Unmanned Systems Technology 036

26 Dossier | Saab Sabertooth AUV recovery platforms are. With smaller vessels using simple cranes, most people would limit such operations to sea state 3 or 4, Siesjo notes, but dedicated dive ships boasting heavy-duty launch & recovery equipment can recover ROVs and AUVs up to and even beyond sea state 6. Energy storage and distribution Saab assembles its battery packs from lithium-polymer cells supplied by Kokam. They are mounted inside the two pressure hulls, along with the battery management system. Siesjo notes that there are some off-the-shelf battery packs that are oil- filled and pressure-compensated, and therefore don’t require the protection of a pressure hull, but that Saab decided to build its own and put them inside the hulls to get maximum buoyancy from the system as a whole. While most sensors draw their power straight from the battery bus in order to make the most efficient use of the available energy, the power conditioning electronics also include a DC-DC converter that provides voltages required by the computers and other loads. The standard auxiliary supply is at 24 V, while there is also an unregulated 250 V supply for more power-hungry loads such as cleaning brushes and manipulators. Because they are inside the pressure hull, the power conditioning hardware is standard industrial ‘dry’ equipment, the main challenge associated with which is cooling. If it is possible to mount a piece of equipment in direct contact with the hull, it benefits from efficient cooling by conduction. “If on the other hand you end up mounting it in the middle of a pressure hull then you will be warming the air in there and creating problems,” Siesjo says. One solution to this are heat pipes incorporated into the Sabertooth. A typical heat pipe is a sealed copper tube containing a wicking structure and a working fluid, with an evaporator section at one end and a condenser section at the other. The evaporator is thermally coupled to a heat source, such as a component that needs cooling, while the condenser is connected to a heat sink, such as the wall of the pressure vessel that’s exposed to the surrounding water. The heat turns the liquid to a vapour in the evaporator, and the vapour travels to the condenser, where heat can flow out into the water. Lastly, the vapour turns back into liquid and returns to the evaporator. Saab builds its own wiring harnesses and generally uses waterproof electrical connectors made to its specifications by Scorpion. Siesjo describes them as metal shell connectors of very high quality. “In general, connectors are one of the main sources of problems on underwater vehicles – they always have been and probably always will be,” Siesjo says. “There are various brands and types of connectors that our customers prefer – or hate – so we often end up supplying vehicles with customer-specified connectors. For the core equipment that we couple up ourselves though, we use Scorpion connectors.” February/March 2021 | Unmanned Systems Technology The Sabertooth’s substantial polymer side plates are prominent here, along with apertures in the front fairing for navigation sensors including sonars and cameras (Courtesy of Saab) This floating dock surface deployment and recovery system is part of a set-up that includes a subsea garage to enable full decoupling from the support vessel (Courtesy of Modus Seabed Intervention)