Manta – the Royal Navy gets its first extra-large autonomous submarine

In early March this year, the First Sea Lord revealed that the RN will receive its first large unmanned submarine. Here we look at the background to this project and how the platform will be used to test technologies and develop doctrine.

Defence accelerator

In April 2019 the MoD published a competition notice seeking proposals to develop an autonomous version of an existing extra large unmanned underwater vehicle (XLUUV) to be procured under its Defence and Security Accelerator (DASA) programme. The contract was placed with Plymouth-based Marlin Submarines Ltd. (MSubs) in September 2019 although not publicised until this year.

The XLUUV is a two-stage project totalling just £2.4 Million. Phase 1, worth £1M, is for the delivery of the vehicle that can complete basic seaworthiness and autonomy tests. Phase 2, worth £1.4M, covers renting the vehicle from the contractor for two years and conducting more complex mission testing. The idea behind such “accelerator” projects is to trial concepts quickly in a “fast to fail” process of elimination. By making small investments in potentially risky but innovative projects the MoD hopes to get successful new technology to the front-line faster. However, there is high confidence that this XLUUV will be just the first iteration of a successful concept that offers a partial solution to both ASW challenges and the RN’s lack of SSNs.

The DASA specification called for the contractor to base their proposal on and exiting platform in order would reduce costs and speed up delivery. The submersible would need to carry, deliver and recover test payloads of and least 2 cubic meters and 2 metric tonnes, have intelligence-gathering and ASW barrier capability.

MSubs are a British design and manufacturing success story and have designed a range of small manned and unmanned submersibles for military and commercial purposes. In 2011 they built a Mobile Anti-Submarine Training Target (MASTT) for the US Navy. This low-cost UUV for Anti-Submarine Warfare (ASW) training remains in service today. MSubs also developed the Mobile Under Sea Test Laboratory (MUST), a 12-metre UUV that was used in an ASW exercise by the RN in 2012 and have supplied Dry Combat Submersibles (special forces delivery vehicles) to the US Navy. Harnessing this specialist UK expertise to the benefit of the RN is long overdue and this is the largest contract awarded by DASA to date.

MSubs’ experienced workforce at their Estover, Plymouth factory has built a succession of small submersibles over more than 10 years and has been able to complete the DASA project quickly. Like all existing plans, the COVID-19 pandemic may impact the trials timetable but MSubs say their software designers continue to work at home and a small team is currently working to complete the vessel. It has been possible to practice social distancing and there is only space for one person to work inside the submarine at a time regardless. The vessel has already been moved from the factory to Thales’ UK Maritime Autonomy Centre at Turnchapel Wharf in Plymouth from where the trials will be conducted. The original launch date was set for mid-April with the Phase 1 testing completed by the end of September and it may still be possible to keep to this schedule. As a prototype, it will not be painted black like an operational UUV, but instead will be white which can aid recovery from the seabed in the event it should become lost.

XLUUVs have many potential future roles but for the RN their first employment is likely to be ASW training particularly for simulating small conventional or AIP submarines in littoral waters. It could also conduct trials to de-risk aspects of future submarines – the SSN(R) or Dreadnought designs.

Exclusive photos of the Manta XLUUV pressure hull under construction at MSubs facility.

Manta is based on the existing S201 manned submersible.


When the RN officially announced the DASA project it caused confusion by stating it would be 30-metres long. A vehicle of this size is an aspiration for the future but Manta will be considerably smaller, based on the existing 9-metre S201 manned submarine. The S201 has been retrofitted with an autonomous control system to allow unmanned operation and the outer hull modified. The craft is fuel cell powered and can be operated for about a week without charging. The original intention was for 3-month endurance and a range of about 3,000 nautical miles. MSubs say that Manta could probably achieve this but only by filling the payload bay with additional fuel cells. As a testbed that will only be deployed on trials over for short distances at first, the endurance requirement is not critical.

The exact payload that the vehicle will carry has yet to be decided but will likely include passive sonar, EO cameras, electronic warfare (EW) and communications electronic support measures (CESM) equipment. The Manta project and the future UUVs that will follow should help to stimulate industry to take more interest in developing compact, low power sensors for the RN since existing frigates and SSNs have relatively generous space and power supplies available. The smaller European conventionally-powered submarines have to be more careful with power consumption and so there is some international experience of these design constraints. The market for miniaturised sonar arrays to be carried by unmanned systems looks set to expand rapidly in future. The advent of UUVs is also seeing a revival of research into non-acoustic detection methods that were mostly abandoned at the end of the Cold war. The RN has already experimented with this technology on its SSNs that may include wake detection systems. These sensors tend to be small, modular and have much lower power requirement than sonars, being well suited to equipping UUVs.

A major consideration for the project has been compatibility with Type 26 and Type 31 frigate mission bays. Given the range constraints of even XLUUVs, in future, they are likely to be carried into theatre onboard warships for launching at sea. UUVs have the potential to hugely extend the ASW and ISR reach of just a single ship. The Type 26 has a 15 x 20m mission bay and sophisticated mechanised Handling System (MBHS) that can lower a payload of up to 15 tonnes into the water. At least to begin with, the Type 31 will have virtually no ASW capability but this could be mitigated by embarking UUVs. Launching and recovering them via a hatch from the mission bay below the flight deck promises to be more challenging.

This is MSubs much larger 70-tonne “Moray” concept which could be the basis of more sophisticated future RN XLUUV iterations. The X-planes at the stern are suited to bottoming the submarine on the seafloor, are quieter and provide more precise control than a conventional cross-form rudder arrangement. The fin is useful for mounting sensors and communication equipment.

The ‘bare-bones’ Manta makes an interesting contrast with the ‘Gucci’ 45-tonne, $50M Boeing Orca XLUUVs being purchased by the US Navy. Boeing are a world leader in the development of large UUVs, having already gained experience with their Echo Voyager. The Orca will build on the Voyager design but lengthened to 25m with an 8-tonne payload bay, making it too large to deploy from a frigate or destroyer.

Uncharted waters

As this is such a novel vessel, new operating procedures and a concept of operations must be developed almost from scratch. Besides assessing the platform’s performance, this project will help the RN to define how operational XLUUVs will eventually be launched and recovered, navigated and deployed. There are also more complex future challenges around water space management, deconfliction and IFF as well as considering the moral and ethical frameworks for using ‘robot submarines’. Communicating with UUVs, especially in real-time remains one of the biggest challenges as the transmission of high bandwidth data through water is difficult or only possible at very short range. Like all submarines, the UUV will increase its chance of being detected when if it must rise to the surface to communicate via satellite or radio periodically.

DASA provides the funds and financial oversight but personnel from the Defence Science and Technology Laboratory (DSTL), the RN and MSubs will work together to run trials in the South Coast Exercise Areas (SCXA).

Stage 1 trials with test Manta’s ability to diving and maintain a set depth, following waypoints and its navigation accuracy. It must demonstrate situational awareness and the ability to detect and avoid obstacles. It must also deploy a small 250kg payload such as a mini UUV or a mine (although payload recovery is not part of the specification). Stage 2 will test its communications abilities and how it may be tasked, re-tasked or abort its mission remotely. Trials with various sensor payloads and power cells will also be conducted and its radiated acoustic signature measures across its operating envelope.

Manta offers a very inexpensive technology demonstrator and pathway for the RN to have operational UUVs within 5-10 years. Its shortage of manned submarines and the increasingly contested underwater battlespace means much is at stake for the RN with this relatively low-profile project.