Submarines operate beneath the ocean’s surface for scientific, reconnaissance, or military purposes. A central question is their submerged endurance. This depends on propulsion systems and the finite resources needed to sustain the crew.
Powering the Dive: Propulsion Systems
The ability of a submarine to remain submerged for extended periods is primarily determined by its propulsion system, with two main types dominating modern fleets: nuclear and conventional diesel-electric. Nuclear-powered submarines utilize a nuclear reactor to generate heat, which produces steam. This steam then drives turbines for propulsion and also generates electricity for all onboard systems, including life support. This method offers a virtually unlimited power source, freeing the submarine from the need to surface for fuel for many years, sometimes even decades. This allows nuclear submarines to travel at high speeds for long durations and remain submerged for months at a time.
Conventional diesel-electric submarines rely on a hybrid power system. On the surface or at snorkel depth, they use diesel engines to propel the submarine and recharge batteries; these engines require atmospheric air, so they must surface or extend a snorkel mast. Once submerged, electric motors powered by batteries provide silent operation. Their underwater endurance is limited by battery capacity, typically requiring them to surface or snorkel every few days to recharge. While some advanced conventional submarines use fuel cell systems to extend submerged time, they cannot match the prolonged endurance of nuclear vessels.
Sustaining Life: Air and Water
Maintaining a breathable atmosphere and a supply of fresh water are fundamental for crew survival during prolonged submerged missions. Submarines generate oxygen primarily through electrolysis, a process that uses electricity to split water into hydrogen and oxygen. Distilled seawater is used for this process. The oxygen is then released into the submarine’s atmosphere, while hydrogen is vented overboard. For emergency or supplementary oxygen, chemical oxygen generators, often referred to as oxygen candles, can be used.
The removal of carbon dioxide (CO₂) and other potentially harmful gases is equally important. Submarines employ CO₂ scrubbers, which use chemical absorbents like soda lime or lithium hydroxide canisters to chemically trap and remove CO₂ from the air; some advanced systems use regenerative liquid absorbents that can be recycled. Fresh water for drinking, cooking, and hygiene is produced through desalination, typically using distillation or reverse osmosis. Modern distillation plants produce a significant volume of fresh water daily for crew needs and equipment cooling. These energy-intensive life support systems are more readily supported by the abundant power of nuclear submarines.
Human Endurance and Provisions
While technological systems can sustain a submarine’s environment for extended periods, the human element introduces practical limitations to submerged endurance. The finite supply of food and other consumable provisions for the crew represents a primary constraint. Modern nuclear submarines are typically provisioned to sustain their crew for about 90 days. Food storage includes a mix of frozen, dehydrated, and canned goods, with fresh produce becoming scarce after a few weeks at sea. The amount of food carried can be substantial, with thousands of pounds of various provisions needed for a multi-month mission.
Beyond physical sustenance, the psychological and physiological impacts of prolonged confinement in a closed environment also play a role. Submariners operate in a space with limited natural light, constant artificial lighting, and a lack of varied scenery, which can contribute to stress, fatigue, and monotony. While crews undergo rigorous training and selection processes, the mental well-being of individuals is a factor in determining mission length. Maintaining morale and addressing the psychological effects of isolation are ongoing considerations for extended deployments.
Operational Considerations and Resupply
Even with advanced technology that allows for prolonged submerged operations, submarines eventually need to interact with the surface world due to various operational requirements. Mission duration, communication needs, routine maintenance, and equipment checks necessitate surfacing or returning to port. While a nuclear submarine could theoretically remain submerged for years based on its power source, practical and strategic considerations dictate shorter deployment cycles.
Submarines require periodic resupply of food, spare parts, and other consumables. While primary resupply typically occurs in port, at-sea resupply is possible, though it often requires the submarine to surface. This can involve cargo planes air-dropping supplies into the ocean for retrieval, or helicopters performing vertical replenishment by lowering cargo onto the surfaced submarine’s deck. These operations are complex and require precise coordination. Additionally, crew rotation, medical needs, and the overall wear and tear on the vessel during extended patrols contribute to the eventual need to return to a base for comprehensive support.