Hypersonics are a growing sector that hopes to launch missiles at speeds not just faster than sound, but even faster than that. To reach speeds beyond Mach 5, oxygen dispersion strengthened and refractory metals that can withstand extreme heats are crucial. Increasingly, hypersonics researchers are turning to additive manufacturing (AM) to 3D print parts from these unique materials. In particular, 3D printing technology is seeing real utility in the Hypervelocity Wind Tunnel 9 at Arnold Engineering Development Complex in White Oak, Maryland.
Early 3D Printing Efforts at Tunnel 9
Tunnel 9 is used by the Air Force for hypersonic ground testing and the validation of computational simulations. The facility team originally purchased a desktop material extrusion machine seven years ago but was unable to produce anything of substance. In 2019, Tunnel 9 system and nitrogen supply engineer Samuel Gigioli set about rebooting the program. This time around, the printer was used to 3D print plastic mounting rigs, fixtures, gauges, and other tooling.
“The Terminal High Altitude Area Defense (THAAD) missile is designed to intercept and destroy ballistic missiles inside or outside the atmosphere while they are in their final, or terminal, phase of flight. The THAAD model shown here was tested at AEDC’s Hypervelocity Wind Tunnel 9 facility in White Oak, Md., to collect high-accuracy static stability and drag data. Once demonstrated, the same test model and instrumentation were used in a test in the von Karman Facility to confirm complementary test capabilities exist between the two AEDC facilities.” Image courtesy of U.S. Air Force.
This led to the purchase of two more 3D printers, including vat photopolymerization machine for detailed parts and a large-format filament extrusion machine. By 3D printing in-house, the group is able to obtain components in just hours instead of weeks. Gigioli said, “There have been a few prototypes applied to the tunnel. In fact, a very critical aero-optic part was prototyped with our resin printer, but most prints are rigs, test fit gauges or other miscellaneous parts.”
3D Printing Refractive Metals at Tunnel 9
By February 2019, the group was pursuing more advanced AM research, specifically a project using laser powder bed fusion to 3D print refractive metal hardware at Tunnel 9. The goal was to cut lead time and costs for high-temperature parts used in the tunnel.
“These Tunnel 9 parts endure very cyclic, high-pressure and -temperature conditions, so the lifespan on these parts is significantly shorter than other parts in the tunnel,” Gigioli said. “These parts can experience up to 3,000 degrees Fahrenheit. This is why refractive metals are chosen, as they are metals that can withstand extreme heat with relatively little deformation.”
Gigioli leads the team, caring for materials, devices, and printers, while working alongside Tunnel 9 Chief Facility Engineer Nicholas Fredrick. One contractor prints the parts, managing the production workflow, while an additional contractor provides the printing powder and performs material characterization.
“Pictured is a petal orifice liner created at Arnold Engineering Development Complex Hypervelocity Wind Tunnel 9 in White Oak, Maryland, using additive manufacturing, otherwise known as 3D printing. The liner is integral to high Mach number tunnel runs at Tunnel 9 and can experience temperatures of several thousand degrees. The refractive metal part has an outside diameter of around 3 inches, an inside diameter of around 2 inches and is nominally 3 inches in length. Additive manufacturing machines at Tunnel 9 are also capable of much larger prints, in the ballpark of 18 to 24 inches in any direction.” Image courtesy of U.S. Air Force.
Among the parts printed by the team is a petal orifice liner key to high Mach number tunnel runs. Placed between two burst diaphragms in Tunnel 9, the component is a part of an assembly that operates as a high-speed valve in the tunnel. The 3D printed liner performed better than the traditionally manufactured counterpart, according to Gigioli.
“The part is cheaper to manufacture, quicker to manufacture and deliver, and is more resilient to the cyclic temperature loading,” Gigioli said. “The AM process creates very unique microstructures within the material and, as a result, the part’s structural and thermal properties are different than traditional wrought billets metal.”
Next, the team will test a 3D printed particle separator for filtering airborne particles to optimize mass flow in the tunnel. Once finished, the component will undergo testing in high Mach conditions circa September.
Close up of the petal orifice liner created at Arnold Engineering Development Complex Hypervelocity Wind Tunnel 9. Image courtesy of U.S. Air Force.
“Imagine if we scale this process up to multiple tunnel parts or even entire systems,” Gigioli concluded. “Now some DOD components are interested in printing test models or leading-edge models. I hope to continue leading Tunnel 9 down this path and increase our success through this technology.”
Also participating in the hypersonic arms race are Purdue University, Astro America, Orbital ATK, Raytheon, and VELO3D, GE, and Lockheed Martin, among others. If it feels like the weapons sector couldn’t get more advanced than it already is, you’re probably right. The world’s most funded militaries are basically pushing their technology to the edge of an asymptote at this point, but can always find new ways to spend public tax dollars. And, as they do, AM will be a key enabling technology.