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If engineers can corral liquid hydrogen, reshape pressure waves, and make fuel from algae, future airline passengers will travel around the world at hypersonic speeds in strange-looking aircraft. (Reaction Engines Ltd/Adrian Mann)

The Perfect Airplane

Fast, green, and quiet. Come on, brainiacs, you can do it.

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A radical new airliner design proposed by Deutsches Zentrum für Luft- und Raumfahrt (DLR), the German equivalent of NASA, won research funding this year from the Brussels-based European Commission. The DLR’s Spaceliner is a hypersonic rocketplane, capable of flying from Germany to Australia in 90 minutes. Coupled to a detachable, recoverable booster that would burn liquid hydrogen and oxygen, the craft would launch from Frankfurt vertically, like an ICBM. Its 50 passengers would experience several minutes of weightlessness, after which the vehicle would glide down to land horizontally, just like the space shuttle. In Sydney, the Spaceliner would be attached to a new booster and fired off once again into the blue, and would arrive back in Frankfurt an hour and a half later.

The astounding craft is part of Europe’s FAST 20XX project. FAST stands for “Future High-Altitude High-Speed Transport,” and 20XX means that nobody knows when (or if) such a thing could be built, much less placed into service. Plainly, FAST 20XX inaugurates a brave new epoch in aircraft design.

But in fact the project is symptomatic of a worldwide trend, an epidemic of futuristic conceptual designs. Starting in the early years of the 21st century, government agencies and officials have been dreaming up all kinds of exotic high-performance specifications for a new generation of airliner. And why not? The average speed of civil air transport has not changed much since the Boeing 707 entered service on October 26, 1958 (with Pan American World Airways). For the next 50 years, passengers plugged along at the same old 600 mph. That might seem acceptable, even ideal, for a hop from New York to Chicago, but on a nonstop flight from San Francisco to Hong Kong, 600 mph translates into 12 hours of pure tedium.

In our era, the chief attraction of air travel is speed (as opposed to such bygone considerations as fun, luxury, and glamour), so above all else, the long-haul airliner of the future will have to be fast. And so last year, when NASA set goals and issued contracts to academic and industry study groups for its next generation of aircraft, the so-called N+1, N+2, and N+3 (the last of which is to be placed into service between 2030 and 2035), the grandest item on its agenda was a 100- to 200-passenger supersonic transport. However, the craft would have to satisfy several other requirements as well: meeting essentially the same airport noise reduction and emissions goals that NASA was proposing for subsonic aircraft, and of course somehow eliminating or at least reducing the volume of the dreaded sonic boom. In other words, it would have to be fast, green, and quiet.

Meanwhile, Japan’s NASA equivalent, the Japan Aerospace Exploration Agency (JAXA), was investigating a supersonic transport concept that was even more ambitious. The Japanese were after a 200- to 300-passenger Mach 2 vehicle that would be environmentally friendly, highly fuel-efficient, and safer and more comfortable than present airliners, plus its long-distance business-class fares would have to be comparable to those of current airlines. And the Japanese wanted to accomplish all this by roughly 2020. Further into the future, by about 2025, JAXA planners were looking to introduce a Mach 5 airliner.

But the prize for truly advanced design programs—as well as for best acronyms—belonged unequivocally to the Europeans. In addition to the FAST 20XX, they had LAPCAT I and LAPCAT II (Long-Term Advanced Propulsion Concepts and Technologies), along with ATLLAS (Aerodynamic and Thermal Load Interactions with Lightweight Advanced Materials for High Speed Flight). The three programs, coordinated by the European Space Agency, aimed to produce two vehicles: one to fly at Mach 5, the other at Mach 8. Of the two, the Mach 8 craft seemed far less likely to get much beyond the idea stage. For one thing, it resembled a flying dustpan: a wedge consisting of a long and broad air intake scoop followed by tail fins, and little else—no windows, for example (see middle image, opposite). For another, as the European Space Agency itself admitted, while the Mach 8 vehicle “seems feasible, the fuel consumption during acceleration requires a large fraction, severely affecting gross take-off weight.” Meaning that the Mach 8 hypersonic aircraft might be very speedy, but not able to actually go anywhere.

“People are pretty casual about throwing around a Mach number here, a Mach number there,” says Joseph Schetz, a hypersonic flight specialist at Virginia Tech in Blacksburg. “But every time you add a Mach number, you move into a different regime. It’s like night and day.”

Subtracting three Mach numbers from the apparently doomed Mach 8 vehicle leaves us with the Mach 5 concept, which as it happens was the masterpiece item in the hypersonic transport design game. The European Space Agency had commissioned a British research and development firm, Reaction Engines Limited, to do a three-year evaluation of a “Configuration A2 Mach 5 Civil Transport.” This at least looked like an airliner, although it too was windowless. Hypersonic flight—Mach 5 and above—generates enough heat to melt conventional airplane windows. But who’d need them when every seat had a 400-channel entertainment system? Anyway, the A2 was supposed to carry 300 passengers from Brussels to Sydney in 4.6 hours, and, most impressive of all, it would be powered by liquid hydrogen, so it would leave no ugly trail of carbon emissions.

This then was the Impossible Airliner, politically correct and guilt-free, which was to say, superfast, quiet, and with zero carbon footprint. Well, not really all that quiet: It would produce “takeoff sideline noise” of more than 100 decibels a quarter-mile away, and it would fly “via North Pole and Bering Straits to avoid supersonic overflight of Eurasian land mass.” But you can’t have everything, even for a one-way ticket price of €3940 (about $5,500).

The question is whether you can have any of it. All of these extremely advanced design concepts rest in large part—if not wholly—on other equally advanced design concepts: materials and technologies that also have to be invented, tested, proven, and then fused with still other vaporware in what engineers commonly referred to as a “highly integrated vehicle concept,” whatever that means.

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