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51 UUVs | Insight compared with the 1% variability for most glider AUVs. It works by changing the vehicle’s weight in water. When on the surface, the engine draws water in, which causes it to descend. At the bottom of its descent, it pushes the water back out, causing it to rise. “The comparatively large buoyancy engine allows the MOD2 to carry more payload instruments, and it can adaptively alter ballast by recognising when water densities around the vehicle change,” Turner says. “It adjusts its buoyancy using a store of reserve buoyancy to overcome those changes. “The engine itself is essentially an open-ended piston, with seawater at one end and the vacuum of the pressure vessel at the other. When it pushes out, it expands the volume of the vehicle; when it draws in it lessens that volume, with one opening where water moves in and out.” The engine generates about 12 lb of force, for horizontal speeds of between 0.7 and 2 knots. The MOD2’s capabilities will be showcased in late spring this year, when Exocetus will demonstrate it to a number of other universities in Chesapeake Bay and off the coast of Delaware, in the US. On the West Coast of America in 2018, Boeing sent its Echo Voyager XLUUV (extra-large UUV) to the AltaSea marine research campus in Los Angeles port, where several companies and universities conduct work relating to oceanographic exploration and study. The intention is to use it to mature the company’s technological capabilities in providing a platform that can operate at sea for months at a time. “Echo Voyager returned to sea in 2018 for continued sea trials,” says Dan Tubbs, Boeing’s deputy director of autonomous maritime and mission systems. “These are designed to expand the operational envelope from the initial test series, with deeper dives, longer endurance runs and greater sea states.” The 50-tonne XLUUV is powered by COTS diesel generators and has a 1000-gallon fuel capacity. These charge lithium-ion battery packs when it is on the surface, and the current packs enable more than two days of subsea operation between recharges. The system’s modularity of design allows extra batteries or the integration of future, higher-capacity battery technology to increase undersea duration further. As competition grows, increasingly large UUVs will be developed, bringing evermore innovative combinations of payloads and capabilities. Unmanned Systems Technology | February/March 2019 The engine itself is an open- ended piston, with seawater at one end and the vacuum of the pressure vessel at the other Boeing’s 50-tonne Echo Voyager UUV is powered by COTS diesel generators and has a 1000-gallon fuel capacity (Courtesy of Boeing)