Nuclear submarines are expensive. Meet the US Navy solution

The Manta Ray prototype extra-large unmanned underwater vehicle (XLUUV) completed full scale in water testing off Southern California in March 2024. The Manta Ray can harvest energy from the seas around it, potentially allowing it to stay on task for very long periods - Northrop Grumman

Right now, the US Navy only operates big, nuclear-powered submarines that can travel thousands of miles at high speeds across the Pacific and Atlantic Oceans and linger for months in conflict zones. But nuclear subs are complex, expensive and maintenance-intensive.

So while the US Navy requires 66 nuclear attack submarines, it currently operates just 50 – and numbers are set to decline as America’s shipyards grapple with a desperate labour shortage that has delayed the construction of new vessels.

New classes of robotic sub could help close the Americans’ undersea gap. And we just got our first detailed look at potentially the most powerful of these robo-subs: the Boeing-made Orca. A photo shared on social media by the former commodore of the US Navy’s drone-sub testing squadron depicts the Orca’s huge central payload module – which will house sensors, weapons and possibly smaller drones.

The diesel-electric Orca was around 50 feet in length before the US Navy got its hands on the design. The service added the 35-foot payload module with doors on the top and possibly on the bottom. With the module, an Orca displaces around 85 tons of water. It can sail autonomously as far as 6,000 miles, periodically surfacing and raising antennae to communicate with its faraway operators.

Orca would also, like a conventionally-powered manned sub, run its diesels while at the surface to charge up its batteries. Battery charging while at or just below the surface is the great time of danger and weakness for a manned submarine – this is a major reason why the US and Royal navies insist on nuclear power for manned subs, despite its terrific cost. But higher levels of danger are much more acceptable for an unmanned platform.

The US Navy has one Orca in its testing squadron and is expecting to take delivery of another five this year: Britain is also working on a similar, though smaller, robo-sub. The US fleet could acquire dozens of the $45-million Orcas in the coming years and deploy them for missions that don’t require a manned submarine – and could free up the manned subs for other tasks. Surveillance, for instance – or even minelaying.

There’s a chance the Orcas could handle anti-shipping missions, as well – provided US industry can develop a suitable torpedo, and US Navy testers can devise effective tactics. The US fleet’s standard sub-launched torpedo, the Mark 48, is 19 feet long and normally launches from a horizontal tube. The bulky weapon might be incompatible with an Orca’s payload bay.

There are smaller torpedoes in the works, including a new seven-foot model Northrop Grumman has been developing. Whether and when the Americans arm their Orcas with these smaller weapons remains to be seen.

As surveillance assets, the Orcas could have company. While Boeing and the US Navy develop the Orca, Northrop and the US Defense Advanced Research Projects Agency – DARPA, the Pentagon’s fringe-science agency – are working on a separate robotic submarine type, the Manta Ray. It has obvious surveillance applications.

The Orca has a conventional, tube-shaped submarine hull that suits its conventional diesel-electric propulsion. Official photos of the Manta Ray, snapped during testing off of Southern California this spring, reveal a wide winged vehicle that’s 85 feet long and displaces 85 tons of water. The wide hull suits the Manta Ray’s primary means of propulsion: the sub propels itself by changing depth and using its winged body to “glide” through the water. It harvests energy from the sea around it both to run its systems and to make it rise and fall through the water, potentially allowing it to stay at sea for very long periods.

“The Manta Ray program seeks to develop [underwater unmanned vessels] that operate for extended durations without the need for on-site human logistics support or maintenance,” DARPA explains.

There are regions of the world’s oceans the US Navy would like to patrol, but where it doesn’t have a lot of support infrastructure. Think southern, Arctic and African waters. Fuel-free propulsion “provides … energy security in remote and austere operating environments,” the DARPA team explained.

The Manta Ray on the surface in tests earlier this year. The 85-foot submarine 'glides' through the sea by varying its buoyancy to rise and fall - Northrop Grumman

To achieve fuel-independent propulsion for a vehicle meant to lurk in the dark depths of the ocean – meaning solar power is also out of the question – DARPA is borrowing technology from the green-energy industry. One of the energy-harvesting methods being investigated would see the Manta Ray anchor itself to the seabed and deploy a “kite” holding a turbine, which would automatically seek the depth of water above with the strongest tidal current flowing. As the tide flowed through the kite turbine, this would generate electrical energy to top off the sub’s batteries.

Other energy-gathering systems would involve using heat from the surrounding water to melt and expand wax in internal chambers, which could store energy mechanically by compressing gas. This would cause the Manta to lose buoyancy and sink, so driving its glide propulsion. Then the gas would be permitted to expand again, potentially driving generators for electricity and causing the Manta to rise and glide on.

It seems that these methods won’t produce high levels of power or speed, however, so the Manta will probably need to be put into the sea somewhere fairly close where it’s wanted. DARPA has a plan for that, however: the Manta breaks down for carriage in five standard shipping containers, allowing it to be transported to where it’s needed quickly.

“Shipping the vehicle directly to its intended area of operation conserves energy that the vehicle would otherwise expend during transit,” says Dr Kyle Woerner, DARPA project director. “Once deployed, the vehicle uses efficient, buoyancy-driven gliding to move through the water. The craft is designed with several payload bays of multiple sizes and types to enable a wide variety of naval mission sets.”

It might take a while to fully prove the Manta Ray works – or doesn’t. If it works, and is worth the cost, the US Navy could take over the project and order an operational version of the robo-sub to join its growing fleet of Orcas. The Orcas might handle missions requiring weapons and higher speed and power, while the Manta Rays stick to unarmed surveillance missions.

Whatever form the US Navy’s future fleet of robotic submarines takes, it’s clear American naval planners have options. And they’ll need them as they scramble to fill an undersea gap left by the rapid decommissioning of older manned subs.

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