Seen from the side, the experimental X-59 aircraft unveiled on December 12 by NASA and Lockheed-Martin will greatly appeal to Blake and Mortimer fans, as it resembles the Swordfish, the submersible, supersonic and radio-controlled combat aircraft at the heart of their first adventure! Unlike Edgar P. Jacobs' comic strip, the device's huge, very thin nose was not designed for aesthetics, but for a very specific reason: to reduce the intensity of the "bang" produced when it breaks the sound barrier.
The US aerospace agency's goal with this demonstrator, whose first liftoff is expected later this year, is to prove that it is possible to produce a commercial supersonic aircraft quiet enough to be allowed to fly over continents. The example of the X-59 produced by Lockheed Martin for NASA is unique, and is not intended for mass production. "Noise is clearly one of the limiting issues for future civil supersonic aircraft," says Quentin Bennehard, a research engineer at ONERA, the French aerospace research centre. Fifty years ago, in 50, U.S. regulations banned the supersonic overflight of a civilian aircraft over land, which reduced the number of commercial routes open to Concorde."
Shockwaves
Contrary to popular belief, the supersonic boom is not a one-off effect of an aircraft breaking the sound barrier, but a continuous noise produced as soon as its speed exceeds the speed of sound (about 1200 km/h). "The aircraft travels faster than the sound waves it produces, resulting in shock waves produced from the nose of the aircraft to the rear of the fuselage," explains Quentin Bennehard. As these shock waves travel to the ground, they combine, add up, and produce a sudden, violent noise that can be disturbing. For the Concorde, which flew at Mach 2, this produced a bang of about a hundred decibels, which could be mistaken for an explosion." The noise is therefore continuous, but since it is a shock wave that travels with the plane, the boom is heard gradually by all the populations that are under the plane's ground track.
The rare passages in supersonic flight by fighter jets in France, as was the case over the Paris region in September 2020, sometimes cause concern among populations.
The X-59, unveiled in its flight livery on January 12 in Palmdale, California, in a Lockheed Martin hangar. GARRY TICE/LOCKHEED MARTIN/REUTERS
The solution chosen by NASA to mitigate the supersonic boom after several decades of research was to work on the shape of the aircraft, to reduce the shock waves as much as possible, and to act so that they do not add up as they propagate through the atmosphere. The result was a very slender X-59, with a nose measuring one-third of the total length of the aircraft. Its entire geometry has been designed to reduce pressure peaks that accentuate shock waves, especially on parts where there are discontinuities, such as the junction between the wings and the fuselage, or the engine inlet. For this reason, the engine air intake was installed above the fuselage so that all sound waves would travel upwards, not to the ground. In the same way, the cockpit windows are installed in continuity with the fuselage to avoid causing a rupture. Without direct forward vision, the pilot will have to take off and land with images projected on a large flat screen in the cockpit.
Wind tunnel validation
Still with the idea of reducing noise pollution for the population, NASA has chosen a slightly lower cruising speed than that of the Concorde, Mach 1.4 instead of Mach 2. The power of the bang is amplified by the speed, as well as by the size of the aircraft.
The general shapes of the X-59 are finally quite close to those that had been explored by the European Rumble program, which studied from 2017 to 2020 solutions to reduce the intensity of the supersonic boom, with Airbus, Dassault Aviation (*), ONERA and DLR (the German space agency). "Since Concorde, we have been working on this issue for a long time," says Quentin Bennehard. We also recently had a collaboration with NASA on the subject of the supersonic boom, which ended in 2021, to compare the calculation codes and their validation by work in wind tunnels."
The shape of the X-59 is designed to reduce the amplitude of the supersonic boom perceived when flying over inhabited areas. ROBYN BECK / AFP
With all these innovations, NASA engineers predict that the "supersonic noise" of the X-59 will be around 75 decibels, or the sound of a car door slamming a few meters away. The gain should be huge compared to the Concorde's 105 dB, which was close to a thunderclap! The in-flight measurement campaign will take place in 6 different geographic regions of the United States, to take into account variations in terrain and weather that are known to influence the intensity of the "bang". NASA engineers are confident that the X-59 will be quieter than the pre-existing supersonic aircraft, but part of the measurement campaigns will aim to verify that this modified noise will be acceptable to the populations it flies over.
(*) The Dassault Group owns the "Figaro"