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EDWARDS, CALIFORNIA -- Engineers here are on the fast-track, readying the next flight of NASA’s X-43A, a super-sleek, high-speed craft powered by a scramjet engine.
Earlier this year, the unpiloted 12-foot-long, 5-foot-wide surfboard-looking vehicle howled its way into the history books. The X-43A reached its test speed of Mach 7 -- seven times the speed of sound, or about 5,000 miles per hour. In doing so it set a world-record speed for “air-breathing” flight, the rocket technology advanced by NASA’s Hyper-X program.
The X-43A’s air-breathing scramjet “breathes in” oxygen from the atmosphere rather than toting along an oxidizer, mixing it with a cache of onboard rocket fuel to produce combustion and forward thrust.
Being the hypersonic air-breather it is, the X-43A also caused some hyperventilation among project leaders when they watched the vessel tear itself apart on its inaugural flight on June 2, 2001. On that day the X-43A never reached test conditions.
But on a successful second flight, the X-43A flew freely for several minutes following scramjet engine operation. The vehicle's supersonic combustion ramjet, or scramjet, ignited as planned and operated for the duration of its hydrogen fuel supply.
Now it’s full speed ahead to Mach 10.
Blistering temperatures
Here at NASA’s Dryden Flight Research Center, the third X-43A is undergoing “short-stack integration,” explained Joel Sitz, X-43A project manager at the center. The craft is being outfitted and tested for a Mach 10 mission in the September-October time frame, he told SPACE.com .
Jacking up the speed will mean the vehicle will see higher heat loads than those observed on the Mach 7 flight on March 27.
“At Mach 7, the front leading edge of the vehicle would see about 2,400 degrees Fahrenheit. At Mach 10, it’s probably twice that…twice the heat load essentially,” Sitz explained.
Those blistering temperatures will be tamed by special thermal protection applied to the Mach 10 vehicle, Sitz said. “The coatings that we are using were sort of a mini-research experiment in itself.”
Higher degree of confidence
Like the two previous tests, the next X-43A is mounted to the front of a modified Pegasus XL booster. The combo is hauled skyward from Edwards Air Force Base under the wing of NASA’s B-52B carrier plane. Once unleashed from the carrier plane, the Pegasus will rocket to 100,000 feet above Earth where the X-43A separates and then powers itself to Mach 10.
At Mach 10 -- or 10 times the speed of sound -- the X-43A is traveling at about two miles per second. That’s in the range of 7,500 miles per hour. Speeds over Mach 5 are defined as hypersonic.
“It is definitely easier to start with a successful Mach 7 [flight] and go to Mach 10. Had we not been successful at Mach 7 we would have had a big job ahead of us,” Sitz said. “Predicting the aerodynamic forces should be pretty straightforward. But the thermal is the challenge.”
Thanks to the triumph in March, wind tunnel data, design tools and predictive skills have been validated. “So we have confidence that they are telling us the right things. Designing a control system around that…you’ve got a higher degree of confidence,” Sitz added.
The X-43A research vehicles are built for one-way, non-recovery flights. Heavily instrumented, they end their mission dropping into the Naval Air Warfare Center Weapons Division Sea Range off the southern coast of California.
Fin, fin finale
The X-43A’s maiden voyage into the hypersonic realm ended in failure in 2001. That attempt to reach Mach 7 was over in seconds following Pegasus ignition. Fins on the booster broke off, taking the X-43A for a ride all over the sky except in the right direction.
For one, the Pegasus/X-43A was deployed from the B-52B at 25,000 feet. The Pegasus is normally dropped at 40,000 feet. “The fins ended up being more effective than we thought…and we just drove it out of control,” Sitz said.
“We mispredicted the aerodynamic forces on the fins. That was a huge contributor,” Sitz said.
Furthermore, there’s a black art to calculating forces around a craft flying transonic. While a vehicle isn’t moving at supersonic speeds, air moving around an aircraft might be, producing small shocks that can influence aircraft control.
“You might get one answer in the wind tunnel…and it could be something completely different in flight,” Sitz said. “You get vortices and shockwaves dancing around.”
Trying to be a dance partner and sense what transonic moves are going to occur ahead of time is tough sledding. “NASA has been trying to do it for 30 years. And I don’t think we’ve got it down yet,” Sitz explained.
End of the program?
The upcoming Mach 10 run of the X-43A appears to mark an end of the program. The seven-year, approximately $250 million Hyper-X program was created to provide unique "first time" data on hypersonic air-breathing engine technologies.
“Right now the program on paper ends when we’re done with Mach 10,” Sitz said. Shifting budget priorities at NASA have dictated closure of the program. A follow-on X-43C project has also been cancelled at this point, he said.
NASA's Dryden Flight Research Center is responsible for X-43A flight-testing. The space agency’s Langley Research Center in Hampton, Virginia manages the program.
ATK-GASL (formerly MicroCraft Inc.), based in Tullahoma, Tennessee, built both the X-43A and its engine. The Boeing Company's Phantom Works in Huntington Beach, California designed the thermal protection and onboard systems. The booster is a modified Pegasus rocket built by Orbital Sciences Corporation of Chandler, Arizona.
Moving along
“NASA, in my opinion, needs to continue hypersonics research. We need to stay involved in this. Scramjets look pretty promising from a technology perspective,” Sitz observed. “What we have to prove now…can you combine them with turbojets? Can you combine them with rockets?”
Sitz admitted that some individuals argue that integration of the technology is too big a problem - a problem that can’t be solved in a practical sense. “And some people say you can do it, and have the designs that show it can be done. So that’s where I’d like to see NASA aeronautics go.”
For right now, Sitz concluded: “We have the world’s fastest air-breathing aircraft. We’re going to fly at Mach 10 … even though it’s only for a few seconds. We’re moving along.”
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