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25 including its Iridium modem, an Ethernet radio and the GNSS receiver. Each of those is connected to an antenna inside an RF-transparent fin atop the next section back, enabling data uploads, real-time comms and navigation updates when the UUV surfaces. Just forward of that antenna fin are two control surfaces that provide pitch control. The main CPU also controls the USBL transceiver, a sound velocity sensor, a drop-weight for emergency resurfacing, and a survey magnetometer from sister company Ocean Floor Geophysics. Of the control surfaces, Crees adds, “The turning radius has to be less than 30 m at 1 m/s forward speed, so the planes are quite small. The turning rate isn’t as quick as on other, smaller UUVs, but that’s a key and intentional part of the design.” Inside the hull under the antenna fin is the payload ‘canister’, with serial and Ethernet I/Os for connecting a range of survey sensors. It also connects to a BlueComm 200 optical modem for short- range data uploads. For payload integration, 1 litre and 1 kg of dry volume are available, with 27 litres and 5 kg of wet volume free for additional payloads. Almost any other payloads can be accommodated by adding new, modular hull sections. Situated under the payload canister is a Sprint-NAV 300 INS-DVL from Sonardyne for readings on inertial navigation and velocity over the seafloor, plus the suction anchor and its winch. In the middle of the vehicle is the fuel storage module, which contains two hydrogen tanks, one oxygen tank and two variable ballast tanks for containing the distributed exhaust water from the fuel cells. The aft tail section contains the fuel cell module, as well as the thruster motor and the aft control surfaces for steering the UUV. “The battery pack is a 40 kg cylinder in front of the electric motor, beneath the fuel cell. The tail skins come off the bottom with a couple of screws and a locking pin, for swapping out or charging the battery in situ,” Crees says. “It’s a 4.5 kWh battery, enough to run the stubby Solus-LR at a nominal speed for about 8 hours, meaning a full day of sea trials. By comparison, the fuel cell gives between 180 and 250 kWh of energy.” Hydrogen power The Solus-LR’s power system is charged by a proton exchange membrane (PEM) fuel cell system, which relies on hydrogen gas as the fuel and oxygen gas as the oxidiser. During normal operations, the UUV consumes 64 g/kWh of hydrogen and 505 g/kWh of oxygen, to provide 250 kWh of useable energy between refuelling stops. “Initially we conducted a study to compare the relative performance capabilities, benefits, drawbacks and so on for different fuel cell systems, versus using just lithium-ion batteries to power the UUV,” explains Reuben Meikle, senior mechanical engineer and project leader for the fuel cell. “At the time, we wanted 180 kWh of energy, so we wanted the highest possible energy density in the smallest possible physical volume. These targets went directly towards how large and potentially unwieldy a UUV we’d end up with, from having to accommodate the size of the onboard energy or reactant storage.” Size and weight were critical considerations, as syntactic foam must be added to keep the UUV neutrally buoyant. If the company had used only batteries with no range extender, the Solus-LR would have ended up being much larger, especially after installing the required foam. “Also, we couldn’t use a direct methanol fuel cell, as they need about two-and-a-half times as much oxygen as a PEM fuel cell of equivalent power,” Meikle explains. “They also produce CO 2 , and compressing that exhaust gas Unmanned Systems Technology | February/March 2020 The 8.5 m length of the Solus-LR means a trailer is needed to take it down a boat ramp, or a gantry crane to lower it into the water (Courtesy of Cellula Robotics)