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

84 run the engine at higher rpm to give less time for knock to be initiated. “Another way to solve knocking though is to have the combustion event happen quicker,” Hilbert says. “Chemically speaking, knock happens because of end-gases. As a flame front emanates from the spark plug, the gases in the cylinder that aren’t immediately burned become hotter and more pressurised as the flame front pushes them out towards the edges of the combustion chamber. “With a smaller bore engine running at a high rpm, there’s less time for that to happen and less room in which it can happen, because there’s less distance for the flame front to travel. That’s another reason why we’ve gone with an I3 rather than, say, a twin for the displacement, power and fuel type we wanted for this engine.” Further stability of the fuel-oil mixture has been achieved by consulting with VP Racing Fuels, who worked on heavy-fuel formulations, as well as choice of oil with Cobra and Power4Flight. Crankshaft philosophy As mentioned, the crankshaft has received particular attention from the early stages of the A99’s design process. A key goal has been to minimise the number of parts, as crank assembly and longevity can become more and more problematic as the number of cylinders goes up (in the absence of a monolithic shaft). “You can do a twin-cylinder crank in maybe three or four parts, no major headache there, but when you’re trying to get a three-cylinder crank perfectly spaced with the pins 120 º apart, trying to get all the bearings concentric with each other, the part count really goes up,” Hilbert observes. “Starting at the front, you have the nose, then the front bearing journal followed by the first crank pin with its counterweights, then another bearing journal, then the middle crank pin, third bearing journal, back crank pin, rearmost bearing journal and shaft section. “Originally, that all started as a seven- piece crank, but we already knew how tough that could be to assemble, align and afford. There are also potential durability issues from having so many June/July 2022 | Uncrewed Systems Technology Dossier | Cobra Aero A99H What the main bearings look like after being pieced together. One of the two holes (one for oil, one for fastening) is visible on the lower right The A99H’s water jacket design maximises cooling efficiency for heavy- fuel handling and helps vaporise the heavy fuel before combustion Endurance tests signify a direct causal relationship between how hot an engine runs and its TBO. Cobra and Power4Flight expect the A99H’s real TBO to stretch for thousands of hours