Navies Look to Uncrewed Systems to Counter Threats Beneath the Waves

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May 21, 2024

An extra large unmanned undersea test vehicle, photo courtesy of GAO/ Navy Program Office

An extra large unmanned undersea test vehicle

Photo courtesy of GAO/ Navy Program Office

By Kiran Suman-Chauhan, Nicolas Jouan, James Black

Even as the world focuses on land wars in Israel, Gaza, and Ukraine, threats to maritime security are proliferating quickly. Backed by Iran, the Houthis continue to threaten international shipping in the Red Sea. Russia and China are both investing in icebreakers to project power into the thawing Arctic. And Beijing has stepped up controversial efforts to assert claims in the East and South China Seas—building artificial islands, chasing off foreign ships with its navy, and deploying a “maritime militia” of fishing boats to harass those who oppose it.

These threats are not confined to the surface of the sea, either. There is a growing awareness of just how vulnerable critical sub-sea cables and pipelines are throughout the globe. This changing threat landscape begs the question: how do navies achieve broader, more systematic coverage of the seas without spiralling financial and human costs? One possible answer might lie in uncrewed underwater vehicles. UUVs, as they are known, have been used for decades in deep-sea exploration and research. More recently, technological progress has enabled advanced military applications of UUVs, which are being rolled out in support roles, including minesweeping, bathymetry, search and rescue, and surveillance. Direct use in combat is a growing possibility, with recent reports of Houthi rebels using armed UUVs to threaten shipping, and the Ukrainians using UUVs to strike Russia's Black Sea Fleet.

Technological progress has enabled advanced military applications of UUVs, which are being rolled out in support roles.

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This extended use is driving both new threats and opportunities for innovation. The recent announcement by French shipbuilder Naval Group of their contract to produce a 10-ton unmanned combat underwater vehicle for the French Navy reflects growing global interest in autonomous maritime capabilities. The world's major nations all have ongoing programmes to develop and acquire extra-large, unmanned underwater vehicles (XLUUVs). The United States' Orca, the United Kingdom's Project Cetus, Germany's Modifiable Underwater Mothership, Russia's Sarma-D, and the multiple unnamed projects of China, South Korea, and India are all XLUUV programs that are either under construction or in testing.

Although most XLUUV projects are officially unarmed, this is quickly changing. The recurring theme in XLUUV design is modularity. The American Orca is designed to accommodate a “variety of large payloads”, while the Royal Navy has issued a call for proposals for “all kinds of payloads” for Project Cetus. Naval Group's project is the first XLUUV to be officially labelled as a combat vehicle, but it is not alone. Although China's programmes are extremely secretive, at least one design appears to have torpedo tubes. Even more concerning, both Russia and North Korea claim to have tested nuclear-armed autonomous torpedoes. While these may not technically qualify as XLUUVs, they represent growing interest in uncrewed underwater systems with unambiguously offensive capabilities.

Several issues limit the effectiveness of today's combat XLUUVs, however. Advancements in areas like energy and propulsion are needed to increase endurance and mobility, and an asset operating in the deep sea requires higher levels of autonomy than an uncrewed vessel on the surface. Because radiofrequency signals cannot penetrate deep water, 'swarming' is challenging, and it is also difficult to keep a human 'in the loop' to, for example, decide upon or abort a strike. This raises technical and ethical challenges—though authoritarian states or groups like the Houthis may be less concerned about the latter than democratic nations.

Still, UUVs could be beneficial enough that these barriers are worth trying to overcome. One big consideration is cost asymmetry. Navies around the world want to transform their future fleets to increase combat mass, and autonomous systems are critical to those plans. Concentration of force into a small number of high-value assets—such as aircraft carriers and nuclear-powered submarines—is a big problem for modern navies. Western navies already struggle to crew, maintain, and keep available such sophisticated assets, let alone replace losses. Uncrewed systems might form part of the answer, being cheaper, expendable, and easier to replace if destroyed. The recent Australian Fleet Review, for example, committed to buying large quantities of optionally crewed surface vehicles at relatively low cost.

We have already seen how low-cost uncrewed capabilities with proportionally high potential impact have helped Ukraine in the Black Sea. Six unmanned surface vehicles (USVs) costing £250,00 each recently sank a Russian warship worth £50 million. Alongside anti-ship missiles fired from land, the threat to Russian warships from uncrewed systems has allowed Ukraine to nullify the sea control of a fleet many times the size of its own. This has forced Russian ships back from the coast or into port, where they are vulnerable to air strikes, and reduced the threat of amphibious assaults towards Odessa. In turn, the pushback has allowed Ukraine to free troops for other fronts, and to challenge Russian attempts at an economic blockade.

The prospect of projecting power while limiting human casualties is one that today's navies are keen to realise, especially given recruitment issues.

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Though the above example of low-cost uncrewed capability use has benefitted the West, the growing hostile use of UUVs could turn the tables. The United States and its allies have enjoyed near-global naval access and supremacy in the post–Cold War era, but as smaller hostile states and nonstate actors like the Houthis increasingly get their hands on UUVs, regional power balances at sea could begin to shift, demanding new countermeasures.

Weaponising expendability is not a new tactic. From ancient fireships to the suicide bombers of modern terrorist groups, it has been a tactic employed throughout history. Autonomy, though, is potentially transformative, removing the need to expose or expend human life. The prospect of projecting power while limiting human casualties is one that today's navies are keen to realise, especially given recruitment issues. The rise of autonomous systems is an opportunity to change the underwater environment: to achieve distributed sensing and lethality through flexible, affordable UUVs; to augment and help protect smaller crewed fleets; and to provide added mass and resilience in the face of increasingly attritional warfare at sea.

Uncrewed, combat-capable vehicles are transforming warfare in the air and on the ocean surface now. They could do the same for underwater combat in the very near future.


Kiran Suman-Chauhan is a research assistant, Nicolas Jouan a senior analyst, and James Black an assistant director at RAND Europe, a non-profit research institute and part of RAND.