US and UK navies fire a 3,700-pound torpedo from a British robot submarine

On September 13 at the British Underwater Test and Evaluation Centre in Scotland, a British extra-large unmanned submarine launched a roughly 3,700-pound US Navy Mk 48 heavyweight torpedo. The Royal Navy's XV Excalibur carried the weapon on a mount bolted beneath its hull because the weapon will not fit inside the platform's internal payload bay. The US Navy and Royal Navy called the launch a "historic first for allied undersea warfare" (U.S. Navy press release). Project Broadsword, the AUKUS Pillar II program that approved the work in February 2026, ran from approval to firing in seven months.

The test, called Trial Iron, sat inside Project Broadsword, the AUKUS Pillar II program that connects US, UK, and Australian teams to work on advanced capabilities including AI, autonomy, quantum, and undersea systems. The Department of the Navy Rapid Capabilities Office, the Royal Navy, OPNAV N97 (the undersea warfare directorate), the Defense Innovation Unit, and Naval Undersea Warfare Center Keyport jointly approved Broadsword in February 2026. By September, a US kinetic weapon was firing from an allied autonomous hull. The seven-month gap between approval and firing is the result both navies are pointing to (TechRadar Pro coverage).

What the trial actually demonstrated is bounded and concrete. According to the US Navy release, the test "validated the mechanical, electrical, and software integration" of the Mk 48 with Excalibur's mission system. It proved that a heavyweight US torpedo can be married to a British autonomous platform with its interfaces aligned, not that either navy can now fight from robot submarines. "This successful test marks a historic first for allied undersea warfare," Adm. Daryl Caudle said. Robert Gaucher framed it the same way: allied capability, not combat capability.

The external mount is the visible limit on what that loop can carry today. Extra-large uncrewed underwater vehicles (XLUUVs) are designed for long-endurance missions such as intelligence, surveillance, mine countermeasures, and payload delivery. Excalibur was not built around a torpedo tube. The Mk 48, the US Navy's standard heavyweight torpedo since the 1970s, is roughly 19 feet long and weighs about 3,700 pounds, with a diameter of 21 inches. Sticking it under the hull works for a controlled test in friendly waters, but it changes the platform's hydrodynamics, signature, and recovery profile. The two navies know this; the workaround is the finding.

That finding shapes the next round. The follow-on program, called Project CUTLASS, is planned to use Anduril's Dive-XL XLUUV rather than Excalibur (TheDefenseNews). CUTLASS is being scoped around a platform with internal weapons bays and modular payload sections, which would let a Mk 48 ride inside the hull rather than hang beneath it. If the seven-month Broadsword loop is the template, CUTLASS is the chance to test whether the loop also lets the allies build the right platform, not just adapt the wrong one.

The same loop, however, has critics. Pillar II has political critics in all three legislatures who read fast integration as a way to spread sensitive technology faster than export-control regimes and political review can absorb. A seven-month integration window is a feature when the work is rigorous; it is a failure-mode risk when the interfaces get compressed. The external carriage is a visible signal: the alliance is moving faster than its current platforms were designed to move.

The test is not a doctrine. It is one integration firing, in friendly waters, of a known weapon carried unconventionally. The next milestone is whether the Broadsword-to-CUTLASS hand-off can run on the same clock without forcing the same workaround.