Robotic Operations
	Introduction The Three Sisters AUVs Superman AUV Tug ROV Drillette ROV Distant Docking Station Spiral Sensor Array Sensor Fleet Sensors Tools Docking with Habitat Power Mobility Structure Navigation

Mobility

Propulsion

Propulsion systems for AUVs and ROVs are different because their needs vary and they have different amounts of power available. The current design, therefore, includes the following considerations:

AUVs

They will be equipped with two horizontal thrusters that will provide heading control. Two vertical thrusters for vertical control might be desirable, although the lack of power might rule out this option. Depth control might be therefore achieved by slight changes in buoyancy instead. It is possible to use a dynamic system that compensates for pressure at any depth through a complex system of valves, ballast tanks and compressed air. Unfortunately, a dynamic system requires a large amount of space and may not be reliable in maintaining proper buoyancy, so this option might have to be ruled out at the end. It is important to point out that our AUVs will not need as much vertical mobility, yet they will need to travel relatively long distances. Thus, focusing on horizontal propulsion will yield better results.

ROVs

The ROVs will need great maneuverability and accurate displacement along the vent fields. Therefore, vertical propulsion will be of utmost importance for their control. Two horizontal thrusters provide heading control and four Nektor (TM) propellers for vertical maneuverability. The Nektor propulsion system was designed to provide maximum underwater agility by using incredibly thin and flexible propellers. At the time, Nektors have been tested in the Pilotfish ROV, and they have provided a response rate up to 20 times faster than regular thrusters, as well as a greater angular acceleration. (see Nekton Research) In theory, four Nektors can provide omni-directional thrust (360 degree propulsion), just enough to suit our goals, but we believe that horizontal thrusters will be better for traveling long distances. The Nektors will be placed at regularly spaced intervals along the hull's perimeter (45, 135, 225, and 315 degrees).

Nektors -- 10 kW of power (approximately 13.5 hp) are enough to move a 150 kg. vehicle. This will depend, however, on the actual design of our vehicle as well as its weight.

General Considerations

The unmanned underwater vehicles (UUV's) will at all times travel at speeds of around 1 to 2 knots (1.8 to 3.6 km/hr). This is intended to minimize power consumption and provide a longer battery life. According to Prof. Kimball of the MIT hydrodynamics laboratory, great propeller efficiency at these speeds is not a great concern, although might be desirable. In sum, energy efficiency differentials depending on different kinds of propellers at slow velocities are depreciable, and that is why most AUV teams do not take propeller design into consideration when building their AUVs or ROVs.

Other issues to consider when designing and placing the thrusters and nektors is the sea-floor. Turbulence caused by the propellers might damage some geological features as well as sea-life.

Additional information on propulsion with Nektors at great depths