A British Excalibur uncrewed underwater vehicle (UUV) has fired a U.S. Mk 48 heavyweight torpedo in what is said to be a first-of-its-kind test for both countries. Though just a proof-of-concept so far, this is a significant new step for the United States and the United Kingdom toward future operational fleets of armed UUVs.
The U.S. Navy and the Royal Navy conducted the armed UUV test at the British Underwater Test & Evaluation Center (BUTEC), a maritime test range in Scotland, on September 13. Personnel from the Pentagon’s Defense Innovation Unit (DIU) and the Naval Undersea Warfare Center Keyport Division (NUWC Keyport), as well as unspecified industry partners, also took part, per a U.S. Navy press release.
The test was also carried out under the umbrella of Pillar 2 of the trilateral Australia-United Kingdom-United States (AUKUS) defense cooperation agreement, which was first announced in 2021. AUKUS’ Pillar 2 covers a wide range of advanced technology cooperation, with particular focus on uncrewed platforms and autonomy. Pillar 1 of the agreement centers on support for Australia’s establishment of a nuclear submarine force.
For the September 13 test, the Mk 48 torpedo was slung under the Excalibur UUV. The nearly 40-foot (12-meter) long, 19-metric-ton submersible drone has a large internal payload bay, but it is reportedly not large enough to accommodate the Mk 48, which is the standard heavyweight torpedo in U.S. Navy service today. The 19-foot-long, 21-inch-diameter weapon has been successively upgraded over the years to expand its capabilities and improve its resistance to enemy countermeasures.
The Royal Navy officially unveiled Excalibur, which was developed by U.K.-based MSubs under Project Cetus, last year. The external torpedo launch capability was the result of two additional efforts, called Project Broadsword and Trial Iron, according to the U.S. Navy release.
The September 13 test “successfully validated the mechanical, electrical, and software integration of a U.S. kinetic payload onto an allied autonomous vessel, proving that allied forces can rapidly achieve weapon interchangeability,” the Navy’s release adds. “Approved in February 2026 by the Department of the Navy Rapid Capabilities Office (DONRCO), the project was executed in less than seven months.”
How the Excalibur UUV was controlled during the test and whether the torpedo was launched at a test target of some kind are unknown. TWZ has reached out for more information.
“BROADSWORD is a great example of how AUKUS Pillar II drives interoperability in across the alliance. Demonstrating the launch of a US torpedo from a UK extra-large unmanned underwater vehicle (XLUUV) is an important early step in developing lethality in our undersea autonomous systems,” U.S. Navy Vice Adm. Robert Gaucher, Director of Submarine Programs, did say in a statement. “We look forward to continuing to leverage the AUKUS partnership to keep pushing the potential warfighting advantages of unmanned undersea systems.”
“This successful test marks a historic first for allied undersea warfare,” U.S. Navy Adm. Daryl Caudle, Chief of Naval Operations, also said in a statement. “Proving that we can seamlessly launch a U.S. heavyweight torpedo from a Royal Navy autonomous underwater vehicle validates our shared vision for true interchangeability.
“This is a really important moment in our journey towards building a hybrid navy, which is where the Royal Navy will sail with crewed ships and submarines alongside uncrewed and autonomous vessels,” Luke Pollard, U.K. Minister of State for Defence Readiness and Industry, separately said in an interview with LBC radio. “It is a substantial increase in our lethality, to show that torpedoes can be fired not just by the Astute class hunter-killer submarines that we have in the Royal Navy, but also now by our uncrewed systems.”
The U.S. Navy and the Royal Navy employ various tiers of UUVs operationally today, but primarily for mine-sweeping and reconnaissance and surveillance missions. Future fleets of submersible drones capable of carrying out attacks on other vessels will create entirely new operational possibilities. Operated closely together with crewed surface warships and submarines, torpedo-toting UUVs could be sent ahead into riskier environments and offer an immediate option for engaging any threats they might find. Even a bigger UUV like Excalibur would also be more challenging for opponents to spot and track than a traditional attack submarine.
Excalibur is also in a category of larger and more capable submersible drones that offer significant endurance and higher degrees of autonomy. Adding in the ability to launch torpedoes, in turn, then opens the door to new concepts for employing these UUVs in groups more independently of crewed companions. In the future, for instance, armed Excaliburs or similar UUVs might be deployed to a particular maritime choke point to not just monitor for enemy movements, but actively hold them at risk.
In addition, in general, UUVs offer a way to expand total undersea warfare capacity in a more cost-effective manner compared to acquiring and fielding more traditional submarines. The ability to have more presence below the waves at a lower price point also translates to more persistence across large areas, something that could be especially valuable across the broad expanses of the Pacific, as well as other bodies of water globally. This presents additional targeting challenges and uncertainty for opponents.
“In securing the North Atlantic, especially against those nations that wish to do us harm, like Russia, the ability for us to now say it’s not just where our submarines are that you need to worry about – it is about how we create greater deterrence and question marks in the eyes of our adversary, with uncrewed systems there as well,” U.K. Minister Pollard said in speaking with LBC.
Pollard’s remarks here speak to particular considerations the British government is faced with just in its backyard. This includes the so-called Greenland-Iceland-United Kingdom (GIUK) Gap, which is the main path through which Russian Navy submarines transit to and from bases in the northwestern end of that country and the Atlantic proper. There is also the English Channel and the nearby Baltic Sea. The Royal Navy has limited capacity with its crewed submarines to monitor this and other critical regional chokepoints. UUVs could then help take pressure off its fleet of just six Astute class submarines, helping free those boats up for the myriad other missions they are tasked with, including carrier escort and special operations support.
“By converting commercial and allied uncrewed undersea hulls into high-end strike assets, the partnership drastically alters the cost-exchange ratio in contested maritime environments,” the U.S. Navy’s press release today notes, as well.
There are still many questions still to be answered about future larger UUV operations, including just how they will be deployed and recovered. Video the U.S. Navy released today shows that the torpedo-armed Excalibur was launched into the water in Scotland using a commercial truck-mounted crane. U.S. shipbuilders Huntington Ingalls Industries (HII) and General Dynamics National Steel and Shipbuilding Company (NASSCO), among others, have been working on new options for launching and retrieving larger drone submersibles from surface warships. External carriage on the hulls of ships and/or submarines might also be an option in the future.
Additional torpedo and other munition options for UUVs beyond the full-size Mk 48 could also be on the horizon. The U.S. Navy has invested heavily in compact and lightweight torpedo designs that could be well suited for employment from underwater drones like Excalibur.
The U.S. Navy says it is working with its international partners on another similar UUV capability demonstration, but this time involving an Anduril Dive-XL, under what is dubbed Project Cutlass. This effort “will tackle advanced payload integration data, command and control architectures, and tactical employment scenarios to further mature uncrewed undersea capabilities,” according to the U.S. Navy’s release.
The Royal Australian Navy is already in the process of acquiring and fielding a militarized version of the Dive-XL, called Ghost Shark. It’s also worth noting that the Dive-XL evolved from the earlier Dive-LD design. The Dive-LD is in at least limited operational U.S. Navy service now, a fact that was revealed by Iran’s capture of one of these submersible drones earlier this month, as you can read more about here.
The U.S. Navy is also continuing to push ahead with plans to field a force of Orca XLUUVs from Boeing. In August, the service announced that an Orca had conducted a 1,000-nautical-mile transit in the Pacific the preceding month as part of the ongoing development of that design.
Other countries are pursuing larger UUVs, as well. China has been making strides in this regard and is in some ways ahead of other competitors globally.
Excalibur’s ability to launch a standard Mk 48 heavyweight torpedo is already an important show of progress now toward a shared goal of a future underwater ecosystem that includes submersible drones that are able to attack other vessels.
Contact the author: joe@twz.com
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