Large drone submarine joins British fleet

The Royal Navy has taken ownership of XV Excalibur, a 12-metre Extra-Large Uncrewed Underwater Vehicle described by the Ministry of Defence as the largest autonomous submarine ever trialled by the service.

Developed under Project Cetus by the Submarine Delivery Agency in partnership with MSubs Ltd, Excalibur displaces 19 tonnes and is intended as a testbed for both autonomy and future payloads. According to the Ministry of Defence, the project aims to build confidence in uncrewed underwater operations and help shape how such systems might complement crewed submarines in the coming years.

Excalibur was formally named in May at HMNB Devonport and will now undergo two years of sea trials. The SDA noted that it has already exceeded several of its original design specifications during acceptance testing.

Earlier milestones include its participation in Exercise Talisman Sabre in August, where the Royal Navy controlled the vessel in UK waters from a remote operating centre in Australia. The event formed part of AUKUS Pillar 2 work on advanced technologies and marked the first demonstration of UK–Australia XLUUV interoperability.

The programme also carried out a world-first trial of Infleqtion’s “Tiqker” quantum optical atomic clock on board the vessel. The SDA stated that the device offers improved long-term timing stability for navigation without reliance on external signals, supporting covert endurance. The trial took place six months ahead of schedule.

Now formally transferred to Royal Navy ownership, Excalibur will continue to be supported by the SDA’s Autonomy Unit through a test and evaluation programme designed to understand how autonomous underwater vehicles could operate alongside nuclear submarines in future force structures.