It's time to talk about the alternative to nuclear submarines for the Royal Navy

Telegraph

03-05-2025

During my 20 odd years at sea in the Royal Navy, there were two things you simply did not say. One was 'I think corvettes are great, we should have loads of them' and 'why don't we bring back diesel-electric submarines'.
The reasoning was simple – do not do or say anything that might jeopardise exquisite platforms, such as nuclear powered submarines, aircraft carriers and high-end warfighting escorts. And it wasn't just banter. I know at least two officers who had their careers ended for suggesting (in writing) that a modular corvette would be a useful thing to have. I suggested in my undergraduate degree thesis that getting rid of our diesel submarines was a mistake. If I had pulled a stunt like that as a commander, it would have ended badly.
The notion makes sense. Why wouldn't the Navy want the best kit – platforms that can cover the entire spectrum of conflict from influence operations through the grey zone and up to a shooting war?
The problem comes when you want the best kit but the system doesn't give you enough cash to support those noble ambitions. In the case of Defence, that sum has been insufficient now for over thirty years which is why in nearly every service and branch we now have some really good stuff but nowhere near enough of it.
As an example our nuclear powered attack submarine fleet, the hunter killers, is woefully thin. We currently have five, with two in build. The shambles that is their maintenance programme means that of the five, we have two that are operational. For parts of last year, that number was zero: no working attack boats. You don't have to be Alfred Thayer Mahan to know that when things get heated, these assets are going to be near the top of the response menu. But not from alongside they're not. And you don't need to be Admiral Hyman G Rickover, the father of the US nuclear navy, to know that number should actually be twelve. This gives you four boats to use (the three to one ratio is an immutable law of complex ships), one for working with the carrier, one for UK work, one for the high north and one in the Mediterranean would be a good start.
We actually had fifteen such boats when I joined back in 1990 but that was the end of a sustained period of 4 per cent of GDP being spent on defence. In the current climate, where increasing from 2.3 per cent to 2.5 per cent in two years time is seen as a success, I would wager large sums that we will never achieve that number again. We don't have the money, the yards or the people. Neither do we have the courage to kick in the Treasury's doors and change the culture there.
So what is the solution? Well maybe we have to break one of the golden rules and consider reverting, some would say regressing, back to a mixed fleet of nuclear (SSN) and conventional (SSK) powered submarines.
Diesel-electric submarines, so called because this is how they are powered, have some significant advantages over their nuclear powered brethren.
When operating just on batteries, they are whisper quiet. I went up against a German SSK in an exercise off the south coast and it had the ability to just disappear. One minute you would have really solid tracking and the next … gone. It was quite unnerving. This acoustic advantage over nuclear powered submarines is not as marked as it was in the Cold War with SSNs now able to cool their reactors without the use of 'noisy' pumps, but it is still there.
They are also cheap. You can buy four for the price of one SSN. This is the main reason why so many navies have these as their submarine of choice. This simplicity also leads to a better availability ratio. I mentioned three to one for SSNs, for SSKs it is just over two to one, ie you would get two working boats from your outlay of four you purchased. The Dutch, Norwegians and Germans all have excellent options for us to buy off the shelf should that ever be politically acceptable.
SSKs are also smaller. An Astute class SSN displaces 7,800 tons when dived: the US equivalent, the Virginia class, slightly more. The AUKUS submarine that will one day replace our Astutes will be over 10,000 tons. Meanwhile, most SSKs displace less than 3,000 tons, some a lot less. This and the fact that they don't have a reactor means they can operate in places that are too shallow for an SSN. Operating areas like the Baltic and the Gulf become more easily accessible.
It also means they can go alongside in places that would not allow a nuclear powerplant alongside. A nuclear submarine's superpower is that nobody knows where it is, but that isn't really true of a diesel sub: it runs on the surface most of the time if it wants to actually get anywhere. The effect of abandoning secrecy and going alongside with flags waving is the same as it is in any warship and should not be underestimated. It's also fun, and in an era where recruiting and retaining is difficult, doubly so in submarines, this is important.
Perhaps the real advantage though would be their ability to restore operating experience that has been gradually eroded by having so few working nuclear powered boats. SSK crews are much smaller, generally less than 30, but rapid rotation and short, punchy patrols would improve experience levels across all departments and reduce the amount of luck currently needed to be posted to a working boat in order to advance your career. From a leadership perspective there is no doubt that commanding a couple of smaller ships made me better when it came to driving a frigate but it's broader than that; it would thicken everything.
There is, of course, a catch. And it's a huge one. At some point, no matter what technology you are using, or how good your batteries are, you have to charge them back up. This involves coming up to periscope depth and putting up a snort mast that allows your diesels to breathe so that you can recharge your batteries. At the very moment you are most visible, with a mast up that can be detected by radar, you are also making the most noise. Modern technology means that the gap between snorts is increasing – in some boats it can be over a month – but to achieve that you have to run at very, very slow speeds. At these speeds, your ability to get into a position to be of any use, particularly if the target is also moving, is hugely reduced.
Non diesel 'air independent' propulsion technologies are impressive, but they can't overcome physics: and good luck getting that kit refuelled/recharged anywhere but at a specially equipped naval base. A good indication how practical these options are is provided by the fact that diesels are always included in the design as well.
Meanwhile the nuclear powered SSN can sustain high speeds, fully submerged, virtually indefinitely. They might not want to, for tactical and acoustic reasons, but they can, and it's a game changing advantage and the overwhelming reason why the US and UK went nuclear-only all those years ago and why Australia wants to do so now.
So what would we use SSKs for?
They would be no good operating with the Carrier Strike Group because they couldn't keep up with the group unless they ran on the surface the whole time. Similarly they aren't a lot of use for attacking surface warships or even merchant shipping out in the open ocean, as these move too fast for a submerged SSK to get into a firing position unless the captain is lucky. They would be of almost no use in the High North against Russian nuclear submarines operating up there. Under ice work can be done by SSKs but isn't recommended.
They would, however, be useful for protecting Critical National Infrastructure around the UK, an ever-increasing vulnerability. Their shallow draft makes them eminently suitable for some Special Forces work and acting as an underwater sentry off, say, the submarine base in Faslane, Scotland. In fact, pre-positioned and operating at slow speed, they would be excellent for any chokepoint work.
We could use them for minelaying should that be a box we ever want to open again. They would have utility in protecting our nuclear deterrent subs at choke points and be invaluable for training future submarine captains and acting as an aggressor for training our SSNs and anti-submarine frigates. As mentioned before, they could work in places like the Baltic and Mediterranean – although this is offset by how many allies we have there who already have SSKs and would probably rather we concentrated on fixing our SSNs.
If money was no object you would simply have a proper number of SSNs. But it is an object, and given that we cannot afford a reasonable number of SSNs, it's time to consider having some affordable SSKs alongside our limited SSN force (note that I said affordable – our last attempt at SSKs, the disastrous U boats, were not even cheap).
In my view the advantages SSKs provide in terms of providing experience and training coupled with the odd tactical advantage in certain situations would be enough to offset the huge operating disadvantage that limits them. We also need more mass, and this is a relatively quick and easy way of achieving it. In fact, we really need to get better at selecting capabilities one rung down from exquisite across the board. The Type 31 Frigate programme gives me hope that we might be getting better at this.
But I will not be thanked by a system that is working to get 'improve nuclear' to feature high up in the Strategic Defence Review for bringing this up just now. I'm afraid I am cynical enough to make that a reason to do so. For once, I would love to see nuanced discussions like this take place based on the strategic and tactical operating requirements rather than fear of the Treasury. But that's not the world we are currently in.
Perhaps I should write something on Corvettes next.