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64 Digest | Stone Aerospace Sunfish AUV be rugged enough to survive for long periods of time, even if a thruster or other mission-critical component failed. To that end, the chassis needed sufficient mounting space for redundant thrusters, so that if one or two were damaged or lost a signal link, the AUV could still follow its optimal path back to the launch or recovery point. The hull’s exterior is made from carbon fibre panels. While the material is popular across unmanned vehicle designs, primarily for its high strength-to-weight ratio, Stone Aerospace cites its drag efficiency as being an even bigger driver behind its selection. “The choice of hull material and shape has also been heavily affected by our target for the highest instrument count possible in the least volume, so we’re constantly looking for ways to integrate sensors into the vehicle more tightly,” Stone adds. “By and large, we go with the smallest sensors possible, and where we can’t fit them in a cylindrical pressure vessel, we put them outside – so the carbon fairings of the hull prevent them from inducing drag. “There are newer hull designs we haven’t released yet, but what we have now is primarily the optimisation of drag and weight. It’s taken a huge amount of CFD and CAD simulations to arrive at this point.” An internal pressure equivalent to 1 atm is maintained, although the company has considered using a pressure-balanced oil-filled hull architecture for extreme missions such as exploring the Challenger Deep. However, the 1 atm target is key to the Sunfish’s neutral buoyancy in all orientations (with added proprietary technology for modelling how to retain that buoyancy when installing and arranging new internal components). A series of redundant O-ring seals and numerous fasteners serves to seal in that pressure. Propulsion and power The Sunfish’s electric thrusters have been designed and built in-house to ensure a range of mission-critical parameters for long-range autonomy. The first of these is energy efficiency: with COTS thrusters, the company determined that perhaps 25- 28% overall efficiency could be achieved. However, Stone says, “We’ve been reaching more than 50% efficiency with our latest thruster designs. They’re all brushless DC motors using ceramic bearings.” The need for redundant thrust, roll, pitch and yaw governed the number of thrusters on the AUV and how they are oriented. The standard design has two thrusters that act as the prime sources of propulsion, with dual redundant attitude thrusters installed for every degree of freedom, to enable manoeuvres through environments that tethered ROVs cannot go without significant risk of snapping their umbilicals. The attitude thrusters consist of a horizontally oriented motor-propeller December/January 2021 | Unmanned Systems Technology Cylindrical pressure vessels are used wherever possible to house sensors and other electronics