Evolution or revolution: maybe it’s a bit of both

Underwater operations remain a core output that navies and their parent countries use to secure national interests and create international influence. Such operations are, of course, conducted covertly, with submerged platforms: this is the unique selling point, and it endures because the world’s oceans remain opaque to sensing technologies trying to see into them from above, on, and below the surface.

In contemporary underwater operations, unmanned underwater vehicles (UUVs) have joined submarines as tools for generating this output. With such operations becoming more important as global security becomes increasingly more unstable, the requirements to stay submerged and move at speed underwater have become more important. Yet for submarines or UUVs, there are different reasons and options when assessing why and how to do this.

Broadly, there are two submarine types, defined largely by their power and propulsion systems. While overall operational requirements for both types are generally the same – stay hidden, and move and act quickly and covertly – how they do this is different.

Most of the world’s submarines are conventionally powered: using what is generically termed ‘diesel-electric’ propulsion, these boats – generally known as SSKs – combine traditional submarine ‘power supplies’ of diesel engines and lead-acid batteries. Today, technologies such as air-independent propulsion (AIP) and Lithium-ion (Li-ion) batteries are added or used instead.

A select number of the world’s submarine navies operate nuclear-powered attack submarines (SSNs). These boats generate power and propulsion using nuclear reactors designed for underwater operation. Operating an SSN flotilla is underpinned by significant financial, industrial, and supporting infrastructure investment – a core reason why very few submarine-operating navies opt for SSNs.

An SSN’s core capabilities underline, however, why navies and their parent countries pursue submarine capabilities, whether through SSN or advanced SSK programmes. Staying hidden, moving covertly over relatively large distances at relatively high speeds, and conducting various activities unseen where and when required are the main reasons navies invest in submarines. SSKs can generate these outputs too, but not with the sustained submergence, endurance, and flexibility of an SSN.

Over the last decade or so, these requirements have become even more pronounced as state-based rivalry has re-surfaced around the world. In both the Euro-Atlantic and Indo-Pacific theatres, the first stirrings of this re-surfacing rivalry could be sensed underwater, with the Russian Navy and Chinese People’s Liberation Army Navy increasing submarine operations.

Over the same timeframe, what has also surfaced in the military world is the use of uncrewed systems. For navies, UUVs were introduced largely to provide surveillance capability around key locations, like maritime choke points or ports and naval bases; and to conduct mine countermeasures (MCM) operations, enabling ships, submarines, and sailors to remain at a safe distance outside the minefield.

However, as more is learned about UUVs – in terms of what industry can provide, and what capabilities navies require – these roles are evolving. Navies are now looking to use UUVs to conduct surveillance at distance, to operate in an ever-more integrated manner with crewed and other uncrewed platforms in the underwater and other domains, to carry a wider range of effectors including (potentially) kinetic weapons, and to conduct these tasks with greater speed and sustainability.

For any underwater vessel (SSK, SSN, or UUV), the onboard power and propulsion system is the primary source of such sustainability and speed, alongside the submerged stealth that is the raison d’etre for possessing the capability.

Evolution to revolution

When asking the question of whether developments in underwater propulsion are evolutionary or revolutionary, the answer is probably ‘both’.

For submarines, whether SSKs or SSNs, power and propulsion systems have generally developed in an evolutionary manner, with improvements made to nuclear reactors, to fuel-based propulsion systems, and also to batteries as more is learned about them underwater and as more developments are made ashore in industry – including in the commercial world.

In the world of uncrewed systems, more revolutionary developments may be found – not only through the accelerating understanding of what can be done with UUVs and how, but what other options exist to do different things with UUV design and technology given the simple fact there are no humans on board.

The propulsion options for underwater vessels may differ depending on the operational requirement, but also on the vessel type.

As regards submarines, “In terms of the options you’ve got, they’re the same that have been there for 5-10 years,” Alex Walchester, a senior naval architect specialising in the defence and security sectors at UK-based naval design house BMT, told European Security & Defence/Maritime Defence Monitor in a 5 September interview. These options’ development trajectory has remained largely constant, Walchester added.

For conventional submarines, diesel-electric capabilities are being enhanced by improvements in battery and AIP technologies. Lead-acid batteries have been the traditional SSK battery type, but new chemistries – notably, Li-ion batteries – are being assessed and used. The Japan Maritime Self-Defense Force (JMSDF) is one such navy leading the exploration of Li-ion battery options.

“It’s all about the battery management system and how you keep the battery safe [while] getting the power output you need; it’s marrying up the battery chemistry with the safety and the power output in a manner that meets your requirements – because you want power delivered in a certain manner, which suits some chemistries more than others,” said Walchester.

Submarines versus smart phones

Commercial industry plays a crucial role in developing battery technology, particularly through financial investment. “Li-ion and other batteries are being developed for commercial applications: in terms of investment and dollars spent, its order of magnitude is different between what’s being developed on submarines and what’s being developed just for smartphones, battery banks etcetera,” Walchester explained.

For navies, combining technologies can add capability value. “Batteries are always trying to be part of your solution, but AIP might enhance that,” Walchester added. “AIP gives you underwater endurance that batteries can’t …. The fuel type used in AIP systems is probably far more power-dense than a battery would be, so it allows you to stay submerged for longer.”

Technological developments to improve the efficiency of the fuel cells that power AIP systems may also extend a boat’s power output and submerged endurance, Walchester explained.

For SSNs, nuclear propulsion technology is a long-trodden, well-established development path. A potential area for change is in the development of different reactor types, said Walchester. Micro-nuclear reactors (MNRs) are one focus area with molten salt reactors (MSRs) being another. Some European navies have begun assessing nuclear propulsion options for surface ships, given growing operational requirements and increasing costs of traditional fuels. MNRs may offer more affordable nuclear propulsion. However, all naval platform design and capability choices bring trade-offs.

“If you go smaller, you’re probably not going to get the same amount of power,” said Walchester. “In my opinion, with MNRs, you may end up in a place where you’ve got a system that’s similar to an AIP system, but the difference is your AIP system needs to be refuelled.” So, in sum, MNRs might not offer the capability of an SSN, but could support a capability step-up for navies seeking to step on from an SSK.

As regards UUVs, power and propulsion developments offer potential for design and capability revolution, said Paul Burke, BMT’s senior business development manager for submarines, and a former UK Royal Navy submariner.

“The interesting thing in the last ten years or so is the UUV angle, where you could actually consider more technologically advanced power systems, because you don’t have the people element, so you can take a bit more risk in the propulsion system area,” said Burke. For example, he continued, “There’s been very little, if any, investigation of how MSRs would work in the maritime or in the underwater environment – but that might be something that could be tested [in UUVs] without threat to human life.”

Evolving capability requirements for UUVs will, however, shape the propulsion – and even technological and operational – options that could be considered. For UUVs, Burke explained, “This transition has been happening over quite a few years. As you change from that short-term activity – send something out, bring it back, recharge it easily – to where you want to have something as a capability going out into the deep, doing a long-term activity, it’s going to be an endurance issue.” This endurance relates to powering not only the UUV, but the sensors, communications systems, and other mission-relevant capabilities.

“There are lots of factors that could be considered to get the right power units in there… There are a whole load of [propulsion capability] options to consider too,” Burke added. Looking at how UUV batteries could be recharged, Burke explained: “SSKs must still come up and snort every so often. Why can’t you have a long-endurance extra-large autonomous underwater vehicle [XLAUV] that has to surface and stick a solar panel up? …. Can you use plug-in charging points on the ocean floor?”

“There are lots of things that can be thought about for how to maintain a UUV [on station] for a longer period of time,” Burke added.

Understanding requirements

Key to many such decisions, said Burke, is for navies to clearly understand and set out what it is they want and need the platform to deliver: this applies to submarines of both types, and also to UUVs.

“It all depends on the requirements for that platform …. The question is ‘what is the right thing for the task I want to do?’” asked Burke.

“The nice thing about a submarine is it can do loads of things. However, depending on what type of things you want it to do, that addresses your calculus for the propulsion requirement. It’s the same for UUVs,” Burke explained. Requirements to consider in any such calculation include the endurance, sustainability, and speed needs, plus planned sensor and weapons fits. “Then it’s a case of asking ‘how do I fit that within the specification?’ … [and] whether it’s the best or most efficient option – in other words, financially effective,” Burke added.

Such design, capability, and operational requirement choices, including power and propulsion, are perhaps most striking when it comes to submarines. “The nice thing about nuclear propulsion is it gives you multiple options; the nice thing about non-nuclear is it’s a lot cheaper, because a lot of the infrastructure needed – even for MNRs – is probably more expensive than conventional-type means of power,” said Burke. “So, sometimes it’s a cost-benefit decision a country needs to make.”

“If you’re just looking at submerged endurance, or just want a really high top speed … there are multiple ways of getting there with conventional propulsion systems,” Walchester added.

According to Burke, emerging UK requirements for UUVs may well start to reflect those of its submarine capability – namely, set within governmental and defence strategy requirements, operating UUVs that potentially can do a lot of different things, and can respond at speed to the need to do these things, including at distance.

“Within the UK, we’ve traditionally had a requirement for being able to operate at range. To achieve that range, you need to have speed, and you need to have endurance, and then be able to deliver multiple types of operations,” said Burke. While the UK’s optimum underwater solution for meeting these requirements is an SSN, given its multiple sensors and capabilities, a UUV does not fully fit this bill. However, Burke continued, “A UUV can deliver some of the other effects required, like indications and warnings, tripwires etcetera. So, there’s definitely capability that could be easily delivered by these longer-range, XLAUV-type capabilities.”

To meet such requirements, UUVs will need sustainable, reliable propulsion.

Revolution versus reliability

The use of tried-and-tested technologies for submarine propulsion raises a key consideration when assessing options for trying something new with UUV propulsion. While the absence of crew onboard may enable testing of some alternatives, this very absence can impose limitations. The important lesson learned here is reliability, said Walchester.

“As you don’t have people there to manage the system or manage defects, your reliability requirement goes up,” Walchester explained. “With XLAUVs, when you start incorporating diesel propulsion and snorting etcetera, you start looking at very reliable propulsion systems, and you effectively end up playing it safe.” “There’s a safety and battery management aspect …. [It] is something people are aware of in the commercial and naval worlds,” he continued. “It’s reliability versus capability.”

What the commercial world has learned regarding reliability in monitoring and maintaining oil and gas pipelines seems relevant to deploying and operating UUVs at distance. “You have something that just sits there: you don’t need to worry about it; it’s not going to break down; and it doesn’t need a massive, expensive maintenance period,” said Walchester.

As UUVs become more capable and usable, there is the added element that they may be seen as less expendable. Deploying a UUV to conduct MCM operations instead of a ship or diver, or to act as an anti-submarine warfare (ASW) picket across a choke point instead of a submarine, underlines an original focus in uncrewed system operational concepts – prospectively trading their loss to save human lives. However, with naval activity increasing and crewed naval platforms expensive to build and operate, UUVs can offer sustainable capability and mass. For example, XLAUVs are viewed as potential forward-deployed sensing pickets to support high-value assets like carrier strike groups (CSGs) on the move, even providing land-attack or anti-ship strike capabilities to sanitise CSG transit corridors.

“You get this juxtaposition with an uncrewed platform: it’s new, it’s novel, it’s cheaper than a submarine, so you want to be able to trial new technologies on it; but at the same time, you don’t want to lose it, you don’t have people there to adapt to a situation when it goes wrong, and you’ve possibly got slightly less confidence in the system to adapt itself to overcome any problems,” said Walchester. “So, you end up placing quite stringent reliability requirements on your XLAUV, particularly if it’s off doing long-distance operations in the North Atlantic where the environment is not always favourable due to poor weather conditions.”

Stealth, simple

As with submarines, stealth will remain the primary driver of UUV operational requirements, going hand-in-hand with sensing output to maximise value in the covert capability. “It’s the underwater domain, it’s submarines and underwater vehicles: they operate underwater for a reason – stealth. So, stealth capability is always very high up in system requirements,” said Walchester. “It’s what it’s there to do, and everything else is effectively there to support that.”

The enduring importance of stealth in underwater operations raises the issue of what are the primary questions a navy should ask when assessing power and propulsion options for underwater vessels.

“It’s an assessment of the threat, and therefore what you’re trying to achieve… It’s from high-level strategy all the way down to ‘OK, so what’ll actually achieve that?’” said Burke.

This assessment, Burke explained, includes whether threats to interests will be surface- or sub-surface-based, whether the requirement is merely to sense the threat (including maintaining long-term surveillance of it) or to consider if – and how – to prosecute it, including whether that is offsetting the threat or physically attacking it. Next comes the question of generating the availability of safe and capable platforms to conduct any required tasks.

Thinking through each factor in this equation drives the propulsion requirements.

There are ‘bigger picture’ elements to consider here too, Walchester added. These can include how the submarine or UUV must interact with other sub-surface assets and assets in other domains.

There is also real-world operational reality. “If you look at Ukraine and some of the lessons learned… there’s what you’re doing in peacetime, and then there’s – if things go wrong, and you’re in a situation you’d rather not be in – if someone is shooting at you, how does the kit degrade over time as elements of your network or fleet slowly degrade, require maintenance, get damaged, or are lost etcetera,” said Walchester. “You need to think about the big picture, and then you need to think about the big picture after it’s been roughed up a little.”

As regards how a ‘roughed up big picture’ might re-align underwater platform power and propulsion requirements, Walchester said, “I think mass and survivability will become increasingly important, with the requirement for the number of platforms and sensors growing.” Here, UUVs will be vital to adding mass in the short term, but still complementing crewed submarines that will continue to bring capability and flexibility that cannot be replicated currently by UUVs, he added.

Dr Lee Willett

Dr Lee Willett is an independent writer and analyst on naval, maritime, and wider defence and security matters. Previously, he was editor of Janes Navy International, senior research fellow in maritime studies at the Royal United Services Institute, London, and Leverhulme research fellow at the Centre for Security Studies, University of Hull in the UK.