Supersonic Flight: The Shape of Things to Come (Part 1)

SSBD at Valiant Air Command Warbird Museum

Concorde, known for its supersonic, trans-Atlantic flights of yesteryear, was in the news just past week, as a French court overturned manslaughter convictions and upheld civil damages in relation to the Air France 4590 crash in 2000. To some, this story might seem an afterthought to the dashed hopes for supersonic flight.

The dream of commercial, supersonic transports capable of safely and cheaply whisking business passengers through a geographically dispersed day, breakfast in London, lunch in New York City, and back to London for dinner, is nearly as old as supersonic flight itself. By the mid-1950s, mainstream publications like the Saturday Evening Post were breathlessly predicting, “No doubt we’ll be flying faster than sound by the thousands in a few years.”

The closest that commercial aviation ever came to realizing that prediction was Concorde. But Concorde, never able to fly profitably even in the best of times, was done in by steadily increasing fuel costs, decreased travel after 9/11, and a deadly accident. It’s been nearly a decade since Concorde’s last revenue flight in 2003.

SSBDLess than a decade ago, test flights by the Shaped Sonic Boom Demonstration (SSBD) aircraft renewed hope for commercially viable supersonic flight. New designs for commercial supersonic aircraft turn up at the major international air shows regularly and increasingly target the lucrative business-jet market, the so-called supersonic business jets (SSBJ). These designs owe much to the ground-breaking technology established by the SSBD.

In the days prior to this year’s Farnbourough International Airshow, media outlets were rife with rumours of an announcement that had NASA joining forces with some combination of Gulfstream, Boeing, or Lockheed-Martin to bring a version of the X-54 SSBJ to market by 2030. Despite the frenzied anticipation, no such announcement was made.

(NASA photo)

At Le Bourget 2011, the Euro-conglomerate European Aeronautic Defence and Space Company (EADS), the parent company of Airbus, announced the Zero-Emission High Speed Transport (ZEHST), a technology demonstrator featuring three separate propulsion systems—turbofan, ramjet, and cryogenic rocket engines—that will take to the skies in 2050. A second design, the SonicStar, a supersonic business jet with a more immediate timetable of first flight in 2021, was also announced at Le Bourget 2011, presented by British aerospace startup HyperMach. Both designs claimed to incorporate engineering solutions for addressing an aspect of the fundamental physics of supersonic flight that has accompanied every breaking through the sound barrier since Chuck Yeager’s 1947 flight, the sonic boom.

The passage of an aircraft through our planet’s ocean of air is often likened to a boat moving across the water. The boat’s passage is marked by waves of reflected energy as its hull pushes through the water. Visualizing a boat’s wake is misleading to a degree because we only see the two-dimensional “V” spreading out from the boat on the water’s surface. In reality, the mechanical wave and its associated pressure increase are also spreading out underneath the boat. In physics, a mechanical wave is one that requires a medium—water for the boat, air for the airplane—in which to travel, as opposed to an electromagnetic wave, which can propagate in a vacuum.

SSBDNose2The mechanical waves generated by an aircraft are sound waves, miniscule increases in pressure generated by the aircraft’s body pushing and compressing air molecules as it passes. Below the speed of sound, these pressure waves radiate outward from the aircraft in three dimensions. As an aircraft approaches the speed of sound (761 mph at sea level and 59 F), the pressure waves are only moving slightly faster than the aircraft, so they bunch up together just in front of the aircraft’s surfaces (cf. “Supersonic Revolution” by Richard Hallion, Aviation History, July 2011): the nose, the wing leading edges, and the empennage. As an aircraft reaches the speed of sound, the bunched pressure waves actually combine into a big single shock wave. That shock wave is heard as a sonic boom.

SSBDdougThe sonic boom is characterized by the dramatic increase in pressure at the aircraft’s nose and by a steady decrease in pressure to below atmospheric levels at the aircraft’s tail. After the supersonic aircraft passes, the pressure sharply increases to return to atmospheric levels. The two pressure increases—one at the aircraft’s nose and the other after it passes—produce the sonic boom’s double-bang sound. This sequence of increase–linear decrease–increase in pressure gives the wave its characteristic shape (up, down, up) and name: the N-wave.

In an interview for the NOVA program Supersonic Dream on Concorde, James Hamilton, at one-time the Director-General of Concorde, compared the sound of an aircraft approaching at subsonic speeds with the surprise arrival of sound associated with a supersonic aircraft: “You hear nothing until you get all the noise collected together, as it were, and when that happens, instead of a getting a continuous rumble of noise, you get a very sharp boom.”

SSBD at Valiant Air Command Warbird Museum
SSBD at Valiant Air Command Warbird Museum

Federal Aviation Regulation 91.817 prohibits flying over land at supersonic speeds for civil aircraft in the United States. The regulation specifies that aircraft “will not cause a sonic boom to reach the surface within the United States.” In the mid-1960s, the FAA, NASA, and the U.S. Air Force conducted a six-month experiment into the effects of sonic booms on a human population by performing supersonic overflights of Oklahoma City. Using a mixture of F-101s, F-104s, F-106s, and B-58s, the experiment, initially designed to provide favorable support for continuing the development of an American version of Concorde, the supersonic transport (SST), subjected the residents of Oklahoma City to 1,253 sonic booms. In the end, public outcry led to the early cessation of the experiment and, ultimately, a class action suit that the government lost.

After it was prohibited by law, the inability to conduct overland supersonic flight restricted the usefulness and profitability of Concorde by limiting it to a handful of transatlantic routes.

The X-54, ZEST, and SonicStar are being designed to make use of sonic boom mitigation techniques, techniques that will attempt to reduce the pressure wave that reaches the ground by shaping the shock wave that emanates from an aircraft traveling faster than sound, techniques that were pioneered by the SSBD.

Next Wednesday, we’ll take a closer look at this aircraft with looks that only an engineer could love.

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