74 Dossier | NEX Power 15 kW GenSet gas micro-generator compressors on a separate coaxial shaft in order to drive a propeller at a lower, more efficient rpm. “We machine the turbine from a proprietary material selection at the moment, which like the impeller is fine for prototyping and small batches, but again this is something we’d cast for higher volumes and that would open a few approaches for coating the cast turbine to improve its thermal efficiency,” Najafi says. “We’re not settled yet on which approach we’d take, but we used a number of coatings back in our CHP days to improve the microturbine-boiler’s thermal efficiency, so we’re investigating which could work best in the context of aerospace customers. Thermal barrier coatings such as ceramic coatings could preserve the integrity of the components well.” The turbine is welded to the shaft by Electron Beam Services in the UK, with whom NEX has worked for many years for their specialisation (in electron beam welding) and quick adaptability to the requests made of them over the term of the partnership. Exhaust gas evacuates behind the turbine section via a conventional pipe of high-grade stainless steel, sized to avoid back-pressure, overheating, and noise and vibration that could negatively affect the turbine. Potentially helpful exhaust coatings or materials are to be investigated based on use-case (because these must take aircraft integration into account). Foil bearing The 15 kW GenSet’s shaft runs in two bearings, both of which are foil bearings: one placed at the end of the alternator and one located between the compressor and turbine. Although NEX Power has benefited greatly by eliminating oil from the microturbine through use of foil bearings, the company observes that foil bearings remain rare across UAV engines owing primarily to the challenge of engineering them well. “Air bearings are very sensitive to manoeuvres of the platform they’re on, so unless they’re designed very well, they can find it impossible to resist a very sharp and sudden movement by the aircraft – it takes a very specific design of air bearings to handle such moments,” Najafi says. “They are, however, a common sight in APUs, which often sit centrally on an aircraft’s fuselage and use air bearings frequently, but the main engines on passenger aircraft change position more frequently and sharply than APUs, so it’d be harder for air bearings to handle the loads in those engines.” NEX Power designed its foil bearing in-house, with the help of contracted consultants initially, after which a few years of r&d followed (including rounds of prototype testing and iteration). The general design of such bearings consists of a sleeve, a top foil and a bump foil. The top foil runs in close contact with the shaft and, hence, is coated in a Teflon-like material for minimal stiction and scratching. The top foil must also be sized exactly to sufficiently handle the engine loads without overheating. “Say, for instance, we go from a 2 cm length air bearing to a 5 cm one, it’ll be way better at keeping the shaft stable, but it’ll also generate a lot more resistance and more heat, which in turn will rapidly degrade the coating, causing severe damage thereafter,” Najafi explains. “Designing these parts, selecting the coating, optimising the gap between top and bottom foil, and getting the length of the sleeve and the thermal stability of the overall bearing right are the factors we needed to balance together to get the air bearing right, and all of them must be recalculated whenever you change your engine size, engine power or shaft size.” Electric power The alternator has been sized to accommodate a range of battery August/September 2025 | Uncrewed Systems Technology The combustor is an annular design (like in other microturbines we have featured), built by bending, forming and welding stainless steel sheet
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