In the second of a series of articles focussing on the Astute class SSNs, we look at the overall design of the boat, its propulsion and accommodation arrangements.
Innovations
Despite their elongated development and construction time, the Astute class submarines represent a major jump in capability over their Trafalgar class predecessors that were based on 1960s and 70s technologies. The Astutes reflect the global trend towards larger vessels are the biggest SSN the RN has operated, having 7,800 tonnes dived displacement and measuring 97 meters in length. Increased automation has reduced the core crew requirement by 25%, down to 98 compared to the 130 of the Trafalgars plus there is also dedicated additional space for a further 12 sea riders, specialist augmentees or an embarked military force. Accommodation standards are improved slightly and in particular, the Astutes avoid the need for ‘hot-bunking’ as each man has his own bunk. Despite being larger than the Trafalgar there is very little increase in actual living space and the Astutes are still quite cramped for such a large boat as the reactor and propulsion system take up more than half of the volume of the pressure hull.
Weapons capability has been improved with 6 torpedo tubes and stowage capacity for 38 Spearfish torpedoes or TLAM. (compared with the 5 tubes and 30 weapons of the Trafalgars). The latest Spearfish Mod 1 is a very effective weapon and gives the Astute a substantial uplift in submarine or ship-killing capability over the Spearfish Mod 0 and Tigerfish Mod 2 that previously equipped RN SSNs.
In the post 9/11 world in which Astute was developed, anti-terrorism and special forces operations assumed greater importance and the RN successfully argued for the design to be modified to carry a Dry Deck Shelter. This allows special forces personnel and swimmer delivery vehicles to be delivered covertly ashore from the boat while submerged. This addition of this excellent capability had a significant impact on the design and is the reason for the distinctive and slightly ‘hump-backed’ profile of the after casing. The Astutes also have a more basic SF insertion option with an inflatable Zodiac raiding craft stored under the forward casing that can be manhandled onto the deck and launched when the boat is surfaced. Whether the cost and delay the DDS modification entailed was justified is debatable, seen in the context of today’s environment where countering peer competitors means the tiny SSN force is mostly employed in their core ASW and intelligence-gathering role. (For more details on the DDS, see previous article here).
Astute-Class-Submarine-General-Arrangement-1One of the most significant innovations that impact the operation of the boat is the optronic periscope. Until Astute, all RN submarine had periscopes containing mirrors and prism arrangement that allowed the commander to directly observe the view above the waterline when the boat is at shallow depth. The periscopes penetrated the pressure hull, were sited in the centre of the control room and raised and lowered into a well below. The new system is contained within the fin and the CM010 electro-optical sensors gather high-definition digital imagery quickly, reducing the time the mast is above the water when the submarine is potentially vulnerable to visual or radar detection. The sensor head unit features 3-axis stabilisation which gives a much more stable and clear picture, even if the boat is pitching or rolling when at periscope depth in rough seas. Instead of only the operator being able to observe the scene, the new method can conduct a rapid 360º all-round look, and whole command team can review the imagery at leisure on screens in the control room long after the periscope is lowered.
This changes the dynamics of intelligence-gathering, operating in close proximity to shipping and offers a major advantage. With new technology comes a learning curve and personnel adjusting to using an optronic periscope was unfortunately a small contributing factor to HMS Ambush collision with a merchant ship off Gibraltar in 2018.
The Astute is controlled using a small joystick which replaces the heavy control columns of previous of submarines. More importantly, the steering, depth and propulsion are operated through a digital Integrated Platform Management System (IPMS). IPMS makes submarine operation inherently safer and control more precise by automatically preventing the boat diving to deep, broaching or losing trim. Autopilot and machinery monitoring helps reduce the crew numbers and the helmsman’s workload. The IPMS has multiple backups and revisionary modes which ensure it can be overridden in emergencies or should it fail.
Astute is covered by more than 39,000 anechoic tiles that absorb active sonar and reduce noise radiated from within the boat. The chemical composition of the tiles is classified but they are an elastic material containing tiny air pockets and optimised to attenuate the typical sonar frequencies of homing torpedoes. Development of the tiles is not simple as they must withstand great pressures as the boat goes deeper, compressing the air pockets, affecting the frequencies absorbed. Tiles are not new to Astute and were introduced RN boats in the 1980s but are likely made from a new compound and it appears that a reliable adhesive has finally been found. Previous boats frequently lost tiles washed off during operations which is detrimental to stealth, required repair and offered the opportunity for analysis by adversaries if recovered.
Propulsion
The PWR2 nuclear reactor around which the design was developed incorporates the core-H design which will not require refuelling for more than 25 years. This benefit should save future expense and increase availability, although some of the early Astutes may have to be extended in-service and refuelling could be required if their SSN(R) replacements are not ready in time. Pressurised Water Reactors are an inherently safe design because the hotter the water used to cool the reactor becomes, the slower the nuclear fission reaction becomes, thus making it self-regulating.
In common with the USN the RN uses Highly Enriched Uranium (HEU) as fuel for its submarine reactors which results in very high power density. The power of Astute’s reactor is classified but is estimated to be over 100MW. Nuclear propulsion is especially sensitive and details are more closely guarded by the UK government even than the production of nuclear warheads. This is partly because aspects of the designs are based on technology shared by the US that come with very strict classification conditions.
The enriched uranium handling facility at AWE Aldermaston stores and processes HEU for initial fabrication into reactor fuel for the UK Naval Nuclear Propulsion Program (NNPP). It is estimated the UK has about 80 years worth of HEU to support the NNPP at the current rate of consumption. HEU fuel from AWE are processed and the submarine nuclear reactor cores are fabricated and test-assembled at Royce in Derby before being transported to Barrow for final assembly and insertion into the Astute hull ring.
The reactor core is contained within the Reactor Pressure Vessel (RPV) but only takes up a small part of the reactor compartment which is filled with shielding, pumps and complex pipework. Radiation protection is provided by polythene blocks which shield the upper half of the RPV which can be visually inspected via a thick glass window in the access tunnel above. The bottom of the RPV sits in a water tank surrounded by lead plating. Shielding is so effective that most submariners receive less radiation during a patrol than they would get from background radiation ashore. The superheated radioactive steam from the reactor passed through the primary circuit and the energy is transferred by heat exchangers to two secondary circuits. This steam is then piped to the engine room where it drives the two steam turbines and two turbo-generators.
The nuclear propulsion system has backup systems. Diesel generators can be run either when surfaced or using a snort induction mast at periscope depth and provide electrical power to the Emergency Propulsion Motors (EPM) that can drive the propulsor at slow speed. This mode is used for short ‘cold-moves’ when the reactor is shut down (such as for short transits between Faslane naval base and the weapons depot at Coulport). If the steam turbines, gearbox, shafts or propulsor were damaged or failed there is a Secondary Propulsion Motor (SPM) colloquially known as the ‘egg whisk’. This is a small thruster can be lowered from inside the aft free flood space to provide a few knots in an emergency and could be run from batteries for a short period. The thruster can be rotated to provide steering and may also be used to assist with manoeuvring in harbour.
The large rudders have a total area of about 30m2 and are controlled with a complex mechanism fitted around the main shaft line that also actuates the hydroplanes. The aft hydroplanes are and can be moved independently to contribute to steering and stability, giving a wider safe operating envelope in the unlikely event of a rudder or hydroplane jam. The aft free flood area can be sealed and pumped out when in harbour to allow for maintenance of the hydroplane and rudder actuators. The forward hydroplanes are on one shaft and the actuators under the forward casing can be accessed easily for maintenance as they are above the water line when surfaced.
The diagram above does not remotely do justice to the complexity of a nuclear submarine that has multiple interconnected systems of systems. The vessel has to withstand enormous subsea pressure, support life for months without surfacing, move, and fight all as quietly as possible. Besides the enormous sophistication of nuclear propulsion, the boat relies on its critical hydraulic system for steering, plane control opening and closing valves, and weapon doors. The high-pressure air system is needed to control buoyancy and trim tanks and the low-pressure air system provides emergency breathing for the crew. There is also a complex electrical system that includes a large bank of lead-acid batteries as a backup power supply. The crew also rely on ventilation, air purification, oxygen and water-making systems to sustain life below the waves.
Escape
In the event of the submarine becoming disabled, and sinking in waters shallower than crush depth, the Astutes are fitted with two escape towers capable of operation with the boat lying at angles up to 30% off horizontal. The forward escape tower (FET) allows one man to escape at a time while the aft chamber is larger and has a two-man capacity. The larger aft hatch is also used when in harbour for embarking stores.
Both towers have a hatch at the bottom where the escapee enters wearing survival suits and breathing apparatus. The hatch is closed and water is slowly admitted into the chamber until full and equalised with surrounding sea pressure. The upper hatch is then opened and the escapee acends to the surface. In theory, it sounds simple but is not for the faint-hearted, fraught with dangers and only really viable if the submarine has sunk at very shallow depths. Rescue by a submersible that can mate with the escape chamber hatches is a much more realistic hope of survival and the NATO Submarine Rescue Service (NSRS) based at Faslane is always available to respond to distressed submarine incidents.
NOTE: All information in this piece is derived from sources already in the public domain and due to the particularly sensitive nature of submarine design, is inevitably vague or incomplete in places. In a future article, we will examine the Astute class weapons and sensors.
Main image: HMS Ambush, Mediterranean 2016, via NATO Marcom.
no astute for australia. realistically.
More realistically likely to be SSN(R).
yes
I have been wondering broad stroke as the international picture changes as extra capacity is needed for the RAN boats then perhaps it could be used to generate more hulls for the RN. I think 12 even 16 boats is a realistic number for the RN. Especially if missile technology pushes maritime strategy towards one of denial. We could keep pace with the PLAN.
Larger numbers of subs are realistic for land powers like the previous Soviet Union and in WW2 Germany. Its hard to think the RN competing branches will let the submariners grab all the new construction funding
If you look at production we can realistically sustain around 15 boats build every 25 years if there are no gaps ordering or big issues. So probably our maximum fleet of SSNs would be 11 unless we went for longer lifetimes than 25 years or new production. Capability.
RAN did want an in production design , it’s perfect in the circumstances. They even want to fit them out in Australia and Cammel laird were sub contracted on Astute hulls . It’s just a matter of will , and I guess we will know this month sometime
This is interesting: https://www.abc.net.au/news/2023-02-04/marles-hints-at-three-way-aukus-submarine-design/101931608
Quote:
Why?
It’s what Australia will most likely end up with, to the point that I think it’s pretty much a done deal.
It keeps the yards running through the “valley of death” period between the end of production of one class and the production of the SSN (R). It also “scales” the program and the economies that brings with it. The RN can push out procurement of a boat or boats until after a boat or two has been produced for Australia whereas, with no exports, funding would be needed to ensure back-to-back procurement. As stated in the first article in the series, you don’t just turn sub production on and off.
An extra one or two boats might not seem much, but it will add around a decade of work for the yard ensuring the workforce remains in place until SSN(R) is ready for production.
Put your money on Astute being Australia’s next submarine.
Agreed. The Australian paper Australian Financial Review ran an article this week speculating that the Australians may be lining up to buy Astutes HMS Agamemnon and HMS Agincourt. This would provide some much needed cash for the government, and would push the UK’s new boats to positions 8 & 9. An additional benefit would be apparently to fill a production hole in the Barrow-in-Furness pipeline.
The RN badly needs Agamemnon and Agincourt, the Aussies will have to wait in line or lease LA Class boats from the US. In reality, neither we or the US have a capacity to build Nuclear powered boats for anyone other than ourselves at present.
ASTUTE needs PWR2 which is now out of production by RR.
PWR3 doesn’t fit into the ASTUTE hull.
So it is all nonsense talk.
Ben Wallace was very clear that there would be no more ASTUTES built.
What this maybe saying is that the drumbeat can be upped and faster hull production for SNR can be achieved directly after the new boomers hulls are fabricated with the outfitting being done in AUS. It may be that SNR is more advanced in design than we realised. Building the hills to a commercial drum beat would cut costs.
Although AUS then risks creating a series of camels that they then have to support themselves.
I posted this earlier with link with but it was quarantined when I tried to edit it.
From ABC news Australia 04/02/23
Oh, I agree the core design will be shared.
Ben Wallace already said as much.
My issue is that USN will want something and RAN will want an all singing g and dancing version. Whereas RN has boomers so a more austere specialist version is likely their want/need. Risk is that RN ends up with something very big and very expensive and locked into it like F35z
I see what you are saying but maybe, just maybe the USN does want, (or think it needs), a more austere boat to complement Virginia/SSN(X).
‘Wallace said as much’ . Thats because hes has the Treasury and their acolytes in MoD whispering in his ear. You would have to take all the US equipment and systems to go with it and it would ‘only’ have the UK reactor. The US Virginias already have the main non power/reactor forward systems built as a ‘block’ outside and then installed on a raft inside the hull.
If they go down this path it will lead to tears and wasted money. Britain couldnt even agree on a common destroyer platform- a much simpler design- with the French/Italians
I’d tend to agree it will end in tears and the needs are very divergent.
RN will want an austere crew whereas USN will want 2-3x that crew level.
Yes I would imagine the RN would not want to go above the 98ish crew of an astute, but USN SSNs have crews of around 130.
” Acolytes” Haven’t heard that since the days of UKDJ when Herodotus used it to belittle a really funny bunch before he was banned for having multiple accounts and a huge Ego problem and spending way too much time on there “Lording” it over everyone else who dared to comment. I wonder what became of him ? Cheers Phil, still in Wales.
A design that worked with common components. Just sayin’ 🙂
A common reactor would only be the section that makes sense (It was done once only)
Theres is some technology shared but for obvious reasons Britain wants its own reactor development and build, maybe theres a ‘conversation’ coming up on that.
I think Rolls Royce marine provide the pump jet propulsors for USN already
It’s a American reactor. The UK tagging on that it’s somehow theirs is retarded.
Do they have the time to wait till 2040 for this paper boat to enter service?
For construction reasons and to avoid HEU uranium restrictions their obvious choice is the nuclear version of Frances Suffren with LEU ( they have spent A$900 mill on this already for the diesel powered Barracuda version).
For political reasons this is a no no.
Im inclined to take everything the politicians there say as blather. The enriched uranium would breach their own signatory to nuclear material treaties.
why? Australia has had HEU safely and reliably since our first research reactor in the went live in 1958 fueled with it, we slowly reduced the UEL level over time in line with trends to reduce risk – reaching LEU level in 2006 – That reactor has now been replaced.
Theres a production hole at Barrow ?
They have already started on hull 2 of new Dreadnoughts!
Realistically the pressure hulls won’t be in fabrication for that long.
It is all very well retaking those workers to other yard jobs but there will be pressure hull fabrication experience and team fade.
Given the glacial pace of these things then, yes, there may well have a pressure submarine hull production hole sooner than some might think.
This is glossed over by the overall build Gantt chart which dribbles on forever with the outfitting and static testing.
Actually, it’s Hull 3 they are working on now. Thought you would have known that ?
Yes, thats right . News was out 10 days ago
https://www.navaltoday.com/2023/02/10/construction-kicks-off-on-uks-3rd-dreadnought-class-submarine/
There is no product positions 8-9 as the dreadnoughts are being built.
There are already talks suggesting the boats would be built in the UK then sailed to Australia for final fitting out. Aussie engineers will fly to the UK to join in the build and learn on the job. RAN crews will do the same.
Can’t be done as PWR2 can no longer be used once PWR3 begins to be installed in the ‘boomers’.
This is a wonderful article, thank you. It has often been stated, especially recently as Nuclear submarines grow more and more complex, that placing 100 human beings in a steel tube and transporting them at depth, at speed, in the Oceans is a far more complicated process than placing a man on the moon. Reading this article goes a long way to explaining why this is true. As an ex skimmer when the my early world had HMS Dreadnought and Sputnik I am in absolute awe of the submarines, and spacecraft for that matter, that now exist. As for the crews of submarines, well it almost looks cosy until the realism strikes you about their existence at sea. I am full of admiration for these people
To be honest, I sometimes wish I was back on Torbay at times, this land stuff is a terrible bore, at least we all smelled the same. Cheers, Phil.
It just needed some vertical tubes……….
http://www.dmitryshulgin.com/wp-content/uploads/2016/05/Virginia-Payload-Module-VPM.jpg
SSN(R) getting them
https://www.navalnews.com/naval-news/2023/01/britains-new-attack-submarine-to-be-first-with-vertical-launch-system/
I just don’t understand why they were missing from Astute. USN were getting rid of torpedo tube launched TLAM. It isn’t a small hull so there is plenty of room. Mystery.
But were the USN getting rid of tube TLAM back when the main layout was frozen on Astue? As part one made clear that was a long long time ago.
Mk41 has been in service since 1986. Which way do you think USN thinking on missile munitions was going?
Additional tube launched TLAM IV was ordered in 2014. It hasn’t been a problem so far. Besides, money talks – it may just mean larger orders need to be placed
There is also the French option (naval version of Storm Shadow). 1,000km range from a torpedo tube. There are alternative options.
That was nearly 10 years ago. I love how so many here go really out of their way to defend poor Royal Navy decisions. The USN Virginia class was designed in the 1990s. If costs are a problem you go with the item available in bulk.
The USN may be building in bulk ( I think they have run out of states names) but it seems each Virginia is more expensive than UK Astutes
Also last year reconditioning of TLAM was ordered updating our existing stock to the latest version.
I always thought the T class had room for 20 reloads plus 5 if the tubes were loaded and the As had 30 reloads plus 6 in the tubes,
so T was dramaticly better at 30 stowage and astute a bit better at 36 stowage
Brilliant platforms but built at a glacial pace and with only 7 being built that means no more than 2 on active deployment at any one time.
They are our most capable ASW assets but there are too many missions for them; SSBN protection, CSG protection, ASW in the North Atlantic and cruise missile carriers.
Hopefully the arrival of T26 (if provided with a land attack missile in its Mk41’s) will release the Asute’s from being wasted as £1.3bn cruise missile carriers.
There has been little to no real strategic thinking about naval needs since the end of the Cold War. Imagine the RN 2 large destroyers / cruisers in the Med and Indian Ocean both with a silo full of cruise missile and our own recce constellation. We could reach most of what we would want to ‘strike’ on the planet.
Or fit out a few A400’s to carry cruise missiles. With tanker support they can quickly get where they are needed without the cost of the 6/7 ships (and 10 crews) needed to keep 4 ships on station.
For the Med yes. Further out, no.
Don’t forget you need assets to place. You need flight plans. And so on. With tankets and transports you are talking the cost of an escort give or take. You can just move a ship at sea.
You also need permission to overfly other nations airspace..which is not that easy.
A400s with cruise missiles? Its just biggles stuff.
Greenham Common AFB used heavy trucks for their cruise missiles, need to be closer , well just drive or even better use A400 to take the vehicles, otherwise use a ship if there’s ocean nearby.
Ukraine has shown how useful shoot and scoot can be for firing missiles
No its not.
https://www.youtube.com/watch?v=a0jL1d7MscM&ab_channel=USAMilitaryChannel
Makes as much sense as the RAF in the 1990s flying down a hostile runway centreline and dropping cratering munitions. Do it in the dark so they cant see you coming !
Oh.
Perhaps a very ignorant question as I was not a silent service type but how grossly expensive would it be to retrofit Astute (at least the last few platforms) with VLS…requiring lengthening the hulls of course. I guess I am asking if it is even practicable.
Why?
The idea of firing a VLS from a silent boat is a bit nutty as it gives anway its location straight away.
Yes, it makes sense to get a first punch into an oppose landing or such like.
You’d be better off putting VLS into T31 as that would be relatively cheap and easy to do.
Yet the USN does it. TLAM has a range of 900nm. How many states can say guarantee the oceans from their coast up to say, for example, 450nm is clear of submarines?
But you are safe putting it on a 7000-ish tonnes of ship that is highly visible? Really? And take it a step further a ship like T31 that is under armed and can’t look after itself.
Unless you have been to sea on one your comments here are flawed. Not going into detail but A Boats are very quiet. BUT diving depth and speed are not good. The conning tower cannot break through ice like the T class. Aus please ask Groton CN to build for you not Barrow.
That is in the design and not in the build?
The issue with speed was fixed after #1. And is certainly fixed in the B2 boats which are subtly different.
Um. You serve in a ship not ON it. But hey ho……….
Is that a requirement for Australia in Indo-Pacfic…. to break through ice ? Apart from the Ballistic subs for say Russia to fire missiles attack subs should stay away from the surface let alone the sea ice when detection is even easier
No mention of the pump-jet propulsion?
Everyone has it now for nuclear boats. Another British invention thats immediately taken up. The Aussies wanted it too for their diesel Barrcudas where the advantages arent obvious
Where is the suction pump or what that term to pull in the water to spin the propellers?
The Junior Rates’ Mess as illustrated in the photo very quickly became the Senior Rates’ Mess. As more and more roles onboard that were once the domain of Chiefs and POs are ‘de-enriched’ and taken on by JRs, it means their space is, frankly, inadequate. Queues for meals often stretch the entire length of 2 deck. It’s also not unusual to find people spread out through compartments due to the simple fact they cannot sit down in their own mess. By contrast, knock on the door of the Senior Rates’ Mess and step-in and it’s quite common to find the compartment occupied by two or three people at most.
Hot bunking is still quite common as well, as whichever boat is currently at sea is often sent as many trainees (those who have just left phase 2 training, who need to re-qualify in certain positions etc.) as they can possibly take which inevitably leads to some people doubling up. The bunks themselves also leave something to be desired. At one point a couple of years ago the A-boat I was on was carrying somewhere in the region of 120 people. This is a reality often factored into the submarine’s logistical planning and means every storage space for food is rammed full, including the fridges and freezers. The latter do break as it is, but filling them literally to the top of the compartment really doesn’t help.
I mentioned the above as they struck me as examples of reality versus intended design/expectation that probably wouldn’t be known to those reading the article without any experience of submarines.
Thanks – interesting to hear first hand experience.
Remember this study was supposed to only take 6 months.
Times almost up !
https://www.navylookout.com/the-royal-navy-to-examine-the-balance-between-investment-in-submarines-and-warships/
Was completed this week along with the wider Australian defence review which was completed last week, results to be announced in US with a press conference of the three leaders in mid to late March.
There are 3 wash basins in the photo; a 50% budget increase? 🙂
In my view the best way to secure the AUKUS contract for the UK is to offer boat 7 of the Astute Class (Agincourt I believe) to the Australians. Once Agincourt is completed and commissioned, send it to Australia with a British crew and they train the Australians in it operations and maintenance which would probably take a couple of years before the boat becomes fully Australian operated. This would give the Australians time to build up the support infrastructure required repair and support nuclear subs.
Meanwhile for the RN, I suggest that the last 3 T-boats be refurbished and activated to act as the 7th boat (2 of them) for the RN with the 3rd acting as the reserve for the RN and the RAN. Due to the age of the T-Boats the RN should essentially operate them close to home (North Atlantic). The 3rd T-Boat when sent to Australia can act as a training boat in Australian wateerd for additional Australians submariners once the Agincourt is fully crewed by Australians.
Once the next generation SSN(R) boats are approved and production authorized, the Australians orders should comprise 1 of every 3 boats constructed in the first 2 batch orders, after that RN and RAN orders per batch of 4 should be equal (50:50) all the projected 16 boats required by the RN and RAN are delivered.
Just my 2 cents on how to get SSNs quickly to the RAN while ensuring the UK gets the AUKUS contract.
That depletes Royal Navy forces. Really stuipid idea.
I don’t know what you mean by stupid idea. Currently the RN essential has only 5 hulls operational if you count the lat To boat which is being decommissioned soon and replaced with Anson. The last 2 Aboard Agamemnon and A gin our t are 2-4 always from being delivered to the RN.
So if the last An boat such is due in say 4 years time is passed to the RAN then to ensure a UK win on an 8 boat contract, what’s wrong without? Especially since my proposal essentially secures the RN a minimum of 7 hulls (in actual fact 8, due to availability of 2 older T-boat hulls, albeit of reduced capabilities to the A-boats), which is more than currently available to the RN.
The gentleman below @Bubblehead uses term “forward deploy” for placing an Ad boat with RAN. Call what you want, sale, lease, forward deploy, joint operation, the goal is the same, secure Australian order without reduction in the current number of deployable boats which to all intent and purposes will not exceed 6 for at least 8 years.
My proposal retains and in fact exceeds the projected deployable boats for the RN, yes with a caveat that the 7th boat consists of 2 older class of boats with lesser capabilities (that’s why I suggest 3 boats to support that 7th function, and limiting them to close to home role and training).
*Sorry Typos A-Boat not An Boat. Similarly with T-boat no To boat*
the solution is to forward deploy a platform to the asia-pacific for several years like the RN has done with the TY23’s in the Gulf. That way you can power project in that part of the world and also develop the skills the RAN will need to operate and maintain nuclear platforms. Hull availability prevents us from leasing one to them. Remember number one role of the SSN is to protect CASD.
Money does a lot of things. Show RR a billion $ and they’ll restart PWR2 production for more subs. It’s a business decision. If they have a customer for it, they’ll do it. Quite simple. Build 2 more Astutes in the UK, then MOVE the tooling for the forward end to Australia so they can build 4 to 6 more. Mate them with the aft end produced in the UK and shipped to Australia.
Modify the design to incorporate US CMS and weapons, and even insert a plug in the Australian-built versions for VLS if required. It’s just engineering and cost.
UK gets to extend production, keep highly qualified shipyard workers employed, and produce a 2nd line of nuclear reactors.
US gets to sell their CMS, Mk48 torpedoes, sub-Harpoon missiles and TLAM.
Australia gets their SSNs, initially from the UK, but later domestically built once their workers are up to speed, which will take at least 10 years.