In Spring 2020 the RN received its first extra-large uncrewed underwater vehicle (XLUUV). Here we look at how trials and experimentation with Manta are progressing and some of the wider considerations about bringing uncrewed submarines into frontline service.
Since we reported on Manta’s imminent delivery in March 2020 the vessel has been conducting a series of trials and experiments operating from its base in Plymouth. This is a joint effort between manufacturer MSubs, the MoD’s Defence and Security Accelerator (DASA), the Defence Science and Technology Laboratory (DSTL) and the RN’s underwater battlespace and submarine delivery teams.
This £2.4M DASA-funded project will complete the first 2 phases in April 2022. Phase 1 covered delivery and basic seaworthiness and autonomy tests. Phase 2 covers two years of mission testing of gradually increasing complexity. Negotiations have begun for phase 3 which is likely to see the submarine more closely integrated with the navy MARCAP (Maritime Capability) team and conducting trials that relate more directly to the operational environment and de-risk future technologies for SSN(R).
Like most elements of the undersea naval domain, precise details are limited but MSubs say that over the trials period they have gradually ironed out initial bugs and the submarine has now become very reliable with accurate depth control to within 20 cm achieved. Phase 1 trials proved Manta’s ability to dive and maintain a set depth, following waypoints, maintain navigational accuracy and detect then avoid obstacles. The specification also called for the ability to deploy a small 250kg payload such as a mini UUV or a mine (although payload recovery is not part of the specification). MANTA is really more ‘automated’ than ‘autonomous’ at this stage with AI-enabled decision-making capability in development.
As the platform performance has been improving, MANTA has been fitted with sensors including sonars and cameras. Two-way communication is possible while submerged at modest ranges. Inevitable challenges have been encountered during the testing phase, for example, rudders or hydroplanes have been damaged or jammed after hitting debris or weed. This experience has fed back to engineers who have enhanced the design to make it more robust and there are multiple redundancies in the solid-state computing systems on board. The radiated acoustic signature has been measured across its operating envelope and reduced where possible. MSubs retain ownership but MANTA is indemnified against loss allowing increasing risks to being taken to test its capabilities.
There is similar work going on in Southampton especially in the civil sector but Plymouth is developing into the UK centre of excellence for practical maritime autonomy. MSubs has expanded and now employs around 100 staff. Besides their portfolio of UUVs, they have a contract to supply Swimmer Delivery Vehicles to the United States Navy and will be well-positioned to bid for future RN XLUUV procurements. MANTA was assembled in Plymouth with the majority of its content sourced from UK manufacturers.
The RN has been operating small UUVs in various forms for almost two decades in the short-range reconnaissance, survey and mine warfare roles. The XLUUV offers a big step up in terms of potential capability but also presents a series of dilemmas in how they should be deployed operationally.
The First Sea Lord recently placed the undersea domain in the top 3 priorities for the RN. There is little chance of the manned SSN fleet ever exceeding 7 boats so uncrewed systems are clearly the only realistic way to supplement the all-too-thin ORBAT.
Manta is the start of a long developmental road and there are two key technical issues – communications and the fuel source, that are more difficult to overcome for UUVs than for above-water uncrewed systems. At present, there are big limitations on the bandwidth and range of underwater communications. For UUVs this means the amount of real-time intelligence data that can be sent back is restricted unless they surface, potentially compromising their mission. Commands from the mission controller to the UUV may also be limited to short or medium range. This is also a hurdle that must be surmounted if future UUVs are weaponised and a human needs to remain in the decision-making loop for weapon release.
Manta runs on Lithium-ion battery cells and range is dependent on battery life. If future XLUUVs are going to conduct missions that last several days or weeks then there are challenges in developing affordable and practical fuel sources. The highly sophisticated Boeing ORCA XLUUV being developed for the USN uses diesel-electric propulsion, similar to a conventional SSK but this is bulky and expensive. Hydrogen fuel cells are another alternative but dangerous hydrogen and oxygen have to be stored in pressurised bottles and the bi-products disposed of.
Batteries are clearly very attractive for the sake of simplicity and current technology is fine for small or medium size UUVs with a modest endurance. It may be technically possible to build a lithium-ion battery cell to fit in an XLUUV that could provide 5-10 MW hrs of power but such energy-dense cells present a major fire risk. If an XLUUV with such a battery caught fire while being carried on a ship it will be virtually impossible to extinguish and would have to be quickly dispatched overboard before it burned through steel decks. There is clearly a complicated safety case for carrying XLUUVs in the mission bay of a Type 26 frigate for example, before even addressing the challenges of launch and recovery in anything but the calmest sea states.
One of the big decisions that the designers of the SSN(R) which will replace the Astute class will have to make fairly soon is whether the boat will have a ‘mission bay’ to launch and recover future LUUVs or XLUUVs. This would add complexity and size and will present a difficult trade-off between the potential to greatly extend the reach of the SSN while compromising on size, cost and possibly stealth. Sticking to small disposable UUVs that can be launched via or torpedo tube or something marginally larger may be a better option. It is possible small UUVs for reconnaissance and decoy purposes may already be deployed onboard RN submarines.
The 25-metre ORCA is ultimately intended for the long-range independent mission, although this capability has not been fully proven. The RN’s first generation of operational XLUUVs are probably around 5-10 years away from deployment and will likely be considerably smaller than ORCA.