A Lockheed Martin concept for a low-boom supersonic airliner. Image: NASA/Lockheed Martin
It’s been almost 11 years since the last supersonic passenger flight flew, but the dream of vast swaths of land and oceans faster than the speed of sound isn't dead yet. Researchers with NASA are working to define a new standard for lessening sonic booms, and success would remove one hurdle to the return of supersonic commercial aircraft.
The Aérospatiale-BAC Concorde remains legendary, and it's hard to believe that it was first developed in the 60s. The sleek passenger aircraft made its first supersonic passenger flight from London’s Heathrow airport to Bahrain on January 21, 1976; its last flight, a October 24, 2003 trip from New York’s JFK airport to Heathrow, came after years of declining ticket sales and at a time that the aviation industry as a whole was facing declining revenue.
While cost is another huge factor, the sonic boom problem is a major reason why supersonic commercial flights have always been over water, and why they aren’t more common today. (As for cost, opening up more routes could potentially make the economic equation more favorable.)
The problem is that the sonic boom doesn’t occur just once when an aircraft first reaches Mach 1 or the speed of sound. The sonic boom follows the flight path. So a supersonic commercial flight from New York to Los Angeles would produce a loud booming sound that would reverberate diagonally across the country, disturbing folks on the ground.
But several NASA aeronautics researchers are hoping to change that. At the American Institute of Aeronautics and Astronautics Aviation 2014 event in Atlanta this week, they will share the progress they are making in overcoming some major hurdles of supersonic passenger travel.
Among the main goals of this team’s research is to generate crucial data for developing a low-boom standard for civil aviation, including the procedures and requirements that may lift the ban of supersonic flights over land.
Even if it wasn't exactly an economic success, there's no denying the Concorde was a marvel. The plane could carry 100 passengers, along with a crew of six, more than 4,400 miles at a cruising speed of about 1,350 miles per hour. At an altitude of 60,000 feet, that translated to about Mach 2, or twice the speed of sound. The aircraft’s fastest transatlantic flight was on February 7, 1996; it flew from New York to London flight in 2 hours, 52 minutes, and 59 seconds.
Flights between New York and major European cities like London and Paris were among the Concorde’s most common routes and indicative of its flight profile’s dominant characteristic: the Concorde could only fly at supersonic speeds over unpopulated areas, which largely means over the world’s oceans. That’s because of the sonic boom.
As a plane (or any object for that matter) moves through the air, air molecules are forced aside to get out of its way. When a plane moves faster than the speed of sound, this pressure differential forms a shock wave, which you can visualize as being similar the wake of a boat moving across a calm lake. A cone of pressurized air forms as a result of the shock wave, and the cone moves outward and rearward of the plane’s flight path in all directions until it hits the ground. The sharp release of air pressure after the buildup of the shock wave is heard as the sonic boom.
"Lessening sonic booms… is the most significant hurdle to reintroducing commercial supersonic flight," said Peter Coen, head of the High Speed Project in NASA's Aeronautics Research Mission Directorate at the agency's headquarters in Washington.
But decreasing sonic booms is just one hurdle barring supersonic flight. "Other barriers include high altitude emissions, fuel efficiency and community noise around airports,” said Coen.
NASA used F/A-18 supersonic fighters to test to effects of low-intensity supersonic booms on people on the ground. Image: NASA/Jim Ross
NASA engineers at agency centers around the country are tackling the sonic boom problem, including working on developing low-boom aircraft. Researchers are also working out how to quantify the loudness and annoyance of the sonic boom by asking people to listen to sounds in a noise test chamber.
Recently, at NASA's Armstrong Flight Research Center in Edwards, California—a place unique in that pilots are allowed to make supersonic flights over the base—residents explored ways to gauge the public’s response to sonic booms in a real-world setting. Similar work is going on at NASA's Langley Research Center in Hampton, Virginia, where volunteers are listening to and rating how annoying they find sonic booms.
"They each listened to a total of 140 sounds, and based on their average response, we can begin to estimate the general public's reactions," explained Langley acoustics engineer Alexandra Loubeau.
Other studies are focusing on predicting the sonic boom and what design approaches might reduce it.
"We are working to understand the worldwide state of the art in predicting sonic booms from an aircraft point of view," said Mike Park, a fluid mechanics engineer at Langley. "We found for simple configurations we can analyze and predict sonic booms extremely well. For complex configurations we still have some work to do.”
To this end, wind tunnels are an important tool. Like supersonic aircraft of the past, including the Concorde, most designs engineers are testing are needle-nosed and sleek, with a delta wing or highly-swept wings—a basic shape that produced lower sonic booms.
Supersonic research has reached a point, NASA and industry engineers say, where a practical low-boom supersonic jet is within reach. It would be incredible if this pans out. Hopefully ticket prices will drop from the thousands it cost to fly on Concorde. Otherwise most of us will be stuck on longer, cheaper subsonic flights.