Supersonic planes are notoriously noisy, but NASA engineers think they can reduce the thunderous boom these planes produce into a barely audible thump by cleverly shaping the aircraft to minimize how it reflects sound waves.
The raw structure of a prototype of such a plane, the X-59, has just been assembled at the facilities of NASA contractor Lockheed Martin in Palmdale, California. The 99-foot-long (30 meters), 24,000-pound (10,000 kilograms) one seater might take to the sky as early as the end of 2022, paving the way for a new era of supersonic aviation.
There is no way of not noticing a supersonic fighter jet zooming over your head; the sonic booms are not only loud, but they create vibrations that you can feel. As the plane bursts through the air, it creates soundwaves. But because the plane travels faster than the speed of sound, it surges ahead leaving the waves in its wake crashing into each other. The boom the waves produce, akin to a gunshot, can rattle furniture and even shatter glass.
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For example, the supersonic boom produced by the iconic Concorde, the so far only supersonic passenger aircraft in history (retired in 2003), reached 105 decibels, about as loud as a nearby thunderstrike.
The X-59, in comparison, should make no more noise than a car door slamming 20 feet (6 meters) away, according to NASA.
"The amplitude of [the sound wave generated by our] airplane is probably five to eight times lower than that generated by the Concorde," David Richwine, NASA's deputy project manager for technology for the Low Boom Flight Demonstrator project, told Space.com. The plane's NASA designation is the X-59 Quesst experimental plane, with Quesst short for Quiet SuperSonic Technology.
"We're trying to generate a much more mild, much lower amplitude shock wave and also create a longer rise time to that shock wave on the airplane so that the sound waves don't come together and create the loud boom as they do on existing supersonic planes," Richwine said.
This noise reduction might in the future persuade regulators to allow supersonic planes to fly over inhabited areas. So far, because of the disruption caused by the supersonic boom, supersonic air travel is only permitted over the oceans.
But how can this noise reduction be achieved? Richwine explained it took a lot of careful computer modelling to engineer the entire shape of the plane in a way that minimizes the shock waves created as the plane bursts through the air.
"The most obvious thing is that our plane has a longer nose than, for example, the Concorde," Richwine said.
This long nose, however, created other technical challenges the engineers had to solve. The smooth and gradual shape of the nose prevents the cockpit of the X-59 from having a direct view of what's in the front. Instead, the pilot looks at high-definition screens that are fed video input from an external vision system.
"The Concorde had a droop nose that allowed the pilots to see the land so that they could actually see where they were landing," said Richwine. "Thanks to the technical progress that had been achieved since the time that the Concorde had been developed, we could make use of the high-definition cameras and TV screens that we have today. That enabled us to develop a 'see the land' capability that is much lighter and simpler."
Richwine said that most people would probably not even notice if the X-59 flew over a busy city. In a quiet rural area, they might notice what Richwine described as a "quiet thump."
"It wouldn't be startling or bothering you," he said.
NASA hopes the X-59 could pave the way for a new era of supersonic travel that could see people zoom across continents in half the time it currently takes. After the plane takes to the sky for the first time probably at the end of next year, the space agency will run an extensive test campaign that will see X-59 fly over selected communities in the U.S. After each flight, local residents will be asked to answer questions about how much they had noticed the re-engineered supersonic sound.
Aircraft manufacturers could then use the technologies developed as part of the X-59 project to develop larger commercial planes that could carry up to 120 passengers.