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Mastering Microturbines

Will Hasting
By Will Hasting Director of Aviation and Power Turbine Solutions, Velo3D

Extreme complexity is inherent to jet engines of all sizes, from those on a Boeing 777x to ones that power the smallest drone. While drones obviously need less turbo power than a jet, the microturbines that drive them remain a serious design and manufacturing challenge for engineers who have to cram all that complexity into a much smaller package.

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This 3D-printed diffuser contains low-angle, internal passageways manufactured with Velo3D’s support-free AM technology.

The general rule of designing any jet engine is to keep it straight and round.
Whatever introduces variation into the dimensions of an engine is going to cut away at performance.

Drone microturbines, until very recently, have traditionally involved significant hand fabrication and many different piece-parts. Not only does this increase manufacturing and assembly costs, it leaves the engine more susceptible to performance variation.

Workarounds to fix such issues can result in design compromises, weight gain, and excess fuel burn. No wonder, then, that drone prices have remained high: a single agricultural- or military-size aircraft can cost between hundreds of thousands to many millions of dollars.

This state of affairs has created a window of opportunity for advanced metal-AM equipment manufacturers, who are experiencing an uptick in inquiries about the cost-effectiveness of printing an entire small turbine in the build chamber of a 3D printer.

Drone makers are taking their cue from commercial-aircraft engine manufacturers who have increasingly been using AM for part consolidation and lightweighting (witness advancements in 3D-printed engine components over recent years).

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KW Micro Power’s generator is small enough to fit into a suitcase.

Just like the major aircraft manufacturers, drone companies’ ultimate goals are more efficient engines that burn less fuel, allow larger payloads and provide a wider flight range.

Despite the claims that additive manufacturing can print anything, many current AM machines are nevertheless still not up to the task of creating fine details deep inside sophisticated microturbines.

Such was the experience for KW Micro Power, whose design for a microturbine included a diffuser comprised of a titanium disc 10-inches in diameter and 4-inches tall with an interior labyrinth of low-angle exhaust-gas channels.

The company approached several 3D-printer manufacturers—and was turned down because the internal channels were deemed impossible to print.

Why? Because most metal additive manufacturing technologies can’t make features that are less than 45-degrees from level—without scaffold-like supports that keep a workpiece from drooping and warping during the build process.

Though expensive and time-consuming, it’s accepted industry practice to machine or grind these supports away post-build.

The KW Micro Power design contained inaccessible stator vanes deep inside—leaving no possibility of support-structure removal after 3D printing.

Newly developed, support-free AM technology from Velo3D, however, provided the capability to print the near-horizontal surfaces inside entirely without supports.

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Before and after images of a stator ring illustrate how technology from Velo3D eliminates the need for interior supports (in red) during the 3D-printing process.

The AM build was completed, first run, with the final part 37% lighter than the original design with significant performance improvements and reduced operating stresses.

A smooth surface finish also contributed to that improved performance by optimizing the air-flow without post-processing: the lower the surface roughness, the less air friction will eat away at engine efficiency. Fine-feature resolution allows for intricate cooling passages and fuel-delivery channels.

The latest system’s print-setup software, continuous build management and quality-assurance monitoring contribute to the highs levels of part consistency demanded for turbines of all sizes.

Microturbines fit into current AM build-chamber dimensions nicely; on the way are larger systems capable of manufacturing engines for helicopter and business jet applications, as well.

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