NASA Flights Advance Celestial Schlieren Imagery for Supersonic Aircraft

When NASA's next X-plane takes to the skies, it will produce some pretty cool images.

Thanks to the completion of a recent flight test series at NASA's Armstrong Flight Research Center in California, the agency is a step closer to being able to visually capture the shockwaves of NASA's future Low Boom Flight Demonstration aircraft, or LBFD.

The LBFD will demonstrate the ability to fly at speeds beyond Mach 1 without creating the loud, disruptive sonic boom typically associated with supersonic flight. When this happens, around 2022, imagery to confirm that the future X-plane's shockwaves match NASA's predictions will need to be captured using a technique called schlieren photography.

The technique was used in a series of flights in 2016 called Background Oriented Schlieren using Celestial Objects, or BOSCO, at NASA Armstrong. BOSCO validated the use of a special hydrogen alpha filter, and positioned cameras to use the sun as a background, to visualize shockwaves from supersonic aircraft eclipsing the sun 40,000 feet from the camera. Placing the cameras on the ground enabled the use of full-sized telescopes, which were used to maximize the size of the sun image on the camera.

However, LBFD will be flying at higher altitudes around 60,000 feet, and in order for shockwave data to be captured at a high quality, images will need to be taken at closer range, by equipment onboard a chase aircraft. This means the photography equipment will need to be small enough to fit in a small wing pod, but still have the ability to take high-quality images of shockwaves.

The recently-completed second phase of BOSCO flights, or BOSCO II, accomplished just that.

In addition to validating the quality of smaller equipment, BOSCO II successfully applied this photography method from a range of 10,000 feet, similar to the range needed for an air-to-air system when LBFD flies, according to BOSCO II Principal Investigator Mike Hill.

"The main objective here was to see what the image looks like at close range, including what kind of shockwave structure we can make out," Hill stated.

"We needed to use our new compact camera system in order to get an idea of the quality of the images of those shockwaves using a smaller system."

Whenever an aircraft flies supersonic, or faster than the speed of sound, it produces shockwaves that we eventually hear on the ground as a loud sonic boom. This is the driving factor behind the Federal Aviation Administration's restriction on supersonic flight over land. NASA, which has conducted decades of supersonic flight research, has worked with Lockheed Martin to complete an initial aircraft design called Quiet Supersonic Technology, or QueSST, which features a mitigation of those shockwaves to sound more like a quiet thump.

NASA intends to demonstrate quieter supersonic flight through the LBFD, and should the quiet thump of the shockwaves prove to be within acceptable limits to the FAA and communities on the ground, according to predicted sound levels, it may open the future to supersonic flight over land on a commercial level, potentially cutting flight times in half.

While NASA has used computational fluid dynamics to predict how those quieter shockwaves will travel through the air, validating these predictions will require researchers to visually observe the shockwaves through schlieren imagery.

"There are different concentrations of hydrogen atoms caused by varying magnetic fields on the sun's surface, and where there's a higher concentration of hydrogen atoms, we see more light, while lower concentration shows less light. The hydrogen alpha filter works by allowing only the wavelength of light, emitted by hydrogen on the sun's surface, through," Hill explained.

"This is what gives the sun's surface the granular texture we need to be able to get these images."

The BOSCO II flights were flown using an U.S. Air Force Test Pilot School T-38 aircraft, as well as a NASA F-15. In order for accurate images to be captured, pilots had to be in a precise location at a low altitude of 10,000 feet, directly between the cameras on the ground and the sun, and all while flying faster than Mach 1.

"This wasn't an easy task for our pilots, but they hit the mark," Commercial Supersonic Technology Sub-project Manager Brett Pauer noted.

"In the first series of BOSCO flights, we were trying to hit a spot that was about 300 feet in diameter. For these flights, however, since we had to shoot at a closer range, we needed to hit a spot that is one quarter of that. We're talking about a spot in the sky that's under 100 feet in diameter."

Now that flight tests have confirmed the quality of the images taken on a smaller photography system, and provided insight into how to optimally operate these imaging systems at close range, flightworthy hardware can now be developed and integrated into a high-speed NASA chase aircraft to be able to capture similar images when LBFD takes flight.

BOSCO was flown under NASA's Commercial Supersonic Technology project, which operates under the Aeronautics Research Mission Directorate.

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