US defence research agencyDARPA has developed Sea Hunter, a prototype unmanned submarine tracking vessel with the ability to autonomously patrol the seas for months on end at a fraction of current costs. Julian Turner investigates what this could mean for conventional submarine warfare.

Image: DARPA

Even by US military standards, the excitement surrounding Sea Hunter, a prototype unmanned submarine tracking vessel developed at a cost of $20m by US defence research agencyDARPA, is startling. Variously described as “a highly autonomous unmanned ship that could revolutionise US maritime operations” and “a new vision of naval surface warfare”, the drone was developed through the agency’s Anti-Submarine Warfare Continuous Trail Unmanned Vessel (ASW ACTUV) programme.

The Class III unmanned surface vessel (USV) has the potential to traverse thousands of kilometres of open ocean for months without a single crew member aboard and at a fraction of current costs – estimates range from $15,000-$20,000 a day compared with $700,000 a day to operate a destroyer. According to DARPA, Sea Hunter could ultimately lead to a whole new class of ocean-going vessel and eradicate the need for larger manned warships, transforming conventional submarine warfare.

“ACTUV represents a new vision of naval surface warfare that trades small numbers of very capable, high-value assets for large numbers of commoditized, simpler platforms that are more capable in the aggregate,” said Fred Kennedy, director of DARPA’s Tactical Technology Office (TTO).

“The US military has talked about the strategic importance of replacing ‘king’ and ‘queen’ pieces on the maritime chessboard with lots of ‘pawns’, and ACTUV is a first step toward doing exactly that.”

Measuring 132ft in length and capable of 27 knots, Sea Hunter is the world’s largest unscrewed ship and is designed as a trimaran, the name given to a multihull vessel comprising a main hull and two smaller outrigger hulls (or ‘floats’) attached to it with lateral beams.

A successful joint test with DARPA’s Towed Airborne Lift of Naval Systems – meaning Sea Hunter could handle communications relays, and intelligence, surveillance, and reconnaissance sensor packages – was followed by open-water testing of the vessel’s sensing and autonomy suites.

In 2017, three tests took place to integrate the suites and use them to comply with International Regulations for Preventing Collisions at Sea during operationally realistic scenarios. In August 2017, Sea Hunter conducted at-sea tests with a mine countermeasures (MCM) payload. DARPA and the Office of Naval Research (ONR) also conducted tests in order to prove a key design element: the vessel’s flexibility to handle diverse missions by switching among modular payloads.

“We are already working on autonomous control”

DARPA completed its ASW ACTUV project in January and transferred the demonstration vessel to ONR the following month. ONR will develop the prototype as the Medium Displacement Unmanned Surface Vehicle (MDUSV) and test Sea Hunter’s automated payload and sensor data processing, new mission-specific autonomous behaviours, and autonomous coordination among multiple USVs.

“ACTUV’s move from DARPA to ONR marks a significant milestone in developing large-scale USV technology and autonomy capabilities,” stated TTO programme manager Alexander Walan. “Our collaboration with ONR has brought closer to reality a future fleet in which both manned warships and capable large unmanned vessels complement each other to accomplish diverse, evolving missions.”

“We are already working on autonomous control, a challenging area that is key to maturing MDUSV and delivering it to the fleet,” added Robert Brizzolara, ONR programme officer for MDUSV.

Pending test results, Sea Hunter could enter service for the US Navy in 2018 in a variety of roles. The drone’s stated purpose is to locate, track enemies, and engage submarines, primarily using a high frequency fixed sonar array, but MCM testing suggests mine countermeasures could be an option.

“Both roles make good sense for an unmanned system such as Sea Hunter,” writes Joseph Trevithick in The Drive. “A group of MDUSVs could more readily search a wider area for both hostile subs and mines and similar underwater hazards.

“The drone boats could also scout well ahead of manned ships for the enemy… and get close to particularly high value assets, such as aircraft carriers or amphibious assault ships.”

“A group of MDUSVs could more readily search a wider area for both hostile subs and mines and similar underwater hazards”

Submarines pose a growing threat to the US Navy’s surface ships. Air-independent propulsion technology means diesel-electric submarines are quieter, and therefore harder for sonar to detect. They can also remain submerged for significantly longer periods of time than their predecessors. As DARPA’s Fred Kennedy alluded to, smaller unmanned boats would also be more cost effective than relying entirely on larger warships, aircraft or aerial drones to search for and counter threats.

Able to endure up to 90 days at sea without a crew and with an estimated range of 10,000 nautical miles, fleets of unmanned ships could also provide 24/7 protection for inlets, harbours and other sensitive maritime locations, and deliver missiles, torpedoes and other supplies to vessels at sea.

In an interview with Reuters in 2016, former US Deputy Defense Secretary Robert Work said he believed USVs in general, and Sea Hunter in particular, could revolutionise maritime operations.

“This is an inflection point,” he said. “This is the first time we’ve ever had a totally robotic, trans-oceanic-capable ship.”

“I would like to see unmanned flotillas operating in the western Pacific and the Persian Gulf within five years,” he added, going on to emphasise that such fleets would always be under human control. “There’s no reason to be afraid of a ship like this.”

The US Navy clearly believes that unmanned ships have the potential to effectively track and engage enemy submarines for extended periods; plans already in the pipeline include equipping drones with anti-submarine weapons and additional sensor suites to gather visual and electronic intelligence. In addition to flexible mission capabilities, multiple autonomous fleets can potentially be operated by a single land-based crew, a budget-conscious alternative to conventional, personnel-heavy fleets.

“This is the first time we’ve ever had a totally robotic, trans-oceanic-capable ship.”

The fact that China, Russia and North Korea are looking to develop their submarine fleets is another point in Sea Hunter’s favour. A sister vessel, Sea Hunter II, has been ordered by the US Navy at an initial cost of $35.5m; operational testing of the original craft is a real possibility by the end of 2018.

The Drive reports that in February 2017, Captain Chris Sweeney, deputy director of Surface Warfare for Aegis and Ballistic Missile Defense, told USNI News that the service was considering forming an “experimental squadron” consisting of Sea Hunter, the first-in-class USS Zumwalt stealth destroyer, an Arleigh Burke-class destroyer, and a littoral combat ship for the purpose of field evaluation. Some analysts believe Sea Hunter may even participate in real-world missions this year, beckoning forth a new era of naval operations dominated by fleets of drones operated by land-based crews.