Purdue University
The term “supersonics” means aircraft that break the sound barrier at Mach 1, such as the now retired Concord and the up-and-coming Boom Supersonic XB-1, and extends to around Mach 5, five times the speed of sound and over 3,500 mph at sea level.
When the upper speed is reached, the hypersonics environment is entered. This is when weird things start to happen.
Researchers at Purdue University in Indiana are learning that this unusual environment can be a perfect proving ground for metal 3D printing. Recent work done by the team demonstrates that advanced additive manufacturing technology is now capable of producing fully dense, end-use parts with robustness that surpasses traditional methods like casting. Parts that can create hypersonic conditions and live to tell the tale.
“There’s a line you cross at around Mach 5,” said Purdue Associate Professor Carson Slabaugh, whose 20-person team at Zucrow Laboratories has been studying high-speed combustion systems since he started his lab at Purdue in 2015.
Slabaugh added: “When a vehicle flies that fast, extreme compression and heating of the air flowing around and within the fuselage occurs. At Mach 5, it’s about a six-fold increase in the temperature and a pressure increase of a few hundred times. That kind of thermal and mechanical loading causes the regime of aerodynamics and structural mechanics to completely change compared to lower-speed systems.”
To achieve powered hypersonic flight, with a controllable aircraft, unmanned vehicle or missile, you need to add an engine, which is where it gets really intense.
Above Mach 5, the temperature of atmospheric air as it rushes by is thousands of degrees, and the pressure is several hundred psi. The air itself can even become chemically reactive if you fly fast enough. The extreme flow conditions become a challenge to any vehicle-propulsion system whose thrust comes from burning fuel.
Purdue University
Carson Slabaugh
To meet this challenge, Slabaugh and his team partnered with Velo3D to ‘print’ fuel injectors with complex geometries that achieve very high fuel-air mixing performance. Conventional fabrication methods could not have produced such parts, especially with the high-strength metal superalloys needed to survive the extreme testing conditions.
“Through our partnership, we helped Velo3D to understand the design requirements for high-speed combustion systems and they taught us how to better design for additive manufacturing,” said Slabaugh. “This is the sort of mutually beneficial relationship we build with industry partners as we solve the challenges associated with advanced technology transition.”
Designing and manufacturing parts for hypersonic conditions, let alone attempting to fly vehicles going at such speeds, is understandably expensive. Much of the research and development for developing hypersonic capabilities has been supported by NASA and defence-level budgets.
The cost of flight testing the systems means major limits on what can actually be measured for research purposes. Engineers are recreating the conditions experienced during hypersonic flight and proving out engine components in ground-test facilities, like the one at Purdue for which Slabaugh’s team is building a rocket which will never leave Earth.
“We engineer components that will experience hypersonic environments while going zero miles per hour and staying bolted to the ground,” said Slabaugh.
Methods such as computational fluid dynamics (CFD) and fluid-structure interaction (FSI) can be used to simulate material and structural behaviour in response to the flow of air or liquid. This allows engineers to optimise designs before ever manufacturing anything.
“One fundamental problem is that we can’t reliably predict the flow and flame conditions within the engine at hypersonic conditions,” said Slabaugh.
Slabaugh talked about how the team went about solving this problem, saying: “What we’re working on with Velo3D is basically a very large 3D printed burner, which will be used to create the hypersonic environment on the ground in a test cell.”
To create a hypersonic vehicle on the ground, you must build a rocket engine with a large, converging-diverging nozzle and a supersonic plume of extremely hot gas. Then whatever component is being tested is put inside that plume.
Slabaugh said: “With Velo3D, we’re designing the injectors for that combustor to produce very specific turbulent flow fields that mix fuel at a certain rate and allow us to stabilise a very powerful flame in a very compact volume. This creates the conditions for all the things we’re going to test downstream.”
Purdue University
3D printed components
The ability to quickly 3D print a variety of injector geometries for the test combustor, in this case made out of Hastelloy X, a high-strength, high-temperature superalloy that cam withstand a hypersonic environment, enabled the Purdue team to rapidly identify which design worked best.
The engineers subtly varied the flow passages of the injector with five different designs. It was just a matter of tweaking the STEP data file that the VELO3D Sapphire system’s print-preparation software accommodated automatically. The designs were printed out and ran through a gauntlet of hypersonic-relevant test conditions.
In just two weeks, the team was able to isolate the highest performer that had all the stationary and dynamic features they were looking for. The high-performance injector met the critical parameters that the engineers valued most for performance of the combustor, flame power, and flame stability.
The next step for the team is to now assemble a large array of injectors into an even more powerful combustor. Velo3D is consulting further with Zucrow labs to help them take advantage of its build-anything-you-want capabilities, by integrating the injector set into a single piece, 3D printed component.
From there, the engineers will continue to refine and assemble a complete combustor system, aiming for full-scale hypersonic test capability in late 2022.
