The torpedo weighed more than 1½ tonnes and ran nearly six meters long. The submarine that fired it had no crew aboard. XV Excalibur, the Royal Navy’s 12-meter experimental uncrewed submarine, has launched a U.S. Navy Mk48 Mod 4 heavyweight torpedo on a test range off the northwest coast of Scotland, the first time a Royal Navy robot submarine has fired a torpedo.
The weapon was an inert, recoverable version of the Mk48, not a live warhead. The shot came during a three-week experiment run with the United States Navy under the AUKUS partnership of Australia, the United Kingdom and the United States.
The Royal Navy announced the trial in a release published Sept. 30, 2026. It states: “For the first time a Royal Navy robot submarine has successfully fired a torpedo in a milestone trial pushing the boundaries of undersea warfare off Scotland.” The firing took place “at the British Underwater Test and Evaluation Centre (BUTEC) near the Kyle of Lochalsh on the northwest coast of Scotland,” according to the release, which does not give the date of the shot itself.
“The importance of this achievement cannot be overstated – this paves the way for a step change in underwater lethality.” — Lt. Cmdr. David Iwanek, Royal Navy, UK trial director
XV Excalibur, a 19-tonne testbed
Excalibur is not a warship. The Royal Navy’s naming announcement of May 15, 2025, made at HMNB Devonport, describes “the 12m experimental vessel, displacing 19 tonnes and at two metres across,” and says the craft “was built by Plymouth-based MSubs, who specialise in automated submersibles.” The vessel is the product of “the three-year long Project Cetus.”
That same 2025 announcement set the boat’s role plainly: “As a demonstrator, the vessel will not perform operational duties but will shape future concepts.” It said that “over the next two years, Excalibur will carry out extensive sea trials.” The Sept. 30, 2026 release confirms Excalibur entered service in May 2025, putting the torpedo trial roughly 16 months into that two-year program.
Firing a U.S. Mk48 Mod 4 from a drone
The weapon came from the U.S. inventory. Per the Royal Navy release, Excalibur was “adapted with a specialist management system to successfully fire and control US Navy Mk48 Mod 4 Heavyweight Torpedo.” The release describes the Mk48 as “weighing over 1½ tonnes and nearly six metres long.”
The U.S. Navy’s Mk48 heavyweight torpedo fact file states that the Mk48 “is used by all classes of submarines as their anti-submarine warfare (ASW) and anti-surface warfare (ASuW) weapon,” lists a 21-inch diameter and liquid-propellant propulsion, and notes that its latest variant, the Mod 7, was developed jointly with the Royal Australian Navy. The fact file does not cover the Mod 4 variant named in the Royal Navy release.
The trial used a practice round. The Royal Navy states the experiment “used an inert, recoverable version of the heavyweight torpedo,” so no live warhead was fired.
Six months, two navies, one range
The Royal Navy release says “the trial became a reality in just six months.” Naval News, reporting the announcement on Sept. 30, 2026, gives the trial’s codename as Project Broadsword/Trial Iron and ties it to “the AUKUS agreement between Australia, the UK and USA.” The release lists the participants as the Royal Navy, the United States Navy, the UK’s Submarine Delivery Group Autonomy Unit, MSubs, and QinetiQ, which operates the BUTEC range. The work falls under AUKUS Pillar 2, which the release describes as focused on “rapid capability development.”
The Royal Navy frames the result in cost terms. The release states that “it shows that the US and Royal Navies can eliminate the need for costly and time-consuming bespoke vessels or systems to bring undersea combat power to bear.” That is the service’s claim; what the trial demonstrated, per the same release, is that an uncrewed vessel could be adapted to fire and control an existing U.S. heavyweight torpedo.
Gen. Sir Gwyn Jenkins, the First Sea Lord, said in the release, “In just six months, British and American teams, supported by industry, have delivered a credible fighting capability.”
More shots are planned. According to the Sept. 30 release, “This launch is the first in a series of trials initiated by the United States that will increase in scope and complexity.”
Top image: A Mk 48 heavyweight torpedo on public display. XV Excalibur itself is not shown. Image Credit: Chris Light — CC BY-SA 4.0 / Wikimedia Commons
This article was produced with the assistance of AI and reviewed by Combat Rated editors prior to publication.