Unmanned Systems Technology 021 | Robot Aviation FX450 l Imaging Sensors focus l UAVs Insight l Liquid-Piston X-Mini l Riptide l Eurosatory 2018 show report l Zipline l Electric Motors focus l ASTS show report

68 precise buoyancy control at any depth. To dive deeper therefore, more gas can be vented and the craft can dive without using as much power. It does mean though that the operation of the battery is an integral part of the control system. The battery will have its own onboard battery management system that will control the rate of flow of gases through the battery and so control the buoyancy. This data then has to be integrated into the control system in a closed-loop algorithm. The new battery system is being tested on the midrange 7.5 in platform, and the plan is to develop versions for the smaller A-size system and the larger 22 in one. On the motor side, the company chose COTS components that were cost- effective rather than the most efficient. The main benefit came from having the motors externally mounted so that they run down to 6000 m without a shaft seal. The motor is rated to 350 W but typically uses 2 W to generate 2 knots of propulsion. That does mean though that the craft can run at up to 10 knots for a short time with the alkaline batteries. “In the longer term we are looking at quieter custom designs,” says Smith. “Self-noise is critical for some applications.” These include surveillance to avoid acoustic monitoring but also to avoid the noise of the craft potentially interfering with its own sensors. The physical design uses a coupled elevator and rudder with active roll control and stability from three fins. The dorsal fin provides the rudder function while the lateral fins provide elevator control. The team initially used a commercial propeller, then designed an internal propeller optimised for 3-4 knots. This was initially 3D-printed to test out the performance in the prototype and then moved to injection moulding, as better materials can be used, says Smith. The body of the craft can be used in a number of ways for different payloads with dry or flooded sections, and the ability to combine both is attractive, says Smith. For example, for a towed sonar array the basic penetrator sensors can be mounted in a wet section at the rear, with cameras in a mid-body dry section. That means the camera can also include onboard storage, something that is absent in a lot of the deeper sea cameras that are built for remotely operated vehicles that stream video back to an operator. August/September 2018 | Unmanned Systems Technology New battery technology As a result, Riptide is looking at new types of battery power. It is working with L3 Open Water Power on an aluminium-seawater battery designed to fit into the smallest UUV. The craft would have to be longer, at 30 in (762 mm), in the A-size design to accommodate the aluminium battery. The company is modelling larger, 7.5 in (190 mm) diameter designs that would provide 2.5 kWh of power, rising to 300 kWh for the 22 in (559 mm) version. That would provide a 1000-mile range for a system without a sensor, travelling at 2.8-3 knots for up to a month. However, the energy density of aluminium-seawater technology is lower than with alkaline or lithium cells, limiting the speed to 3 knots. But a key advantage is that it is pressure-tolerant. A UUV typically operates by slightly positive buoyancy, and it goes deeper by moving downwards under power, which takes energy. The aluminium cell generates hydrogen as a by-product as it consumes the aluminium to generate power, so by controlling a vent controls the buoyancy of the vehicle. That means the system can be designed to give a The UUV features a flexible battery architecture that can use commercially available alkaline AA cells Everyone wants something different, so we have set things up to give that flexibility and not make every integration hard