Sierra Turbines
Drives Innovation
for Microturbines
Manufacturing gas microturbines
to be 40X more efficient, 10X more
power density, and 50% lighter in weight

Roger Smith is breaking the rules of manufacturing. The chief executive
officer of San Jose, California-based Sierra Turbines Inc., he’s discarded
the industry’s long-held belief that additive manufacturing (AM) is limited
to prototyping and low-volume work. In fact, Smith plans to additively
manufacture 95% or more of his microturbine components even when
he reaches large-scale production.
Smith doesn’t care that he’s discarding several other industry beliefs, among them the notion thatoverhauling microturbines every 40 to 50 hours is acceptable. Smith is unhappy with much of thesmall-engine status quo. A microturbine has historically been a simplified version of its full-sizecounterpart; cost restrictions have always prohibited manufacturing complex features that would
give microturbines improved performance. With a little radical thinking and a hefty dose ofadvanced technology, he’s prepared to change it.

Two 3D-printed turbine combustors that serve as thecore of Sierra Turbines’ small engine. They are smallenough to fit in the palm of a hand and were built
layer-by-layer with 3D metal printing. Printing theseengine components took about 50 hours vs. traditional
manufacturing that would have consumed months.

One of the key pillars of this advanced technology in Smith’s arsenal is the metal AM system frommanufacturer VELO 3D, located just a few minutes south of Sierra Turbines in Campbell. Says Smith,“We opened the company in 2017, and within the first year or so had pretty much scoured the planetfor a 3D-printing process to produce our parts. That’s when we learned about VELO 3D. We were allquite surprised—and pleased—to discover they are just down the street from us. It makes our work
together that much easier.”
The “we” in this story is Smith’s team of scientificand technical professionals. Some have gas turbine
expertise; others are extremely knowledgeable insoftware and control systems (Smith himself spent 19years with Apple before launching Sierra Turbines).
All have worked in Fortune 100 product environments.
Together they boast a combined 70 years of industry
experience, with each team member sharing the same
goal—to design and manufacture better solutions forthe burgeoning microturbine market.
Microturbines are suitable for a variety of applications,
Smith explains. First and foremost of these is unmanned
aerial vehicles, or UAVs. Where gas turbine engines
weighing many thousands of pounds push commercial
and military aircraft through the skies every day,microturbines perform a less dynamic, though justas important, task: powering the electric motors thatkeep UAVs aloft.

“For future gas turbine
development, we aim toleverage the power of
additive manufacturing
to integrate features suchas an efficiency-boosting
recuperator, printed-insensors, and morenovel insulating andcooling geometries.”

They can also be used to charge the batteries that drive those motors, as range extenders, or in hybrid
UAVs that alternate between batteries and motors. These same functions apply to land-based power
generation needs, where microturbine-based systems not much larger than a washing machine serve
as auxiliary power units (APUs) in backup generators and other standby electrical-generation needs.
Cross-section of a Sierra Turbines engine combustor highlighting the complexity of the interior. Notice the thin
air foils and smooth lattice that helps the combustor effectively atomize fuel prior to combustion. With traditional
manufacturing, this specific part would have required the assembly of 61 discrete components, but 3D printing
consolidated it into a unified geometry with fine details and features that are unattainable via any other method.

The Aurelius Mk1 Project
Sierra Turbines named their microturbine “Aurelius Mk1,”
drawing on that historical period when engines were named
after Greek or Roman mythology. Inspired by the Rolls Royce
Olympus engines that first powered the Concorde supersonicairliner, Aurelius also happens to be the name of Smith’slate father. Sierra Turbines intends to honor that namesakeby challenging the status quo. The time between overhaul(TBO) for most small turbine engines averages 40 to 50 hours.
Smith intends to raise that value to 1000+ hours, on par withcommercial aircraft. He also plans to reduce engine weightto a kilogram or less, cut noise, and improve fuel efficiency,
all while making the turbines easier to manufacture. These arelofty goals, yet ones shared by practically everyone making
microturbines. He’s confident that his team will succeed
where others have failed, however, through a host of design
improvements only made possible with advanced metal AM.

Benchmarking Sierra Turbines to Market Incumbents
Engine design and performance metrics
Sierra Turbines’s Engine

Market Incumbent’ Engine

Time Between
Overhaul (TBO)

1000 hours

40-50 hours

Part Count

Consolidated unicore

60+ discrete parts

Weight

50% weight reductionvaries

Power Density

2000 watt hrs. per kg

200 watt hrs. per kg (lithium ion)

Smith sees his efforts as part of a bigger picture, work that will contribute towards a revitalization of thecountry’s manufacturing sector. “My thoughts are these,” he says. “If we as a country are to start building
products in the U.S. again, we must develop better ways of designing and fabricating products. This
includes less reliance on tooling, most of which has been shifted to China, but also an embrace of Industry
4.0 and other change-making technologies. Metal 3D printing is one of these change-makers. It’s far
less wasteful than traditional manufacturing and presents endless opportunities for part consolidation,lightweighting, greater product efficiency…the list goes on.”One example of this new approach to design efficiency is the Aurelius combustor. It’s the heart of any
microturbine and the test case by which Smith would determine which providers were able to deliver on
the promise of additive. The component contains hundreds of small, oddly shaped holes, delicate meshlike webbing, dozens of internal cooling channels, and a series of thin, exceedingly tall walls. And where
the initial design contained 61 discrete parts, that number fell to a consolidated “unicore” engine after
Sierra Turbine’s designers gave it an additive makeover with computational modeling software tools.

Key Differentiators
It’s this highly sophisticated design that Smith presented to various additive equipment manufacturers,
challenging them to build the part from Hastelloy® X, a high-strength, low-creep metal that’s favoredby commercial-aircraft manufacturers. The heat-resistant superalloy is also vital to Smith’s reliability
and fuel-efficiency goals, as it will allow his engine to reach temperatures previously unattainable inmicroturbines. “The hotter you go in any gas turbine, the better the performance,” he says.

fig. 3afig. 3bfig. 3c

Cross-sections of the 3D printed engine core. 3a illustrates the necessary support structures if printed onincumbent AM systems, making this a poor business case for AM. 3b illustrates the value of VELO3D’ssupport-less printing process, with the blue portion highlighting a smoothly blended lattice geometry,
designed with nTopology’s optimization software, which has the unique ability to spatially vary lattice size
and thickness to maximize engine output and performance. 3c is the final printed part off the Sapphire printer,
after it was removed from the build plate.

“Metal 3D printing is one ofthese change-makers. It’s far
less wasteful than traditional
manufacturing and presents
endless opportunities for partconsolidation, lightweighting,greater product efficiency…
the list goes on.”Smith isn’t naming names, except to say that“everyone else failed” in their attempts to build thethin-walled, high-aspect-ratio combustor.
“The team at VELO 3D worked with us on the design,
tweaking things to make it more manufacturable,
but it was their Flow software that was perhaps thebiggest enabler,” Smith says, “It contains a numberof special tools that provide valuable feedback
throughout the entire print-preparation process.”
He noticed a range of hardware differentiators aswell. “It was obvious to me early-on that the VELO3D
platform offers a different approach to laser powderbed fusion (LPBF),” he adds. “For starters, their noncontact recoater blade is especially well-suited to printthin-walled sections—in some areas, our walls are lessthan one millimeter thick, and these tended to break
off on other 3D printers.”Smith also notes VELO3D’s ability to eliminate supports on shallow overhangs. This attribute provides forinternal features that would otherwise be impossible to produce, as well as the smooth surface finishes
printed by the Sapphire metal AM system. Each of these reduces the need for secondary machining andother finishing processes, something the team at Sierra Turbines wishes to avoid wherever possible
Finally, the system’s quality assurance software, Assure, documents all the critical sensor data that affectpart quality and produces a comprehensive build report for traceability. Armed with data points concerning
protrusions, oxygen level, filter life, and laser alignment, Sierra Turbines has confidence that the part built wasindeed a good part. “This level of quality assurance is what will move the AM industry forward,” notes Smith.

Taken directly out of the Assure build report and showing actual data from the Sierra Turbines project,see data related to Peak Height and Filter Recirculation. Peak Height measures concerns against protrusions,and the Recirculation Filter measures the life of the filter during the build. Both show no concern as thesensor readings are under the warning threshold.

Reducing the Pain of Traditional Manufacturing
The cost and energy savings by the consolidation of 61
parts into one part cannot be overstated. Production
and transport of different raw materials, manufactureof individual parts using different processes, shippingof individual parts, tedious assembly work, joints ofdissimilar materials, reliance on extra seals or fasteners,
and more: All of this waste is simply eliminated by theintegration of many parts into one printed part which
subsequently requires minimal post-processing.
“3D printing eliminates a great deal of manufacturingpain,” Smith says.

Machine health snapshot from the Assurebuild report, showing that all critical sensors
are calibrated. Laser alignment, laser focus,
and beam stability have always beenmajor concerns for the AM industry as it’scommon for alignment to drift during builds.
Assure captures this data to communicate
confidence to the customer.

Ready for Takeoff
Smith is just getting started. Once the combustor has been thoroughly tested and benchmarked,
he intends to pursue additional performance improvements. He’s also planning to work on the
microturbine’s rotating components, an unorthodox move that many aerospace pundits would agree isbeyond the pale. Here again, Smith is determined. “VELO3D believes that you can use additive for full-scale
production, and so do I,” he says. “For future gas turbine development, we aim to leverage the powerof additive manufacturing to integrate features such as an efficiency-boosting recuperator, printed-insensors, and more novel insulating and cooling geometries.”
Nor is his decision solely a matter of lower cost, shorter lead-times, or abbreviated supply chains.
By allowing Sierra Turbines to consolidate dozens upon dozens of components into a single 3D-printed
piece, one with lower mass and greater mechanical integrity than the welded and assembled alternative,
support-free printing technology is enabling this small company to meet its design goals. Smith fully
expects to meet or exceed his TBO goal of 1000 hours, among other objectives.

“My design team is freed fromthe constraints of traditional
manufacturing and even existing
metal AM technologies such thatthey can focus purely on defining
the geometry needed to maximize
performance and differentiation.”
Roger Smith
Founder and CEO,
Sierra Turbines

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