WELCOME to the most unpleasant room at NASA. The sonic boom simulator at Langley Research Center in Virginia may have a comfortable sofa and a soft rug, but the sound system is vicious. A hundred speakers and subwoofers hidden in the walls can shake the floor and rattle your eardrums as they blast out the thunderous noise of a plane breaking the sound barrier.
NASA uses the room to understand how annoying sonic booms are. Life is full of irritating noise, from the drilling of roadworks to your partner’s snoring. Where do the bangs and rumbles produced by a supersonic aircraft rank?
You might think we already know the answer. After all, fighter jets have been zipping around faster than sound for decades, making a noise like two quick-fire rifle shots. The same goes for Concorde: the famously graceful supersonic airliner produced booms powerful enough to crack windows.
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But it has been more than 40 years since Concorde’s first flight, and engineers at NASA and elsewhere now have some nifty ideas for making booms less shocking. If they can do enough to muffle the din – and prove it doesn’t annoy anyone – then perhaps supersonic air travel can be reborn. “Airliners have been stuck at the same speeds since the 1960s,” says Peter Coen, head of the NASA research team on the case. His goal is to quietly speed things up.
Concorde was a technological marvel. Aerodynamicists still drool over the curves of its wings and engine inlets. It didn’t just break the sound barrier; it smashed it, with a cruising speed of 2100 kilometres per hour, nearly double the speed of sound at its cruising height. This meant it could carry you from London to New York in under 3 hours – if you were well heeled enough to afford a ticket. It remains the only faster-than-sound airliner, apart from Russia’s short-lived .
In other ways, the plane was a nightmare. It guzzled fuel, leading one environmentalist to describe it as a “vulture, spewing black smoke” in 1977. The environmental issues – together with the fatal Paris crash in 2000 – dented the plane’s image. But the real damage had been done years earlier. “What killed Concorde was mainly not being able to fly supersonically over land because of the sonic boom,” says , a curator at the National Air and Space Museum in Washington DC. This reduced its utility, and all airlines except British Airways and Air France cancelled their orders. Concorde was eventually mothballed in 2003, taking the dream of mass supersonic air travel with it.
Well, almost. A few companies, such as , have been developing small supersonic passenger jets for years. Recently, a new player called , pledging to offer supersonic flights across the north Atlantic for the price of a business class ticket from a standard airline. Joe Wilding, co-founder of Boom, says the firm’s research suggests there is a ripe market. But Boom is aptly named: its planes won’t be significantly quieter than Concorde.
In the 1970s, there was a serious effort to dial supersonic flight noise down. “For five years, pretty much all the top aerodynamicists in the world were working on this problem,” says , a pioneer in sonic boom design at Cornell University in New York.
George and his colleagues came to realise that they were up against a serious physical limitation. Planes create shock waves as they move through air and drag it forward. These shock waves can’t travel faster than the speed of sound, called Mach 1. It is the speed at which particles vibrate and pass energy from one to another, and it varies depending on air temperature and pressure. When a plane flies faster than this, the shock waves begin to bunch together, like water piling up in front of a boat.
It turns out that these shock waves typically coalesce as they propagate outwards into an “N-wave” sonic boom: two sharp changes in pressure, one upwards, as the plane shoves air out of the way, and one downwards, as air rushes back. Our ears perceive these pressure changes as sound.
All sorts of weird and wonderful designs to dampen the boom were tried in the 1970s. Imaginative new wing shapes were popular. They could be tiny or huge, stacked on top of each other or dramatically swept back. Some designers even proposed , to see if ionising the air might help. “There were all these exotic configurations,” says George. “But they just didn’t work.”
To do better, engineers needed a finer-grained understanding of how air flows. That is governed by what would become the Navier-Stokes equations, first written down by Claude-Louis Navier in 1822. Trouble is, these are sensitive to minuscule changes: even a tiny puff can change the overall picture. And accounting for the movements of air one particle at a time takes a lot of computing, so aerodynamicists are forced into approximations.
Concorde’s designers were using slide rules, so they had to make a lot of approximations. But now we have supercomputers, and over the past decade engineers have developed a much more detailed picture of what happens to shock waves rippling off the hull of a supersonic plane.
Based on those insights, NASA thought it could design a plane that spreads the shock waves out so that they don’t coalesce into such a sharp boom (see diagram). The agency used computers to precisely model the airflow across hundreds of designs – moving the engines and sculpting the wings to create eddies that cancel out other eddies from the nose. They still can’t follow the shock waves all the way to the ground or account for the subtle effects of turbulence, which can increase the severity of booms. But NASA now has a design called the QueSST (Quiet Supersonic Technology) X-plane, which it reckons will produce a boom that sounds about 40 decibels quieter than Concorde’s.
The proof of the plane will be in the flying, but already there is cause for optimism. The Japanese Aerospace Exploration Agency (JAXA) has been pursuing a similar boom-muffling project, and in 2011 it used a balloon to lift a proof-of-concept glider from Esrange Space Center in Sweden into the stratosphere. When dropped, the glider accelerated past the sound barrier. A low-hovering blimp bearing a microphone measured the noise as about half as loud as a typical N-wave boom. Buoyed by this success, JAXA strapped an experimental, unpiloted low-boom aircraft to a rocket in 2013. It malfunctioned and crashed (with an exceptionally large boom), but a second attempt, in 2015, flew successfully – and as quietly as expected.
However, quiet does not necessarily mean less annoying. Just think of someone snoring again: not loud, but not pleasant either. The real question is how we perceive sonic booms.
We already had a rough idea for N-wave booms. In 1968, a survey of 3000 people in Oklahoma City found that 56 per cent were “seriously annoyed” after six months of exposure to booms produced by .
NASA’s sonic boom simulator – that room with the concealed speakers at Langley – can help estimate whether muffled booms will irritate people. By subjecting people to different volumes of sonic boom, accurately reconstructed as they would be heard in a home, Coen’s research team have established a basic threshold for annoyance, and shown that the X-plane’s boom should be below it. But only with real test flights can NASA hope to get a true picture. “We have done everything we can with computer models,” says Kevin Shepherd, the recently retired head of NASA’s structural acoustics branch.
A real test flight of the X-plane is now being planned for. NASA agreed a contract with Lockheed Martin last year to produce a detailed design by the middle of this year. The next step, building and test-flying a small-scale aircraft, will require more than $200 million from Congress, NASA estimates.
These real-life tests are doubly important because of a potential sting in the X-plane’s tail. In softening the boom at frequencies we can hear, it will shift more shock waves to inaudible frequencies. Neat, you might think, except those frequencies are just the right kind to shake buildings. No one yet knows if that might be enough to make rooms vibrate, creating a noise or jiggling objects off shelves.
Anyway, dampening down the boom might prove to be the easy part, says Stephen Trimble at news and data services firm FlightGlobal: supersonic planes also have an engine problem.
Engine failure
Jet engines used to work by sucking in air, mixing it with fuel and burning the result to generate thrust from the stream of hot exhaust. More modern engines split the air into two streams, with the majority bypassing the combustor and providing almost free lift.
These high-bypass-ratio engines are efficient, but supersonic jets can’t use them – they must be rather wide to accommodate the two streams of air, which doesn’t befit a fast, sleek aircraft. That means the X-plane will have to use the old-style engines, which is one important reason why it will end up using lots of fuel, even if it is made from advanced, lightweight materials (see diagram).
Worse, those engines make a terrible racket because most engine noise comes from the exhaust mixing with cooler air. This is the crucial problem that supersonic planes face, says Trimble. “A supersonic aircraft’s engines must be a lot noisier, particularly on take-off when you need maximum thrust,” he says. International regulations that cap noise levels at take-off are becoming more stringent. So even if the descendants of Concorde make a softer boom, their engines might be too loud to take off legally.
Yet we are too in love with the idea of supersonic flight to stop trying. Just look at the planes the European Space Agency has had on the drawing board for years, some of which are meant to transport passengers at eight times the speed of sound. When it comes to supersonic flight, we will always dream.
This article appeared in print under the headline “Hushed up”
Article amended on 1 March 2017
We clarified the relative noisiness of the sonic booms
