LIKE fish, subs have always used fins and tails to steer through the murky depths. But space technology is offering an alternative that will make them not only more manoeuvrable but also sleeker.
Two marine engineers in the US say that by adopting steering technology used on orbiting satellites, submersibles can ditch their hydroplane fins and rudders 鈥� and the associated hydrodynamic drag that eats fuel.
What鈥檚 more, the space-age control system will make it easy to achieve one of the toughest underwater manoeuvres: staying put. This might be useful, for instance, for scientists who use submersibles to shoot pictures of deep-sea volcanic vents 鈥� also known as black smokers 鈥� on the seafloor.
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As effortless as staying still might seem, it鈥檚 tough underwater, where small cross-currents can make a sub drift. While hydroplane fins can normally compensate for this, the maximum steering force they can exert is proportional to the square of the sub鈥檚 velocity. So at speeds of less than one knot, they can have almost no effect at all. Consequently, it鈥檚 very tricky to make the minuscule manoeuvres needed to keep a sub still.
But Craig Woolsey, an ocean engineer at Virginia Polytechnic Institute in Blacksburg, and Naomi Leonard at Princeton University in New Jersey propose replacing fins and tails with spinning 鈥渞eaction wheels鈥� 鈥� similar to those used to keep communications satellites and even the Hubble Space Telescope pointing in the right direction.
The principle is simple enough. A motor rigidly fixed inside the sub exerts a torque on a steel wheel, causing it to spin. But in doing so, the wheel exerts an equal and opposite torque on the sub, causing it to spin in the opposite direction. It鈥檚 a bit like the way the engine that turns a helicopter rotor tries to turn the chopper in the opposite direction: this is only prevented by the tail rotor, which opposes the torque.
So if a sub is fitted with three discs mounted at right angles, one for each axis of motion, the vehicle鈥檚 orientation can be precisely controlled by carefully altering the discs鈥� spins to create a resultant force. In a cross-current, the sub would be able to point its nose into the current to maintain a steady position, says Woolsey.
Remotely operated and autonomous underwater vehicles usually have numerous steerable propellers that work in concert to control the position of the vehicle. But a sub with reaction wheels would require only one propeller.
And the new guidance systems do not have to be big and heavy. Provided the motors can provide enough acceleration, a relatively small set of reaction wheels would do the trick.
Unlike fins and rudders, the wheeled guidance system is kept safe and dry within the sub鈥檚 hull, so it is better protected against collisions, fouling and corrosion. 鈥淚t means the vehicle is less prone to failure and much more streamlined at moderate speeds,鈥� says Woolsey. Such subs would be good all-rounders, capable of travelling at speed as well as staying stock-still.
Woolsey鈥檚 simulations have proved that the wheels will work in principle, he reports in the journal Automatica (vol 38, p 2053). His team is now building a prototype for real tests.
Another advantage of the internal steering system is that unlike a flapping rudder it won鈥檛 stir up the sub鈥檚 surroundings. 鈥淐ertainly with remotely operated vehicles it鈥檚 a well-known problem. As you鈥檙e trying to control your position and attitude, you鈥檙e kicking up sediment from the seabed and can鈥檛 see anything for a while,鈥� says Steve McPhail, who works on the University of Southampton鈥檚 Autosub. The craft is all set for an autonomous expedition under the Antarctic pack ice next year.
