Posted on 03/25/2004 9:42:40 PM PST by BenLurkin
EDWARDS AFB - More than two years after their first unsuccessful attempt, NASA researchers are preparing for a record-setting flight of an air-breathing hypersonic scramjet vehicle. The unmanned X-43A research vehicle is scheduled for a one-time flight Saturday. If successful, its engine will be the first air breather to operate at Mach 7, seven times the speed of sound.
"It's taken us 2½ years to get back to this point," said Vincent Rausch , Hyper-X program manager . "What we're talking about this Saturday is an aviation first, an air-breathing engine capable of Mach 7."
The X-43A is a small, flat, streamlined aircraft that consists largely of its supersonic combustion ramjet - or scramjet - engine and supporting systems. This scramjet engine is the focus of the experimental flight in the $230 million program.
Scramjets are "simple in concept, extremely challenging in implementation," said Griffin Corpening , chief engineer for the X-43 project at NASA's Dryden Flight Research Center.
In ramjet and scramjet engines, air is scooped into the engine duct, then forced through a combustion chamber, where fuel is mixed in and ignited. This produces energy, which is then forced out the rear of the engine as thrust.
The technical challenge with a supersonic version is the difficulty in mixing fuel and igniting air traveling through the combustor at Mach 3. In the case of the X-43A, the combustor is only about three feet long, and the section in which actual ignition occurs measures only about six inches, said Randy Voland , a propulsion system developer with NASA Langley Research Center, another NASA center involved in the Hyper-X program. This leaves mere milliseconds for the fuel mixing and ignition to occur.
"It's like lighting a match in a hurricane," he said.
To solve this problem, the X-43A's engine uses a series of smaller combustion regions to hold the air longer and ignite it. These pockets act like a pilot light on a gas oven to ignite the rest of the airflow, Voland said.
Scramjets have an advantage over conventional rocket and turbojet engines in that they are more efficient. These air-breathing engines do not have to carry oxidizers on board to mix with fuel, instead using oxygen from the air. For space launch purposes, this means lighter launch vehicles with more payload capacity.
"You get more bang for your fuel dollar with a scramjet than with a traditional rocket," Corpening said.
To get the X-43A to the Mach 7 speeds necessary to test the scramjet engine, the research vehicle is propelled by a Pegasus rocket booster.
The X-43A is attached to the nose of the rocket booster via an adapter. The entire assembly, known collectively as "the stack," is carried aloft beneath the wing of NASA's workhorse B-52, the same carrier aircraft that launched the X-15 rocket planes.
For Saturday's flight, the B-52 and its payload will take off from Edwards Air Force Base and head for the test area over the Pacific Ocean, the Naval Air Warfare Center Weapons Division Sea Range.
The B-52 will carry the X-43A stack to 40,000 feet before dropping it about 50 miles off the California coast. Once free of the bomber, the rocket booster will light its engine and carry the research vehicle to 100,000 feet and Mach 7. At that point, the assembly will pitch over to level flight before the rocket booster and X-43A separate, about 90 seconds from the time they were dropped from the B-52.
Once free of its booster, the X-43A scramjet will ignite and run for approximately 10 seconds.
"Ten seconds of data will tell you whether or not six years of trying was worth it," said Joel Sitz, X-43A project manager.
In that brief span, "we would like to see positive acceleration" from the engine, Corpening said, meaning the engine produces more thrust than the drag created by the airframe.
Finally, the X-43A will splash down in the Pacific, some 450 miles offshore. Neither it nor the Pegasus rocket booster are intended to be retrieved.
The entire process, from the drop from the B-52 until the X-43A hits the water, is expected to take 12 minutes.
During the first flight of the X-43A, in June 2001, the Pegasus rocket booster and X-43A went out of control before separating. The vehicles were intentionally destroyed by controllers, with the rocket booster and research vehicle dropping into the ocean without completing the experiment.
The ensuing investigation found problems in the models used in predicting the aerodynamic characteristics and the rocket's fin actuator system.
The Pegasus rocket booster, chosen in large part because it was an operational launch vehicle with a proven track record, provided too much energy to drop the stack at a more typical 40,000 feet. Therefore, for the first flight, the assembly was dropped at 24,000 feet. The denser air at this altitude created greater stress on the rocket's control fins than predicted, causing them to malfunction.
To address the problem for this flight, some 3,300 pounds of solid propellent was removed from the booster rocket, decreasing the power produced and allowing the higher altitude drop.
Stronger mechanisms also were added to control the rocket's fins.
Small modifications, unrelated to the cause of the mishap, also were made to the research vehicle.
"We took a different frame of mind," Sitz said. In the first flight, the team's focus was on the research vehicle, not the booster rocket.
"Really, we have to treat the whole vehicle as one vehicle," he said.
What makes the X-43A unique to scramjet testing is the integration of the hypersonic propulsion system with the airframe, Corpening said.
"It is the first really serious opportunity to fly a hypersonic scramjet vehicle," a concept that has been approached periodically for some 40 years, he said.
While the scramjet engine itself has been successfully tested in wind tunnels, the system integrated with an airframe has yet to be tested in actual flight conditions, in which the thrust produced by the engine and the drag of the airframe must be balanced.
"Therein lies the real benefit in the research objective of what we're doing," Corpening said.
"What we haven't done is anchor it with the reality of flight," Rausch said. "We're going to do that Saturday."
The data collected during the brief test flight will validate the tools used to design and evaluate future scramjet engines.
"This is cutting edge," Rausch said. "We're going out in the flight regime where nobody has flown a vehicle like this before."
In addition to the historic first of the X-43A vehicle itself, the flight also will mean first-time flights of the booster, the difficult separation at Mach 7 and the trajectory of the flight. All together, the project is a high-risk but high-payoff endeavor, Rausch said.
"It's difficult in that we have a string of things that have to happen well," he said.
Should Saturday's flight prove successful, a flight of a third X-43A is scheduled for the fall, Rausch said. This one, with slight modifications, would travel to Mach 10.
Although the next version of the vehicle in the Hyper-X program, the X-43C, was canceled, recently NASA has a continuing interest in hypersonic flight research, Rausch said.
The agency is working with the Air Force on a similar program called the single-engine demonstrator.
The flight may be delayed if Saturday's weather conditions do not cooperate. The aircraft, mated to the B-52, cannot handle high turbulence or fly through visible moisture.
Anyone gotta boat?
The first test vehicle, B-52 with Pegasus and X-43A.
If successful, its engine will be the first publicly acknowledged air breather to operate at Mach 7, seven times the speed of sound.
Had to fix that :)
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