NASA’s X-59 experimental aircraft is unique in that it is designed to fly faster than the speed of sound, but without a loud sonic boom. To verify the X-59’s ability to fly supersonic and produce only faintly audible “thumps,” NASA needs to be able to record these sounds from the ground. The agency recently completed testing to understand the equipment and procedures needed to create these records.

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

“We’re trying to answer questions like how many people do we need to go out every day and maintain these devices, how do we get data back, how many vehicles are needed — all those things are related to how we operate,” he said. Forrest Carpenter is the principal investigator for the third flight series, known as CarpetDIEM III. “We’re learning to dance now so when we go to the big dances we’ll be ready to go.”

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 Mach 1.4 at altitudes between 40,000 and 53,000 feet . Over three passes, the F-18 performed a special reverse dive maneuver to simulate a soft sonic boom; one of them reached a perceived noise level of 67 decibels, a measure of the perceived noise of the aircraft to an observer on the ground.

“We expect X-59’s sonic boom to be around 75 decibels of perceived loudness,” said Larry Cliatt, subproject manager for Quest’s acoustic validation phase. “It’s much quieter than Concord, which gets over 100 decibels.”

To measure these very quiet sonic booms, ground-based recording systems used during CarpetDIEM flights were calibrated to measure a perceived sound intensity of approximately 50 decibels; This is equivalent to being in a room where the refrigerator is running.

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

“We can’t have 70 different people in each toolbox,” Cliatt said. “We needed to find a way to automate this process.”

In addition to being designed to withstand desert conditions, extreme heat in summer and cold in winter, recording systems are also designed to be resistant to damage caused by wild animals such as rodents, jackals and foxes.

“As we get to Phase 2 of the Quest mission, we plan to make these sonic boom recordings for nine months,” Cliatt said. “We need equipment and operations that will facilitate such a long deployment.”

Another lesson learned: Setup time for recording stations was just under an hour, compared to the expected 2 1/2 hours. Based on system performance, the team will evaluate whether they need to visit all sites on each day of Phase 2 testing.

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 planes to fly outside Edwards AFB airspace to conduct some of the CarpetDIEM testing

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.