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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.

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.

The A2, for example, was to be powered by four Scimitar engines, a unique and new dual-mode design that incorporated a built-in heat exchanger to keep the turbines from melting at hypersonic intake temperatures. The engine needed two modes because the A2 would pass through two distinct flight regimes: The first included take off, acceleration to Mach 2.5, and landing. For that, the Scimitar would work like a conventional jet engine, with turbines compressing the intake air, mixing it with fuel, and igniting the mixture to produce thrust. But operation at hypersonic speeds caused the temperature inside the intake to reach as high as 1,800 degrees Fahrenheit—a death sentence to turbine blades. Hence the need for precooling, which the Scimitar would accomplish in two ways: through the low temperature of the liquid hydrogen entering the combustion chamber, and by means of a built-in heat exchanger that directed precooled gaseous helium into a diffuser that, like an air conditioner’s evaporator coils, reduced the temperature of the air passing through it.

Of course you pay a price for all this. “Heat exchangers tend to be heavy and complicated, and they can leak,” says Schetz. In its description of the system, Reaction Engines wrote: “The incorporation of lightweight heat exchangers in the main thermodynamic cycles of these engines is a new feature to aerospace propulsion.” In other words, the precoolers also had to be filed under the category of “stuff to come.” Still, the company at least had a heat- exchanger test facility in place by December 2005 at the Culham Science Center in Oxfordshire, England, and has built a number of prototype precooler modules. And Richard Varvill, the company’s technical director and chief designer, had an answer to Schetz’s weight objection.

“The weight issue we’re addressing by having very thin precooler walls and small-diameter tubes,” says Varvill. “The tubes are about a millimeter diameter, made of a certain nickel-based alloy. The mass target for the heat exchanger is one and a quarter tons. They are heavy; they certainly add weight. But if you push the engineering to its limit, you can get an acceptable weight.”

There was a reason for all the complication of the precooled, dual-mode engines. Their main advantage, says Varvill, “is that they are good from rest to hypersonic speeds, whereas alternatives such as scramjets and so forth are not capable of that. So to get to Mach 5, you’d have to have two different engines on the same vehicle. And that certainly has major weight and cost implications.”

The precooler, then, was key to the appeal of the Scimitar engine concept, and therefore of the A2 itself. Varvill’s optimism that the device will work is based on the amount of theoretical modeling and experimental testing the company has already done. “We’re now going to the next level, which is to actually make a precooler [that] will be running in front of a jet engine in a couple of years’ time,” he says.

As for the A2’s zero carbon footprint, that too rested on the success of future technological developments, in this case a method of producing large amounts of hydrogen cleanly and greenly.

But if the A2 is not quite quiet and not quite green, at least there are nearer-term alternative technologies being developed and tested—the Quiet Supersonic Platform, for instance, which  originated in 2000 as a Defense Advanced Research Projects Agency program. Its objective was to reduce the sound of a sonic boom to the point that supersonic flight over populated areas would become unobjectionable. To that end, DARPA contracted with Northrop Grumman, which proposed modifying the nose section on one of its F-5E fighters, thereby shaping the sonic boom into one less disruptive to the ear. (The classic sonic boom is a pressure wave with two sharp peaks in rapid succession, like a capital “N.” A shaped wave would have the first peak looking more like the lowercase version: n.)

Northrop Grumman conducted its Shaped Sonic Boom Demonstration out of Palmdale, California, on August 27, 2003, with mixed results. According to the bevy of microphones in place near Harper Lake (an hour’s drive northeast of Edwards Air Force Base), the resulting boom was measurably less intense than that of an unmodified jet of the same type that flew through the same airspace moments later (see “The Boom Stops Here,” Oct./Nov. 2005).

On the other hand, the reduction in intensity wasn’t perceptible to a mere human. “I was out there and listened with my own ears,” says an experienced NASA sonic boom listener (who wishes to remain anonymous to preserve his relationships with NASA contractors), “and to tell you the truth, I couldn’t hear all that much difference.”

Progress in the boom-busting business has been slow and incremental. But even if the sonic boom could be changed from a loud clap to the sound of rolling thunder (nobody thinks it can be eliminated entirely), the public’s stomach for even soft sonic booms is an unknown and is further subject to the vagaries of politics.

“Politicians have overdone these things,” says Joseph Schetz. “In the public mind, the sonic boom was going to mow down buildings and knock over cows, kill whales. It’s literally like slamming a door.”

Matters are even worse when it comes to the touchy matter of alternative fuels. Researchers have proposed all sorts of sources—soybeans, sunflower seeds, babassu nuts, coconuts, palm oil, and algae—for biofuels. Some of these wild potions have even been tested in flight, in genuine, honest-to-God airliners.

In December 2008, an Air New Zealand Boeing 747 departed Auckland carrying a blend of 50 percent Jet-A and 50 percent jatropha oil. (The jatropha plant’s seeds, when crushed, yield an oil usable as fuel.) Over two hours, one of the jet’s four engines ran on the blend and performed normally. A little over a week later, on January 7, 2009, a Continental Airlines Boeing 737 accomplished essentially the same feat over Houston, Texas. (Its particular blend was 50 percent Jet-A, 47.5 percent jatropha oil, and 2.5 percent algae.) Separately, Japan Airlines was planning to launch a Boeing 747 running on a biofuel component blended of one percent algae, 15 percent jatropha, and 84 percent camelina oil. (Camelina oil comes from an oilseed plant that also produces vegetable oil and animal feed.) Air France, to complete the picture, was contemplating the most radically chic and stylish fuel of all, made from little Roquefort cheese morsels rolled in crushed walnuts. (Not really.)

“The most attractive one, but at high cost, is algae,” says Schetz. “It grows very fast. It would be genuinely renewable. Plus, if you tailor your feedstock, you might end up with even more attractive fuel. You can’t tailor what’s in crude oil.”

The airliner of the future, then, is a combination of design studies that may or may not result in practical devices and vehicles, chancy schemes for reducing sonic booms, and alternative fuels that may be too expensive to produce on a mass scale. In light of which, one has to wonder if the prospect of propelling thousands of people daily across vast distances at tremendous speeds while bothering no one and leaving the environment no worse off is anything more than a dream. While such a goal doesn’t seem to violate any known law of nature, there are other laws that need to be considered: Murphy’s Law, Hofstadter’s Law (“It always takes longer than you expect, even if you take into account Hofstadter’s Law”), and the Almost-Law-of-Nature, which states that research-and-development costs are always far greater in the end than they were expected to be in the beginning. (The Concorde supersonic transport, for example, was six times more costly to develop than it was initially projected to be.)

Even so, hypersonics might be inching toward reality. David M. Van Wie of Johns Hopkins University’s Applied Physics Laboratory was an organizer of a 2008 international conference on hypersonic systems and technologies, from which emerged a distinct message. “The big takeaway we’re able to observe right now is that these hypersonic technologies are moving out of laboratories and into flight test demonstration,” says Van Wie. “Technologies that have been for years and years studied in wind tunnels, and by people doing analysis, are now being explored in flight experiments. Not at the level of airplanes yet, but in drones and rocket-propelled test vehicles.”

So when will we be flying the airliner of the future? The Europeans already have the answer: by the year 20XX.

Ed Regis is the author of seven science books, most recently What Is Life? Investigating the Nature of Life in the Age of Synthetic Biology (Oxford University Press, 2008).

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