CAPTAIN NEMO would be green with envy. The US Navy is developing stealth submarines that have exchanged propellers for artificial muscles that let them slip silently through the water like a fish.
A metre-long prototype has already been tested. It uses shape memory alloys (SMAs) as muscles, bending and flexing the hull to generate thrust. 鈥淚nitially we are focusing on unmanned underwater vehicles,鈥� says Othon Rediniotis at Texas A&M University, College Station. But the potential is there to put people in them, he says.
The aim is to find stealthier alternatives to noisy propellers that leave highly visible wakes, says Roshdy Barsoum at the Office of Naval Research, which is funding the research. Copying the way fish move was a natural starting point, he says. 鈥淭hey tend to shed less-turbulent vortices than propellers.鈥�
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The unusual property of shrinking when heated and expanding when cooled makes SMAs ideal for artificial muscles. 鈥淥ne of the benefits of SMAs is that they have very high power densities,鈥� says Rediniotis. This means they can produce a lot of force for their size and weight, so actuators can be made smaller and lighter.
The hull of the vehicle is divided into six rigid sections like vertebrae, each capable of being deflected with respect to its neighbouring sections. SMA wires, made out of an alloy of nickel and titanium called nitinol, separate each section and are counterbalanced by springs (see Diagram). When the SMA wires are electrically heated they contract, causing that section of the spine to bend. When the current is turned off, a built-in cooling system brings the SMAs back down to below the critical temperature. This makes the wires expand to their original length, with a little extra help from the spring.
By carefully controlling the currents used to heat the SMA wires in each section, a range of wiggles can be produced. Making the tail flick further one way than the other causes the vehicle to turn. 鈥淲e have been able to flap the tail as fast as five oscillations a second,鈥� says Rediniotis. He refuses to speculate how fast it could travel.
While other attempts have been made to make fish-like machines that propel themselves by flicking their tails, they have all relied upon noisy motors and complex linkages, says Barsoum. One of the advantages of using SMAs as actuators is that they involve fewer moving parts, while allowing you to exert very large forces. 鈥淎nd they don鈥檛 fatigue because it鈥檚 a natural phase change, like the transformation from water to ice,鈥� he says.
One of the biggest challenges that lies ahead is finding a suitable way to power the SMAs, says Rediniotis. The prototype was linked to a power cable, he says, because batteries would have been too big.
鈥淲e are currently working on developing SMA-powered actuators that are fuel powered,鈥� he says. Fuel cells, which are quiet, are obvious candidates. Fuels such as hydrogen also have higher energy densities than batteries, so a fuel-powered vehicle could be lighter.
Barsoum also hopes that a fish-like propulsion system will be more energy efficient, since it will produce less-turbulent wake vortices, experience less drag and so use less fuel. Attempts to pit fish-mimicking systems against propeller-driven vehicles have so far failed because the two methods proved too complex to compare head to head.
Until Rediniotis builds an untethered version it will be difficult to know if the idea will work. And for that we will have to wait until 2003, he says.