96 requiring a further increase in airframe structural mass in order to carry the extra weight of the recuperator core.” To strike the right balance therefore, lower weight has been purposely prioritised over higher effectiveness, although modifications have been made to the engine cycle to compensate for that. The exact effectiveness of the recuperator, as well as the exact overall engine pressure ratio, are proprietary, although Aant Farm says values have been selected to minimise maintenance and to optimise efficiency and engine mass for 5-8 hour missions. Exhaust The exhaust ducting is made from Inconel 625, although the company anticipates substituting 300-series stainless steel in the future to reduce cost. A key point here is that exhaust temperatures are lowered once heat is removed by the recuperator, so super-high temperature alloys are not wholly necessary. “We will also probably install baffling in the future to properly distribute the exhaust along the microtubes from front to back,” Seegers adds. “We plan to incorporate that into the next design iteration based on testing of the current design, both with and without baffles. If the difference proves minimal, we won’t use them.” While noise generation from the exhaust is not yet a major concern, future hot-flow path design iterations might incorporate tuned metal fibre felts to attenuate noise. This is something Seegers and Weier have designed for business jets, APUs and research engines. “The compressor inlet is currently the source of most of the engine noise,” Seegers notes. “In fact, depending on customer needs, we may offer a tuned inlet attenuator.” Alternators As mentioned, the TPR72 has a frontmounted starter/alternator bolted directly to the gas generator bearing housing. It features 12 stator poles and four neodymium rotor magnets, which are bonded directly to a steel core shaft and then over-wound with pre-tensioned unidirectional carbon fibre tow. “Different versions of the starter alternator are offered; we’re currently testing 50 V and 450 V units,” Seegers comments. Although still at the concept stage, the TA65-1 turbo-alternator will feature a 60 kW liquid-cooled main alternator (as well as the same 5 kW starter/ alternator as in the TPR72) in place of the gearbox and propeller. That main alternator is of a slotless design, directly driven at 76,000 rpm by the power turbine. “To help minimise losses, Hiperco 50 is used in the core stack, and litz wire is used in the windings, which are directly cooled by the heat transfer fluid that flows axially down the stack,” Seegers notes. At the time of writing, both the main and starter alternators were designed to operate to 20% overspeed. The alternator is predicted to be 96% efficient, and will deliver its 60 kW maximum continuous power at 708 V AC (RMS). Gearbox mechanics and lubrication The gearbox design is nearly identical to that on the original TPR80, and in fact uses gearsets left over from that programme. The overall design philosophy has been to produce a reduction drive with the lowest possible parts count, using as many commercially available parts and production processes as possible, to give a minimum life of 3000 hours and maximum flexibility in the available reduction ratios. “We chose a two-stage design with twin layshafts that one might call a twin star-gear arrangement,” Seegers comments. “Two stages instead of one make for a gearbox with less frontal area. “All the gears are made from hardened and ground alloy steel. The teeth are first cut using a gear hob, then throughhardened for the highest possible bending strength, and finally carburised for maximum surface hardness and contact stress resistance prior to precision grinding of the involute profile.” The gearbox has a reduction ratio of 22:1, as per the TPR80’s original sizing for the Hunter UAV programme. Gearbox (and therefore power turbine) speed is sensed by a threaded monopole sensor that reads August/September 2023 | Uncrewed Systems Technology Bescast produced the investment castings for the gas generator turbine rotor (pictured) and the power turbine rotor
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