On March 19, the guided-missile submarine Florida* *fired more than 90 of the roughly 120 Tomahawk cruise missiles that took down Libyan air defenses, clearing the way for NATO strike planes. It was the major-combat debut for America's fleet of "SSGN" subs. Each of the four vessels packs up to 154 Tomahawks, making them some of the world's most powerful warships.

But there's a problem. The SSGNs, commissioned in the last five years, are actually modified ballistic-missile submarines dating from the 1980s. Around 2026, their nuclear power cores will wear out. At that point, the Navy must replace the subs ... or lose a huge portion of its missile firepower. But building new submarines the size of the SSGNs could cost up to $8 billion apiece, nearly half what the Navy spends on ships every year. In other words, way too much.

Fortunately, Electric Boat in Connecticut, the Navy's main submarine-builder, has a plan. Instead of designing new SSGNs from scratch, Electric Boat intends to pack the current Virginia-class attack submarines with extra missiles – and give them new eyes and ears in the form of sophisticated underwater and flying robots. The meaner, smarter Virginias wouldn't carry as many missiles as today's SSGNs, but at just $2 billion a pop, the Navy could afford many more of them.

The *Virginia *missile-boat plan is key to preserving the Navy's overwhelming firepower advantage. It's also the subject of my latest feature for AOL Defense.

The four current SSGNs were still being modified when, in 2003, Navy officials approached Electric Boat with the germ of an idea. The brass had crunched the numbers and knew it would have a submarine missile gap in the 2020s. Could Electric Boat tweak the *Virginia *boats then in production to duplicate some of the SSGNs' capabilities?

Electric Boat took a hard look at the *Virginia *design and concluded that the 377-foot submarine could be "stretched" to carry more than its standard loadout of 12 Tomahawks. "We looked at a variety of length plugs to see how much the ship can take without changing its performance," program manager John Biederka said. Ninety-four feet was the optimal extension. With the additional space, the *Virginia *design could accommodate the same seven-foot-diameter missile tubes as on the SSGN – though only four of them, compared to 22 on the older sub. Each tube packs seven Tomahawks, potentially boosting the Virginia's missile arsenal to 40.

That's a far cry from the SSGNs' 154 Tomahawks. But with at least 10 Virginias tentatively slated to get the hull extension starting around 2019, the Navy will have what Electric Boat vice president John Holmander called a "more distributed" missile-sub fleet, rather than the capacious but concentrated four-boat force it has today. Plus, the Navy plans to equip the Virginias with new robots that can help the smaller missile boats make better use of their weapons.

First up: the so-called "large-diameter Unmanned Underwater Vehicle," projected to enter service in 2020. Three feet across and torpedo-shaped, this UUV will carry sensors and, eventually, its own weapons. The large UUV "will gain access to places that manned platforms cannot – minefields, shallow water, the sea floor," said Capt. Duane Ashton, head of the Navy's underwater robot development.

The step after that is adding an Unmanned Aerial Vehicle that can launch from a Virginia's large-diameter tubes and spot targets for the vessel's Tomahawks. "Combined with a submarine's traditional ability to provide a stealthy and persistent source of weapons in even the most access-constrained littoral environment, an organic [Unmanned Aerial System] will provide submarines a fully organic capability to detect, identify, precisely locate and quickly strike modern [Surface-to-Air Missile] engagement radars," wrote Owen Cote (.pdf), an analyst at the Massachusetts Institute of Technology.

The Virginia hull-extension is firmly in the Navy's planning but hasn't been funded yet. The undersea robot is funded but its development has barely begun. The sub-launched aerial drone is still just a concept. It could take all three to effectively replace today's SSGNs.

Photo: Navy

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