Nuclear powered submarines provide the best submerged endurance but are costly. Air Independent Propulsion submarines come a distant second but are better than diesel electric submarines. Let us compare the three submarines’ performance and their powerplants. The aim was to compare how much time a submarine can stay submerged between periscope depths using air independent propulsion methods including batteries at a particular speed in no comms scenario. We were comparing endurance of three classes of submarines as in

• Nuclear Powered Submarine
• Air Independent Power / Air Independent Propulsion and
• Diesel Electric Submarine

The comparisons will be in a speed range assuming they do nothing more than the speed range. The submerged endurance (including slower loiters for hearing what is around them but not higher than that speed) will be done at three speeds

• >5kn,
• 10-15 kn and
• 25 kn+

Diesel Electric Submarine

Second Taigei class Submarine JS Hakugei

These submarines operate by using their diesel engines at surface or periscope depths (using a snorkel) to charge a battery system. Their submerged endurance is entirely dependent on how long the batteries hold charge and how much peak power they can provide. Longer charge means they can loiter along longer and higher peak power allows for burts of high speed operation. Examples: Kilo class, Indian Scorpene class (Kalvari class submarines), Lada class etc.

The last 2 Soryu class submarines and Taigei class submarines use Lithium Ion batteries that bring about a monumental increase in performance, matching that of AIP submarines while also providing higher power settings for longer than AIP.

Performance

• >5kn: Can spend upto 2 weeks just lurking around only on battery
• 10-15kn: Maybe a week, usually less for older subs
• 25kn: Time counted in hours maybe a day at the most.

Air Independent Propulsion Submarine

Chinese Type 039B submarine which has Stirling engines for AIP

This type of submarine has diesel engines and batteries but do have a secondary powerplant that can operate while submerged by using onboard consumables/fuels. There are several types of AIP systems that are in service today, here they are listed below.

• MESMA: This is an old French system that burns ethanol and pressurized oxygen to turn steam turbines that can either drive the propeller or charge the batteries for quiet operation. This is basically like a nuclear submarine but with a conventionally fired boiler. They should be able to replenish some oxygen using hydrolysis but they are stuck with carrying diesel and ethanol i.e. two fuels for operations eg. Agosta 90B of the Pakistani Navy
• Fuel Cell: These submarines use liquid hydrogen and liquid oxygen and pass it through a Polymer Electrolyte Membrane where the two combine producing electricity. The power generated is relatively low so it is more suited for slow to medium speed running at depth. Higher speeds require batteries which limit endurance. There is a sub-type seen on Spanish S-80 class and Indian DRDO AIP to be fitted on Kalvari class where Ethanol and Oxygen are first reacted to produce Hydrogen and Oxygen and that is fed into the fuel cell. eg. Type 212 and S-80 classes
• Stirling engines: This is a closed cycle engine that uses a hot end and a cold end to heat and cool a gas that drives a piston producing power. These boats use Diesel and onboard Oxygen to produce the heat, thus only 1 fuel type needed and sea water as a heat sink. Oxygen could be generated from sea water but it becomes a chicken and egg problem as it would require more power to generate the gases than the power the gases will produce eg Type 039 and Gotland class submarines.

Performance

• >5kn: Can spend 3-4 weeks if not a month & a half just lurking for newer subs, at this time food and maintenance are limiters.
• 10-15kn: Multiple weeks of usage in transit not a month though
• 25kn: Can run for a day or 2-3 depending on the size of AIP fuel/chemical stores

Nuclear Powered Submarine

Borei-M class SSBN of the Russian Navy

Let me start with a spoiler, Nuclear Powered Submarines have a nuclear reactor, not a lot of people know that. Sorry. These boats use nuclear reactors to heat water that drives steam turbines which either drives the props or drives generators or both. They have a lot of reserve power and only need refuelling once a decade. These submarines can maintain high power and thus speed settings for days-weeks-months if they dont run out of food/consumables or develop maintenance issues before that. So if a boat is fresh out of a refit, she can keep going for as long as the commander wants.

Performance

• >5kn: Limited by food & maintenance
• 10-15kn: Limited by food & maintenance
• 25kn: Limited by food & maintenance