Since test pilot Chuck Yeager broke the sound barrier in 1947, engineers and scientists have dreamed of ever-faster aircraft. Now, they face one of their toughest challenges yet: sustaining hypersonic flight -- going five times the speed of sound or more -- for more than a few minutes.

In a nondescript hangar at Edwards Air Force Base in the Mojave Desert, a team of aerospace engineers has been putting the finishing touches on a lightning-quick experimental aircraft designed to fly above the Pacific Ocean at 3,600 mph. A passenger aircraft traveling at that speed could fly from Los Angeles to New York in 46 minutes.

On Tuesday a key test is set for the unmanned experimental aircraft X-51A WaveRider. It will take the aircraft -- attached to a B-52 bomber's wing -- from Edwards to about 50,000 feet over the Pacific Ocean near Point Mugu. From there, its high-speed journey at Mach 6 is expected to last only 300 seconds, but that's twice as long as it's ever gone at that speed.

Aerospace engineers say that harnessing technology capable of sustaining hypersonic speeds is crucial to the next generation of missiles, military aircraft, spacecraft - and even passenger planes.

"Attaining sustained hypersonic flight is like going from propeller-driven aircraft to jet aircraft," said Robert A. Mercier, deputy for technology in the high speed systems division at the Air Force Research Laboratory in Ohio. "Since the Wright brothers, we have examined how to make aircraft better and faster. Hypersonic flight is one of those areas that is a potential frontier for aeronautics. I believe we're standing in the door waiting to go into that arena."

NASA and the Pentagon are financing three national centers across the country to study hypersonic flight. The Pentagon's research arm, known as the Defense Advanced Research Projects Agency, or DARPA, calls hypersonic flight "the new stealth" for its promise of evading and outrunning enemy fire. The effort to develop hypersonic engines is necessary because they can propel vehicles at a velocity that cannot be achieved from traditional turbine-powered jet engines.

The Pentagon believes that hypersonic missiles are the best way to hit a target in an hour or less. The only vehicle that the military currently has in its inventory with that kind of capability is the massive, nuclear-tipped intercontinental ballistic missile.

Other means of hitting a distant target, such as cruise missiles and long-range bomber planes, can take hours to reach their destination.

When pressed for an example of the need, military officials often point to a 1998 attack when the U.S. militarytried -- and failed -- to kill Osama bin Laden. Navy vessels in the Arabian Sea lobbed cruise missiles at training camps in Afghanistan, hitting their targets - 80 minutes later. By then, Bin Laden was gone.

But with a hypersonic missile, such as the technology being tested on the WaveRider, "the attack would have been cut to just over 12 minutes," Richard Hallion, a former Air Force senior advisor, said in an Air Force Assn. report about hypersonic technology.

The Pentagon itself is funding six major hypersonic technology programs. Over the last 10 years, the Pentagon said it spent as much as US$2 billion on hypersonic technologies and supporting engineering.

The WaveRider program is estimated to cost US$140 million, according to Globalsecurity.org, a website for military policy research.

Yet the funding has turned up few positive results.

One of the more recent attempts was in August 2011 when DARPA carried out a test flight of an arrowhead-shaped unmanned aircraft, dubbed Falcon Hypersonic Technology Vehicle 2. It's designed to travel at 20 times the speed of sound. The launch had received worldwide attention and much fanfare, but minutes into the flight, searing high speeds caused portions of the Falcon's skin to peel from the aerostructure and the flight ended prematurely.

Engineers at Boeing Co.'s research center in Huntington Beach and Pratt & Whitney Rocketdyne in Canoga Park thought they were on the right track with the WaveRider program in May 2010 when the WaveRider made its first flight. In that flight, the WaveRider sped westward for about 143 seconds at 3,500 mph before plunging into the ocean as planned.

But in June 2011 in another WaveRider flight a lapse in airflow to the jet engine caused a premature shutdown.

After the flight, Charlie Brink, the Air Force Research Laboratory's program manager, said the WaveRider attempted to restart but was unsuccessful. "Obviously we're disappointed and expected better results," he said at the time. "But we are very pleased with the data collected on this flight."

Brink and his team try again on a test flight scheduled for Tuesday. The WaveRider will fall like a bomb for about four seconds over the Pacific before its booster rocket engine ignites and propels the nearly wingless aircraft for 30 seconds to about Mach 4.5, before being jettisoned.

Then the cruiser's scramjet engine, notable because it has virtually no moving parts, ignites. The ignition sequence begins burning ethylene, transitioning over about 10 seconds to JP-7 jet fuel - the same fuel once used by the famed Lockheed Corp.-made SR-71 Blackbird spy plane.

The WaveRider is expected to accelerate to about Mach 6 as it climbs to nearly 70,000 feet.

After 300 seconds of flight, the WaveRider is set to break up after splashing into the Pacific, as planned. There are no plans to recover the WaveRider.

The cruiser is designed to ride its own shock wave. That's how the X-51 earned the WaveRider nickname.

"The X-51 is a technology feeder to larger, more sustained flight times," said Darryl W. Davis, president of Boeing Phantom Works, which built the cruiser. "The hope is to advance the state of the art."

Dora Musielak, an adjunct professor of physics at the University of Texas at Arlington whose research focuses on high-speed propulsion, said aircraft like the WaveRider are crucial to commercial planes one day flying nonstop at high speeds from one side of the earth to the other. Other than the turbojet-powered Concorde retired in 2003, commercial transportation has not advanced beyond the speed of sound.

"It is always a dream to see an airplane fly faster," Musielak said, noting that there's still an enormous amount of hypersonic development work ahead. "Once the military proves out the concept, hypersonic transport becomes a step closer to reality."