BEYOND THE BOOM

Through the revival of the X-plane series, NASA has embarked on a quest to create an aircraft that can travel faster than the speed of sound without causing sonic booms associated with supersonic flight.

In April 2018, NASA awarded the aerospace and defense company Lockheed Martin the $247.5 million contract to build the experimental X-plane, which is expected to take flight in 2021. Smaller than today’s medium-sized commercial aircraft, the data-collecting X-plane will be 94-feet long, with a wingspan of 29.5 feet, similar to the popular Boeing 737. According to NASA, when traveling at 55,000 feet it is expected to cruise around speeds of Mach 1.4, which is approximately 940 mph.

Getting a plane to produce a thud as opposed to a thunderous blast broadly means controlling the strength and position of the shock waves produced by the plane as it zooms through the air. Unlike conventional aircraft designs where the shockwaves unite as they expand away from the airplane’s nose and tail, the X-plane is designed with a uniquely-shaped hull, sending those shockwaves away from the aircraft in a way that prevents them from coming together. Instead, the much weaker shockwaves reach the ground separated, which will be heard as a quick series of soft thumps. Today, current regulations, which are based on aircraft speed, ban supersonic flight over land. The sound the X-plane generates, NASA says, should be at 75 perceived-level decibels, or “about as loud as a car door closing,” substantially taming the sound waves.

Designed for a single pilot, the cockpit will be reminiscent of the rear cockpit seat of the T-38 training jet famously used for years by NASA’s astronauts to stay proficient in high-performance aircraft. Upon completion of phase one in 2021, initial test flights will be conducted by Lockheed Martin pilots to ensure the design is safe to fly and meets all of NASA’s safety requirements. NASA pilots will take over the controls in 2022 during phase two.

“A supersonic manned X-plane!” stated Jim Less, NASA Research Pilot and Deputy Chief Pilot for Low- Boom Flight Demonstration. “This is probably going to be a once-in-a-lifetime opportunity for me. We’re all pretty excited.”

Less and his boss, Nils Larson (the other primary NASA research pilot for the X-plane), are sharing their input with NASA engineers and Lockheed Martin when it comes to the design of the cockpit and the development of the simulators they will use for flight training while the X-plane is under construction. “It’s pretty rare in a test pilot’s career that he can be involved in everything from the design phase to the flight phase, and really the whole life of the program,” Less said.

Phase three from 2023 to 2025 involves the community response. Test flights will begin from Armstrong Flight Research Center in California before expanding into four to six cities around the United States. During this phase, all of NASA’s aeronautic research centers will play vital roles in the missions conducted.

How will NASA researchers be able to measure and confirm the velocity of the shockwaves? The answer is in imagery captured using schlieren photography, a technique used in a series of flights in 2016 called Background Oriented Schlieren using Celestial Objects, or BOSCO. 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’s image on the camera.

The X-plane will be flying at higher altitudes so in order for shockwave data to be captured in 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 X-plane 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.”

The BOSCO II flights were flown using a 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 X-plane takes flight.

“There are so many people at NASA who have put in their very best efforts to get us to this point,” said Jaiwon Shin, NASA’s Associate Administrator for Aeronautics. “Thanks to their work so far and the work to come, we will be able to use this X-plane to generate the scientifically collected community response data critical to changing the current rules and transforming aviation!”

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