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Dreams of fast commercial airline travel recur every so often. Over the past year, we’ve seen talk from NASA about a new generation of X-planes to demonstrate technologies needed for a supersonic Concorde replacement. And Lockheed Martin is discussing a commercial transport derivative from its hypersonic weapons and spy plane programs. The world is more than ready to hear such talk; for many, it’s frustrating, even baffling, that airline travel (Concorde excepting) has been stuck at speeds just over Mach 0.8 since the dawn of the jet age.
Alas, the latest dreams are no more real than previous ones. The obstacles to fast jet travel remain very high, and if anything they’re getting higher.
First, supersonics—planes that fly at least Mach 1, typically Mach 2 or faster.* Concorde, the roughly 40-year-old supersonic icon from the jetpack-and-flying-car era, offers a crucial lesson for anyone wishing to follow its footsteps. Thanks to high fuel burn and other high operating costs, supersonic travel calls for first-class ticket prices. Yet the number of those willing to pay the fare needed for supersonic flight is too small to justify such a jet in all but a handful of markets. It made sense for New York to Paris or London, but other markets simply don’t offer the critical mass of high rollers.
Concorde was developed and built entirely with U.K. and French government funding, but only 14 entered service. Even though the airlines that operated them got their planes for almost nothing, they were hard-pressed to make any money with them. Flying from London to New York takes about seven or eight hours today; it took Concorde about three hours and 30 minutes.
Since Concorde entered service, there has been progress in reducing the noise footprint associated with supersonic travel. But the problem with supersonic flight has little do with the boom made by these jets. You could make the boom go away and still be faced with something less sexy: economics. Very little has been done, or could be done, to lower the high costs associated with high-speed flight.
In fact, thanks to the evolution of jet engine design, the economic divergence between conventional jet travel and supersonic flight has increased since Concorde’s day. Bypass ratios—the ratio of air passed around a jet engine core to the air burned in the core—have greatly increased. This ratio is a key determinant of jet engine fuel efficiency, along with noise and emissions reductions.
Supersonic jets, however, need to use relatively low bypass engines. Concorde’s Olympus engines were zero-bypass engines, otherwise known as turbojets (rather than today’s turbofans). A supersonic jetliner today would still need something with a ratio around 1-1 or 2-1.
When Concorde entered service, typical conventional jetliners used turbofans with much smaller bypass ratios, typically around 4-1. The latest generation of engines, now entering service on Airbus’ A320neo and entering service next year on Boeing’s 737 MAX, offer greater than 12-1 bypass ratios. As a result, today’s jets are about 70 percent more efficient than the original jetliner engines of the late 1950s and early 1960s.
So, every time a new generation of high-bypass engines is introduced on a subsonic aircraft, we move further away from supersonic travel economics, in terms of both technology and relative fuel burn. The efficiency difference between the subsonic jetliners of 1976 and Concorde was narrower than the difference would be between a conventional jetliner in 2020 and a next generation supersonic design. In other words, the difference between a subsonic ticket price and a supersonic ticket price will have increased, too.
Another change that has worked against fast jet travel is the level of service and technology provided for premium customers. In Concorde’s time, first-class jet travel involved a reclining chair, a good meal, and a shared screen showing a movie. Today, most quality airlines offer lie-flat seats, personal in-flight entertainment, electrical connections, and perhaps even a small work area. Travelers carry laptops, and most international jets have good internet connections.
In other words, a passenger today has much less incentive to pay more to travel faster. He’s completely connected to his office, can enjoy the latest entertainment on a personal screen, and sleep in a (relatively) comfortable bed. He might even look forward to escaping the office in his well-provisioned cocoon. None of this was true in Concorde’s day. And a new supersonic jetliner, like Concorde, would probably just offer basic reclining seats. Space is at a premium on supersonic jets, due to the need to minimize drag.
Hypersonics, or greater than Mach 5 flight, are even more exciting than Mach 2 supersonic flight. But solving the economic and technological challenges associated with hypersonics has always seemed about 20 years away, making it the cold fusion of the aerospace industry.
The hypersonic airliner dream achieved its greatest fame as the Orient Express, proposed by President Reagan in his 1986 State of the Union address. This was meant to fly 25 times the speed of sound at low-Earth orbit, traveling from Washington to Tokyo within two hours. It was to enter service before 2000. Yet most work associated with this project was abandoned years ago, after almost $2 billion was spent.
With hypersonics as with supersonics, much depends on the propulsion system. Regular jet engines won’t work beyond Mach 2.5 or so. Instead, a scramjet (supersonic combustion ramjet) is needed, possibly in conjunction with another jet engine needed to accelerate the aircraft to the speed at which a scramjet can work. The scramjet concept has been around since the 1950s, but sustained flight testing has proven difficult. The technology has been compared with lighting a match in a hurricane: It works oh so briefly before failing. The engine tends to fail after a few minutes, and just over six minutes is the all-time world record.
Unlike supersonics, however, hypersonics depend on a host of other new technologies that still need to be developed, particularly thermal management systems and materials that can survive intense heat.
Lockheed Martin and other defense contractors will likely be able master these technologies in time. First, we’ll see hypersonic weapons such as cruise missiles, which are used once and don’t require the elaborate safety systems needed for passenger transport. Then, we’ll see surveillance planes, such as Lockheed Martin’s proposed SR-72. These will be capable of multiple uses but probably still unmanned.
Eventually, after 50 years or so, safe hypersonic passenger transport will be feasible. At that point, we’ll merely have to contend with the enormous costs associated with this technology. And of course conventional jetliners will have continued on their path toward ever greater efficiency, meaning the ticket-to-cost ratio between fast and conventional air transport will be enormous.
Pursuing large supersonic and hypersonic transports may be futile, but some form of fast transport is inevitable. There will likely be a market for a supersonic business jet, such as the one proposed by Aerion; the top end of the business jet market is virtually price inelastic. There may conceivably even be a market for a small supersonic transport (20-40 seats) that effectively skims off the world’s most elite airline traffic.
The best indicator of the viability of fast transport concepts is a company’s willingness to spend its own cash. Those smaller aircraft involve private-sector money. The bigger concepts, by contrast, are purely funded with government cash. In the case of Lockheed Martin, this makes sense; the company is in the business of making weapons and surveillance aircraft that could use high-speed technology. But neither Lockheed Martin nor any other large aerospace contractor will spend any of its own money on large high-speed transport development. The risk-reward ratio is unattractive, at best.
This raises the difficult question of NASA’s high-speed research. Much of the agency’s aeronautics research involves long-term technologies that potentially enable more efficient and clean flight. But then there’s NASA’s supersonic transport work, which basically aims to subsidize a tool for the wealthiest people in the world. As industry observers have noted before, “Mach 2 ... Taxpayers Zero.”
Correction, April 29, 2016: This piece originally misstated the definition of a supersonic plane as flying faster than Mach 2. While supersonics typically fly Mach 2 or faster, technically it’s supersonic if it flies faster than Mach 1. (Return.)
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