The U.S. Navy has not built diesel-electric submarines since the late 1950s, instead investing itself into an all-nuclear powered submarine force. Will a slew of modern advancements in AIP (air-independent propulsion) technology prompt the Navy to reconsider its stance? 

A conventional, diesel-electric submarine generates electricity that drives the motor, which then powers the propellers. There are major disadvantages to this design: a diesel-electric submarine must frequently surface to recharge its batteries, and cannot sustain high speeds for prolonged periods of time. 

In the mid-1950s, the U.S. Navy shifted its research and development focus to nuclear propulsion. As with any revolutionary new technology, there were some serious growing pains; it would take a while for the first nuclear-powered boats to consistently exceed the reliability and effectiveness of the latest and most advanced diesel-electric designs. But over the coming decades, it became clear that nuclear propulsion handily outperforms its diesel-electric counterpart across numerous fronts: notably, it offers virtually unlimited range and travels at much higher sustained top speeds despite being markedly quieter. The U.S. Navy quickly began to phase out its fleet of conventional boats in favor of the all-nuclear force submarine force that it currently operates. 

For decades, it seemed like a foregone conclusion that the Navy would never again return to conventional propulsion methods—however, recent strides in air-independent propulsion (AIP) technology pose a fresh challenge to what has been the Navy’s ironclad commitment to nuclear propulsion. AIP, in short, is a method of propulsion that allows the submarine to operate without direct access to outside air. There are three main ways that this can be accomplished on modern submarines: closed-cycle steam turbines, the Stirling system, and fuel cell technology. Each of these methods has its own set of benefits and design challenges, but AIP offers a generalizable set of advantages: AIP submarines generate substantially less noise than both their nuclear and diesel-electric counterparts, stay submerged for relatively long periods of time, and are generally much cheaper to produce than nuclear boats. Moreover, many older conventional submarines can fairly easily be retrofitted with AIP systems, giving them a new lease on life.

So, should the Navy revisit conventional submarines on the basis of AIP technology? The two remaining weaknesses of AIP, relatively limited endurance and lower sustained speed are wholly or partially negated in littoral and coastal waters. Though the Navy has historically prioritized the global-power projection capabilities of nuclear submarines, this doesn’t mean that there isn’t room for a small, cheap defense force composed of AIP submarines. 

On the other hand, there is no specific need for it. The countries currently building AIP submarines—including China, Japan, and India—all share one thing in common: their navies have littoral, coastal and regional missions that justify the investment into AIP technology. The United States, by contrast, is largely engaged in global-power projection missions spread out across the Pacific Ocean, Atlantic Ocean, and Indian oceans; those types of operations are better suited for nuclear-powered submarines. AIP technology is rapidly shaping up to be a potent, cost-conscious alternative to nuclear submarines—just not for the U.S. Navy. 

Mark Episkopos is a national security reporter for the National Interest.

Image: Reuters