Ohio-class SSGN USS Florida transits the Suze Canal into the Red Sea last November. Reportedly the sub fired Tomahawk cruise missiles at Houthi targets, and made use of the Dry Deck Shelter (DDS) mini-sub docking bay visible fitted to her casing -

There are calls for the Royal Navy to acquire a class of submarine it’s never had before – a nuclear powered guided missile sub (SSGN). This is a large nuclear powered submarine, not unlike a nuclear deterrent boat but with a large number of vertically-launched cruise missiles rather than a smaller number of nuclear-tipped intercontinental ballistics. It’s a very good, but very expensive, idea.

Expect the cost to overwhelm the benefit. The Royal Navy is continuing to shrink as it struggles to afford new frigates, destroyers, amphibious vessels, support ships and submarines in adequate numbers to sustain the roughly 80-ship front-line fleet.

Many conventional and nuclear-powered attack submarines – SSs and SSNs, respectively – carry some cruise missiles, including the Royal Navy’s current Trafalgar- and Astute-class SSNs. But they carry only a few cruise missiles, and fire them horizontally from their torpedo tubes, limiting the pace and volume of the shooting.

The idea behind an SSGN is to optimize the boat’s strike potential, by arming with a lot of cruise missiles – and firing them from vertical launch tubes that can salvo scores of missiles in a span of minutes. An SSGN is bigger than an SSN, the same size as a ballistic missile deterrent sub (an SSBN), and it can fit several vertical tubes for cruise weapons into the same space as a single intercontinental nuclear missile. This means a lot of cruise missiles.

It’s this fast, concentrated firepower, packed into an all-but-undetectable stealthy platform, that makes SSGNs such an attractive asset for the most sophisticated navies. It’s their cost – billions of dollars per boat – that limits their use to only the richest navies. At present, only the US and Russian fleets operate SSGNs.

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At least one think-tank wants the Royal Navy to join that exclusive club.

“The submarine service should be de-risked by … procuring an additional Dreadnought as a missile submarine (SSGN) to provide extra deep strike,” William Freer and Dr. Emma Salisbury wrote in a new report for the London-based Council on Geostrategy.

The Dreadnought class is, of course, the Royal Navy’s new class of nuclear ballistic-missile deterrent submarine. The Royal Navy is building four Dreadnoughts – each displacing 19,000 tons and carrying up to 12 Trident ballistic missiles – for a total cost of $39 billion. The first of the huge new boats should commission in the early 2030s.

If the Royal Navy did build a fifth Dreadnought and fit it with non-nuclear cruise missiles instead of nuclear ballistic missiles, it would follow in the wake of the US Navy. In the late 1990s, the Americans reduced their fleet of Ohio-class ballistic missile boats from 18 to 14. Instead of dismantling the four excess hulls, they rebuilt them into SSGNs.

Each Ohio-class SSGN is armed with a whopping 154 cruise missiles. In 2011 the SSGN USS Florida fired at least 90 missiles at targets in Libya, largely destroying the Libyan air-defence network in one blow (some other vessels fired Tomahawks also, including a British sub). This cleared the way for follow-on attacks by warplanes and drones, which largely suppressed Gadaffi’s armed forces.

And the Ohio SSGNs are not only missile platforms. The space left by removing the massive nuclear missiles also allowed them to have extra accommodation and dedicated lock-out chambers, allowing each boat to embark a force of 60 special-operations troops for sustained periods: usually Navy SEALs, as the special operators need to be trained frogmen to deploy from the sub while still underwater. Britain’s submarine-borne special operations unit is the Special Boat Service.

Ohio SSGNs are also fitted for the attachment of one or two Dry Deck Shelters (DDS) – docking bays for mini submarines. This allows the SSGN mothership to lurk in deep water while the SEALs get inshore by mini-sub, travelling longer distances than they could practically swim.

The fact that the cruise missiles and/or SEALs have arrived by nuclear powered submarine means that they can operate deep inside the reach of opposing navies and air forces without anyone being any the wiser. The undetectable nature of nuclear powered subs is the reason people spend so much time and money putting things on them – it is a unique capability possessed by only first-rank nations.

Even so, America’s SSGNs are all already nearly 30 years old and will decommission over the next few years as their steel hulls accumulate stress from repeated dives. The decommissioning of the SSGNs will result in a precipitous decline in the US Navy’s overall cruise-missile capacity – a decline the American fleet hopes to partially reverse with new, longer variants of the Virginia-class SSN. The current Virginias have 12 vertical launch tubes in addition to their torpedo tubes, but the plan is to equip forthcoming ones with an additional 28 vertical tubes in a new hull extension section.

The Block V, Block VI and Block VII Virginias will thus each carry 40 cruise missiles in vertical launchers, in addition to their torpedo tubes. The Americans are betting that these “light” SSGNs – potentially 20 of them, commissioned over the next decade – will bridge the cruise-missile gap until a new class of large SSGN can enter service in the 2040s.

If the Royal Navy can’t afford the $10 billion cost of a single Dreadnought-based SSGN – and there’s every sign it can’t – it could make do with a new variant of the Astute SSN that, like the newer blocks of the American Virginia, could have a longer hull and space for the Royal Navy’s first undersea vertical missile launchers.

This plan is already taking shape. The new SSN the United Kingdom, the United States and Australia are co-developing for the Australian navy under the AUKUS initiative should also replace the seven planned British Astutes. There are reports that this new AUKUS submarine will have vertical launchers for cruise missiles.

How many vertical launchers the design includes is unclear right now. It’s that figure that makes the difference between the new boats functioning as SSNs or SSGNs. If the Royal Navy appreciates the benefits of an SSGN but is also justifiably worried about the cost, it could compromise by squeezing as many cruise missiles as possible into the SSN-AUKUS.

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