Ground tests of NASA’s X-59 aircraft show promising advances in recording silent sonic bursts, paving the way for quieter supersonic travel. NASA’s X-59 experimental aircraft is unique in that it is designed to fly faster than the speed of sound without making loud sonic booms.

To verify the X-59’s ability to fly at supersonic speeds while emitting only soft sonic “thumps,” NASA needs to be able to record these sounds from the ground. The agency recently completed testing to understand the equipment and procedures required for such recordings.

Revolutionary terrestrial recording systems

NASA’s Carpet Identification in All Measurements (CarpetDIEM) flights investigated the quality and reliability of next-generation ground-based recording systems for X-59 tests, focusing on how to deploy the systems and retrieve the data they collect. In total, researchers set up 10 microphone stations in a 30-mile stretch of desert near NASA’s NASA Flight Research Center. Armstrong in Edwards, California.

Preparation and testing stages

“Questions such as how many people are needed for the daily maintenance of these devices, how do we get the data, how many vehicles are needed, are all related to our business.” Forrest Carpenter is the principal investigator for the third series of flights, known as CarpetDIEM III. “Now it’s like we’re learning how to dance so that when it comes to the big dance, we’re ready for it.”

The X-59 itself is not yet flying, so CarpetDIEM III tests using NASA Armstrong’s F-15s and F-18s included 20 supersonic flights at speeds of Mach 1.15 to 1.4 at altitudes of 40,000 to 53,000 feet. During three flights, the F-18 performed a special reverse dive maneuver to simulate a silent sonic boom; one of them reached 67 decibels, a measure of the noise a jet plane makes to an observer on the ground.

Sonic Boom Volume Down

“We expect the X-59’s sonic boom to be no more than 75 decibels in volume,” said Larry Cliatt, subproject manager for Quest’s acoustic validation phase. “It’s much quieter than Concorde, which was over 100 decibels.”

To measure these remarkably quiet sonic booms, ground-based recording systems used during CarpetDIEM flights were calibrated to measure sound levels of approximately 50 decibels (equivalent to being in a room with a refrigerator running).

CarpetDIEM III also validated the use of Automatic Dependent Tracking Broadcast, an existing technology used on all commercial and most private aircraft to transmit speed and position data. This system triggers location recording systems to start recording.

“We can’t have 70 different people at every fixture,” Cliatt said. “We needed to find a way to automate this process.”

Durability and efficiency

Recording systems are designed to withstand desert conditions, extreme summer heat and winter cold, as well as resist damage from wild animals such as rodents, coyotes and foxes.

“As we move into phase two of the Quest mission, we plan to make these sonic boom recordings for nine months,” Cliatt said. “We need to have the tools and operations that can facilitate such a long deployment.”

Another lesson learned was that recording station setup time was just under an hour compared to the expected 2 and a half hours. Depending on the performance of the systems, the team will evaluate whether they need to visit all sites daily in the second phase of testing.

Cooperation and coordination

The team also learned the coordination and documentation processes required for such studies, both with internal organizations such as NASA’s Armstrong Environmental and Safety Offices and with external parties, including:

• U.S. Bureau of Land Management, which allows public lands to be used for testing purposes
• Law enforcement officers helping protect the testing area
• Federal Aviation Administration allowed NASA aircraft to fly outside Edwards AFB airspace to conduct some of the CarpetDIEM testing

Look ahead

To prepare for the second phase of Quesst, the researchers plan to hold a hands-on session in 2024, taking into account all lessons learned and best practices from the three phases of CarpetDIEM.