Weird Science | How X-59 could break the sound barrier without too much sound

Two decades after the last civilian flight by a supersonic aircraft, the iconic Concorde, NASA and Lockheed Martin on Friday (early Saturday in India) unveiled a new model that aims to conquer one of the key issues with such aircraft. Rather than the powerful sonic boom that has been the bane of the Concorde and other supersonic aircraft, NASA’s X-59 Quiet SuperSonic Technology (QueSST) airplane produces only a sonic “thump”. The operative word here is “quiet”.

The noise from a sonic boom not only disturbs civilians on the ground but can also cause damage to structures if the boom is large enough and the aircraft is flying low enough.

To be sure, noise was not the only reason that led to the discontinuation of civilian flights on supersonic jets. In fact, the British-French Concorde was allowed to fly at supersonic speed only over water during its 34-year run from 1969 to 2003. Only one other such aircraft has ever been used on civilian flights: Russia’s Tupolev Tu-144 (1968-77). It was discontinued for technical issues such as cabin noise, and lack of enough routes. The decision to retire the Concorde in 2003, meanwhile, is believed to have been hastened by a devastating crash three years previously, which killed all 109 people on board and four on the ground.

Supersonic aircraft continue to be used as fighters in some countries. In India, the venerable MiG-21, operating since 1963, is supersonic, which is being replaced by the Sukhoi Su-30MKI. But will such aircraft ever fly passengers again? In the US, NASA hopes that this will happen once the sound problem is fully addressed.

A sonic boom releases sound pressure waves within an imaginary cone, and the sound can be disturbing for anyone in the path of that cone

How a sonic boom forms

An analogy commonly used to explain the concept of a sonic boom is that of waves in water. Toss a pebble into a pond and you see small waves in the form of ripples. Sail a boat at slow speed in a river and the waves it generates will spread out in front and behind. As the boat moves ahead, each wave in front will move out of the way. But when a ship sails at a speed faster than the waves themselves, it keeps generating new waves that will reach waves farther ahead before those can get out of the way. A number of waves thus combine to form a single large wave, which we know as a wake.

An aircraft similarly creates sound waves in the air it travels through. Much like the ripples in the water, sound waves already created usually have time to move out of the way of waves formed subsequently. Except when the aircraft is travelling faster than the speed of sound (taken as 343 m/sec or 1234 km/hr). When the aircraft is supersonic, it causes sound pressure waves to radiate outward much faster than they can move out of the way. The motion of the aircraft crowds the waves together, their merger creating a powerful shock wave from which a huge sound is released. That is the sonic boom.

The boom is not heard everywhere in the vicinity. The sound pressure is released through an imaginary cone extending downward from the aircraft’s nose. Suppose you are on the ground when a supersonic aircraft flies overhead. Initially, you hear nothing because the aircraft is travelling faster than the sound it is generating. After it has travelled some distance ahead, the sound pressure cone reaches you, and you hear the sonic boom.

Mitigating the boom

Theoretically, there can be several approaches for suppressing the sonic booms. A key aspect is the shape, of the aircraft as well as the waves it produces. Experiments in this field started as early as 2003 when a government-NASA-industry collaboration flew a US Navy F-5E plane with a modified nose. The focus at that time was on business jets.

The experiment was not really quiet, science writer Tony Feder said in a 2007 article in Physics Today, but it did match predictions of reduced sound. Feder quoted Lockheed Martin engineer John Morgenstern as saying “It’s difficult to get rid of the sonic boom… What we do is change the shape of the waveform to make it far less audible.”

Victor Sparrow, an acoustical physicist at Pennsylvania State University, told Feder: “How do we shape the airplane to have a number of smaller shocks instead of one larger one? The idea is to delay the coalescing of the little shock waves into [a larger] wave.”

Those challenges are addressed with computations using fluid dynamics, various design modifications, and practical validation. On the X-59, the wing is specially designed to enable it to interact with other features of the aircraft, such as its long nose and top-mounted engine. This helps control the location and strength of shockwaves, NASA said in 2019. Flight tests began in 2022.

The road ahead

In a statement in January 2022, NASA expressed the hope that the ban on commercial supersonic flight over land can be lifted once the loud sonic boom is replaced with a softer sonic “thump”. The X-59 is expected to fly at 925 mph (nearly 1500 kph) while generating that thump.

Now unveiled, the X-59 has been designed to provide data to show that the boom has indeed become a thump. NASA will survey what people hear when the aircraft flies overhead. Public reaction will be shared with regulators so that they can consider lifting the ban on supersonic flight over land.

Kabir Firaque is the puzzles editor of Hindustan Times. His column, Weird Science, tackles a range of subjects from the history of inventions and discoveries to science that sounds fictional, but it isn’t.

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