A team of scientists at Johns Hopkins Applied Physics Laboratory, supported by the Air Force Research Laboratory's Air Force Office of Scientific Research (AFRL/AFOSR), currently leads a collaborative research and experimentation effort that could aid development of hypersonic systems.

"[Hypersonics] capability is so important [to] DOD's need to deter and defeat the U.S.'s great-power competitors, China and Russia," said Michael E. White, principal director for hypersonics (OUSD(R&E)), in a May 3 DOD press release.

While reports indicate that the Chinese and Russians have developed hypersonics, Heidi Shyu, undersecretary of defense for research and engineering, said "transformational hypersonic strike weapon capabilities are being developed to provide the U.S. with long-range, responsive, survivable, and lethal weapons to defeat adversary targets of strategic importance in a matter of minutes."

Realizing this capability and the need for development is inspiring research into mature, long-lasting hypersonic systems. Hypersonics research studies the physical phenomena surrounding objects when they are moving at Mach 5 (five times the speed of sound) or greater.

Through several basic research programs managed by AFOSR, APL has successfully developed a testing device called Boundary Layer Transition, or BOLT, which will provide much needed data to improve hypersonics.

There's a Future in Hypersonics
In addition, this program is simultaneously enhancing the STEM pathway by using the BOLT project to train students and future hypersonic scientists. University students in graduate programs were involved in flight test and beyond, said Dr. Brad Wheaton, BOLT principal investigator and chief scientist in the force protection sector at APL.

"Those students [we]re not just in a lab studying phenomenology," Wheaton said. "They were facing real-world design problems, schedules, the need to make decisions, the realization that you can't have everything perfect when you're trying to do something hard and fast."

Universities partnering in the effort include University of Minnesota, Purdue University, and Texas A and M University, which along with NASA Langley and CUBRC in Buffalo, N.Y., each provided wind tunnel facilities for testing. This experimentation resulted in 10 entries taken from seven wind tunnels over an 18-month period prior to flight test at Esrange Space Center in Sweden. Multiple tunnels were utilized because they provided different Mach number capabilities, sizes, disturbance/turbulence levels, etc. Because some were university facilities, students involved in the wind tunnel testing could do more over a longer period, allowing them to learn more and providing a boost to hypersonics workforce development, Wheaton said.

"BOLT has kind of forced academia to think of complicated geometries for this problem rather than just simple conical structures, which is what has been studied a lot," said Wheaton, "-and to start to think about how they can design tools that would predict physics on flows that are really complex-flows that we really don't understand very well."

Dr. Sarah Popkin, program officer for high-speed aerodynamics in AFOSR and whose purview includes the BOLT project, highlighted another academic science team involved with BOLT. The Hypersonic Flight in the Turbulent Stratosphere team, or HYFLITS, provided stratospheric weather data at the BOLT flight test. A long, suspected contributor to influencing the nature of boundary layer transition is turbulence in the stratosphere that a vehicle is flying through-and also particulates, like dust.

The University of Colorado at Boulder leads HYFLITS in collaboration with University of Minnesota and Embry-Riddle University as part of an OSD Multidisciplinary Research Program of the University Initiative highly competitive grant, "Integrated Measurement and Modeling Characterization of Stratospheric Turbulence." The AFOSR High-Speed Aerodynamics research area has managed this grant since 2017.

"[Academia's involvement in BOLT] was a paradigm shift in hypersonic flight experiments," Popkin said. "This-especially in a country that is trying to get better and more frequent with hypersonic testing, you need to start training your workforce as soon as possible-so, this allows the Air Force, the U.S., to start building that workforce that we need so much."

BOLT also leveraged contributions from AFRL's Aerospace Systems Directorate and international partners: the Swedish Space Corporation; Germany's national center for aerospace, energy and transportation research (DLR); and Australia's Defence Science and Technology Group (DST) providing the flight computers.

"It's a unique opportunity to combine the capabilities we have with the capabilities they have to do this science experiment," said Wheaton about the collaboration.

This leverage echoed White's comments from a Feb. 24 DOD press release: "We want to make sure that we fully leverage the talent and industrial base not only in the United States, but with our allies, and then offer capabilities that will be compatible with our allies for potential future applications."

Flight Experimentation
In June, the effort culminated in a flight experiment. The project has attempted to better understand the mechanisms driving boundary layer transition in flight.

"BOLT was never just a flight test," Wheaton said. "It was a research project effort. To understand the physics that we're interested in, we have to look at our computational modeling; second, our wind tunnel testing, which we did a lot of and gathered a lot of good experimental data; and then the flight test was the third piece of that triad."

The flight experiment, which had been delayed since May 2020 because of Covid, took place on June 23 at Esrange Space Center, located above the Arctic Circle in Sweden on a vast, unpopulated tract of land with a facility that has been operational since 1966.

Two staged sounding rockets were due to be boosted to an apogee of just over 174 mi. altitude-conducting the first experiment on ascent between Mach 4.7 and Mach 6, and in the final few seconds of descent, between Mach 6.7 and 7.4. The first stage flew straight as expected. However, sometime between when the first rocket left and the second rocket lit, an unexpected coning motion began happening and then corkscrewing, which generated a lot of drag and prevented it from flying as high or as fast as was desired, only reaching supersonic speeds. Post-launch analysis is ongoing between APL, AFRL and DLR, the launch provider.

The Nuts and BOLTs
A boundary layer is the layer of gas or liquid, invisible to the naked eye but seen using optical imaging techniques, that immediately surrounds an object.

Popkin explained that as an object flies through the air at any speed, a so-called boundary layer exists that causes friction and produces heat. But with objects traveling at hypersonic speeds, exceeding Mach 5 (five times the speed of sound), the heat is so high that the material typically used to build aerospace vehicles would melt or burn; so, high-temperature materials must be used instead.

"But to understand exactly which materials can survive on which vehicles in which conditions for what durations, you really need to understand the heating, or heat flux, that is experienced," Popkin said.

She explained that boundary layer transition is a sort of gateway between when air flow over an object is laminar, or smooth and not allowing much heat, and when it is turbulent, or swirling in circles with as much as eight times the heat transfer-a transition that inevitably occurs as air flows over an object.

"Being able to understand when, why, how that happens is the motivation behind any boundary layer transition research," she said. "It comes down to that heating. If you don't understand it, then you basically have to overdesign to make the vehicle's skin maybe twice as thick, which turns into a vehicle that is twice as heavy, or there's not as much internal volume [to] put in more subsystems for whatever that system's mission is, whether it's computers, sensors, warhead."

BOLT was more focused on the phenomenon itself than design, clarified Wheaton.

"While BOLT wasn't designing a particular vehicle or certain response, it's dedicated to understanding the physics of boundary layer transition so that we can develop tools that will predict it better," Wheaton said.

While BOLT was preceded by an earlier boundary layer transition research program called HiFIRE (Hypersonic International Flight Research Experimentation), another program, BOLT II, succeeds BOLT. The research behind BOLT II steers more toward studying turbulence as it relates to boundary layer transition but uses BOLT's mechanical design and team members, such as some of the same graduate students. The BOLT II launch is projected to take place in March or April 2022 at the NASA Wallops Space Station on Wallops Island, Virginia.

White noted in the February press release that it's an exciting time to be in this field, not only to work on cutting-edge technologies, but also to be a part of protecting national security in a big way.

Related Links
Air Force Research Laboratory
Learn about missile defense at SpaceWar.com
All about missiles at SpaceWar.com

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