NSL Update 12-20-01
The US Navy is developing "stealth submarines" that have no
propellers but are equipped with artificial muscles, enabling them to swim
silently through the water like a fish. A meter-long prototype (3.25-feet),
built by Texas A&M University, can flap its tail like a fish.
Propellers are submarines' weak spot as they cause noise and a wake of disturbed
water, leaving the vessels vulnerable to enemy sonar or aircraft. Fish, however,
have evolved in favor of efficiency, using muscles to move their tail and fins
and propel themselves sleekly through the water. The Texas prototype comprises a
hull divided into six sections like vertebrae.
They are rigid but each can be deflected with respect to their neighboring
sections. They are pushed in and out by wire "muscles" made from shape-memory
alloys. These alloys are a novel mixture of strong, resilient metals that
contracts when it is heated beyond a certain temperature and then expands,
recovering its original shape, when it cools back down below that point.
The prototype's wires, made from nitinol, an alloy of nickel
and titanium, are electrically heated, which causes them to shorten and thus
pull the section in. A built-in cooling system then brings the wires down
to below their critical temperature, which, with help from a spring, causes them
to expand and thus push the section out again.
By carefully controlling the heating and cooling and
coordinating the movements of each section, the prototype can be made to wriggle
forward, fish-like.
The top rate, so far, is five tail-flaps a second, although the researchers are
tight-lipped about what speed that gives.
One of the biggest challenges is finding a power source.