It’s a bird...It’s a plane...It’s hypersonic flight
After the moon landing in 1969, scientists next set their sights on Mars, but the rising costs of the Vietnam War stalled the Apollo program’s future and put the idea on hold for decades.
Now, fifty years later, scientists are once again trying to get humans to Mars and are also developing super-fast spy planes and designing aircrafts that could cross oceans in mere hours. Those goals all call for travel more than five times faster than the speed of sound, called hypersonic flight.
Reaching those speeds means overcoming metal-melting heat and turbulent air, according to Javier Urzay, a senior research engineer at the Center for Turbulence Research. The center was originally created as a consortium between Stanford and NASA Ames Research Center more than 30 years ago for predicting turbulent flows, and is directed by Parviz Moin, a professor of mechanical engineering. In an office adorned with white boards crammed full of mathematical equations, posters of rocket ships and a model of NASA’s X-43 hypersonic aircraft, Urzay puzzles over basic questions that would allow us to command travel at hypersonic speeds.
“If in the future we are going to go to another planet… there is always going to be a stage of the trip that will involve hypersonics,” said Urzay. “Always.”
Astronauts on the moon-orbiting Apollo 10 mission hold the current record for fastest human travel, free falling into Earth’s atmosphere at nearly 25,000 mph — more than 30 times faster than the speed of sound. Closer to our planet’s surface, NASA’s X-43 unmanned hypersonic aircraft earned the trophy for fastest powered aircraft on record, briefly flying at over 7,300 mph — 9.6 times faster than sound — before crashing into the ocean.
But for humans to master flying at such extreme speeds — and bring it to consumers — we need a better handle on the basic physics of hypersonics.
Turbulence and propulsion
Part of Urzay’s work focuses on how air flows around an object that is traveling at hypersonic speed. He is looking for better ways of predicting how turbulent air in Earth’s atmosphere will flow around aircraft, and how they can be built to withstand those forces.
Predicting these flows, however, currently takes immense computational power. Urzay explained that turbulent air flows are complex at many scales much like a view of the Grand Canyon. The canyon is carved out in large basins — but part of its beauty is in smaller details, such as rock colors. He has been developing ways of simulating both the large and small scales of hypersonic turbulent air flow with less computational power, an advance that could hasten our understanding of — and ability to achieve — hypersonic flight.
But while it’s useful to understand what happens at hypersonic speeds, another part of the problem is getting these aircrafts moving that fast in the first place.
“How can we put an aircraft with an engine and have it accelerate from zero to hypersonic speeds and then land?” Urzay said. “Those are very different capabilities.”
Hypersonic aircrafts will likely need two types of engines, and switch between them during flight, Urzay said. One would be a traditional jet engine, which only works with air flowing at slower speeds. At higher speeds, the way the air flows in and around a jet engine changes, and would prevent it from generating thrust.
Instead, hypersonic speeds require what is called a scramjet — a relatively simple box that relies on supersonic air flowing through the engine. The idea is that the traditional engine would get the aircraft up to a certain speed, and then the scramjet would take over.
To date, the only manned, winged, and powered aircraft that has ever achieved flight speeds entering the hypersonic range was the X-15 research rocket plane developed and tested in the 1960s. The X-15 was crewed by just one pilot and had to be deployed from a carrier aircraft in flight. More recently, NASA’s unmanned X-43 aircraft was boosted by a rocket and its scramjet engine worked for only about 10 seconds.
Urzay is hoping his work on the fundamental physics can help make scramjet engines more effective and get these aircrafts flying under their own power.
Inspiring the next generation
Urzay thinks the ebbs and flows in funding for hypersonic research may have hindered educating the new generations. Researchers in the 1960s were more proficient thanks to the steady stream of funding into programs such as Apollo. Scientists who became interested in hypersonics much later weren’t privy to the same training, because many labs were dismantled and graduate courses cancelled.
“New generations, including myself, we need to reeducate ourselves just to do these things,” he said.
With the new surge of interest, Urzay decided to reach out to the next generation of scientists. He began a new class at Stanford in Spring 2018: ME 356, Hypersonic Aerothermodynamics. He took students through the history of the field to better understand the state of today’s hypersonic research and taught them the basics of hypersonic flows — what are they and what are their physical characteristics? Students also acquired a sense of where today’s technology stands and where it needs to go next to explore the expanses of space.
Urzay said he wants to foster his students’ interest in problems that he thinks will “transcend our generation.” He recalled that his students were enthralled by basic questions that would make travel at hypersonic speeds — including into space — a reality.
“There is always an urge inside of us that pushes us to look above us and say, ‘it is all unknown up there in space, and we must keep exploring it in our quest for answers to our own existence,’” Urzay said.