NASA, though best known for its spacecraft, also has a pretty sizeable fleet of aircraft under its belt. It has a venerable tradition of X-planes, where ‘X’ stands for experimental. It started back in 1946 with the X-1, which became the first aircraft to travel faster than sound. Three-quarters of a century on, its new plane, the X-59, also aims to break the sound barrier – but this time it’s going to do it quietly.
The speed of sound has always caused headaches for aircraft designers. The reason lies in the nature of sound itself. When anything from a handclap to a rocket disturbs the air, it causes pressure changes that spread out like a wave. The speed of this wave depends on the properties of the air, but under normal conditions it’s around 1,200km/h (750mph).
“All aircraft change the pressure in the air around them as they fly,” explains Peter Coen of NASA’s Langley Research Center. The consequences depend on whether the aircraft is flying slower or faster than the sound it produces.
“In a typical subsonic aircraft, the pressure changes are gradual, [so] air molecules ahead of the aircraft sense the pressure change before the aircraft reaches them,” Coen continues. “But if an airplane flies faster than sound, the molecules upstream don’t know that it is coming.”
From the point of view of those molecules, all the sound waves the aircraft has been pushing ahead of it arrive at once. “The pressure changes happen instantaneously in what is called a shock wave,” Coen says. “A shock wave, from the nose of the aircraft for example, travels outward in all directions and merges with other shocks, from the wings or cockpit window… The result of this is two large, distinct shock waves that we hear on the ground as the two booms of a sonic boom.”
While we may only hear the sonic boom briefly, it’s actually produced continuously for as long as the aircraft is supersonic. People at different points under the flight path will hear it at different times – and when they do, they’ll all jump out of their skin in surprise. That’s why, back in the 1970s, the United States and many other countries imposed an almost complete ban on supersonic flight over their territories.
This situation is unlikely to change unless the sonic boom is reduced to an acceptable level. This is where Coen and his team come in. He’s the mission integration manager for NASA’s Low-Boom Flight Demonstration project. Their aim is to produce a viable supersonic design that’s no more disruptive to people on the ground than an ordinary aircraft. That would’ve been unthinkable 50 years ago, but advances in computer-aided design mean it’s within our grasp today. The result, a collaborative effort with Lockheed Martin, is the X-59 – a proposed test vehicle dubbed QueSST (for Quiet Supersonic Technology).
“The X-59 aircraft is equipped with unique shaping and supersonic technologies,” Coen explains. “A long slender nose, engine placement on the top of the aircraft and its External Vision System are all designed to control the strength and position of the shock waves to produce a softer sound to those on the ground.”
The aim isn’t to eliminate shock waves altogether – which is impossible – but to design the aircraft in such a way that the shock waves are spaced roughly equally along its length. “Because of this, the shock waves do not merge into the double shock boom but are individually weakened and softened,” says Coen.
Although it was designed with aerodynamic considerations first and foremost, the X-59 is a striking-looking aircraft by any standards. Almost a third of its 30-metre length is taken up by the sharply pointed nose, behind which the single-seat cockpit is so carefully moulded into the streamlined fuselage that it’s barely discernible. In fact, the pilot doesn’t even have a forward-facing windscreen – just an HD video display showing the view ahead (that’s the External Vision System that Coen referred to earlier).
All this careful shaping should, according to the simulations, reduce the dreaded sonic boom to a more acceptable ‘sonic thump’. To quantify sudden, sharp sounds, NASA uses a measure called ‘perceived level decibels’, or PLdB. A conventional sonic boom is around 105PLdB, while a car door slamming six metres away is just 75PLdB. That’s the level the X-59 is aiming at. When it’s flying at 1,400km/h (925mph) – around 1.4 times the speed of sound – at a typical cruising altitude, all you should hear is a mild thump no worse than your neighbour slamming a car door.
So far, however, it’s all theory. Only when NASA takes delivery of the X-59 from Lockheed Martin early in 2023 will they be able to see how reality measures up. The test schedule will fall into two phases – careful scientific measurements over NASA’s California test ranges to start with, followed by a community response study over a few selected US cities.
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The latter phase is crucial, because there are subtleties in the way people react to sounds that go beyond measurable quantities like PLdB. Coen and his team hope the X-59’s sonic thump will be acceptable to the public, but they can’t be sure. “Once we get into the community overflight test phase of the mission, we will collect this input from people who are actually on the ground and hear the sound the X-59 makes when it flies overhead,” Coen explains.
Gauging public reaction is critical, because ultimately only this – as opposed to any number of scientific measurements and calculations – will carry weight with aviation regulators. The aim is to persuade them to modify the blanket ban on supersonic overflights, granting an exemption for any future aircraft that might pick up on the X-59’s low-boom design features.
If everything goes the way NASA is hoping, the final years of this decade could see the start of a second great era of supersonic air travel, following the abortive first era that began and ended with Concorde.
The Concorde was operated between 1976 and 2003 by just two airlines, British Airways and Air France. The stringent flight restrictions meant the iconic aircraft was only ever used on transatlantic routes. Concorde was notoriously expensive, of course, but that was largely because it was the first of its kind.
And with such a limited range of available routes, aerospace companies simply didn’t have sufficient incentive to carry out the research that might have made it more economical in terms of fuel consumption and passenger capacity.
In an alternative timeline in which the sonic boom problem never arose, the situation today might have been very different, with supersonic air travel being the norm on all the world’s long-haul routes. Now there’s a real possibility that this could happen in our world, if the X-59 lives up to expectations.
- This article first appeared in issue 368 of BBC Science Focus Magazine – find out how to subscribe here