NASA X-59 milestone pushes quiet supersonic rulemaking closer
The X-59 of NASA reached the peak of its operational performance during a test flight on June 12 altitude of 55,000 feet and Mach 1.4. At face value, this is just one more ordinary test event. But in a bigger picture, it is a key point in a much bigger US aerospace program devoted to noise measurements, community response data collection, and, in the end, a case for future regulations allowing overland civil supersonic flight.
The special feature of the particular flight is pretty straightforward NASA plans to use the X-59 at these conditions during its upcoming “community overflights,” which the agency calls the main goal of the demonstrator. These flights are expected to yield the data NASA is going to present to the Federal Aviation Administration in order to try to change decades-long ban on overland supersonic civilian operations in the United States. Thus, the test itself is not really about reaching a barrier and proving something. The real deal is making progress through the testing campaign in order to make sure that the aircraft is ready for a particular campaign it is intended to conduct.
And there is plenty of work to do yet. According to NASA, much more months of flight tests lie ahead until the first community overflights begin. The test campaign is still progressing with the X-59 being tested at various altitudes and under different conditions. It is an integral part of flight test discipline: before conducting a test campaign aimed to draw conclusions about some bigger issues, one has to make sure the aircraft works fine at all possible conditions.
But more importantly, the next stage of the test program is “acoustic validation.” NASA states that extensive evaluations of the acoustic signature of the X-59 during its supersonic operations will be conducted in order to make sure everything works fine. And it is absolutely critical for the credibility of the whole program. Before the aircraft could be flown over communities and collect data about the people’s responses, the noise footprint has to be studied in depth.
X-59 is a quiet supersonic demonstrator aircraft created by NASA and Lockheed Martin. NASA states that the X-59 uses the design features, which make the size of the acoustic footprint generated by the aircraft much smaller comparing to other supersonic aircrafts. The agency describes the expected effect as “quiet thump” and suggests that people would perceive it similar to slamming a car door rather than the usual destructive sonic boom.
And this is exactly the idea behind the Quiet Supersonic Technology mission. The technical challenge is not just to design and create an aircraft able to exceed the speed of sound. It is the challenge of creating an aircraft capable of generating the entirely new acoustic effect and then testing if it works. In other words, the aircraft, the test campaign, and the eventual surveys all focus on one technical problem: could a supersonic aircraft produce the noise signature considered acceptable?
When the community overflights take place, NASA is planning to conduct the surveys among the residents to learn how they perceive the noise. It is an essential part of the test program because it connects aerodynamics and acoustics with the human response, which is what the regulators need. The measured pressure signature is essential but for the regulatory purposes, it is just a part of the story. The agency is trying to form the comprehensive evidential base connecting aircraft performance, sound measurements, and public response.
Moreover, there is an aspect related to the entire US aerospace industry here. As NASA stated, the data from the X-59 is needed to establish the acceptable noise thresholds for the future overland supersonic flights, and also help develop design tools and technologies for the future quiet supersonic aircraft designs. Though the demonstrator itself is not a passenger airliner, the test campaign is directly relevant to the future design meeting the regulations.
But for now, the issue is much narrower and more technical. The latest X-59 test did not settle the policy question and did not remove the necessity of further testing for several months ahead. But what it does, is bringing NASA close to the sequence, which really counts: envelope expansion, aircraft performance validation, community overflights, public response data gathering, and giving the regulators something more than theory. From the flight test point of view, this is how the headline test flight becomes the foundation for the regulation.
David Whitaker is the associate editor for AMI’s aerospace and drone systems desk, translating flight systems, aircraft programs, spaceflight, and UAV developments into technical stories.
