Is supersonic passenger travel set to make a comeback?

On October, 24 2003, the last Concorde jet went out of service. What began as a promise of supersonic travel for all, ended as a museum exhibit of a false dawn. However, that may be changing with companies such as Aerion and Spike Aerospace looking to take business jets supersonic. At Aviation 2014, an annual event of the American Institute of Aeronautics and Astronautics, NASA presented examples of the space agency’s work on new technologies that could lead to a revival of civilian supersonic travel within the next 15 years.

When the realistic prospect of supersonic passenger travel first came up in the 1950s, it seemed like the logical progression in the field of civil aviation, which had already undergone the sort of growth that the digital field enjoys today. The idea of flying routinely from London to New York in three hours was seen as not only a great boost to the economy, but also a way of bringing the world closer together.

In the 1960s, the Americans, British, French, and Soviets were all keen on developing and putting into service passenger planes that could cruise at over Mach 1. It was a technological race that was regarded at the time as being as great a technological accomplishment as the Apollo Moon landings. There were visions of fleets of supersonic Concordes, Tupolevs, Boeings, and Lockheeds flying around the world wearing the liveries of the great airlines.

Unfortunately, the aviation annus horribilis of 1973 put paid to those dreams. The Soviet TU-144 “Concordski” crashed at the Paris Air Show, the first OPEC oil embargo threw the already borderline economics of supersonic flight into a cocked hat, and the US FAA put a ban on supersonic overland flights in US territory. The result was programs cancelled en masse, empty order books, and a sum total of 20 Concordes that flew with Air France and British Airways because their governments made them. They were beautiful, full of prestige and a brilliant accomplishment, but, on balance, a technology that was a couple of generations ahead of its time.

That seemed to be the end of any supersonic flights except for military aircraft, but aviation technology has come a long way in 40 years, and private firms and government agencies are taking a fresh look at civilian supersonic planes. In 1969, they may have been ahead of their time, but perhaps their time is now arriving.

Building a Concorde Mark II is a lot more than just dusting off the old blueprints and updating them. If supersonic passenger service is to succeed, there are major hurdles to be overcome. The greatest of these is taming the sonic boom. These days, unless they spend a lot of time in the middle of the ocean or near military air bases, few people hear sonic booms very often, but they’re still a window-rattling problem as the air in front of the supersonic liner tries to get out of the way, only to form a shock wave on the nose of the aircraft.

"Lessening sonic booms – shock waves caused by an aircraft flying faster than the speed of sound – is the most significant hurdle to reintroducing commercial supersonic flight," says Peter Coen, head of the High Speed Project in NASA's Aeronautics Research Mission Directorate. "Other barriers include high altitude emissions, fuel efficiency and community noise around airports."

How to solve this is partly a matter of engineering, partly public attitudes, and partly updating FAA regulations that were pretty vague when they were written, so NASA and its partners are taking a three-pronged approach towards coming up with a solution.

According to NASA, the aerospace agency is currently developing the technologies that could be used in civilian supersonic craft by 2025. Since not all sonic booms are equal, one of NASA’s projects involves having members of the public at the Langley Research Center in Hampton, Virginia, listen to a variety of 140 sonic booms and polling their responses.

This study, and similar ones at the Armstrong Flight Research Center in Edwards Air Force Base, California, will help NASA with the second objective, which is sitting down with the FAA and the International Civil Aviation Organization to update the regulations on supersonic flight that have stood since the 1970s. NASA says that the loudness of sonic booms isn't defined, so NASA and its partners are working with the regulatory agencies on what an acceptable noise level might be.

On the engineering front, which takes up the lion’s share of the effort, NASA centers scattered across the United States in California, Ohio and Virginia are trying to understand the nature of sonic booms as well as coming up with aircraft designs that can minimize them.

At the Ames Research Center, wind tunnel tests are conducted to study how altering of the fuselage, wings, engines, engine nacelles, and other components can shape the sonic boom in a way that could lengthen it or spread it out, turning a thunderclap into a loud rumble. The best design so far involves a needle-like nose, a sleek fuselage and delta or highly-swept wing.

Examples of these can been seen in designs from major aircraft manufacturers that NASA is helping to test in its supersonic wind tunnel. One from Lockheed Martin looks a bit like a stretched Concorde with a third engine mounted on the wing, while the Boeing version is remarkable for having two engines set on top of the wings. According to NASA, engine mounting can be an important factor in moderating sonic booms. Mounting the engines above the wings, for example, can send the boom upwards, but this can affect performance.

These designs are undergoing wind tunnel tests at NASA with specially constructed models that reproduce the characteristics of the full-size vehicle at supersonic velocities. This allows scientists to measure the boom signatures at various distances while estimating engine performance, with this data then used to validate and tweak computer models.

Other tests focused on wind tunnel tests of air inlets and exhausts to study engine nacelles and flow configurations and rates at various speeds from subsonic to supersonic up to Mach 1.8, to learn more about how to integrate them into supersonic plane design without compromising performance.

"The purpose of our testing was to measure the impact of the nacelle configurations on the boom signatures," says Don Durston, a High Speed Project engineer at Ames Research Center. "Preliminary results showed that as expected, with Boeing's nacelles being on top of the wing, any small changes there had negligible effects on the boom, Lockheed’s model having the two of the nacelles under the wing, did show a measurable impact on boom; however, that effect was predicted, and could be accounted for in the design process Lockheed used."

"We've convinced ourselves that we have the design tools and we've validated the level we need to design to," added Coen. "We've reached a point where quiet, low-boom overland supersonic passenger service is achievable."

Source: NASA