Small turbines

For applications that need significant power but do not have much space or weight allowance, there haven’t been many options. But that has changed now that Locust Power LLC develops and manufactures high-speed micro-turbine engines from 5 shaft horsepower (shp) to 150shp that can provide thrust for UAVs or generate electrical power as small generators. The small turbines, which rotate at speeds up to 227,500rpm, can run on heavy fuels or natural gas.

Locust Power’s CEO Kirk M. Warshaw says, “With the right collaborators, we intend to revolutionize certain segments of the energy and UAV markets.”

He adds that the Miami company’s turbine engines incorporate advanced proprietary technologies, allowing for a light-weight, compact system with high component efficiencies that can enable engineers to design unique applications.

Recently, AMD asked Warshaw to explain how Locust’s products are made and his thoughts on their applications.
 

AMD – What is the technology from Locust Power?

Warshaw – Very high-speed, heavy-fuel and natural-gas burning gas turbines. We also have integrated key subsystems, such as generator rotors operating at very high rotational speeds and speed reduction gearboxes, with high-speed reduction ratios.
 

AMD – Why do you consider the technology to be disruptive?

Warshaw – The aircraft industry has evolved to the point where practically all aircraft now are powered by gas turbines. No one has designed and integrated many of the subsystems for small engines that are found on much larger gas turbine engines. Locust Power has. The result is our engines will provide a high power-to-weight fraction. Our micro-turbine engines can replace high-noise, high-weight, and high-vibration reciprocating engines in the same power class.
 

AMD – What elements are proprietary?

Warshaw – The size of our turbine engine and the speed at which it operates create a vast array of engineering challenges including thermal issues, the need for durable and unique bearings, and tiny tolerances which make fabrication of our hardware very difficult. Our detailed design of key components, the materials we use, and how we assemble our engines are all proprietary. We do have some patent-protected component parts, including an engine igniter, a part of our combustor system, and our gearbox.
 

AMD – Is one size optimized for auxiliary power units (APUs), another for UAVs?

Warshaw – There is interest in both the turbo-generator and turbo-shaft UAV designs. Of special interest is our 40hp-class turbo-shaft. Locust Power engines are being considered for newly designed UAVs as well as the replacement engine for some existing UAV designs that have durability challenges.

For the smaller 5hp- and 10hp-class engines, applications will likely be turbo-generators, but these engines have also been operated as turbo-shaft [propulsion] engines with speed-reducer gearboxes and variable-pitch propellers or fans for thrust.

The larger versions already have been operating as turbo-shaft engines with bolt-on starter generators that produce 5kW of electrical power. There’s nothing that would preclude Locust from re-configuring these larger engines to serve as turbo-generators in order to extract the available power as electrical energy up to 100kW.
 

AMD – How does this technology compare on performance?

Warshaw – There are several ways to measure performance:

• Power-output-to-weight ratio – The Locust engines are superior in this category.
• Noise – The high-speed Locust engines have an inherent capability of producing less noticeable noise due to lower source and the high frequency of their noise which, attenuates rapidly in the atmosphere.
• Fuel consumption – Our basic versions of the engines are not always as efficient as reciprocating engines. The addition of a heat exchanger (a recuperator) to capture heat from the engine exhaust brings the Locust engines in line with the reciprocating engines.
• Vibration – Unlike reciprocating engines, gas turbine engines are essentially vibration free.
   

AMD – How does this technology compare in cost to the alternatives?

Warshaw – At the present, this technology and products in the micro-turbine class are more costly than reciprocating engines. Once Locust Power engines are produced in larger quantities, we expect our prices will be similar to the current price points. Locust Power will, however, favorably distinguish itself when looking at a total cost of ownership calculation. Since our products have greater reliability and much longer life with extended mean time between failures and mean time between removals, we expect to be a less-expensive alternative over the life of one of our engines.
 

AMD – What is the micro-turbine’s fuel efficiency versus piston-engine alternatives?

Warshaw – The Locust micro-turbine engines in the 5hp to 150hp classes have design brake specific fuel consumption – BSFC (lb fuel/hp-hr.) values that are comparable to equivalent-size piston engines.

The Locust micro-turbines are designed to operate on all heavy fuels (diesel, JP-8, biofuels) and natural gas. This includes starting and operating on these fuels at extremely low temperatures where piston engines have difficulty being started or re-started.

Design steps

AMD – What is the engineering background of your team?

Warshaw – More than a dozen of Locust Power’s employees each have had 25-plus-year prior careers in aerospace design and manufacturing with the likes of NASA, Pratt &Whitney, Rolls Royce, Sikorsky, and the U.S. military. Also critical is the experience some of our employees have in the design and manufacturing of small precision medical devices. These employees are leading the Locust micro-turbine design effort and are also mentoring a cadre of talented younger engineers who work in the design, assembly, and testing departments.
 

AMD – What were your challenges in developing small, high-speed turbines?

Warshaw – The materials, manufacturing processes, design stresses, pressures, and temperatures are all similar to today’s large gas turbine engines and within good design practice. Flowpath temperatures and pressure ratios are chosen conservatively to be compatible with the high rotor speed and small size, and are consistent with good aerodynamics. The challenges encountered have primarily been associated with size scaling as it affects manufacturing and parts tolerances.

Small ball bearings were developed in co-operation with high-production manufacturers and are lubricated with oil. Rotors are made of the best nickel-based cast, equiaxed super-alloys used extensively by large aerospace OEMs and have been achieved through years of close co-operation with the same high-production casting houses.

The engines operate at conventional temperature levels for gas turbines. The integral high-speed generators and bearings are cooled using air or water with the heat rejected to the atmosphere through a heat exchanger. The combustor temperatures were chosen to allow the highly stressed rotor to be uncooled. The bearings are located in the cold section of the engine which is thermally isolated from the combustor hot section.
 

AMD – What is the life expectancy of the components?

Warshaw – These engines are designed to have a life several times longer than conventional small piston engines. This can be achieved because of moderate cycle temperatures and stresses and very low vibration levels. Also, micro-turbines have no rubbing components, unlike piston engines.

Manufacturing details

AMD – What special equipment is needed to manufacture small, fast micro-turbine engines?

Warshaw – The very high temperature and stresses on a micro-turbine require the same super-alloys that are used extensively in large turbines, so many of the machining processes are the same. The difficulty is in the size reduction, which brings the added complexity of very tight tolerances due to scaling. Locust Power was able to marry knowledge of medical device manufacturing together with aerospace component manufacturing. Medical devices are small, have tight tolerances, and significant quality requirements. It is in the intersection of these two industries that Locust found the know-how and suppliers to reliably manufacture its components.
 

AMD – How would you describe your supply chain?

Warshaw – Locust Power’s supply base mostly consists of small, specialty businesses in the U.S. This small business-to-small business relationship allows us, in many cases, to work directly with the person working the machines.

Internally, Locust Power creates assemblies and sub-assemblies in order to keep that key portion of the intellectual property under control, and we have designed and fabricated some tools that we required in the assembly process.

Due to how small our engines are, Locust Power has had to develop alternatives to many of the components that could normally be found off the shelf. However, there are some items where the designers have had to use what is available and make allowances for them. Typically, that includes bearings, seals, bolts, nuts, and fittings. Aerospace-quality components tend to be too large, and generic market components tend to be too heavy or low quality. Where required, we have had to manufacture our own components, but the cost for small quantities is significant.
 

AMD – What makes the manufacture of micro-turbines possible now that couldn’t be done several years ago?

Warshaw – The biggest difference today from when Locust Power was first starting its development of micro-turbine engines is the cost reduction in the more advanced processes. In some cases, this has come by the proliferation of 3D printing. When Locust Power started, casting prototypes was done by using an experimental wax printing process. The machines were temperamental, slow, and extremely expensive to operate. Since then, new, faster, and less expensive machines have come to the market. This has allowed Locust Power to diversify its supply, and today the caster does the 3D printing as part of its regular process without our direct involvement. Also, more advanced machining centers are now within the reach of small, specialized contract manufacturers and therefore available to small entities like us without our having to invest directly in the capabilities.

Another technology that enables us to test many more variations and complex, high-gain, high-risk concepts is the growing proliferation of direct metal laser sintering. DMLS enables us to test more concepts for the same, or in some cases, lower cost. This shortening of the development and testing cycle is critical for a small business.

Locust Power LLC
www.locustpower.com

About the author: Eric Brothers is senior editor of AMD and can be reached at 330-523-5341 or ebrothers@gie.net.