so-called "airbreathers," powered by scramjet engines, might be simple in principle but in practice can be a challenge. Air funneling into a scramjet's intake must reach the right density, and fuel has to be injected and then burn within a few thousandths of a second. Researchers say it's akin to lighting a match in a hurricane. Moreover, the altitude at which hypersonic airbreathers fly can't be so high that they won't have enough air to "breathe," or so low that the denser atmosphere creates drag and heats them until they melt. The body of a hypersonic vehicle needs sharp edges, which minimize drag but also create operational temperatures that could go above 4,000F. The amount of lift a vehicle needs to stay airborne must match its weight. Although hypersonic aircraft don't need heavy oxygen tanks, compressors or turbines, they haven't yet had enough lift to carry pilots, bombs or passengers.

Any aircraft is a working balance of thrust and drag, lift and weight, but "a scramjet is a highly tuned vehicle," says Paul Dimotakis, professor of aeronautics at the California Institute of Technology. "One must get most everything right for it to work."

After nearly 50 years of calculating fluid flows and running wind-tunnel tests, researchers finally tested successful scramjets, first in Russia in the late 1990s, then in Australia in 2002, and twice in the U.S. in 2004. The U.S. tests--joint efforts by NASA and the aerospace industry--were of the X-43A, which, like all scramjets, had to get up to speed before it would work. With "scramjets, you climb to altitude," says John LaGraff, aerospace engineer at Syracuse University, "and let it rip." Rocketed to about 100,000 ft., the first X-43A flew at Mach 6.8 (5,168 mph.). The second X-43A hit Mach 9.68, or almost 7,400 mph. "It flew more like a rock," says David Reubush, then deputy manager of NASA's program for hypersonic flight, "but from it, we learned that all the stuff we'd been doing in wind tunnels for eons were giving us the right answers."

Aerospace | Engineering