NASA Tests Techniques For X-59 Low-Boom Flights
Validating the quiet supersonic performance of NASA’s X-59 QueSST low-boom flight demonstrator will require precise measurement of the shock waves generated by the aircraft and how they propagate through the atmosphere to the ground.
To that end, the agency has flight tested a visual navigation system that will enable a chase aircraft to fly in exact positions relative to the X-59 when both aircraft are traveling at supersonic speed.
The Airborne Location Integrating Geospatial Navigation System (ALIGNS) will be used during acoustic validation testing of the X-59 at NASA Armstrong Flight Research Center in California ahead of community overflight tests starting in 2024 to survey the public acceptance of reduced sonic booms.
A shock-sensing pressure probe attached to the nose of a NASA Boeing F-15 will be the primary means of measuring the X-59’s shock waves. To take accurate measurements, the F-15 will have to fly in and out of the X-plane’s shock waves at precise locations and distances.
“ALIGNS is software that can predict or calculate a specific point on the shock wave of another aircraft. The team can input parameters into the software so that any location on the shock wave can be chosen,” NASA told Aerospace DAILY. “The pilots get directional cues on the ALIGNS display to help them fly the chase plane to that specific point in space. The point is calculated based on a number of parameters, including Mach, altitude, wind, temperature and separation distance.”
For the initial test flights, using a NASA Boeing F/A-18 as the target aircraft, the ALIGNS tablet was used to show the F-15 pilots where to steer, displaying the horizontal, vertical, fore and aft offsets in relation to a point on the F/A-18’s shock wave.
ALIGNS will also be used to enable the X-59’s shock wave pattern to be visualized using a technique called airborne background-oriented schlieren imaging. This requires the F-15 to be positioned in a precise location that places the X-59 directly in front of the Sun when viewed from the chase aircraft.
A special camera in the F-15 will then capture the distortion of the Sun’s image caused by the changes in refractive index resulting from the density changes in the air compressed by the shock waves. This will determine whether the X-59’s shock wave pattern matches predictions.
On the ground, a high-fidelity recording system will gather time, waveform and spectral data related to the X-59’s shock wave signature. Developed by California-based Crystal Instruments, the system will distinguish the low-amplitude thump from other ambient sounds.
The remotely operated system will be deployed first for field testing at Armstrong and expanded to as many as 70 ground recording stations when the X-59 begins its acoustic validation flying at Edwards AFB, California. This will be expanded further to 175 units when the X-plane begins flying over U.S. communities.
The X-59 is designed, through careful shaping, to prevent the shock waves from the airframe coalescing as they propagate down through the atmosphere into powerful bow and tail shocks that produce a loud, N-wave “double-bang” sonic boom.
Instead, the X-plane is intended to produce a sinusoidal sonic thump of about 75 PLdB compared with the Concorde’s 110-PLdB boom. After acoustic validation tests at Edwards, NASA plans a series of community overflights across the U.S. to survey public acceptance of the reduced boom.
The objective of the program is to provide data to the FAA and International Civil Aviation Organization (ICAO) to help enable them to set a noise certification standard that would allow the prohibition on civil supersonic flight over land to be lifted.
NASA in May awarded environmental and transportation planning consultants Harris Miller Miller & Hanson a contract, potentially worth $29 million over eight years, to support the planning, execution and documentation of the community testing phase of the low-boom flight demonstration.