72 understand their case-specific thermal requirements, and request units in volumes sufficient to justify the expense of a custom radiator design. “Honestly, an aviation engine isn’t even necessarily the optimal engine for use in a Genset, because our generators want to run fast and fixed-wing aircraft engines want to run slowly. Theoretically, the DHA120 could run at 55 kW if spun at 9000 rpm,” Ricci adds. “But the problem then is that the ideal engine for us might be something like a high-revving snowmobile gasoline engine – something that runs most efficiently near its redline – but those engines aren’t made by companies with a pedigree in aircraft engine optimisation.” LaunchPoint is now qualifying and characterising some non-aircraft engines to modify them for aviationcritical requirements such as weight optimisation, direct drive (or at least the absence of transmissions that could obstruct mounting of the generator) and automatic digital control. The company is also open to collaboration with UAV engine companies capable of creating an ICE suited to high-speed, hybrid applications. DHA120 axial-flux electric motor The DHA120 measures 317.5 mm in diameter, 45 mm in length and is constructed with dual Halbach array rotors sandwiching an ironless stator, with 60 poles per rotor and stator. “Our ironless rotor and stator give our motor-generator such low inductance that we can push pole counts and frequencies much higher than conventional systems, and that helps make our electric machine lighter because we’ve got lots of thin poles, rather than a few really big, heavy poles,” Ricci says. “It also helps the voltage to stay very well regulated as the load changes.” While the machine is built around 30 pole pairs, each pole in the rotor is a section of the Halbach array, constructed of three permanent magnets: one upper part and two diagonal parts either side of it, making for 180 magnets per rotor, and 360 per DHA120. The air gaps between rotor and stator are typically 1 mm, although they can be adjusted to specify the voltage for different applications or allow more clearance for axial movement in the bearing system. Unlike radial-flux motors, where the air gap is fixed, LaunchPoint’s rotors can simply be moved apart along the stator shaft (or crankshaft) if a reduced bus voltage or increased shaft speed is required. “When we started, people warned us about crankshaft end play – when the crankshaft slides in its bearing with very slight longitudinal movements, not enough to cause piston scraping front or back, but enough to make engineers think the rotor and stator were going to touch,” Ricci recalls. “We’ve designed the axial gap in the generators to be larger than the crankshaft end play in the engines we’ve used in our HPS400s and other Genset products. We’ve tested and found that the engines exhibit acceptable end-play, even when we cycle them to the limit of their TBOs. But if someone wants a different engine that shows some end-play, we can space out the rotors and stator to compensate, albeit with some voltage and maybe performance loss. There is one right now, for instance, which we’ve set for 2 mm air gaps, where maximum continuous power drops to 30 kW.” Another advantage given of LaunchPoint’s axial-flux design is the self-cooling ability in that air gap. As the rotors spin, they entrain air through machined holes on the inner part of the motor, inducing it to spin and blow outwards at high pressure between the rotors and stator with a circular motion (discussed further below). “Integrating a high number of very thin poles is also the key to unlocking six-phase power; conventional, slotted designs are limited to three-phase AC because they can’t make their poles June/July 2024 | Uncrewed Systems Technology The DHA120 axial-flux motor features 60 poles per rotor and stator, two ironless Halbach array rotors, an ironless stator, and a self-cooling method akin to Tesla turbines
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