Perhaps not at supersonic speeds, but NASA is certainly moving closer to building its piloted experimental aircraft capable of breaking the sound barrier without producing sonic booms. Short for Quiet Supersonic Technology, the first large scale X-59 QueSST aircraft has been cleared for final assembly and integration of systems, the agency announced on December 16, 2019.
The decision to clear NASA’s first large scale, piloted X-plane (or experimental aircraft) in more than three decades, has been made following a major project review, known as Key Decision Point-D (KDP-D), by senior managers of the program.
It was the last hurdle in the X-59 development and construction program before it seeks approval for first flight, currently scheduled in 2021, which the officials will decide upon when they meet again in late 2020.
“With the completion of KDP-D we’ve shown the project is on schedule, it’s well planned and on track,” said Bob Pearce, NASA’s associate administrator for Aeronautics, in an official press release. “We have everything in place to continue this historic research mission for the nation’s air-traveling public.”
Our @NASAAero X-59 was cleared for final assembly & integration of its systems. The X-59 is designed to fly faster than sound without loud sonic booms. Data from the flights will help establish new rules for commercial supersonic air travel over land. https://t.co/NdLZau5ffC pic.twitter.com/MJZWKcEsBx— NASA (@NASA) December 17, 2019
Work on the X-59 QueSST, which began on April 2, 2018, is continuing at the Lockheed Martin Aeronautics’ Skunk Works factory in Palmdale, California (U.S.), and is scheduled to run through December 31, 2021.
According to NASA, three major work areas are actively set up for constructing the aircraft’s main fuselage, wing and empennage. Final assembly and integration of the airplane’s systems is targeted for late 2020.
The idea behind X-59 QueSST
Back in April 2018, Lockheed Martin Aeronautics Company was awarded a $247.5 million cost-plus-incentive-fee contract by NASA to design, build and test a quieter supersonic aircraft.
The X-59 is designed to fly at a maximum speed of Mach 1.5 (990 mph) at a cruising altitude of 55,000 feet and create a sound about “as loud as a car door closing”: that of 75 Perceived Level decibels (PLdB). In fact, NASA wants to eventually beat its own projections and achieve 70 PLdB. For comparison, the Concorde was capable of reaching Mach 2.04 at 60,000 feet with a sonic boom noise level of 105 PLdB.
“Our airplane should be way below the threshold that people are bothered by, or they may not even hear it,” said David Richwine, NASA deputy project manager for technology on QueSST, as quoted by the Air & Space Magazine.
The technology behind X-59 QueSST is based on decades of research into supersonic flight and design engineering. By carefully designing the aircraft’s shape and overall configuration, engineers believe they have found a way to manipulate the shockwaves coming off an airplane flying at supersonic speeds so they do not produce such intense sonic booms.
As an aircraft moves through the air, it creates pressure waves, which ripple out in all directions. At speeds faster than sound, the forward-propagating waves stack up and get pushed out in a conical wake. The sonic boom is the sound made by the sudden change in pressure created by those coalescing shock waves.
The idea behind QueSST is to minimize one of the two major contributors to sonic boom: volume and lift. Volume can be distributed along a longer fuselage so that the boom forms more gradually – the long and dart-like shape of the X-59 and other supersonic aircraft reflects that.
The shock wave generated by the elongated X-59 is also elongated, rising more slowly, and thus exerting less pressure on the eardrum, which means the sound is more muted than the booms generated by other supersonic aircraft.
“The shape is driving every other detail of the airplane,” Peter Iosifidis, Low-boom Flight Demonstrator (LBFD) program manager, told Air & Space Magazine.
Low Boom Flight Demonstrator: the project and the mission
Oversight of the development and construction of the X-59 supersonic aircraft falls under the Low Boom Flight Demonstrator (LBFD) project, which is part of NASA’s Integrated Aviation Systems Program (IASP).
Not to get lost in the myriad of acronyms, simply put, the project is aimed at proving the X-59’s ability to create a sonic “thump” instead of a disruptive sonic boom. And in order to prove that the X-59 QueSST technology is truly capable of producing its unique low booms, NASA will conduct two phases of testing.
During the first phase, the X-59’s capabilities will be tested through a series of flights that “expand” its envelope of operations – moving from subsonic speeds to eventual supersonic flight. “Phase 1” will be conducted within the R-2508 Complex at Edwards Air Force Base in California.
The second phase of testing is divided into three parts. “Phase 2A” or acoustic validation will involve testing the acoustic characteristics of the X-59 – the level of sonic “thumps” produced by the aircraft and whether they match NASA engineers’ predictions.
Data will be collected using the agency’s F-15s equipped with sensors flying close to the X-59 (so called “near- and mid-field” probing). Balloons will also be used to collect meteorological data so that researchers can identify the impact of weather conditions on the sonic thumps.
The subsequent “Phase 2B” will continue with acoustic validation (“far-field” and ground measurements), but this time, data collection will move to ground-based sensors and microphone arrays, and a data-collecting TG-14 motorglider that will be positioned farther away from the X-59.
The final, “Phase 2C” will utilize the so called “schlieren imaging” from the air and the ground to capture visual representations of the X-59’s air flow and noise signature.
Once the X-59’s readiness is proven by these flight trials, the experimental aircraft will then be used as part of the Low-Boom Flight Demonstration mission. The mission will involve conducting flight campaigns over a number of U.S. communities to generate data from sensors and people on the ground.
NASA’s researchers hope the community response data will assist regulators with developing a noise-based standard to enable commercial supersonic air travel over land, thus paving the way for future civil supersonic airliners.
Despite the outcome of the mission, one thing that NASA still needs to think through, however, is that commercial supersonic aircraft will likely face a substantial environmental backlash.