NASA’s Quiet SuperSonic Technology Preliminary Design, or QueSST, has brought the agency ever closer to making the Low-Boom Flight Demonstration aircraft, or LBFD, a reality. Decades of NASA research in supersonics have gone into the unique design of NASA’s next X-plane, including numerous efforts under the Commercial Supersonic Technology project, or CST.
These efforts, a number of which are based at NASA’s Armstrong Flight Research Center, have dealt with research in several areas related to supersonic research, including the use of cutting-edge visualization technology to study shockwaves, the use of F-15s to examine methods for improved cruise efficiency, the integration of displays to help pilots monitor the audial effects of supersonic flight and the impacts of the environment on sonic booms. Each area of research goes into realizing the goal of CST and of QueSST, which includes the eventual demonstration of quiet supersonic flight over land.
In April 2016, NASA’s goal of developing a quiet supersonic aircraft took another step closer following a pair of successful first flights in a series demonstrating patent-pending Background Oriented Schlieren using Celestial Objects (BOSCO) technology, effectively using the sun as a background in capturing unique, measurable images of shockwaves.
The tests flown from Armstrong built on other recent NASA tests to further the art of schlieren photography. Schlieren is a technique that can make important invisible flow features visible. Although schlieren has been in use for over a century, recent research by NASA has enabled its application in flight and greatly enhanced the detail of the images that can be obtained. In this case, NASA improved schlieren captured the visual data of shockwaves produced by a U.S. Air Force Test Pilot School’s T-38 aircraft traveling at supersonic speeds.
As a result of the research, the supersonic aircraft and its shockwaves are seen with distinct clarity in front of the solar background. Observing air density changes makes the details clearer.
Visualizing these complex flow patterns of shockwaves produced by a supersonic vehicle will help NASA researchers to validate computational design tools used to develop the LBFD.
In May 2017, NASA also began a series of supersonic flights to examine efforts to improve the cruise efficiency of future supersonic aircraft.
At supersonic speeds, the force of drag that must be overcome is significant. Due to the interaction of flow with the aircraft’s surface, this friction drag contributes about half of the total drag at supersonic speeds. This particular series of flights will explore ways of reducing friction drag and increasing efficiency through new and innovative methods of achieving swept wing laminar flow.
Future supersonic aircraft seeking to achieve a low-boom, such as NASA’s LBFD, will rely on a swept wing design in order to fly at supersonic speeds without producing a loud sonic boom. The swept wing design generally produces crossflow, which is a name for air flow disturbances that runs along the span of the wing, resulting in turbulent flow, increased drag and ultimately higher fuel consumption.
“Swept wings do not have much laminar flow naturally at supersonic speeds, so in order to create a smoother flow over the wing, we put small Distributed Roughness Elements, or DREs, along the leading edge of the wing,” says CST Subproject Manager Brett Pauer. “These DREs can create small disturbances that lead to a greater extent of laminar flow.”
Swept wing laminar flow technology allows NASA to consider wing designs that have low boom characteristics, yet can be more efficient.
The development of advanced tools and instrumentation has also resulted from NASA’s supersonic research. In December 2016, NASA pilots flew with a display that provides guidance to the exact locations where sonic booms were hitting the ground.
This flight series, which marked the second phase of the Cockpit Interactive Sonic Boom Display Avionics project, or CISBoomDA, continued from the project’s first phase, where only a flight test engineer could see the display. With the ability to observe the location of their aircraft’s sonic booms, pilots can improve on keeping the loud percussive sounds from disturbing specific locations or communities on the ground.
“The display is there to minimize the impact of sonic booms on the ground. Sonic booms generally don’t cause damage at higher altitudes, but they can disturb people, and we want to make sure that we are good stewards to the public,” said Pauer. “The use of this software allows pilots to maximize their flight, and still not bother people on the ground, if used properly.”
The display shows the location of sonic booms based on tracking the aircraft’s trajectory and altitude, and is founded on an algorithm designed by Ken Plotkin of Wyle Laboratories, who died in 2015.
The display will ultimately be used to help NASA proceed with supersonic research in a way that minimizes disturbance on the ground and provides detailed guidance information for future of supersonic technology research.
“Flying with the CISBoomDA display was really interesting,” NASA research pilot Nils Larson stated. “It was great to have it in the cockpit, and I think it’s a valuable tool for the future. As a matter of fact, I’ve asked to be allowed to start using the display on my proficiency flights, just so I can keep practicing with it.”
Finally, NASA’s supersonic research, which already spans several NASA centers, extends to other mission directorates within the agency.
In August 2017, NASA’s Kennedy Space Center played host to the second series of Sonic Booms in Atmospheric Turbulence flights, or SonicBAT, continuing from 2016’s successful supersonic research flights flown at Edwards Air Force Base. The project’s second series of flights took place at KSC to be able to study how the region’s humid atmospheric conditions influence sonic booms.
SonicBAT helps NASA researchers better understand how low-altitude atmospheric turbulence affects sonic booms, which are produced when an aircraft flies at supersonic speeds, or faster than the speed of sound. The flight series is a key initiative in validating tools and models that will be used for the development of future quiet supersonic aircraft, which will produce a soft thump in place of the louder sonic boom.
The SonicBAT flights in Florida marked a rare opportunity for NASA’s aeronautics and space operations to comingle, and for Kennedy showcases that center’s transformation into a 21st century multiuser spaceport.
“This shows that, as NASA, we are all striving for the same thing,” said SonicBAT Project Manager Brett Pauer. “We’re willing to work together and help each other in any NASA mission that may be happening, whether it be space-based, which we do a lot of at our aeronautics centers, or the space centers to help us out with aeronautics. I think there’s a great amount of cooperation, even more than may be expected, between NASA centers.”
Peter Coen, CST Project Manager, added. “It seems to me that ‘one NASA’ is the best way to describe the cooperative spirit that makes it possible for teams to reach out across the agency, and receive the kind of support SonicBAT has received from Kennedy Space Center.”
NASA’s supersonic research itself is a multicenter initiative to push the boundaries of aeronautics. The Swept Wing Laminar Flow research conducted at Armstrong resulted from successful wind tunnel testing at Langley Research Center in Virginia. Subscale models of LBFD continue to undergo similar wind tunnel testing at Glenn Research Center in Ohio and Langley Research Center in Virginia. NASA Ames Research Center has been instrumental in assisting with developing advanced schlieren imaging techniques.
The next steps that result from milestones achieved at NASA’s centers throughout the country are sure to be exciting.