Taking a radically different approach to materials, structures and manufacturing saw Australia’s Hypersonix send its aircraft beyond five times the speed of sound.
When an aircraft travels at hypersonic speed – above Mach 5 and more than five times the speed of sound – several basic assumptions around aircraft design begin to fall to pieces.
For the team at Brisbane-based Hypersonix, the first challenge arrived as a measurement: 40 mm. That is how much their 3.5 m DART AE aircraft would expand in length during its planned Mach 5 flight as a result of the heat.
Heat is the main challenge as an aircraft increases its Mach number, said Professor Michael Smart, the company’s co-founder and Chief Technology Officer. Thermal loading becomes the driving design problem.
That led to a complete change-out of the original design, which took the structure from something resembling an aircraft to a design more similar to a boat.
“The classic aircraft structure has a thin skin and underneath it are all these load-bearing structures, like spars,” Smart said. “The skin is really just there to be a smooth, aerodynamic surface that doesn’t really take any load, other than very locally.
“We tried to design a system like that for DART, where the skin was going to get hot, and then use an internal cold structure to take the loads. But it didn’t work because of the enormous stresses caused by this differential between the hot skin and the cold internal structure. It posed completely unsolvable problems.”
The solution came with the acceptance that the expansion of the airframe, which would reach temperatures around 800°C, would not allow an internal, load-bearing structure.
“We changed to a monocoque structure, more like the hull of a boat,” Smart said. “It doesn’t have any internal structure. It’s a shell that takes all the load.”
The shell, 3D printed in nickel-based alloy Inconel, would take the heat and be responsible for its own structural integrity while also carrying tanks, avionics and other equipment internally.
READ: The biggest challenges to hypersonic flight – and how they’re being tackled
The scramjet is the vehicle
The aircraft that made the hypersonic flight on 27 February, under the US Department of War’s Defense Innovation Unit, was known as DART AE. The hydrogen-fuelled scramjet engine was called SPARTAN. At hypersonic speeds however, the separation of parts and structures became difficult.
“It’s all connected together and it has to be highly integrated,” Smart said. “A commercial airliner has engines slapped onto the wing, quite independent of the structure. But with a hypersonic vehicle, everything is interconnected.”
Culturally, the unique engineering challenge suited the small-business Hypersonix model, he said. Hypersonix employs around 60 people, with many of its engineers aged 25-35.
“In our small company, we all interact very closely,” Smart said. “So the highly integrated technology we’re developing suits the size of our company.”
The manufacturing was unconventional, with the SPARTAN engine consisting of just ten 3D-printed components and the airframe made up of around 30 pieces, joined together with Inconel bolts.
Inside the aircraft, the hydrogen fuel was stored as a gas at around 400 bar, ready to be released and injected into the hypersonic scramjet airflow, where the challenge was ensuring it combusted within a microsecond.
“That’s actually the biggest trick in scramjets: getting the fuel and air to burn quickly before it passes out the back of the engine,” Smart said.
READ: How the world’s largest aircraft by wingspan was designed
Flight lessons
Years of modelling and wind-tunnel testing are no replacement for the real thing, Smart said. The data collected from the February flight, which involved sustainable, manoeuvrable flight at speeds greater than Mach 5, offered invaluable vision into the effects of hypersonic flight through the real atmosphere.
Two areas of insight were particularly powerful.
One was what Smart called “boundary layer transition”, referring to the thin layer of airflow against the aircraft that is originally smooth and low-drag before it transitions to turbulent flow and creates greater drag.
“We got some really lovely flight data, which I haven’t been able to show to anyone yet, but where we could see this happening in flight,” he said. “It’s the real thing.”
The other was vibration. Hypersonic aircraft, perhaps unsurprisingly, experience significant vibration.
“We saw some really interesting data on vibration, which is so important for design,” Smart said.
“Going forward, we want to design aircraft that are scramjet-powered to fly to space and do all sorts of things. Knowing what the vibration level is in these conditions is really critical and really flight-only.”
DART AE was a single-use aircraft. However, Hypersonix is working on a reusable next-generation vehicle. That vehicle is intended to ultimately lead to the design and development of a larger, scramjet-powered aircraft capable of delivering satellites into low-Earth orbit, then landing on a runway.
That, of course, will introduce an entirely new generation of engineering challenges. But Smart is confident Australia has the talent to solve them.
“In Australia, we can do awesome things,” he said. “We just need to learn how to take risks when the potential reward is great. I see lots of young people developing that attitude now.”
Register now for the Australian International Aerospace Congress, 22-24 February 2027.
