90 additive manufacturing (AM) was also vital for quickly testing and iterating the cooling system, particularly as the SLS machines could print structures using polymers that are stable at 200 oC. That meant the fans and housings were not only technically accurate demonstrators that could be integrated and tested on the powertrain and dynamometers, but also viable at a production level, given the complex air channel shapes the SLS machines are suited to making. “It helped the speed of our iterations that there are only three parts to the cooling system,” Kahnert notes. “The fins are printed as part of a plate that mounts onto the rotor, and the housing consists of an upper and lower shell, with the cooling passages integrated into the two halves. “In the future, it might be even easier to make those components through injection moulding, assuming we make them from plastic. We’re open to metals or composites, particularly if we get a build or use case with much higher temperatures, but we’d still need the fine geometric sophistication that AM offers.” Power management Acutronic also supplies the PMU, which is a customised version of its SSGNS200A-01. It has been developed through collaboration between EFT and Acutronic, with customisations being concentrated on the control logic, voltage outputs and thermal management. It incorporates a rectification unit for converting the alternator’s AC output into DC to charge the battery. It also provides the means by which the engine throttle is governed in order to manage the voltage. “Operationally, the first function the PMU provides is actually to use the operating battery power to start the engine,” Peryea explains. “We have logic and a series of controls in the rectification unit that will manage the entire engine start-up process from the perspective of the throttle. When it’s given a command from the HCU to crank the engine, it sets the throttle to the right position for starting, then cranks the engine by backdriving the AC alternator with power from the batteries.” Kahnert adds that the HCU communicates with the PMU via an RS232 connection, and essentially sits one level higher up than the PMU, the BMS and the ECU for the AX250 engine. After that the PMU activates a ‘warmup mode’, which is pre-programmed and can be customised for different engines and customers, and targets a specific rpm to enable the engine to warm up without placing any load on it. Eventually, the warm-up mode gives way to a voltage tracking mode, in which rectification is performed and the throttle servo is advanced or retarded to achieve the desired output voltage. The HCU supervises this mode, monitoring key health indicators such as data from temperature sensors across the powertrain to determine whether operations can safely continue, or transmitting data indicative of poor powertrain health to the flight controller to signal whether an emergency descent is needed. “That comes with several feedback loops that the PMU also monitors for gauging safety levels. One of them monitors howmuch current goes into the battery, to make sure we’re not sending too much and risking an overcurrent or associated damage,” Peryea says. “We’re also checking we don’t draw too June/July 2023 | Uncrewed Systems Technology Electric Flytrain is looking for suitable partnerships for flight testing its powertrain, after which it will start considering productionisation We have logic and a series of controls in the rectification unit that will manage the entire engine startup process from the perspective of the throttle
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