Will flight get faster than supersonic?

5/27/2003

WASHINGTON - CIA agents hunting a future Saddam Hussein or Osama bin Laden pass the word:

“The tyrant and his cronies just started dinner.”

Military officials dispatch a warplane based in the United States, and before dinner ends it delivers dessert: laser-guided bombs.

The International Space Station calls for help, and NASA needs just minutes to dispatch a rescue vehicle - not the weeks now necessary to prepare a space shuttle for launch.

Farfetched scenarios are inching toward reality, as the U.S. military reinvents itself to engage distant enemies with lightninglike speed, and the space agency recovers from the space shuttle Columbia disaster.

The Pentagon and NASA have quietly joined forces to fulfill one of the most elusive goals in aviation:

Practical hypersonic aircraft.

“Technology has progressed to the point where we believe that demonstrations of a Mach number per year, reaching Mach 12 by 2012, are within reach,” Dr. Ronald Sega told Congress a few weeks ago.

Dr. Sega, a former astronaut who is the Defense Department's chief technology officer, believes that step-by-step approach will result in aircraft flying many times faster than sound travels.

Sound's speed in air varies with altitude, and is 740 miles per hour near sea level. That's “Mach 1” in aviation units named for Austrian physicist Ernst Mach. Mach 2 is twice the speed of sound, and Dr. Sega's “twelve by twelve” goal would mean vehicles traveling at 8,000 mph within nine years.

Aviators have dreamed of hypersonic flight since 1947, when test pilot Chuck Yeager first broke the sound barrier in a Bell X-1 “rocket plane.” That was aviation's transition from subsonic to supersonic flight. Supersonic flight becomes hypersonic at Mach 5, about 3,750 mph.

U. S. Army researchers first achieved it in 1949 with an unmanned supercharged German V-2 rocket. In the 1950s, the X-15 rocket plane built by the Pentagon and NASA snared the world record for piloted flight, Mach 6.72. A spyplane called the SR-71 Blackbird, which flew at Mach 3, is the world's fastest jet-powered plane.

The Pentagon's interest in hypersonic warplanes, however, eventually took a back seat to pursuit of radar-evading stealth technology, spy satellites, laser-guided bombs, and other weapons.

Only a handful of hypersonic weapons exist, including intercontinental ballistic missiles and tank-busting artillery shells.

NASA's long-standing hypersonics program lost funding year after year to the money-hungry International Space Station, and was grounded in 2001 after its X-43 rocket plane crashed on the inaugural flight.

New needs have drawn those two old hypersonics enthusiasts together in an alliance to build aircraft that could fly anywhere on Earth within a couple hours.

The Pentagon sees hypersonic flight as the key to a smaller, faster, more flexible war machine that is less dependent on overseas air bases that could be closed by shifting political sands.

“The development of hypersonic technology could reduce vulnerability of future systems while potentially providing a flexible capability to strike quickly and effectively deny enemy sanctuary anywhere in the world,” Dr. Sega said.

NASA sees the project as salvation from a crunch caused by the space shuttle Columbia disaster. It exposed a gap in NASA's ability to launch emergency flights to rescue astronauts, and raised concerns about the space shuttle's safety. Hypersonics could be used in a rescue vehicle for future emergencies, and in a new “orbital space plane” to replace the technologically dated space shuttle fleet.

Sean O'Keefe, NASA administrator believes that a new space plane would provide cheaper, more reliable access to earth orbit and the International Space Station than the shuttles. The plane could be launched much like an aircraft, without the weeks of preparation needed for shuttles.

Pentagon envision the hypersonics program as a kind of latter-day Apollo lunar landing program, a highly publicized project that captures the public's imagination.

Some scientists and engineers, however, think that unsolved technical problems put routine hypersonic flight in the pipedream category. The last major review, done in late 2000 by the U. S. Air Force Science Advisory board, made this odd comparison:

“Accepting hypersonics as the wave of the future is somewhat like belief in the Second Coming of Christ,” it said. “One might accept its inevitability, but with little idea when it might actually happen.”

Dr. Sega counters that some of the basic technology for a “12 by '12” timetable is available from NASA and other hypersonics research that dates to the 1950s. And the rest, he adds, can be developed in the Pentagon's one-step-at-a-time approach.

A 2003 report in the journal Science partially concurs, noting: “An array of recent developments - from real-world tests of new engines to improvements in materials and computer simulations - boosts the chances for success.”

For instance, a new hypersonic engine capable of Mach 5 speeds has been ground-tested and is scheduled for flight tests in 2006. That would leave researchers two years to ramp up performance to the Pentagon's Mach 7 goal for 2008.

Piloted hypersonic craft probably will have “combined cycle” engines, named because they use a graduated series of propulsion systems during different stages of flight.

A rocket engine, for instance, would accelerate the vehicle to about twice the speed of sound. Then the rocket would turn off, and a high-power jet engine would accelerate to Mach 10 or so. Jet engines use oxygen from the air to burn fuel. That is a big advantage over rockets, which must carry their own oxygen, and burn extra fuel to move its weight.

The rocket engine could kick in again after March 10 to propel the craft into orbit or sustain suborbital cruising speed.

Among other technological challenges is a light reliable insulation system to protect the vehicle's outer skin from searing temperatures generated by friction as it zooms through the atmosphere.

The Defense Department is adding $150 million annually to its hypersonics program, the exact scope of which has not been revealed.

NASA plans to invest up to $2.4 billion over the next four years in a crew rescue vehicle and other hypersonics research, and plans to resume the X-43 program in a few months. Its goal is Mach 7.

NASA foresees eventual benefits beyond weapons systems and the International Space Station, including “a future fleet of government and commercial hypersonic vehicles, traveling between dozens or even hundreds of `skyports' around the world - and beyond it.”