Unmanned Systems Technology 036

24 Dossier | Saab Sabertooth AUV control and comms electronics and inertial sensors. On the outside of each pod is a layer of pressure-resistant foam, protected by a polymer skin, to give the vehicle neutral buoyancy. All the spaces in the vehicle around the e-pods are free-flooding. The pressure hulls for the 1200 m- rated version are made from aluminium alloy, while those for the 3000 m vehicle are carbon fibre reinforced plastic. To withstand the pressure at depths between 1200 and 3000 m, an aluminium pressure hull would need much thicker walls, and would be bulky and heavy as a consequence. “Using carbon fibre keeps the weight down, so we need less buoyancy, and pressure-resistant buoyancy material is really expensive,” Siesjo explains. Building pressure hulls from carbon fibre is challenging, because it is electrically conductive and will corrode if exposed to seawater in the wrong way, he points out. If the fibres rather than the resin come into contact with metal such as fasteners or inserts, salt water can set up galvanic corrosion of the metal, with hydrogen evolving into the composite and forming blisters, for example. Siesjo emphasises the need to seal it properly, paying particular attention to the sealing surfaces at the interfaces with the pressure vessel end caps, which are usually made from aluminium, although the metal is anodised to provide protection from corrosion. It is important here to prevent direct contact between the carbon and the metal to eliminate the risk of galvanic corrosion of the metal, for example. A pair of polymer side plates provide some protection from impact and a degree of extra buoyancy. They feature circular openings at the front on either side for the lateral thrusters, ahead of which are two vertical thruster ducts that pass through the upper and lower surfaces. Two more openings in the side plates at the rear allow water to flow to and from the forward/aft thrusters. Ahead of these openings is the centrally mounted third vertical thruster duct. Polymer plates are also fitted to the front, and they provide mounting points for relatively lightweight devices such as LED lights, video cameras and forward- looking imaging sonars, including members of Teledyne’s BlueView family. “Then, all around and below, there is space to mount payloads such as sensors and tools,” Siesjo notes. “Exactly where you put those is of course highly dependent on what they are and in which direction they will be used.” Heavier payloads and equipment have reinforced box mounts, and are attached to the framework with bolts made from the same A4-grade stainless steel. Even though the Sabertooth’s shape is a compromise, hydrodynamic efficiency is still very important. “In everything we do, even if we are working on box- shaped ROVs, we want to make the best use of the available power, so we run hydrodynamic simulations,” Siesjo adds. The principal CFD package the company uses for this is Fluent, from Ansys. “The biggest trade-off is ruggedness and flexibility versus range, in that a pure torpedo shape has maybe a tenth of the drag of a box-shaped vehicle, so you really need to choose which one you are going for,” Siesjo says. Its close relative, the military Double Eagle SAROV mine countermeasures vehicle, does not have to be quite so rugged. “That vehicle is quite similar but it is not expected to come into close contact with stuff on the sea floor. Even though it has a similar ‘flatfish’ shape it is more streamlined, because we can use parts that are a bit more delicate,” he explains. While the vehicle itself is not affected much by the sea state, launch & February/March 2021 | Unmanned Systems Technology Operating close to large pieces of seabed infrastructure is a primary mission for the Sabertooth, so it had to be made physically robust (Courtesy of Modus Seabed Intervention via Saab) The main trade- off is ruggedness versus range, in that a pure torpedo shape has maybe a tenth of the drag of a box- shaped vehicle