The weird tool that will tell NASA if the X-59 supersonic jet is truly silent

As Lockheed Martin prepares for the first flight of the X-59, its silent supersonic jet, NASA scientists are finalizing tests to measure the real impact of the aircraft’s aerodynamic shape on its sonic shockwaves. NASA will measure the air vibrations caused by the jet’s sonic boom by surveying residents in areas where the X-59 passes overhead, but it will also need to measure it through scientific means.

These shockwaves will be so small that the American aerospace agency had to reinvent the shockwave-sensitive probes that it previously used to measure sonic booms. Mike Frederick, the lead researcher on the project at NASA’s Armstrong Flight Research Center, tells me in an email interview that these shock-sensing probes were “adapted from existing designs to meet the unique requirements of capturing the X-59’s quieter sonic boom.”

The probes will be mounted on booms placed on the nose tips of F-15B chase planes, measuring the pressure variations in the atmosphere caused by the X-59 in flight as the aircraft generates its sonic booms. Shaped like a long tube with a cone at its end, they contain five ports that measure pressure changes. One port is located at the cone’s tip, and the other four are distributed around its circumference. These sensors detect pressure changes, recording thousands of samples per second to calculate the intensity, duration, and propagation of the shockwaves generated by a supersonic aircraft. 

For the X-59, the probe will also compare the data collected during actual flight tests with predictions from mathematical simulations. Before starting these tests, NASA will conduct measurements to establish fundamental truth—the empirical baseline data that will validate future measurements with the X-59. These measurements will be conducted using an F-15 flying supersonic followed by the F-15B equipped with the new probe to measure the shockwave of the X-59.

A probe designed to measure the almost imperceptible

According to Frederick, “the legacy design relied on long lengths of pneumatic tubing. That tubing tends to attenuate the shock features to a certain extent.” The new ones use high sample rate transducers—a device that transforms one form of energy into another, like a microphone turns your voice into an electric signal—and very short lengths of tubing that do not suffer from the same amount of attenuation as the shockwave travels through them to reach the sensors.

This design is able to resolve very weak shocks more effectively, he says. The small amount of tubing in the probe still has a slight effect on the measured shocks, but it’s thought to be negligible. In a perfect world, Frederick says, they would mount the pressure transducer flush with the cone surface to completely eliminate any attenuation, but the probe isn’t large enough in diameter to accommodate that design.

The current probes also include a heating system to maintain a constant temperature during flight. Frederick tells me that this temperature stabilization is crucial to get an accurate reading. The probe’s pressure transducers have a piezoresistive sensing element, which is a type of sensor that changes its electrical resistance in response to applied pressure or mechanical stress. “The output of this sensing element changes with temperature for a given pressure,” he says, “so, to ensure consistent output for a given pressure, we need to keep the pressure sensors at a constant temperature.” 

This is complex because the probe temperature is affected by the temperature of the atmosphere and the aerodynamic heating of the probe, which occurs when flying supersonic because of extreme air friction. “Without a thermally controlled environment of the sensors,” Frederick points out, “we’d have an unacceptable amount of variation in the pressure measurements due solely to temperature changes.” According to him, this thermally stable environment has been their biggest advancement. “We had to implement some calibration tricks to ensure the very small pressure range met the pressure resolution requirements for X-59.”

The probe design has also been optimized into two versions for different uses. The first version measures shockwaves near their source, at an altitude of approximately 55,000 feet, flying directly behind the X-59. “The near-field probe provides the primary measurement,” he says. This measurement will be compared with the results from NASA’s computational fluid dynamics simulations of the near-field shock signature. The mid-field probe will fly very close to the ground and will capture a secondary measurement that will be compared to the simulations of how the shockwave travels through the atmosphere down to the ground. This two-pronged approach will give a complete picture of how the X-59’s boom generates and propagates but also will improve future simulations with its empirical data.

A new era of commercial supersonic flight

The X-59 is designed and built by Lockheed Martin Skunk Works to demonstrate that supersonic flights over land are feasible without causing discomfort to humans. This experimental aircraft will test a groundbreaking aerodynamic design, reducing costs by using components reused from other aircraft. According to Dave Richardson, director of the X-59 program at Lockheed Martin, “[Reducing the sonic boom] is not based on exotic materials or revolutionary technologies but simply on the shape of the aircraft.” This shape was developed from work NASA conducted in the 1960s with wind tunnel tests.

The X-59’s engine, a General Electric F414-GE-100, generates 22,000 pounds of thrust and is the same used in the U.S. Navy’s F/A-18 Super Hornet, although modified for this aircraft. With this engine, the X-59 can cruise at a speed of Mach 1.42 (approximately 940 mph) at an altitude of 55,000 feet. Additionally, its cockpit lacks a windshield, allowing for a cleaner aerodynamic profile. Instead, pilots use an external vision system, with cameras projecting the surroundings onto screens inside the cockpit—a design that has already received approval from the Federal Aviation Administration (FAA).

The X-59 also stands out for significantly reducing development times and costs thanks to supercomputing and digital modeling, optimizing the design process by avoiding numerous wind tunnel tests. Richardson explains that “[Digital models allow predicting the propagation of shockwaves] from the aircraft to the ground, something that in the past would have required countless wind tunnel tests at prohibitive costs.”

After successfully completing engine tests in early November, the team is now preparing for the final steps before the X-59’s first flight, scheduled for early 2025. Paul Dees, deputy propulsion lead for the X-59 at NASA, notes, “The exact date of the first flight will depend on the success of each test, but we are confident it will be soon.”

Once airborne, the X-59 will fly over U.S. communities, where surveys of residents and sound samples collected with ground microphones will be used to measure both the subjective reactions of humans and the objective variations in sound levels. In addition to the scientific measurements from this instrument, these data will serve as a basis for regulatory decisions by national and international agencies, aiming to lift the current ban on commercial supersonic flights over land. 

If successful, this weird airplane design will be the basic blueprint for the airlines of the future. Private companies will be able to use this public research to build their own designs, following the path open by the thunderous Concorde, but this time tiptoeing at supersonic speed.