Aussie research team to help unpick secrets of hypersonic flight

A hypersonics experiment performed by UNSW Canberra has been selected as a test case for a NASA workshop in the US next year, which will help inform the design of vehicles that can travel at hypersonic speeds.

The workshop will involve two high-speed test cases, UNSW Canberra’s Hypersonic Multibody Aeroelastic eXperiment (HyMAX), and the US Air Force Research Lab’s RC-19 case. The UNSW research is funded by the Australian Research Council (ARC) and the US Air Force. 

What is hypersonic travel, and why is it so hard?

According to the project’s experiment lead, Andrew Neely, hypersonic flight means flight at (roughly) five times the speed of sound (Mach 5) and beyond. 

That’s in contrast to a traditional airliner (like, say, a Boeing 737), which tends to fly at around 80% the speed of sound. Supersonic aircraft, like the now defunct Concorde and some military jets, involve travelling at up to twice the speed of sound (otherwise known as Mach 2). Those are the planes that produce an epic “sonic boom” the moment they exceed the speed of sound.

But hypersonic travel is (obviously) much faster, and introduces a whole new range of tricky engineering problems.

“One of the key differences,” Neely says, “is that you generate so much friction between the air and the vehicle, because the air is ‘sticky’ where it touches the vehicle.” This is one form of fluid-structure interaction – air being the fluid, the aircraft being the structure.

Neely explains it by imagining the air-aircraft interface from the point of view of the aircraft.

“If you imagine the air flowing towards you at five times the speed of sound, it gets brought to a halt where it touches the vehicle. What you’re doing, then, is you’re actually converting kinetic energy into heating up the airflow locally. And a lot of that thermal energy then gets dumped back into the structure of the aircraft and it heats up the aircraft.”

That intense heating effect needs to be mitigated with the right materials – you certainly can’t use traditional aluminium, as in many traditional aircraft. 

There’s another complicating factor, too. Neely says a structure that could travel at hypersonic speeds would need to balance robustness with being lightweight enough to fly.

“For any aircraft, the heavier you make the structure the less payload you can carry, or the less range and manoeuvrability.”

And the imperative to make the aircraft robust enough to manage the aerodynamic load of flight is compounded by the heating problem.

“Not only do you have those aerodynamic loads, you’ve now also got these thermal loads. What that means is that often, if you build a structure, especially out of metallics, those metallics lose strength and stiffness as they get hot, and so they’ll bend even more and expand as well. 

“So, this becomes a huge challenge when you’re designing hypersonic vehicles.”

Then there’s the impact of increasing and decreasing the load on the aircraft – by taking off and landing and taking off again over its life-cycle, as well as higher-frequency vibrations caused by the fluid-structure interaction (FSI). Here, the aerodynamic loads can couple with and reinforce the structural deformations they drive, and result in oscillations that can damage the vehicle.

“If you ask someone to break a paperclip, it’s very hard to snap a paperclip. But if you keep bending it backwards and forwards long enough, it breaks very easily,” Neely explains. 

This is known as structural fatigue, and it applies as much for aircraft as it does for paperclips – and even more so for hypersonic aircraft because of the escalating effects of all that heat coupled with FSI.

Engineers working in the sector aren’t confident, yet, that the computer models they use to evaluate these complicated dynamics and their effect on the aircraft are sophisticated or accurate enough.

Why hypersonic?

That’s not to say that we aren’t designing aircraft and vehicles for hypersonic speed even now. In fact, anything we put into space and expect to come back down again will have to deal with hypersonic speeds.

“When you first re-enter Earth’s atmosphere, or when you enter the atmosphere of Mars, Venus or wherever, that is a hypersonic entry,” Neely says. 

“When they were landing the rovers on Mars there was always the question mark – would the spacecraft withstand entry? And it did because it was well designed.”

Similarly, every time a SpaceX Dragon capsule, having carried a payload into space, comes back down to Earth, it will reach hypersonic speeds at least initially.

Fine-tuning the ability of a vehicle to withstand hypersonic speeds, then, has powerful implications for science and space exploration. But it could also, in theory, potentially help us get to other places on Earth, faster.

“The argument for that is that those of us who live in the Antipodes, rather than spend 20 hours on an aircraft getting from Sydney to London, could do it in, say, three hours.”

Fast travel like this could be revolutionary, not just for transport of people but of supplies and critical infrastructure. 

“The US military’s even talking about transport of infrastructure rapidly to a zone where they need disaster relief,” Neely notes.

But that kind of travel is probably a long way off, if it’s at all possible.

“That’s part of the challenge we’re trying to solve, and fluid-structure interaction, along with aero-elasticity or aero-thermal elasticity, is one of those challenges.”