Uncrewed Systems Technology 044 l Xer Technolgies X12 and X8 l Lidar sensors l Stan UGV l USVs insight l AUVSI Xponential 2022 l Cobra Aero A99H l Accession Class USV l Connectors I Oceanology International 2022

80 Dossier | Cobra Aero A99H has a special muffler we’ve designed specifically to make it quieter than other boxers.” Drag reduction benefits can be gained from the long, thin aspect ratio of the I3, lending it well to aerodynamic cowls, and as a liquid-cooled engine the A99 requires no exposure to external airflow (with a radiator posing far less disruption to aero than finned cylinders) and can largely be packaged inside the hull. One final benefit worth noting is the phenomenon of ‘cross-stuffing’. As the three cylinders share a common exhaust manifold, the pressure pulse triggered by one piston opening an exhaust port is exposed to the other two cylinders’ ports. In particular, that means the cylinder closest to compressing its charge can trap more of it. “It’s the same phenomenon that ideally you get with a tuned exhaust system, where you have a wave that is reflected and comes back,” Vaglienti explains. “But instead of a long tube with your own pressure pulse reflecting and coming back, you’re getting the pulse from your neighbour 120 º earlier in the cycle. Cross-stuffing boosts the power a bit and improves the engine’s fuel efficiency.” Design & AM The first design steps, in early 2018, focused on the cranktrain, with a goal of creating a robust and lightweight crankshaft. The crankcase was then topologically optimised around that, using some of the latest available CAD tools such as HyperWorks to identify and map each individual point of mechanical stress throughout the block, in order to effectively carve away all the metallic material that wasn’t stressed and therefore wasn’t helping to hold the engine together. “We iterated that process three or four times to end up with an engine case designed for maximum stiffness with minimum weight,” Hilbert says. “And around that time we were also starting to dip our toes into metal AM, which enables us essentially to print a full bill of materials in a single build-plate.” The build-plates printed by Cobra’s metal AM systems measure about 10 x 10 in, and feature both crankcase halves and all three cylinders. That enables them, and their cooling jackets and ancillary component inserts, to be extruded to very close tolerances before being moved to CNC machinery for cutting the metal engine parts from the plates. The material is AlSi 10, with 0.1% manganese. Hilbert notes its similar mechanical and structural properties to A356 cast alloy, with some marginally reduced thermal conductivity compensated for by the liquid-cooling system. “The average density of a casting is in the 80th percentile – there’s a lot of porosity in even very good block castings,” he adds. “On the other hand, AM parts like these are 97-99% dense, with much lower part-to-part variability as well.” Metal AM enables complex structures to be built but it is still time-consuming. Printing all the major crankcase parts on a single build-plate helps shrink lead times to 5 days per print, before being fastened together with titanium bolts. Hilbert notes that the build-plate design incorporates some proprietary features to make the job of milling the cylinders and crankcase halves from the print easier, saving considerable time and tooling by not having to create special fixtures for first block-cutting operations. “Before 2017, when the A33i hadn’t June/July 2022 | Uncrewed Systems Technology The intracycle crankshaft torque of the A99H compared with the B100 boxer twin – the I3 engine visibly suffers lower torque ripple than the boxer twin The plenum is additively printed from PA12 Nylon, with the cross- stuffing effect ensuring that each cylinder breathes independently