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66 Dossier | Turbotech TP-R90 and TG-R55 the thousands of hours, and broad multi-fuel capability, potentially enabling significantly reduced operating costs compared with a wide variety of other UAV engines. Operating concept and principles Turbotech’s fuel-based propeller-drive engine and hybrid-electric engine are built around largely similar engineering philosophies and mechanics. Both are single-spool engines (with a single compressor and single turbine), and both are currently rated to 3000 hours of operation between overhauls. The TP-R90 has a dry weight of 64 kg (74 kg with electronics, and fasteners), and runs on a range of fuels, including Jet A-1, diesel, UL91, avgas and biofuels.  It produces a continuous output of 90 kW at its maximum continuous speed (at which the propeller runs at 2272 rpm and the turbine shaft at 80,000), with a maximum power output of 100 kW (with a boost supplied by a starter/alternator), and a specific fuel consumption of between 18 and 25 litres of Jet A-1 per hour at cruise. It measures 1084 mm in length, from propeller governor to end-cap, and 412 mm at its widest diameter (from corner to corner of the gearbox).  The TG-R55 meanwhile weighs 55 kg dry and 65 kg when its electronics and bolts are added, and is 1015 mm long and 412 mm across. It produces 53 kW of continuous DC power at the point of generator output (for an efficiency rating of 26% from the fuel tank to the power output) up to a peak of 55 kW, which can be supplied at anywhere between 400 and 900 V, depending on end-user requirements. Apart from UL91 it can run on the same fuels as the TP-R90. During typical operation, the TG-R55 will consume 15-22 litres of Jet A-1 per hour. Assuming 50 kg of Jet A-1 (for an overall powertrain-plus-fuel weight of 115 kg), it can provide an aircraft with 155 kWh of electricity – a capacity that would otherwise require around 1000 kg of batteries. In both systems, a compressor sitting between the forward and rear sections provides the initial air intake. The air feeds back through an annular channel into the rear heat exchange half. Upon reaching the physical rear end of the engine, the air is directed forwards again (by running into a second, inner set of channels) into the engine’s combustion chamber, where it is mixed with fuel and is burnt. The pressure from the combustion chamber escapes through nozzles that drive the turbine wheel (which is cast from a nickel superalloy). This provides the rotary driving force for the shaft and the compressor, before passing through the exhaust vent at the back of the engine. With the turbine wheel mounted on the rear of the shaft and the compressor wheel mounted in the middle, the forward shaft is where the two systems differ. The TG-R55 has an electrical generator on the front of the turbine shaft, whereas the TP-R90 integrates a gearbox to reduce the speed of the turbine from 80,000 to 2272 rpm. The reduction is carried out by a proprietary arrangement of straight-cut stainless steel gears, and four engine mounting points are located around the periphery of the gearbox enclosure. In the future, the gearbox might be reconfigured to drive a helicopter rotor, if such a version were requested.  The performance, timing and health of the engine are controlled and monitored April/May 2020 | Unmanned Systems Technology The compressed air is cycled and heated through the heat exchanger before entering the combustor (Courtesy of Turbotech) The bearings we use are ceramic roller bearings, so oil isn’t needed to help reduce friction; 90% of it is to cool the bearings