Humans have been capable of flying faster than the speed of sound since U.S. Air Force Capt. Chuck Yeager, piloting the experimental X-1 rocket plane, broke through the "sound barrier" in the skies over eastern Kern County on Oct. 14, 1947.
But breaking that barrier is a noisy business, creating loud sonic booms that can startle dogs, rattle windows and annoy residents in communities for miles around.
As a result, commercial aviation, with a few notable exceptions, has flown its fleets of airliners and cargo jets below that still mystical threshold.
But all that may change. And ongoing experiments in the Kern County desert could help pave the way.
NASA aims to burst through that noise limitation with the development of the agency’s "quiet" supersonic X-plane, the X-59, which could be ready for flight testing as early as 2022.
"The X-59 is designed to have quiet sonic booms that won’t be disturbing to the people, but first we actually have to go out and prove it," Ed Haering, a sonic boom expert at NASA’s Armstrong Flight Research Center, said in a NASA news release.
"NASA," Haering said, "will do that by flying the aircraft and taking real measurements on the ground before we eventually fly it over communities, to make sure that it is as quiet as it should be."
The testing and measurements could help open the door to future faster-than-sound air travel, NASA said in the release. By deploying a microphone array deployed along a 30-mile area near Armstrong, NASA researchers like Haering can measure the sound produced by sonic booms.
NASA’s X-59 Quiet SuperSonic Technology X-plane, or X-59 QueSST for short, will eventually fly over select communities around the United States to demonstrate its ability to reduce that sonic boom to a quiet "thump," NASA said in the release. The data from these flights will be turned over to the Federal Aviation Administration to possibly establish new sound-based rules for supersonic flight over land.
Should that happen, it could be a game-changer for commercial cargo and passenger air travel.
But before these community overflights take place, the X-59 must first undergo an acoustic validation phase, during which NASA will deploy an approximately 30-mile-long array of specially configured microphones to measure the X-59’s thumps, to verify that they are as quiet as predicted.
The tests this summer — named Carpet Determination In Entirety Measurements flight series, or CarpetDIEM — were the agency's "first practice" for the X-59’s acoustic validation flights, NASA said.
The space agency collected the sound measurements using a microphone array on the ground that covers the entire width where the X-59’s quiet sonic thumps can be heard — a measurement area known as the "carpet." The goal of CarpetDIEM was to practice deploying a large-scale microphone array, and gain valuable lessons on the array's ideal configuration, instrumentation and logistics.
The test microphone array was deployed along an area of the Mojave Desert near NASA Armstrong, from which NASA flew an F/A-18 aircraft to produce sonic booms under the designated corridor, which runs from Nevada toward Edwards Air Force Base.
"We chose this area of the Mojave Desert because it’s a nice, wide area under our high-altitude supersonic corridor, where we are able to fly our F/A-18s at supersonic speeds routinely," Haering said in the release. "Here we can learn how to best deploy a sensor array of this magnitude, and the logistics of getting the hardware out here, using it for testing, and getting it back, and logging the data."
The array featured high-fidelity microphones capable of measuring 50,000 samples per second, giving researchers the ability to obtain accurate sound data and assess the loudness of the sonic booms, just as they will measure the much softer sonic thumps from the X-59.
The flight series included engineers, researchers and managers from NASA Armstrong and NASA’s Langley Research Center in Hampton, Va., as well as participants from the Volpe National Transportation Systems Center in Cambridge, Mass., and graduate students and professors from Brigham Young University in Provo, Utah.
"We have a series of microphones set up, and the various sites have different configurations," Juliet Page, a physical scientist with Volpe, stated in the release.
"We have microphones oriented in different configurations, including inverted, vertical, horizontal, some with different wind screens," she said, "and we're evaluating the acoustic performance and the difference between the different configurations in preparation of the X-59."
"It’s cool to come out in the desert and just do all these measurements and just kind of geek out with this technology," Page said.
Overcoming the technical barriers to practical commercial supersonic flight is the ultimate endgame.
And Haering, who has spent 25 years of his 35-year career at NASA researching sonic booms, may know better than anyone what's at stake.
And as he stood beneath the same historic skies where Yeager flew supersonic for the first time, he suggested it's time to move beyond the limitations sonic booms have placed on air travel.
"It’s been 70 years since Chuck Yeager broke the sound barrier," Haering said.
"Now we’re trying to fix it."