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.
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 36 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).
One 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.
Model showing the control room – sonar consoles on are the port side. Not requiring the control room to be centred around optical periscopes (which must be directly under the fin) allows for an optimised layout. (Photo: Navy Lookout)
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.
HMS Astute conducts her first trim and basin dive in Devonshire Dock, Barrow (Oct 2007). On the top of the casing, the fwd and aft escape hatches are clearly visible along with retractable bollards. Note the tiny bridge, relative to the large size of the fin which goes almost the full width of the casing to allow the Special Forces Payload Bay (SFPB) / DDS to mate with the lock-out chamber. The temporary green rack fixed to the casing held lead weights that could be moved around to test trim angles. (Photo: BAE Systems).
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.
Shielded from public gaze, the Astutes feature a new ultra-quiet propulsor design that is constructed as a separate unit which eases maintenance and installation. The cowl is made from composites and reduces acoustic energy emitted from the spinning rotor. The stator blades placed ahead of the rotor improve the water flow reducing turbulence and cavitation. HMS Artful, May 2014. (Photo: BAE Systems)
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.
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.