In a small area – 4 inches by 4 inches, to be precise – in a Naval Postgraduate School (NPS) research laboratory, a uniquely-capable facility is supporting foundational hypersonics research and preparing U.S. Navy and Marine Corps officers to advance, integrate, and employ the sea services’ emerging hypersonic capabilities.
Under the leadership of NPS’ Mechanical and Aerospace Engineering (MAE) department and with sponsorship by the Office of Naval Research, recent improvements in the NPS Supersonic Wind Tunnel (SSWT) laboratory have made the facility fully capable of conducting long duration high supersonic testing and experimentation, advancing NPS’ support to the Navy’s Force Design imperatives, as outlined in the Chief of Naval Operations’ (CNO) Navigation Plan (NAVPLAN) 2022.
“Hypersonic systems provide a combination of speed, maneuverability and altitude that enable highly survivable, long-range, rapid defeat of time-critical, heavily-defended and high-value targets,” CNO Adm. Mike Gilday commented during an early 2022 visit to industry partners developing hypersonic technologies. “Delivering hypersonic weapons continues to be one of the Navy’s highest priorities, which the Navigation Plan makes clear.”
“Our research capability is unique among academic institutions in the United States,” said Dr. Garth Hobson, MAE professor and Principal Investigator for the SSWT. “You’d have to go to NASA or the big Air Force research laboratories to match these capabilities. We can run the wind tunnel for half an hour at a time at Mach 4 and that allows us to do very meaningful experimentation.”
This capability, combined with NPS’ ability to conduct highly-classified research and the intellectual and operational capital of its faculty and officers, positions NPS to be a critical enabler in meeting future force requirements in the realm of hypersonics.
In recognition of its role in educating mid-career officers in warfighting applications and applied hypersonics research, on Dec. 8, 2022, NPS was officially welcomed into the University Consortium for Applied Hypersonics (UCAH), a collaborative network of more than 100 universities and 150 industry partners actively working in the field. Membership in UCAH has already borne fruit, with collaborative efforts underway between NPS, the University of Arizona and North Carolina State University initiated through this engagement.
“Membership in UCAH opens up a wide spectrum of opportunities for NPS to engage in basic and applied research essential to helping the United States remain competitive with our adversaries in this challenging discipline,” stressed Dr. Kevin Smith, NPS Vice Provost for Research.
“The national effort in hypersonics will undoubtedly generate advances in many existing and new technologies with applications that can help solve operational problems of warfighting,” he continued. “NPS participation in UCAH can help accelerate the transition of these technologies to operational applications.”
Supersonic vs. Hypersonic
A Mach number is generally understood as the ratio of air speed to the local speed of sound: “Mach 2” refers to twice the speed of sound; “Mach 3” is three times, and so on.
The speed of sound, however, is not a constant. The “local” speed of sound depends on a variety of factors including the altitude, temperature and density of the surrounding air. For example, the speed of sound at sea level at 59 degrees Fahrenheit is 761 miles per hour. At a height of 20,000 meters and minus 70 degrees Fahrenheit, it’s 660 miles per hour. The term “supersonic” refers simply to a speed higher than the speed of sound.
At high speeds around Mach 5, however, things get rather peculiar. The surrounding air molecules break apart and turn into an electrically-charged plasma with the kinetic energy of the aircraft changing to heat, yielding intense variations in air density and pressure that materialize through a series of shock waves and expansions.
This is hypersonic speed.
“Something becomes hypersonic when it’s in air that can no longer be treated as ‘perfect.’ Things start to react and you start to worry about how hot things get,” explained Ben Nikaido, a computational fluid dynamics expert with NASA’s Ames Research Center who is working on his doctorate at NPS with the SSWT team.
“There really is no absolute line in the sand that says everything beyond this speed is hypersonic and below it is supersonic. It’s a large gray area with a lot of overlap,” Nikaido explained. “For example, the air is so thick at sea level that even when you’re flying something at Mach 2 or 3, you can still get hypersonic effects.”
Mitigating such extreme forces and temperatures for air-breathing aircraft or missiles, let alone maintaining command and control, is no small feat. However, breaking through to the hypersonic side presents a host of tactical and strategic advantages, namely through unmatched speed and difficulty to detect and defend.
“While the U.S. Navy currently uses the Aegis Combat System on Arleigh Burke and Ticonderoga-class cruisers to defend ships at sea, high-energy lasers are becoming more important to our layered defensive against rapidly evolving threats,” said Navy Lt. Cmdr. Brian Curran, a Ph.D. candidate in laser physics and executive director of the Meyer Scholar program. “To lead effectively and fight decisively, NPS is working to develop officers who are technologically competent and confident in the employment of advanced naval warfare systems.”
A Mighty Wind
Thanks to a three-year grant by the Office of Naval Research (ONR) Code 35, Naval Air Warfare and Weapons, ONR’s innovative naval prototypes division, Hobson and his team have spent the last year renovating, reconstituting and recalibrating various components of the wind tunnel.
The complex – a decommissioned engine test cell which dates back to 1956 – now consists of three massive pressure vessels of compressed air at several hundred psi. Operation is powered by a powerful electric motor and multiple compressors that pump the high-pressure air into air dryers before converging in a plenum chamber, from which it blasts at Mach 4 into the 4 inch-by-4 inch test section at temperatures near minus 300 degrees Fahrenheit.
Additionally, with a grant provided by the NPS Foundation, the installation of a specialized air heater allows air speeds to be increased through Mach 5.
Over the next year, the team will focus on modeling and investigating the elemental physics of hypersonics, Hobson says. Drawing extensively on Xerox’s first liquid metal printer and a small powder bed metal printer, NPS engineers are able to fabricate a wide variety of components out of different metals, including aluminum and titanium, predict their performance using computational fluid dynamics, and see how they perform under hypersonic conditions.
Crucial to this process has been the work of NPS Aerospace Engineering student U.S. Navy Ensign G. Forrest Dawe, who has developed a method to measure internal conditions to further streamline the newly-upgraded wind tunnel.
The newly-commissioned ensign is attending NPS as a Shoemaker Scholar, meaning he’s on a fast track to earning his master’s degree right after his undergraduate degree at Boston University and before attending Navy flight training in Pensacola, Fla. He aims to become a test pilot, so not only is he intimately involved in the science of hypersonics, he could be a practitioner of hypersonic flight as well.
“During the course of the supersonic wind tunnel upgrade, our team ran simulations on it and predicted that there would be vortex conditions along the sidewall,” he explained. “So my thesis involved using a probe to measure velocity and pressure along the wind tunnel.”
All of this initial research is in preparation for the third year of the ONR grant for experimenting with hypersonic propulsion. Specifically, the team will develop and model a solution to a vexing problem in hypersonic research – an engine “unstart,” which is the violent breakdown of engine inlet airflow at hypersonic speeds.
“In other words, an ‘unstart’ is when a supersonic inlet reverses supersonic air flow within a nanosecond,” Nikaido explained. On the best of days, this results in power loss of the aircraft and a sudden if not violent yaw.
“We do the research to generate novel ways of preventing this from occurring, all the while learning a lot about the physical underpinning of supersonic airflow.” Hobson said. “There are many challenges associated with hypersonic speeds. The value of our research is not only to develop solutions to improving hypersonic flight, but to do it alongside our military students who contribute operational insight while they gain technical understanding to develop effective concepts of operation.”
Such aircraft, and the leaders ready to employ them, are on the horizon with NPS and its SSWT laboratory playing a critical role in hypersonics innovation through graduate education and research.