There are encouraging signs that we’re finally picking up the pace to realize the promise of hypersonic flight — flight at speeds in excess of Mach 5, more than 3,000 mph. Private sector companies now are pushing to develop this technology, and it’s an exciting time to be in hypersonics because there’s so much interest, both commercial and military.
It’s about time. In Ronald Reagan’s 1986 State of the Union Address, delivered just one week after the Space Shuttle Challenger disaster, he said,
“So, yes, this Nation remains fully committed to America’s space program. We are going forward with our shuttle flights. We are going forward to build our space station. And we are going forward with research on a new Orient Express that could, by the end of the next decade, take off from Dulles Airport and accelerate up to 25 times the speed of sound, attaining low-earth orbit or flying to Tokyo within two hours.”
The president’s words outlined the importance and potential of hypersonic technology — shortening the paths between distances, enabling intercontinental travel to become a “day trip” scenario, and making access to space quicker, easier and cheaper. These advances are possible because an air-breathing hypersonic vehicle (which doesn’t require a heavy tank of oxidizer) is an ideal first stage for an orbital launch vehicle, or even the basis for a single-stage combined-cycle vehicle.
But in the time since President Reagan made his speech, the Space Shuttle flew 110 additional post-Challenger missions and was retired, and the International Space Station has been completed and has been continuously occupied in orbit for over 20 years. But the latter-day Orient Express — the National Aerospace Plane — has failed to materialize. In fact, we’ve moved backwards in one sense: We had commercial Mach 2 flight in 1986 with the Concorde, and we don’t have that now.
For most of the 35 years since 1986, the goal of crewed hypervelocity flight has seemed scarcely closer than it was then. Several obstacles persist and are the subject of vigorous research, including supersonic fuel-air mixing and combustion. A significant aerodynamic challenge involves the transition from smooth to turbulent flow over the surface of the aircraft, and its effects on heat transfer, control, and the predictability of aerodynamic drag.
At hypersonic speeds, much higher thermal loads — higher by half an order of magnitude or more — result from the increased heat transfer due to turbulent flow. Yet more massive thermal protection systems to safely dissipate the heat from that turbulent flow impose significant mass and efficiency penalties. Thus, measuring; understanding; and, if possible, controlling the path from smooth (laminar) to turbulent flow are critical for hypervelocity vehicle design and operation — and recent computational, experimental and flight efforts have pursued these goals.
My work focuses on bridging the gap between ground-based aerodynamic experiments and the external aerodynamics of hypersonic flight, with complementary emphases on characteristics like instability, turbulence, and the impact of complex structural geometries and surface roughness. These are crucial areas of national priority in hypersonic vehicle design, with potentially revolutionary implications for both military and commercial vehicles. Reliable, sustainable Mach 5+ atmospheric flight will not be achieved without addressing these issues.
Computations are vital — you can measure quantities in a very exact way in a simulation that isn’t possible in an experiment, and you can simulate a situation that is very difficult to create in the laboratory. But we do not have the ability, now or on the horizon, to fully simulate even relatively simple flows over relatively simple shapes, so we need to develop simplifications, approximations and models, and join these efforts to experiments. A mentor of mine, Dr. Ivett Leyva (now at the Department of Defense), put it this way: “One such computation stressed available research computing resources by using more than 30 billion grid points and 102,000 cores to simulate a Mach 2.5 flow over the simplest of surfaces — a flat wall.”
My research is funded by the Air Force Research Laboratory, Sandia National Labs, Lockheed Martin, and Northrop Grumman, as well as smaller businesses, including Spectral Energies, based near Wright-Patterson AFB, Ohio. Purdue recently acquired the Hypersonic Pulse (HYPULSE) shock tunnel from Northrop Grumman, which will allow flight simulations at speeds up to Mach 40. I’m also supporting Purdue professors Steve Schneider and Brandon Chynoweth as they design and develop the first Mach 8 quiet wind tunnel in the world, which will be housed in the same facility as HYPULSE.
There’s a synergy at Purdue between production work and basic research. We believe that we are the perfect home for these large facilities of national importance because we have the people and technologies to capitalize on them. We have the structure in place to execute on test programs, develop new experiments and exploit new science at the requisite level of security.
We are in for an exciting next decade-plus in hypersonics. I’d like to see the future President Reagan envisioned, in which I can take my family and fly to Tokyo (or maybe London, as I spent three years at Oxford as a Rhodes Scholar and have missed visiting during the pandemic) in a couple of hours, have sushi (or a pint) for lunch, and get back by dinnertime. That’s a bit facetious, of course, but I do think we’ll see hypersonic business travel within my lifetime.
The more immediate applications are military. An important part of an optimal future for hypersonic flight, for me, is keeping the United States and our allies at the forefront. We are collaborating with a pair of NATO working groups — of which I am a member, along with several other Purdue faculty colleagues — to maintain supremacy in this area.
Joseph S. Jewell, PhD, PE
School of Aeronautics and Astronautics
College of Engineering, Purdue University