Why hypersonics is hard

We’ve heard about advancements in hypersonics – and delays as well – along with concerns about where we stand compared to global competitors. Flight at hypersonic speeds – Mach 5 or above – isn’t exactly new. It’s been studied since the 1950s and tested on the ground and in the air. In NASA’s Hyper-X program, two X-43A uncrewed, 12ft-long test vehicles set airbreathing speed records at Mach 7 and at Mach 10 in 2004.

At NASA Glenn Research Center’s recent Aero Dayz open house, Dr. Robert Baurle, senior scientist for Hypersonics in the Aerospace Systems Directorate of the Air Force Research Laboratory (AFRL), outlined some of the reasons progress in hypersonics has been difficult.

“Hypersonics is complex. If it wasn’t complex, we’d be flying hypersonics a lot sooner than we are now,” Baurle says.

Reusable, crewed hypersonic aircraft have special challenges. It’s difficult to integrate the engine and airframe. “The engine is really part of the aircraft. There’s no siloing the research to try to build a hypersonic aircraft,” Baurle explains. Engines require more research to optimize integration of nozzles and inlets, dual mode ramjets, and turbine cocooning.

More daunting challenges are managing thermal energy with thermally resilient materials and developing advanced cooling systems.

“We have materials that can deal with Mach 5, but what if you want to go to Mach 7, two more Mach increments?” Baurle asks. “It doesn’t sound like a lot, but unfortunately, the energy scales as the velocity squared, so Mach 7 has roughly twice the energy as Mach 5.” And with that energy comes a commensurate increase in heat on the airframe.

Takeoffs and landings add to the challenges. “Things are going to heat up, cool down, heat up, cool down. Structures don’t like that,” Baurle says. That creates a need to design robust, lightweight structures able to withstand extreme aerodynamic forces and thermal conditions without excessive weight.

Baurle says we must build hypersonic research capabilities we don’t have today: “This problem is too big for any one agency to try to do alone. It’s going to take a coordinated national effort.” He adds it will require leveraging research efforts between government agencies and branches of the military. “Industry must work closely with us to make sure we’re doing the right research to enable them to do their part. We’re going to rely on academia to tell us the art of the possible and give us ideas to close gaps we don’t know how to close right now,” Baurle notes.

Hypersonics has another challenge – it requires a lot of capital expenditure for simulation, ground and flight testing, and gathering data. “We really need to keep cost in mind,” Baurle says. “Because if you can solve the technology problems but you can’t afford to buy it, it’s not going to do anybody any good.” – Eric