Supersonic air travel is coming back: The next big challenges

The next big challenge: silencing sonic booms to allow overland flights

Ever since the 1970s, NASA has been wrestling with the math around the pressure waves that create aircraft sonic booms, trying to figure out how to reduce them. And, over the past decade, its engineers think they’ve finally cracked the problem.

“We’re at the point where we think we can design a quiet supersonic airplane,” says Peter Coen, the commercial supersonic technology project manager at NASA’s Langley Research Center. His team is currently working with Lockheed Martin on a $20 million project to design a prototype X-plane with much, much softer booms.

The science of what Coen’s team is doing gets a bit complicated (see this report for the gritty details), but here’s a very basic explanation. When a plane like the Concorde travels at supersonic speed, it creates a bunch of invisible shock waves, sharp pressure disturbances emanating from all the objects that stick out of the plane — the nose, the windshield, the wings, the tail. They’re shaped like this:

These shock waves are all different strengths, and they as they travel through the air, they coalesce into just two powerful waves — a strong positive pressure wave at the nose and a strong negative pressure wave at the tail. This “N-wave”configuration is highly stable and doesn’t decay much as it travels toward the ground. When this wave hits people below, our ears register it as two noisy bangs from each of the two sharp swings in pressure.

“So the trick,” Coen says, “is to keep those shock waves from coalescing into a N-shaped signal.” In theory, a plane with a different shape would create shock waves of more uniform strength that coalesce more gradually as they move through the air. If done successfully, people on the ground would experience a gentle rise in pressure when the wave hits rather than two sharp pressure changes.

Recent experiments have been promising. The Concorde created booms that were perceived to be as loud as 135 decibels on the ground — about as loud as the sound a jet engine makes from 100 feet away. But by experimenting with different shapes, NASA has developed aircraft models that, in wind-tunnel tests, create booms as soft as 79 perceived decibels, roughly comparable to a car passing 10 feet away. Eventually, NASA would like to get that down that to 70 decibels.

The goal is to design an actual plane using these concepts. Here is an artist’s conception of what the new X-plane might look like.

NASA will then plan to build a one-third-scale prototype and conduct the first test flight in 2020. Lockheed Martin has a contract to design a plane. But NASA hasn’t yet selected a company to actually build the prototype. The idea is that they’ll gather data on what sorts of sonic booms the planes actually make, and then the FAA can use that data in deciding whether it makes sense to replace the blanket ban on overland supersonic travel with a noise standard — saying, for example, that supersonic flight is acceptable overland so long as the booms are below a certain threshold. If that actually happened, Gulfstream says planes with quiet booms might be a possibility by 2025 or 2030 or so.

But changing those rules will require wading into the murky world of politics — which is never easy.

The US government could still put a ban on supersonic flight

In theory, the FAA is open to the idea of revising its blanket ban on supersonic flight over land. In 2011, an FAA official gave a public presentation explaining that research on silencing sonic booms has progressed far enough that it may be time to consider a noise standard.

But the FAA is moving very slowly on revamping the rules — in part because it’s focused on other challenges, like overhauling the nation’s air traffic control system. The agency is also waiting for NASA and other companies to demonstrate their quiet-boom technology before crafting fresh regulations.

In their paper for the Mercatus Center, Dourado and Hammond argue that the FAA’s current approach is precisely backward. It would be much better for the agency to set guidelines ahead of time on what type of overland sonic booms would be acceptable — so that companies can have some certainty and know what to aim for in developing new designs. “Right now, the FAA is saying we’ll accept supersonic when we hear what’s acceptable,” says Hammond. “We’re trying to point out the absurdity in that.”

Even if the boom issue gets sorted out, however, there’s still another hurdle for supersonic planes: takeoff and landing. While all supersonic planes would fly at less-than-supersonic (or “subsonic”) speeds around airports, they’d still make a fair bit of noise on takeoff and landing. And that’s where things get a little tricky.

Over the years, the FAA has developed strict standards for the noise that airplanes can make around airports. Aircraft manufacturers have responded to these rules, in part, by doing things like building high-bypass engines with large-diameter fans that propel air out of the engine more slowly and hence reduce the noise from the exhaust.

The trade-off with these high-bypass engines is that they’re not as fuel-efficient at takeoff and the large fans create more drag while the plane’s in the air. That’s not a huge deal for normal aircraft, but it could be ruinous for supersonic jets. If the FAA requires supersonic jets to adhere to the newest, strictest noise standards coming into effect by 2018 (known as Stage 5 standards), those jets will take a major fuel efficiency hit. By contrast, if the FAA merely asked supersonic aircraft to adhere to the standards that were in place back in 2006 (known as Stage 3 standards), Scholl estimates, that would reduce supersonic ticket costs by some 15 percent.

Dourado and Hammond argue that the FAA should allow looser airport noise standards for supersonic jets in the very beginning, at least, to allow the technology to get to market. Engineers can then work on making them quieter. The companies making the supersonic planes agree. “The physics of supersonic aircraft are just so different, so saying you need to meet the standards of subsonic regulations could put a damper on development,” says Aerion’s Miller. “We’re hoping to reach a compromise with the FAA on this, since we’re talking about a new industry that could be beneficial.”

This could prove a contentious subject, however. The politics around airport noise can be extremely dicey (in separate research, Dourado and Raymond Russell have found that most airport noise complaints to the FAA come from just a small handful of people). While a strict airport noise rule wouldn’t necessarily kill supersonic flight, it could increase prices and dampen the market, particularly in the early days.

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