ARTICLE

May 2004

Navy Adjusts Course for Underwater Robots

by Sandra I. Erwin

The Navy is updating its blueprint for future undersea robotic vehicles to reflect recent changes in military strategy. A revised “master plan,” expected in about a year, will emphasize joint-service operations and interoperability between unmanned undersea vehicles and conventional ships.

UUVs would be deployed as underwater scouts and navigation aids. Launched from submarine torpedo tubes, they would search for mines and other underwater threats. Ambitious plans to spread UUVs across the fleet, however, have been toned down by the complexity of the technology and soaring costs.

The existing UUV blueprint, written in 1999, has become outdated as a result of new war-fighting tactics, the state of technology and the planned introduction of the Littoral Combat Ship in 2007. The LCS is envisioned as one of the primary carriers of unmanned undersea vehicles.

Unlike commercial submersibles and small UUVs used for explosive ordnance detection, the Navy’s future undersea robots must operate autonomously, collect, analyze and transmit intelligence to the host ship.

Emblematic of the technology challenges of underwater robots is the LMRS, or long-range mine reconnaissance system. In development since 1999, the torpedo-size LMRS is intended to deploy from submarines, search for mines clandestinely and identify them. Despite significant cost overruns, the Navy has stuck with LMRS and plans to continue to upgrade it with new sensors and communications systems. One fundamental shortcoming in LMRS, however, is that it can only perform one type of mission. With an expected price tag of $20 million to $30 million per two-vehicle system, LMRS is viewed as too expensive for a single-mission UUV.

Seeking to get more productivity out of these pricey robots, the Navy is planning to introduce by 2009 a “reconfigurable” 21-inch UUV, with a modular design allowing for different sensor payloads that would be tailored for specific missions, such as surveillance, navigation or communications relay.

Before the Navy can set any realistic schedules for fielding UUVs, however, it must improve the performance of the batteries that energize multiple onboard systems.

Powering autonomous underwater robots for long-endurance missions is among the toughest problems that researchers face, because batteries generally fall short on range and stamina. The only high-performance batteries that allow LMRS to complete a 120 nautical-mile sortie are disposable lithium-thionyl-chloride packs, which cost up to $250,000. Because lithium is hazardous to the environment, the Navy must pay several thousand more dollars to dispose of each battery after it’s been used. “That makes for an expensive mission,” noted an industry expert.

Other options, such as rechargeable lead-acid and silver-zinc battery packs, do not meet the LMRS performance requirements.

“You can’t achieve range and endurance with rechargeables,” said Capt. Paul Ims, Navy program manager for unmanned undersea vehicles. The rechargeable silver-zinc battery provides about 15 hours of run time. The expendable lithium pack lasts more than 40 hours.

The LMRS prime contractor, the Boeing Company, is searching for better power alternatives. “For future planned upgrades of the LMRS, Boeing is looking at new battery technologies that are more efficient and provide longer operational life,” said a company spokesman.

A rechargeable lithium battery would be desirable, but the Navy is reluctant to bring those aboard submarines, as long as they continue to pose safety risks. According to industry experts, when the lithium batteries are recharged, they have potential for a “rapid discharge,” or what some people would categorize as an explosion.

“When you recharge lithium batteries, they have exhibited on occasion this sort of problem,” one expert said. The risk of a rapid discharge may be acceptable on other types of systems, but not inside a submarine.

Cmdr. Jerry Burroughs, the Navy officer in charge of the LMRS battery certification said that the lithium battery “can be made safe for use onboard submarines.” More tests are expected in the months ahead.

Another technology hurdle ahead for UUV designers is the Navy’s desire to employ these vehicles to “track and trail” enemy submarines. That mission will require a much larger vehicle than the 21-inch UUV, Ims said in a presentation to the Association of Unmanned Vehicle Systems. An “analysis of alternatives” now is under way to study options for a “large-displacement” UUV, Ims said.

The submarine track-and-trail mission requires considerably more endurance and sensors than what a 21-inch UUV can provide, said John Brandes, senior program manager for undersea robotics at Lockheed Martin. The company received a contract last year to develop a 21-inch “mission reconfigurable” UUV, intended to replace the LMRS.

A much larger vehicle would be needed to accommodate sensors, computer processors, data storage devices and the batteries required for track-and-trail operations. A big enough battery to power a UUV for several weeks would fit only in a large vehicle.

Although the Navy has not committed to any specific size, Brandes speculated that the large-displacement UUV would have to be at least 10 tons, compared to the 2-ton MR-UUV.

A vehicle that size would not fit in torpedo tubes, but could be launched from submarines equipped to carry SEAL mini-submarines or similar submersibles, Brandes said. The Navy’s SSGN submarines, which are being refurbished to carry cruise missiles, also could be candidate platforms for large UUVs. The missile tubes are 7.5 feet in diameter and more than 30 feet long. The LCS could serve in that role, as well.

But no matter what size its UUVs, the Navy still must overcome the same difficulty that has beleaguered other robotic programs in the Defense Department: the complexity of making a vehicle operate autonomously, without an operator in the loop.

Unmanned aircraft for the most part are controlled by human operators almost continuously. Ground vehicles are more complicated, because they have to deal with the terrain. As the world witnessed during DARPA’s Grand Challenge robot race in the California desert last month, without an operator in the loop, vehicles have a tough time negotiating obstacles.

“The autonomy issues are similar for all of us,” Brandes said. Under the sea, the UUV doesn’t encounter many obstacles, and has sensors that keep it from hitting the bottom. The problem with vehicles that operate in the ocean’s depths is the poor connectivity of the communications links. “When it comes to communications, things don’t travel through the water as well as through the air,” Brandes said.

To get better communications, the vehicle would have to stay close to the surface, making it more vulnerable to detection. The dilemma for UUV designers, said Brandes, is “do you want constant communications or something more covert? Those are the tradeoffs you deal with.”

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