How loud will the next generation of supersonic aircraft be? Facing a continued surge in design and construction efforts from a handful of companies, the FAA set out guidelines this week for how the new aircraft are expected to perform. Just as interesting as the numbers is the model that was left out of the definitions.
In a Notice of Proposed Rule Making (NPRM) published this week the FAA formally introduced the first noise standards for supersonic aircraft since Concorde flew nearly two decades ago. Most significant is that the FAA does not propose removing the restriction against supersonic travel over land. The sonic booms will not fly overhead. Digging further into the definitions also raises questions about how Boom Aerospace‘s Overture might fit into the framework; it is excluded from this initial release.
The NPRM defines Supersonic Level 1 (SSL1) aircraft as “supersonic airplanes that have a maximum takeoff weight no greater than 150,000 pounds and a maximum operating cruise speed up to Mach 1.8.” The noise regulations proposed scale with the number of engines and are slightly more lenient than similarly sized subsonic planes, “The proposed standards include noise limits that are quieter than the Stage 4 limits at which most of the current subsonic jet fleet operates, though louder than the current certification level of Stage 5 for the same aircraft weights.”
These metrics should help manufacturers like Aerion ensure their designs meet the noise requirements. The AS2 aims for a Mach 1.4 supersonic cruise speed, well inside the envelope defined by the SSL1 rules.
But Boom’s Overture is designed to cruise at M2.2. Despite notes in the NPRM indicating that the FAA’s SSL1 spec is “expected to encompass most of the projects currently under design,” the Boom aircraft is outside the FAA’s SSL1 classification.
The filing indicates that aircraft outside the scope of the SSL1 spec would likely use it as a “starting point for establishing an individual certification basis” and “a foundation for future specific standards once the distinguishing characteristics of the next class (whatever they may be) emerge and can be taken into account.” But there is no specific roadmap from the FAA for Boom to follow today as it develops Overture.
Dan Rutherford, aviation director at the International Council on Clean Transportation (ICCT), suggests there are further reasons for concern about the noise levels. He notes that “no data is provided to justify the proposed limits” while the manufacturer-supplied data is considered a trade secret and exempt from FOIA review. These standards were effectively designed in a black box with no real insight on the methodology, “other than that some legacy NASA modeling was applied.”
Boom is not concerned
Despite its exclusion from the current specifications, Boom is confident that Overture’s certification will not be a problem on the noise front. The company says it is “actively engaged with the FAA” and other regulatory bodies around the globe, hoping to see “thoughtful, practical standards” that will apply to faster supersonic aircraft such as Overture. It is also contributing data to the FAA’s efforts.
Moreover, the company suggests that it is designing for “the existing noise environment at the world’s major airports—those serving cities such as London, New York, and Tokyo.” With the current FAA proposal relaxing the rules slightly from the current standards that could be sufficient for Boom to meet the future spec when it is developed.
Boom also insists that the lack of overland supersonic flight is not a problem. “Overture is designed to fly at supersonic speeds only over water, and our business case does not require any changes to existing regulations, in the US and elsewhere, on overland supersonic flight.” This statement is somewhat belied by potential route maps the company has produced in the past. Still, enough options remain on the overwater routes that those should keep the company and its airline customers busy for a while.
Environmental issues, too
While the noise requirements are one component that Boom and other supersonic manufacturers must comply with the environmental footprint of supersonic flight is another significant challenge. The ICCT identified expected emissions levels for supersonic transport at 5-7x compared to subsonic travel. This raises questions about the supersonic options and regulatory approval.
Boom continues to suggest that a derivative engine option will be sufficient for the Overture aircraft while not identifying a specific manufacturer that will supply that engine. And the fuel burn challenges may further hurt the program’s chances for success. Even with a transition to 100% biofuels – a supply of which is uncertain, especially if crude oil prices remain depressed for any extended period of time – the consumption rate will be significantly higher on a per passenger-mile basis.
Timing continues to slip for Boom
The XB-1 demonstrator will not seek certification from the FAA. As such it is not affected by these proposed noise rules nor by emissions standards. While its test flights are not impacted by the rules it does continue to slip on the calendar.
In mid-February the company celebrated a milestone with the closeout of the wings, bringing it ever closer to full assembly. But in a statement this week a company spokesperson acknowledges that, while rollout of the assembled XB-1 is still expected “later this year,” first flight no longer is. That milestone is now pushed to 2021 at the earliest.
It is unclear how the slipping XB-1 timeline affects Overture’s development plans, though an impact should be assumed. When first XB-1 flight was planned for early 2020 the Overture program anticipated a 2025 first flight and 2027 entry in service. Those likely now slip by at least a year.