NASA’s high-risk, high-payoff Hyper-X Program is ready to attempt its greatest challenge yet – flying a “scramjet”-powered X-43A research vehicle at nearly 10 times the speed of sound.
Officials have set Nov. 15 or 16 for the flight, which will take place in restricted U.S. Naval airspace over the Pacific Ocean northwest of Los Angeles. This will be the last and, by far, the fastest of three unpiloted flight tests designed to explore an intriguing alternative to rocket power for space access vehicles.
Supersonic combustion ramjets – or scramjets – promise more airplane-like operations for increased affordability, flexibility and safety for ultra high-speed flights within the atmosphere and for the first stage to Earth orbit. The scramjet advantage is that, once they are accelerated to about Mach 4 (four times the speed of sound) by a conventional jet airplane engine, it is believed that they can be flown in the atmosphere up to about Mach 15 without having to carry heavy oxygen tanks as rockets must. Also, rockets tend to produce full thrust or nearly full thrust all the time; scramjets can be throttled back and flown more like an airplane.
The scramjet concept is simple: Accelerate the vehicle to about Mach 4 by a conventional jet engine, then start the scramjet engine (which has few or no moving parts) by introducing fuel and mixing it with oxygen obtained from the air and compressed for combustion. The air is naturally compressed by the forward speed of the vehicle and the shape of the inlet, similar to what turbines or pistons do in slower-moving airplanes and cars.
While the concept is simple, proving the concept has not been simple. At operational speeds, flow through the scramjet engine is supersonic – or faster than the speed of sound. At that speed, ignition and combustion take place in a matter of milliseconds. This is one reason it has taken researchers decades to demonstrate scramjet technologies, first in wind tunnels and computer simulations, and only recently in experimental flight tests.
The upcoming flight will be the third of three flights in the eight-year, $230 million Hyper-X Program. The first flight, in 2001, was ended prematurely when the booster rocket veered off course and had to be destroyed before the test could begin. The second flight, in March of this year, was a resounding success. The 12-foot-long X-43A research vehicle was delivered to the proper altitude and test speed, where its scramjet engine started and performed flawlessly for 11 seconds, as planned.
In the process of demonstrating a scramjet-powered airplane in flight for the first time, the March 2004 flight set a world speed record for an “air breathing” (jet-powered) vehicle. It flew at nearly Mach 7, or 5,000 mph. It easily surpassed the previous record set by the military’s now-retired SR-71 Blackbird high-altitude reconnaissance aircraft, which flew at about Mach 3.2.
For the third and final flight, there are several significant differences from the second flight. At Mach 10, the third X-43A vehicle will be zooming westward over the Pacific at approximately 7,000 mph or almost two miles per second. The vehicle will have additional thermal protection, since it will experience heating roughly twice that experienced by the Mach 7 vehicle. Reinforced carbon-carbon composite material is being added to the leading edges of the vehicle’s vertical fins as well as the nose and wings to handle the higher temperatures.
Also for the Mach 10 flight, the booster rocket will launch the X-43A higher (110,000 ft v. 95,000 ft) before it separates and the X-43A starts its scramjet. The X-43A will travel further (about 850 v. 450 miles) before splashing into the ocean.
Ultimate applications of scramjet technology include future hypersonic missiles, hypersonic airplanes, and reusable single- or two-stage-to-orbit launch vehicles.
The final X-43A mission is expected to be the last research mission for NASA’s venerable B-52B “mothership” heavy launch aircraft, which is due to be retired in the near future after almost 50 years of service.
The Hyper-X Program, managed by the NASA Aeronautics Research Mission Directorate in Washington, is conducted jointly by NASA’s Langley Research Center, Hampton, Va., and Dryden Flight Research Center, Edwards, Calif.
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