US Navy should procure SSKs to supplement its SSN fleet

Japan Maritime Self Defense Force’s JS Tōryū (world’s second lithium-ion powered submarine)

Going to get right to it; SSKs (subsurface conventionally powered) are cheap, and SSNs (subsurface nuclear powered) are expensive. US Navy’s newest Block V Virginia class SSNs are $3.4B per boat. Japan’s new Taigei class SSK is arguably the most expensive SSK ever made, and it costs $640M, roughly 1/5th what a Virginia costs. On top of being more expensive, SSNs require a far larger crew due to the complexity of the nuclear reactor and size of the boat required to hold it. A Virginia class SSN has a crew of around 135 personnel, while a Taigei class SSK has a crew of just 65–70 personnel.

US Navy currently procures 2 Virginia boats a year, but wants to increase that to 3 boats a year to ensure there’s no decline in overall numbers. What that equates to is an over $10B a year budget item every single year, just to build 3 boats. Should the US Navy stick to 2 Virginias and then procure 2 SSKs comparable to the Taigei, it could save over $2B a year, and it have effectively zero impact on manning (1 Virginia with 135, 2 Taigeis with a combined 130–140).

A brief explanation into the difference between an SSK and an SSN can be summed up by the power plant driving them. SSKs are reliant on diesel engines that run while the boat is on the surface to charge up battery banks that allow for the boat to then operate while submerged since the diesel engine won’t have access to oxygen. SSNs on the other hand use compact nuclear reactors that generate steam, which in turn cause turbines to spin and generate power.

Don’t let me lead you astray though. SSKs are not some “with this one neat trick, you too can save money” secret over SSNs. There’s a lot that an SSN can do that an SSK can not. Because they don’t the nuclear reactor power plants an SSN has that can run unrefueled for 30+ years, SSKs are limited by the amount of fuel they can carry, and by the power density (how much power can be stored) of the batteries. Once the batteries are empty, the boat has to surface for the diesel engines to run and recharge them, and the diesel engines themselves are reliant on however much diesel fuel the boat has onboard. What that means is an SSK typically lacks the speeds and endurance to keep up with a surface fleet to protect it. They’re typically left as defensive systems that will, park if you will, along key areas and just wait it out, maximizing their endurance on the internal batteries. If they’re trying to keep up with a surface fleet, they just don’t have the power density for both high speed and long endurance, and will have to surface quite often to recharge, something that leaves the submarine at high risk, and causes the surface fleet to temporarily lose one of its most effective assets. This is a big reason as to why the US, UK, and France have gone with purely SSN fleets, as they require the long endurance at max speed while submerged that only an SSN can offer.

So SSKs could be a viable asset for the US Navy with regards to known key locations and waterway chokeholds such as the Suez Canal, Strait of Hormuz, and Korea Strait, but viable doesn’t always cut it, particularly if they’re too lacking to do some of the other missions assigned to SSNs.

With that said, new SSKs are rapidly closing the speed/endurance gap with SSNs. AIPs (air independent propulsion) submarines for example, use liquid oxygen stored onboard the boat to allow the Stirling engine generator to run while submerged, trickle charging the batteries and extending the underwater duration from just days, to weeks. German and Italian Type 212 AIP submarines can stay submerged for up to 3 weeks at a time because of the AIP generator, which is a massive boost to capabilities. They still can’t however maintain that level of endurance while cruising at high speeds. The Type 212s can really only stay under for 3 weeks if they stay under 8kts, where as their top speed is around 20kts, so it’s a very noticeable improvement over traditional SSKs, but still not enough to take on enough roles that the US Navy needs, for it to be a viable solution.

Japan’s recent Taigei class SSK (more so the last two Soryu boats, but semantics aside…) has yet again revolutionized the SSK design. These boats no longer have an AIP powerplant, and in fact can’t run any engines at all while submerged. While that may seem to be a massive step back and not some sort of great step forward, the revolution that has taken place is actually within the battery blocks. SSKs until now have used the traditional lead-acid batteries, akin to what a car battery uses. They’re… fine for energy storage, but battery technology has improved drastically since the first lead-acid battery was created back in 1859. Pretty much every mobile device today is powered by what is a lithium-ion battery. Lithium-ion batteries came to existence in the late 1980s, and have become the mainstay for long life mobile devices due to their massive gains in energy density.

Lithium-ion batteries can store 2 times as much energy as a lead-acid battery of the same physical dimensions (can go up to almost 7 times as much, but going to keep it at the lowest end of the scale to under estimate the difference). For the Type 212 from before, swapping to lithium-ion batteries means it might be able to stay under for 3 weeks while still maintaining 16kts, which is a drastic improvement. But the Taigei class doesn’t just swap out the batteries. Remember, it no longer has the AIP powerplant as well. That space and weight is now taken up by even more battery banks (to the tune of several hundred tons). So while the Taigei class is the exact same size as the Soryu AIP boat it’s based off of, it should have over twice the amount of batteries, with the batteries at a minimum having twice the amount of stored power, giving the Taigei over 4 times the total amount of energy via batteries while submerged vs the Soryu class. Mind you, that’s the low end of the improvement, it could be anywhere from 4x the total energy to 14x the total energy…

You’re now looking at an SSK that quite potentially has both the speed and endurance necessary to adequately augment the SSNs for a large bulk of US Navy subsurface missions. These new boats likely still lack a little too much endurance to truly replace SSNs attached to carrier strike groups to forward screen for threats, and they have no VLS (vertical launch system) cells for deploying Tomahawk missiles against shore targets, so the SSNs can stick to those. But beyond those roles, these SSKs seem like prime candidates to take on a bulk of the US Navy’s subsurface needs of ISR (intelligence, surveillance, reconnaissance), surface warfare, and deploying special forces behind enemy lines. At 1/5th the cost and 1/2 the necessary crew, they become very attractive options, particularly when you start to look at forward deploying them.

The US has (rightfully placed) security concerns regarding the ships’ reactors, and while there are no longer any Tomahawk missiles equipped with nuclear warheads, a lot of ports will not even allow warships to enter if they believe there is even a possibility of a nuclear weapon due to a myriad of local and national laws worldwide. New Zealand for example does not allow nuclear weapons or nuclear-powered ships within their territorial waters. As such, Japan is the only foreign nation that US SSNs are homeported at.

With SSKs like the Taigei being notably not nuclear powered and lacking VLS for no longer existing nuclear weapons, the option to forward deploy more subsurface assets to various regions suddenly becomes viable. It’s over 5500 miles from Hawaii to the South China Sea, but just 2100 miles there from Australia. It’s over 6000 miles from Norfolk to the Suez Canal, but just 600 miles there from Crete. This not only creates more of a headache for an opposing power to track and eliminate US Navy subsurface assets, but it can allow for US submarines to more rapidly deploy to a conflict zone.

With the costs of these SSKs being far cheaper than SSNs and the crew requirements being halved, the US Navy can procure 2 SSNs and 2 SSKs every year without impacting manning. This would save over $2B a year over the current 3 SSNs a year, allowing for the US Navy’s budget to go further than it can in this currently flat budget cycle. 4 submarines a year would also go a long way into not just sustaining US submarine numbers, but expanding it as well to deal with growing threats across the globe. On top of the cost saving and capabilities growth, with smaller crews on these SSKs, the US Navy could in turn create a pathway for early leadership roles via allowing Lt. Commanders to be the skippers of these smaller boats.

The Navy’s begun looking at drone submarines, known as UUVs (unmanned underwater vehicle), which could in theory take all the value out of the SSK fleet. With that said, today at least UUVs are mostly being looked at as just underwater ISR platforms for monitoring enemy surface fleets and tracking enemy submarines. Some have argued for adding a minelaying capability to them down the road, but that’s largely the extent of the combat capabilities being looked at for them. One of the issues in doing so is the environment a submarine operates in. Submarines can’t just sync up to a satellite for high speed communications like a UAV (unmanned aerial vehicle) like the MQ-9 Reaper can, because the water kills the signal before it can reach a submarine beneath the waves. Realistically the only communication method that works with a submarine that’s underwater is LF/VLF (low frequency/very low frequency), and that’s just one way communication from shore to boat. A UUV won’t be able to maintain a proper line of communication with controllers on another boat or on shore, so their ability to go on the offensive doesn’t really exist, and it can’t either, not unless we’re giving the UUV the ability to engage targets on its own, which is absolutely NOT something that should be considered for a multitude of ethical and security reasons.

So the Navy would be left with a fleet of UUVs for ISR, and the 50 SSNs for protecting surface fleets, anti-shipping, land attack, and deploying special forces. That’s… A LOT to ask of just 50 boats, particularly when 50% of those cause the submarine to announce its presence. That’s arguably not enough boats for the mission, especially if a conflict with China and its 350+ ship Navy arises. SSKs would go a long way to shore up US Navy subsurface assets, and would give the SSNs some much needed breathing room given they’re currently being ran ragged with every tasking under the sun. In total, SSKs, particularly lithium-ion powered SSKs, simply make far too much sense for the US Navy to keep turning their noses up to them.

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