Key point: It is no surprise that navies that operate largely around coastal waters are turning to cheap AIP submarines, as their disadvantage are not as relevant when friendly ports are close at hand.
Nuclear-powered submarines have traditionally held a decisive edge in endurance, stealth and speed over cheaper diesel submarines. However, new Air Independent Propulsion (AIP) technology has significantly narrowed the performance gap on a new generation of submarines that cost a fraction of the price of a nuclear-powered boat.
A conventional submarine’s diesel engine generates electricity which can be used to drive the propeller and power its systems. The problem is that such a combustion engine is inherently quite noisy and runs on air—a commodity in limited supply on an underwater vehicle. Thus, diesel-powered submarines must surface frequently to recharge their batteries.
The first nuclear-powered submarines were brought into service in the 1950s. Nuclear reactors are quieter, don’t consume air, and produce greater power output, allowing nuclear submarines to remain submerged for months instead of days while traveling at higher speeds under water.
These advantages led the U.S. Navy to phase out its diesel boats in favor of an all-nuclear powered submarine fleet. However, most other navies have retained at least some diesel submarines because of their much lower cost and complexity.
In the 1990s, submarines powered by Air Independent Propulsion (AIP) technology entered operational use. Though the concept dated back to the 19th century and had been tested in a few prototype vessels, it was left to Sweden to deploy the first operational AIP-powered submarine, the Gotland-class, which proved to be stealthy and relatively long enduring. The 60-meter long Gotlands are powered by a Stirling-cycle engine, a heat engine consuming a combination of liquid oxygen and diesel fuel.
Since then, AIP powered-submarines have proliferated across the world using three different types of engines, with nearly 60 operational today in fifteen countries. Around fifty more are on order or being constructed.