"The better you can predict a heat load, the better you can come up with materials or structure to handle that heat load," said Stuart A. Craig, an assistant professor in the aerospace and mechanical engineering department at the University of Arizona who researches hypersonic aerodynamics.
Development of a larger scramjet engine -- formally known as a supersonic combustion ramjet -- also has been challenging. Although rockets can get a vehicle to hypersonic speeds, they are too large, heavy and inefficient to use in lighter missiles or aircraft.
Enter the scramjet -- an air-breathing engine that can provide the boost needed to reach speeds greater than Mach 5 but is lighter and more efficient. Unlike a rocket, a scramjet does not need to carry its own tank of oxygen to burn with fuel, which is typically a hydrocarbon or hydrogen. Instead, it uses the air in the atmosphere to serve as an oxidizer for the propellant.
"You can't afford to build all these big rockets every time you want to fly a hypersonic glide vehicle or a cruise missile," said George Nacouzi, senior engineer at Rand and an expert on missile development. "It's just not practical."
Scramjets typically start working at speeds of about Mach 5, when the air flow is still supersonic and is highly compressed. NASA's X-43A aircraft program last decade proved that scramjets could work, though on a smaller scale than a typical plane.
Scaling up can be challenging, said Boeing's Bowcutt, who developed his X-51A design while at Rockwell International, which was later acquired by Boeing. Since wind tunnels can be limited in size, engineers must also rely more on computer simulations, which can't necessarily give full verification of a concept in real-world conditions.
But new technological developments have helped make some of these issues easier to solve. In a recent presentation at an aerospace technology conference, Lockheed Martin Skunk Works Vice President Jack O'Banion said increased computer-processing power and digital tools helped the defense giant design a scramjet engine in 3-D for a plane concept known as the SR-72.
Lockheed Martin has said this hypersonic aircraft concept could travel at speeds as high as Mach 6 and be operational by 2030. A Lockheed executive recently disputed speculation that the SR-72 already exists, saying the company's focus was on hypersonic weapons systems. (The name is a nod to Lockheed's stealthy SR-71 Blackbird, which first flew in 1964 and reached average speeds of 2,200 mph.)
With digital-printing manufacturing, O'Banion said the company could integrate the scramjet engine with an "incredibly sophisticated cooling system," allowing the engine to withstand multiple firings for routine operations. No moving parts would be involved.
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"It would have melted down into slag if we tried to produce it five years ago," he said.
With all of these challenges, many researchers say hypersonics probably will be developed first for missiles and later for manned aircraft.
But don't expect to book a seat on a hypersonic passenger jet any time soon, as commercial applications of the technology could be at least 30 years away, said Nacouzi of Rand.
That timeline would depend on the business case for hypersonic travel, which would presumably command premium ticket prices. The example of the Concorde passenger jet isn't exactly promising. An air disaster claimed 113 lives in 2000, temporarily grounding the fleet, but the high costs of operating the jet amid the slower market for air travel after the Sept. 11 attacks were what led to the plane's ultimate retirement in 2003.
"It's much easier to start with missiles," Nacouzi said. "They're simpler than aircraft. An aircraft has much more systems involved."
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