What is the sound of no plane booming?

There’s a famous koan, or philosophical question used in Zen meditation, that asks, “What is the sound of one hand clapping?” An aviation variant of the question is, “What is the boom from a supersonic aircraft that hasn’t been built yet?” It may surprise you to hear that this has real policy implications.

In late January, Boom Supersonic flew its XB-1 demonstrator aircraft faster than the speed of sound for the first time and then promptly retired it. CEO Blake Scholl subsequently said that XB-1 didn’t produce a sonic boom and called for the 50-year-old U.S. ban on overland supersonic flight to be lifted, based solely on this test flight. What’s going on here and why is it best for regulators to ask tough questions before undoing the overland flight ban?

Boom stated that XB-1 achieved “Boomless Cruise” and claims that its Overture aircraft, which it says it’ll bring into service in 2029, will do this, as well. Boomless Cruise was originally the goal of Aerion, a different supersonic startup that went bankrupt in 2021. It’s based on Mach cutoff, a theory which says that under the right atmospheric conditions a slow supersonic aircraft—typically flying between 110% and 130% the speed of sound—might not generate a sonic boom that propagates to the ground. Mach cutoff occurs when the sonic boom is reflected away from the ground due to a density difference in a warmer layer of air below the aircraft.

Given the novelty of the claim—Boom made no mention of a goal to achieve Boomless Cruise before the flight, and the company is literally named after the atmospheric disturbance—some scrutiny is in order. Modeling suggests that achieving Mach cutoff, which is sensitive to speed, altitude, temperature, humidity, and winds, will be tricky under real-world flight conditions.

Penn State researchers found that for certain windspeeds and headings, a 1% change in cruise speed can disrupt Mach cutoff and lead to a focused boom being concentrated down on the ground. That’s because a precise combination of speed, aerodynamics, aircraft mass, and atmospheric conditions is needed to produce Mach cutoff, and that needs to be supported by precise avionics and accurate atmospheric tracking. So, without first being demonstrated under a variety of flight conditions, “boomless” aircraft might prove to be anything but.

Boom did not release any observational data about the XB-1 test flight and such data is needed to help understand how representative the conditions were that XB-1 was operating under. For example, Georgia Tech research found that conditions of low humidity and low temperature lead to lower perceived boom on the ground. XB-1 was flown over the Mojave Desert in late January, which might be ideal for Mach cutoff.

To understand why Scholl may be interested in raising this issue, you need to know a bit about how the economics of supersonic flight intersect with the challenge of sonic boom. How large the market will be for Overture is hotly debated. Boom estimates that it can sell more than 1,000 aircraft to service routes over water. It’s a bold claim given that only 14 Concorde—which had famously bad economics—ever entered commercial service.

How likely are those 1,000 planes? Well, back in 2022, we analyzed the potential market for supersonic aircraft in 2035 using a model that MIT’s Laboratory for Aviation and the Environment developed for NASA. The model used consumer’s willingness to pay for time savings to project the potential supersonic market. We estimated that there could be a market for about 235 Overture-sized supersonic aircraft in 2035, with about 70% of the potential demand in the Middle East and North America (map).

But there’s a catch. That’s an unconstrained forecast that assumes no overland flight ban. If supersonic aircraft can only operate over water, the modeled market falls by 95% to around 12 aircraft. So, right back to Concorde levels—and Concorde was tried on a variety of routes before settling into its core, profitable markets of Paris to New York and London to New York.

This result suggests that there just aren’t enough profitable routes over water to justify the more than $20 billion needed to build a new supersonic aircraft. That’s a problem for a cash-strapped startup that has raised about 4% of that capital to date. Prior to the test flight, Boom said it had identified 600 to-be-announced routes over water on which Overture could operate profitably. Now, Scholl is arguing that Overture will be so quiet that the United States should repeal its ban on overland supersonic flight outright. Conveniently, that would translate to a much larger market for Overture, assuming that the public can handle any sonic boom.

Due to the powerful engines needed to achieve sustained supersonic flight, supersonic aircraft will always be noisier than subsonic designs. According to an upcoming UN report, supersonic aircraft are, on average, expected to emit 25% more noise per flight and about 6 times more noise per seat during takeoff and landing. Still, the Achilles’ heel of supersonic aircraft is sonic boom: An aircraft operating supersonically propagates a shock wave continuously along its flight path. Sonic boom has been found to be deeply unpopular with the public and supersonic flight is banned overland not just in the United States but in most countries. The current ban on overland supersonic flight is enforced via a speed limit for aircraft.

Back to Boomless Cruise. If achieved, this would increase the market for Overture but wouldn’t be a panacea. Flying at 110% the speed of sound will save some time but also increase fuel burn, thereby increasing both fuel costs and greenhouse gas emissions. The German Aerospace Center estimated that achieving Mach cutoff speeds by a commercial supersonic airliner would increase fuel burn about 25% compared with operating at Mach 0.95.

Unfortunately, with XB-1’s retirement, we’re unlikely to get additional data on Boomless Cruise until at least 2028, the target date for Overture’s first flight. To complicate things, we won’t know if Overture’s full-scale, four engine turbofan design, which bears little resemblance to the smaller turbojet XB-1, can achieve Boomless Cruise until after it’s built.

What’s a careful regulator to do? A prudent approach would be to continue to support an international standard for sonic boom intensity, which the UN is aiming to deliver in 2031—not much later than Overture’s target entry-into-service date of 2029. That would supplant the overland speed limit with a robust and objective noise standard for supersonic aircraft. A bit of patience here to project the American public from unwanted noise pollution is preferable to hasty regulatory changes to support a product that may or may not be built, and that may or may not boom.