The nuclear-powered submarine USS Florida was lying in wait, quietly submerged off the Libyan coast, when the order came from then-Secretary of Defense Robert Gates to launch its cruise missiles.It was the evening of March 19. Two days before, the U.N. Security Council had unanimously voted to approve Resolution 1973, authorizing a no-fly zone over Libya aimed at protecting civilians against the forces of Muammar Gaddafi’s repressive regime. To clear the way for the no-fly zone, the U.S. Navy and Royal Navy prepared a barrage of more than 100 precision-guided Tomahawk cruise missiles.

With a capacity of 154 Tomahawks, the Florida — a former ballistic-missile boat converted to non-nuclear missiles in 2006 — was by far the most powerful Tomahawk shooter off Libya. The night of March 19 she fired no fewer than 90 Tomahawks, with deadly accuracy. “I could see their professionalism and determination in their faces,” Chief Fire Control Technician Lee Taylor, from Florida’s strike fire control division, said of his sailors. They hunched over their consoles, carefully managing the underwater missile launches over a period of hours.

Later, then Chief of Naval Operations Gary Roughead was almost cavalier about the Tomahawk barrage. “That went off as we expected it would,” he said.It was the major combat debut for the four-strong fleet of converted Ohio-class missile submarines, or “boomers.” Known as “SSGNs” to the Navy, the former Ohios can also carry scores of Special Forces troops and possess their own stealth and high-tech listening devices to spy on enemy forces. “They provided unprecedented intelligence, surveillance, reconnaissance and terrific firepower, all from the sea,” said Vice Admiral Harry B. Harris, Jr., U.S. Sixth Fleet commander. “They are critical to winning any war against any adversary today and tomorrow.”

But the SSGNs are as old as the Ohios they’re derived from. Submarines only last as long as their nuclear cores. The cores of the 1980s-vintage Ohios — 14 ballistic missile versions and the four SSGNs — will expire starting in 2027.

When that happens the Navy will lose 600 of its roughly 9,000 Tomahawk-capable vertical-launch cells — and arguably the 600 most flexible ones, at that. Compared to surface vessels and other submarines, the SSGNs are “much better in terms of being a better source of Tomahawks,” Owen Cote, Jr., a naval analyst at the Massachusetts Institute of Technology, told Breaking Defense.

Today the Navy and the U.S. submarine industry are scrambling to come up with a plan for replacing the SSGNs and preserving their awesome strike capability.

Skeptics question whether it’s possible, given declining defense budgets and the high cost of submarines. “The sub forces are going to fall below the absolute minimum and there’s nothing anyone can do about that,” Capt. James Hay, a retired Navy submariner, told Breaking Defense.

But Connecticut-based Electric Boat, America’s main submarine builder, has a plan that company officials say could actually improve the SSGN force by replacing a small number of capacious Ohio-class missile submarines (19,000 tons displacement) with a much larger force of smaller Virginia-class vessels (currently 8,000 tons displacement) capable of being in more places at once.

The key to that plan, and to America’s undersea strike capability following the Ohios’ retirement, is a single, brilliantly-conceived bit of engineering. It’s a piece of equipment that is, ironically, mostly empty space.

It’s all about the tubes
The SSGNs have roots in the mid-1990s, when the Pentagon decided that 14 Trident-missile-armed Ohios would be sufficient for the reduced U.S. nuclear force mandated by the Strategic Arms Reduction Treaty II. Instead of decommissioning four redundant boomers, the Navy spent around $1 billion apiece modifying them to fire cruise missiles. Electric Boat performed the work between 2002 and 2010.

Electric Boat tweaked two of the boomers’ Trident missile tubes — each seven feet wide and 40 feet deep — for use as lock-out chambers for Special Forces swimmers. The shipbuilder modified the other 22 so-called “large-diameter tubes” to accept packs of Tomahawk cruise missiles known as “Multiple All-Up Round Canisters,” or MACs.Today Electric Boat builds Virginia-class attack submarines — at the moment, the only submarine class in production for the U.S. Navy. Electric Boat vice president John Holmander told Breaking Defense that a variant of the Virginia could replace the SSGNs starting in 2026. “The Navy came to us [in 2003] for a conceptual solution for how you could potentially get the Virginia to perform the missions of an SSGN,” Holmander said. “What we’ve done at this point is we’ve a developed module … which is 94 feet long and which increases the payload capability of the ship but allows the platform to perform within key performance parameters.” The parameters include speed, endurance and acoustic stealth.

The 94-foot hull extension, inserted aft of the Virginia’s sail, would increase the vessel’s length by more than a quarter and provide space for four large-diameter tubes copied from the Ohios. The Virginia’s payload tubes are nearly as wide as the Ohio’s, but shallower. The Virginia’s current MAC holds six Tomahawks instead of seven.

In-production Virginias starting with the 11th vessel — slated for a 2014 delivery — already have two large-diameter tubes forward of the sail for 12 Tomahawks, replacing the baseline of 12 small, individual tubes. The four additional Ohio-style large-diameter tubes each carrying seven Tomahawks would boost the Virginia’s cruise missile capacity to 40. Though a threefold increase in firepower for the Virginia, that’s barely a quarter the weaponry of today’s SSGNs.For that reason, it’s all but impossible for the Navy to avoid a reduction in overall Tomahawk capacity once 2027 rolls around. But a fleet of smaller, Virginia-based SSGNs — while carrying fewer weapons overall — would enjoy a key advantage over their larger predecessors. They’d be relatively inexpensive (for a nuclear submarine), could be built in large numbers and could be in more than just four places at a time.

“You … end up with a distributed network of these capabilities that’s not concentrated only with four platforms carrying the load,” John Biederka, SSGN program manager for Electric Boat, told Breaking Defense.

The smaller, more widely distributed SSGN fleet can’t happen unless we build more Virginia-class subs than currently planned. Under current projections, production would end after the three “Block V” vessels are finished in 2019. The Block Vs are likely the first Virginias that could include the 94-foot extension, as Electric Boat would need time to tool up for the modification. Therefore only Block Vs would have SSGN-like capabilities.

Holmander said that Block V construction could last up to five years. Based on the current, two-per-year acquisition rate for Virginias, Block V could include 10 ships. The last seven Block Vs, if they become reality, would represent an increase in the Virginia production run from 30 to 37 vessels.

If the stars align, the next-generation SSGN fleet would number at least those 10 vessels, carrying half as many cruise missiles as today’s four Ohio-class SSGNs, but they would be cover a much larger area.

That’s a tradeoff the Navy seems comfortable with. “The SSGN plays a key role in global undersea war fighting,” said Vice Adm. John Richardson, Submarine Forces commander. “The tremendous combat capability the boat brings – land attack missiles, Special Forces, torpedoes — that’s a lot of bets the enemy has to cover down on.”

Budget crunch
Of course, the Virginia SSGN plan is contingent on funding. Navy shipbuilding is being squeezed in the vice of rising ship costs, growing pressure for deep cuts in military spending and, most alarmingly, the imminent need to replace the Ohios in their nuclear deterrence role.After a vigorous internal debate earlier this year, the Navy rejected proposals for a ballistic-missile version of the Virginia — a vessel that Cote said would have been “terrible acoustically” — and opted for a brand-new design from Electric Boat. At an estimated $7 billion apiece, the new boomers pose a “huge challenge,” Hay said. They could easily consume half the Navy’s shipbuilding funds in each of the dozen or so years in which they’re built. That, Cote said, “is causing people to gag.”

But as the most survivable of the Pentagon’s nuclear triad, boomers are non-negotiable. While the Navy and Electric Boat can constrain the design to save some money, any way you slice it the new ballistic-missile boats will be expensive. That shifts the burden to other shipbuilding programs to reduce costs … or face cuts.Electric Boat is working hard to keep down the cost of the Virginia class. Indeed, the addition of the two large-diameter tubes starting with vessel number 11 was part of a savings initiative that also included a new, cheaper sonar housing. That initiative, known as “2 for 4 in 12,” aimed to reduce the cost of a single Virginia to around $2 billion in 2005 dollars, so that the Navy could afford to buy two of them per year no later than 2012. The initiative was successful, and today the Navy plans to purchase two Virginias annually most years until at least 2022.

But adding four new tubes to the Block V Virginias will increase their cost. Unless Electric Boat can find other efficiencies to offset the price of the hull extension and new missile tubes, the Virginia SSGNs could price themselves out of the shipbuilding plan and leave the Navy with an even bigger Tomahawk gap than currently projected.

“The machine of intellectual property continues to generate ideas to reduce cost further,” Holmander said, without specifying exactly what Electric Boat might do to slash the price. He did, however, describe the philosophy that he said underpinned the “2 for 4 in 12″ initiative and could help protect the Virginia SSGNs, as well.”The biggest thing that enabled this to work was the collaboration that exists on our industry-Navy team,” Holmander said. “We invented a process on the baseline Virginia called ‘design-build,’ where we put designers and fabricators in the same room with the Navy during design.” That helped temper design ambitions while also ensuring the fabricators understood the Navy’s goals. “We’ve actually taken that relationship to a new level,” Holmander said.

At the moment, there are no backup plans if the Virginia SSGNs becomes unaffordable. Cote said the Navy has considered converting two more Ohios into SSGNs in the next few years. While that would mean a big short-term boost in Tomahawk capacity, it wouldn’t solve the long-term problem of replacing the Ohios as their reactor cores wear out. If, after 2027, the Navy is to duplicate Florida’s firepower feat off of Libya, it’s the Block V Virginia or bust.

David Axe is a member of the AOL Board of Contributors.