Ever since the last Concorde razored the skies in 2003, there have been no attempts to create civilian supersonic aircraft that could cut fly time in half. But new jets could bring back supersonic flight within the next decade. At least four United States entities are actively designing supersonic aircraft, including Lockheed Martin‘s commercial airliner and newer companies, like Aerion and Spike Aerospace. One plane design, in particular, caught most of the attention in 2020. Boom Supersonic’s XB-1 prototype vehicle unveiled at the startup’s Denver hangar brought the dream of returning to supersonic air travel closer to reality.
The XB-1 aircraft is a one-third scale demonstrator for Overture, the company’s 55-passenger, three-engine future supersonic airliner, which echoes the earlier shape of the Concorde. Except XB-1 offered its creators an opportunity to explore more advanced designs and manufacturing technologies than were available to Concorde engineers. That included additive manufacturing (AM) technologies to produce some of the most complex part designs.
“There are many reasons for choosing that technology [3D printing] over others. There’s a great deal of design flexibility in using 3D printed materials,” asserted Byron Young, Propulsion Structures Engineer at Boom. “Engineers are always trying to implement time-savings into a job. Much of the time and effort in aircraft design goes into joints, the interfaces between components. By designing directly for AM, we can reduce the number of parts and joints, reducing time and net effort. And part consolidation cuts out significant amounts of weight, something that’s a major priority in aircraft design.”
In total, 21 challenging titanium parts were 3D printed for the XB-1, many of them related to channeling air that could exceed 500 degrees Fahrenheit and contain geometrically complex vanes, ducts, and louvers that required a surface-based design approach. Boom relied on metal 3D printing technology from VELO3D’s next-generation laser powder-bed fusion (LPBF) Sapphire technology and worked closely with Duncan Machine Products (DMP) engineers, the supply chain partner that handled both printing and post-processing. The partnership was a success, as the Boom team found that the AM process was more complicated than they had envisioned but could deliver on their original design intent.
Having established a relationship with VELO3D on some trial parts in 2019, Colorado-based Boom chose the company’s Sapphire system to produce several printed titanium components (a right and a left version for many of them) located in critical areas of the plane. These included manifolds for the Variable Bypass Valve (VBV) system that routes air released by the engine compressor to the aircraft’s outer mold line (OML) and exit louvers for the environmental control system (ECS) that cools the cockpit and systems bay. Also, louvers that direct the center inlet’s secondary bleed flow to the OML, two diverter flange parts, and NACA ducts – frequently used in high-speed aircraft to capture exterior air and channel it into the aircraft to cool the engine bays.
Young said he was impressed with Sapphire’s ability to produce the parts’ extremely thin-walled designs accurately, allowing the engineering team to print walls as thin as 20 mil (0.02 inch), with a surface finish that didn’t require additional machining in most cases.
According to new data that emerged from a case study by Boom, the high aspect ratio (height to width) made possible by VELO3D’s non-contact recoater system distributes each new layer powdered metal fused by dual lasers – was another plus. To remove mass, the vanes on the center inlet’s bleed louvers were printed hollow, and the parts were designed with high aspect ratios (very thin walls along long spans). VELO3D’s technology provided the ability to print that very high aspect ratio in this kind of design without excess material to provide strength inside the structures.
One of the project’s most significant challenges was working with the 3D printed parts’ titanium material. AM engineers from DMP said there is less loss of strength at high temperatures with titanium than aluminum or carbon fiber. However, the lightweight, extremely heat-resistant titanium, widely used throughout the aerospace industry for critical components, also has a reputation for being delicate and difficult to work with no matter how it is manufactured.
“This was a learning process on all sides,” said Gene Miller, an Applications Engineer at VELO3D. “Boom designed a part family that was new to us, really pushing the envelopes for weight reduction and thin-wall geometries, and we had a lot to learn as far as printing these components out of titanium and what to expect from the physics of printing. How is it going to move? How is it going to shift? What can be printed without supports, and what areas needed to be supported, so the result is nominal?”
However, VELO3D’s Sapphire system is built with a semiconductor mindset on quality assurance, ensuring repeatability and dependability throughout serial manufacturing. This proprietary AM process optimizes the print parameters and sequences to produce robust titanium parts, reducing the amount of internal stress in the substrate as the material is built up, indicated Miller, diminishing the possibility of cracking by mitigating internal stresses formed during cooling.
Once Boom’s titanium parts were 3D printed, engineers easily sliced off the build plate with sawing or wire-cutting Electrical Discharge Machining (EDM), an electrothermal production process. DMP machinists claimed post-processing was relatively straightforward, compared to parts made in other AM systems they had previously used.
“After cutting off the build plate, we had very little to do in the way of post-machining apart from minimal support removal,” revealed DMP AM engineer Aaron Miller. “You don’t have any tiny supports in small crevices or hard-to-reach places because the SupportFree technology eliminates the need for those. The parts come out of the Sapphire system almost finished, just needing a little handwork with a screwdriver or grinder. We also ream out pilot holes (on larger parts to be joined together) with a mill to ensure they’re the correct size. It’s part-dependent, but probably just a half-hour of machining per part, which is not a big deal.”
The engineers also explained that the finished parts were heat-treated or hot isostatic press (HIP) processed to enhance fatigue life. Mainly when there are flight components that may be cyclically loaded during takeoff and landing. Supersonic flight introduces many different phenomena and stresses generally unseen with conventional air travel, described Young. The main forces applied are not generally pressure loads from breaking the sound barrier. In many cases, the induced strain is caused by the aircraft’s overall structure flexing around the 3D printed parts.
Moreover, Young described that when parts with dissimilar thermal expansion coefficients are mounted to each other, significant stresses can also result (this includes carbon composites and aluminum in addition to titanium). But designing these 3D printed parts to be very thin and flexible can actually mitigate some of these issues. After the successfully 3D printed titanium parts for the XB-1 demonstrator, we can surely expect Boom to continue collaborations with 3D printing companies to create more challenging parts.
Commercial flights for Overture are scheduled to begin before the end of the decade, with hundreds of potential routes already identified. Two major airlines – Virgin Group and Japan Airlines – have already pre-ordered a collective 30 aircraft. To do this, Boom expects to expand its 100-person workforce to about 500, scale-up for production to over 1,000 manufacturing employees, and estimated development and certification of Overture will cost $6 billion (the company has so far raised over $196.1 million). Aside from taking on the task of successfully 3D printing complex metal parts, Boom is also attempting to manage landing and takeoff without using afterburners and working on optimizing fuel efficiency and minimizing maintenance requirements.
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