• British XV Excalibur robotic submarine has now launched an American Mk 48 heavyweight torpedo
• Project Broadsword completed its first weapon integration test in seven months
• Engineers mounted the 3,700-pound Mk 48 torpedo externally beneath XV Excalibur’s hull

The US Navy and Royal Navy have launched an American Mk 48 heavyweight torpedo from the British XV Excalibur uncrewed submarine during testing in Scotland.

Held on September 13 2026, the launch took place at the British Underwater Test & Evaluation Centre under Project Broadsword, approved only seven months earlier.

It examined whether an American weapon could be integrated with a British autonomous underwater vessel through compatible mechanical, electrical and software systems.

AUKUS Pillar II sets the framework for Broadsword

AUKUS is a security partnership between Australia, the United Kingdom and the United States covering cooperation on advanced military capabilities.

Its second pillar, known as AUKUS Pillar II, focuses on developing and sharing advanced capabilities including artificial intelligence, autonomy, quantum technologies and advanced undersea systems.

Project Broadsword is part of this technology-focused cooperation, with the latest trial applying allied interoperability to an American weapon and British uncrewed underwater vessel.

Approved in February 2026, Project Broadsword brought together the Department of the Navy Rapid Capabilities Office (DONRCO), OPNAV N97, Royal Navy, Defense Innovation Unit, Naval Undersea Warfare Center Keyport and industry partners.

The project connected the Mk 48 with XV Excalibur, an extra-large uncrewed underwater vessel designed for autonomous or remotely controlled operations.

Unlike a conventional attack submarine, Excalibur does not have traditional internal torpedo tubes, so engineers mounted the approximately 3,700-pound weapon externally beneath its hull.

That configuration allowed the teams to conduct the launch while testing the interfaces required between the American torpedo and British underwater platform.

Excalibur had already been used by the Royal Navy as a mobile underwater laboratory for testing technologies and future payloads.

In 2025, the vessel was remotely controlled in British waters from a command centre in Australia, approximately 10,000 miles away.

Those tests gave the Royal Navy experience operating an uncrewed underwater platform without placing personnel aboard the vessel during its missions.

“This successful test marks a historic first for allied undersea warfare…validates our shared vision for true interchangeability,” said Adm. Daryl Caudle, Chief of Naval Operations.

“By integrating weapons, payloads, and mission architectures across allied platforms, we are increasing the flexibility, scale, and combat power of our combined forces and ensuring they are ready to meet the strategic demands of tomorrow.”

The test showed that a U.S. weapon could be integrated with a British uncrewed platform rather than requiring both systems to develop exclusively around individual systems.

Vice Adm. Robert Gaucher, Director of Submarine Programs, described the launch as an important early step toward developing greater interoperability for autonomous undersea systems.

Project CUTLASS will take the integration further

The US Navy plans to continue the work through Project CUTLASS, using Anduril's Dive-XL platform for the next stage of development.

The follow-on effort will examine payload integration, command and control systems, and operational scenarios involving uncrewed underwater platforms.

The Navy said this work will further develop the use of autonomous underwater vessels with different payloads and mission systems.

The September launch completed the Broadsword proof-of-concept, while CUTLASS now moves the effort toward additional integration and operational testing.

Further trials will determine whether the approach can support other weapons, platforms and mission configurations beyond the system used during Broadsword.