Although not as capable as nuclear-powered submarines, AIP technology excels in littoral environments and offers an enticing capability to countries uninterested in global power projection.
Air-independent propulsion submarines, or AIP, are non-nuclear submarines but differ from traditional diesel-electric submarines and offer advantages in underwater endurance and capabilities. By harnessing alternative sources of power that do not require oxygen from the atmosphere—or are air-independent—AIP submarines can remain below the surface for long periods.
AIP submarines offer, in essence, a bridge between traditional diesel-electric submarines and their larger nuclear-powered counterparts in some operational contexts.
AIP Submarines versus Diesel-Electric Submarines: Key Differences
The traditional diesel-electric submarine operates using its onboard diesel engines while sailing on the surface and batteries when below the ocean waves and away from the breathable atmosphere. The downside of this system is that batteries’ electrical charge can delete quickly, necessitating repeated surfacing or snorkeling to fire up the diesel engines and recharge batteries—a critical period when submarines are vulnerable to detection.
Air Independent Propulsion technology allows submarines to reduce the amount of time they have to spend on the surface.
Air Independent Propulsion Technology
AIP systems leverage several technologies to extend their time underwater. One of these technologies is Stirling engines, which are external combustion engines that use liquid oxygen and diesel fuel to generate highly efficient and quiet power compared to traditional diesel engines.
Fuel cells are another technology that produces electricity by combining hydrogen and oxygen within solid oxide fuel cells and producing water as a byproduct. Close-cycle steam turbines are similar to nuclear reactors but use onboard liquid oxygen stores rather than uranium. Modern lithium-ion batteries, which are not strictly air-independent, allow submarines to store greater amounts of electricity and remain submerged for more time.
Depending on the specific technology used and the submarine size and design, AIP submarines can remain submerged for several weeks—a significant improvement over their traditional diesel-electric counterparts.
Air Independent Propulsion: Limitations and Advantages
One of the great strengths of AIP submarines is their reduced acoustic signatures, as they generate less noise than diesel engines and less noise than nuclear reactors, which must be continuously cooled. They’re also the least costly in terms of initial build costs and sustainment for nuclear-powered submarines.
However, AIP submarines’ top speeds are generally lower than nuclear submarines, limiting their ability to cover long distances quickly. However, one of the most significant drawbacks of AIP submarines is their overall endurance, which is restrained by fuel and crew requirements like food. Nuclear-powered submarines, by contrast, are limited only by crew sustainment, i.e., food, and can otherwise remain underwater indefinitely.
Air-Independent Propulsion Technology Advantages Over Nuclear
Air Independent Propulsion submarines are especially useful for coastal defense against shipping and surface vessel operations in littoral waters near coastlines and in situations where blue-water transit and power projection are not required.
Closer to shore, AIP submarines are generally smaller, and low noise levels offer advantages compared to nuclear submarines.
AIP submarines are also particularly well-suited for mission sets that include intelligence gathering and surveillance and deploying or receiving of special operations groups. Perhaps most significantly, AIP submarines offer countries that cannot afford to purchase nuclear submarines or lack the infrastructure to maintain them a particularly attractive alternative.
The Cost Considerations AIP versus Nuclear
AIP submarine program costs are significantly lower than that of their nuclear-powered counterparts and cost anywhere from $500 million to $1 billion per hull. In contrast, nuclear submarines can cost several billion dollars per submarine.
However, initial purchase price aside, nuclear submarines also have very distinct infrastructure and supply chain requirements that add to the cost. Specialized dockyards designed to handle radioactive components, nuclear fuel supplies, and disposal strategies for spent nuclear fuel, increase the operating costs for nuclear submarines significantly.
By comparison, AIP submarines can operate from conventional—non-nuclear equipped—dockyards and do not require specialized fuel handling or maintenance procedures.
Research and development (R&D) costs for AIP technology vary depending on the country and propulsion system. Countries developing indigenous AIP solutions must invest heavily in R&D, increasing initial program costs, but those acquiring technology from established suppliers can reduce these expenses.
Detection and Capabilities: a Comparison
Nuclear submarines offer certain advantages in speed, overall endurance, and firepower. Still, air-independent propulsion submarines provide benefits in detection and maneuverability, particularly in shallow, littoral waters. But there’s nothing better than a nuclear-powered submarine for global reach and power projection.
Combined with the intercontinental ballistics missiles equipped with some nuclear-powered submarines, their reach and firepower are unparalleled.
However, the edge of nuclear-powered submarines in the open ocean is potentially negated in littoral waters, given their relatively large size and more limited maneuverability than air-independent propulsion submarines.
Gotland-Class AIP Submarines
Gotland-class submarine. Image Credit: Creative Commons.
Gotland-class Submarine.
Image of Gotland-class Submarine.
Gotland-class Submarine.
Gotland-class Submarine.
Postscript
Air-independent propulsion submarines are a significant advancement over their conventionally propelled counterparts and offer a cost-effective balance between endurance and cost, particularly for countries that do not have global power projection aspirations.
And while they cannot replace nuclear-powered submarines in terms of overall capabilities, they fill an essential niche in specific littoral applications.
About the Author: Caleb Larson
Caleb Larson is an American multiformat journalist based in Berlin, Germany. His work covers the intersection of conflict and society, focusing on American foreign policy and European security. He has reported from Germany, Russia, and the United States. Most recently, he covered the war in Ukraine, reporting extensively on the war’s shifting battle lines from Donbas and writing on the war’s civilian and humanitarian toll. Previously, he worked as a Defense Reporter for POLITICO Europe. You can follow his latest work on X.
