Water-Powered Robot Explorers

Oct. 11, 2003 -- Unmanned aerial vehicles (UAVs) have proved to be an invaluable assets to the Army troops who need to monitor miles of hostile terrain in Afghanistan and Iraq. But now the Navy hopes to take UAVs to a new — and very deep — level.

Its Office of Naval Research is helping to fund the development of new types of unique autonomous underwater vehicles (AUVs) that could help make monitoring the vast high seas more manageable.

These new experimental AUVs are strikingly similar to the Predator and Global Hawk drones used in the on-going ground conflict in Afghanistan. With slim streamlined shapes and wings, these pilot-less watercraft can be programmed to "glide" through certain routes to gather various bits of information using instruments stored inside their hulls.

But what separates these robotic gliders from other drones and underwater vehicles is that neither of them uses motors or fuel to move through the ocean depths. Instead, they rely upon changes in buoyancy.

In other words, they "swim" when they "sink" from — and "float" back to — the ocean's surface.

Sink to Swim

The Seaglider from the University of Washington in Seattle weighs about 110 pounds and is nearly 6 feet long. Inside the hull are computerized controls, sensor equipment, a GPS receiver, satellite communications system, a bank of lithium ion batteries and an empty reservoir tank.

Before launching the Seaglider, researchers can program the computer with the coordinates of which parts of the ocean it should go and measure. Once the AUV gets its bearing from the GPS satellites, a small electric pump transfers about 100 cubic centimeters of oil from an external bladder into the reservoir, making the Seaglider heavier and dense enough to sink.

As the vessel dives, a small motor pushed the bank of batteries slightly forward, shifting it into a nose down attitude. The water that rushes over the "wings" of the glider pushes the craft forward as it falls through the water.

Once it reaches a pre-determined depth, the process is reversed and the Seaglider begins to rise. As it goes up, the wings continue to give the Seaglider forward momentum, pushing it further along through the water.

To change from a straight-line course, the batteries are rolled from side to side inside the hull. The shift in weight causes the glider to "bank" and turn like an airplane.

The glider travels in this vertical up-and-down "sawtooth" pattern and navigates to its programmed destination using compass readings and "dead reckoning" — figuring out where it is based on how fast it's traveled since its last fix from the GPS satellites.

Once at its destination, the AUV collects the requested data — water temperature and salinity, for examples — using the on-board instruments. The glider then rises to the surface and transmits its finding back to the lab using the Iridium satellite communications system. Researchers can then send back new destinations and instructions to the glider.

Slow, But Long-Lasting

Charles Eriksen, an oceanography professor and one of the developers of the Seaglider, says that such a propulsion system isn't fast. At best, the glider can make about half a knot — slightly more than half a mile an hour.

But since it will use only one-half watt of electrical energy to produce that speed, Eriksen says the Seaglider has a range of "thousands of kilometers" and remain in the ocean gather data for much longer.

"We can operate one of these for a year and across whole ocean basins," says Eriksen.

Driven By Heat

Since one of the greatest limitations of the gliders is the finite amount of energy stored in the on-board batteries, Webb Research in East Falmouth, Mass., has a glider of a slightly different design.

The company's Slocum Glider operates on a similar principle to the Seaglider. But instead of electric pumps to move the oil, its AUV uses a complex system that involves a proprietary, temperature-sensitive material.

Like the Seaglider, oil is pushed into a balloon inside the Slocum Glider's hull. As the oil fills the balloon, it also displaces a specially developed liquid out of the glider's hull the glider into external tubes.

As the glider slips lower into colder ocean water, the liquid contracts into a solid form, pulling more oil into the balloon and sinking the glider lower.

Once it reaches a certain depth, the glider's computer turns a valve, allowing a tank of compressed nitrogen gas to expand and force the oil out of the balloon. The now-buoyant glider rises to the surface, where warmer waters cause the solid material to expand back to liquid form. As the material expands, it fills the empty balloon and compressed the nitrogen tank and prepares the glider for a repeat sinking performance.

Clayton Jones, a project engineer at Webb Research, says that since the propulsion engine is driven by the heat of the ocean, the range and endurance of its AUV is greatly increased.

"You're saving the battery energy for the sensors, navigation and communication equipment," says Jones. "A thermal glider will run for like four years."

Silent Spy Service for the Navy?

In addition to long range and endurance, the researcher says the gliders will be cheap in comparison to traditional research ships. The scientists say each hand-built prototype cost around $50,000 to $75,000 to build — a figure that could possibly be made even lower if a commercial venture undertook production.

But both teams admit that their gliders still need to be extensively tested — especially in deep, open waters. The Seaglider is currently undergoing such trials just off Cape Flattery in Washington. And Webb Research plans for open sea tests of the Slocum Glider sometime in January.

In addition to their own research, the Navy will get into the deep sea act as well. Thomas Swean, Jr., team leader for ocean engineering and marine systems section of the Office of Naval Research says both gliders will be part of a larger Navy exercise next September.

Swean says that the Navy is interested in the gliders' abilities to collect and monitor ocean conditions — capabilities that would be useful in naval conflicts. Collecting information about the waters off a hostile shore would help sailors tune their ships' sonar systems for optimal performance, for example.

What's more, "The gliders almost make zero noise," says Swean. And that would make them ideal underwater spies that could be used to help land special forces troops in advance of any sea-based invasion.

If next year's open water tests go well, Swean says it's possible that the Navy could be using underwater gliders within two or three years.