Conventional Submarine Propulsion - Overcoming the Challenge

While conventional submarines are far more effective in the littoral environment vis-à-vis their larger, more powerful and noisier nuclear counterparts, the limited dived endurance is also their greatest vulnerability

Issue: 2 / 2014By Commodore (Retd) Anil Jai SinghPhoto(s): By US Navy

Perhaps the single greatest limitation in conventional submarine operations is the constraint on their dived endurance or the period they can remain dived without having to recharge their batteries. This duration is determined by the rate of discharge of the submarine’s batteries which in turn depends upon the propulsion, speed, machinery and equipment running on board, and the tactical situation prevalent in the area. It is this limitation with its consequential effect on speed, stealth and concealment that makes conventional submarines vulnerable to detection, particularly in a geographically limited littoral environment. Herein lies the paradox; while conventional submarines are far more effective in the littoral environment vis-à-vis their larger, more powerful and noisier nuclear counterparts with concealment and stealth being their greatest assets, the limited dived endurance is also their greatest vulnerability as it exposes them to detection from the air and from surface platform in a relatively restricted oceanic space.

It is this limitation on dived endurance that has led the enthusiasts of nuclear powered submarines to contemptuously dismiss conventional submarines as “mere submersibles” to quote the legendary initiator of nuclear powered submarines, Admiral Hyman G. Rickover on a visit to an Indian Naval Submarine in December 1982.

It is this limitation on dived endurance that has led the enthusiasts of nuclear powered submarines to contemptuously dismiss conventional submarines as “mere submersibles” to quote the legendary initiator of nuclear powered submarines, Admiral Hyman G. Rickover on a visit to an Indian Naval Submarine in December 1982.

Nuclear submarines are also of two types. There are the strategic ballistic missile submarines (SSBN) also called Boomers and the Attack submarines (SSN). The former are the ultimate instrument of nuclear deterrence as they combine concealment and speed with a weapon arsenal that can destroy the world several times over. During the Cold War, these submarines ensured that the war remained ‘cold’ and in the post-Cold War period have continued to deliver deterrence in an increasingly dangerous world. SSNs on the other hand are designed for speed, endurance and lethality; while their primary role was to tail the SSBNs, they have proved to be extremely effective in an expeditionary 21st century littoral environment with their cruise missile capability and long endurance limited only by the human factor on board. Modern SSNs like the British Astute class do not need their reactors to be refuelled for their entire service life which in effect means they have unlimited endurance. SSNs have repeatedly proved their versatility and reach from the Falklands in 1982 to Libya in more recent times. Nuclear submarines however are extremely expensive platforms to build, operate and maintain.

That apart the limited utility of nuclear submarines in a littoral environment characterised by relatively shallower depths has led almost 40 nations to operate conventional submarines and this number is growing. Vietnam has recently ordered six Kilo class submarines from Russia. Bangladesh has also shown keenness in acquiring submarines, with China apparently willing to oblige. The entire Indo-Pacific region has in fact seen a proliferation of submarines as they provide the maximum bang for the buck in delivering the cutting edge of offensive firepower to any navy and are therefore the most effective instruments of sea denial, i.e. denying the use of the sea to the enemy and particularly if it is a more powerful one.

Early Years

Conventional submarines are propelled by electric motors powered by batteries which are charged by diesel generators on board. Over the years the improved efficiency of the diesel engines, better batteries and power conserving motors have attempted to limit this constraint and ‘indiscretion rate’ which is the total time the submarine is exposed vis-à-vis the total time it is dived in a 24-hour cycle has reduced substantially. However, the challenge to overcome this inherent limitation on endurance, speed and concealment has constantly driven innovation in submarine propulsion technologies. Scientists and engineers have grappled with this challenge and various technologies were attempted to design a system which would not require external air to run the diesel generator engine in what is now known as Air Independent Propulsion (AIP). The pioneering attempt was by a brilliant German engineer Dr Helmuth Walter who designed a system in the early 1930s using hydrogen peroxide as an oxidant. This system was based more upon enhancing a submarine’s dived speed than longer dived endurance. After being fitted and tried on board a class of submarines, the programme was discontinued for various reasons, not least among them being the danger of using hydrogen peroxide on board.

Submarine operations in World War II and the diminishing effectiveness of submarines due to new detection technologies led to the invention of the ‘snorkel’. Although this did not require the submarine to break surface for running the diesels and the batteries could still be charged while dived though close to the surface, it only addressed the problem partially as the submarine still had to expose the ‘snort’ mast as an air intake for the diesels. The advent of radar and sonar and the emergence of maritime patrol aircraft and helicopters with submarine specific detection equipment like dipping sonar, sonobuoys and Magnetic Anomaly Detectors increased the submarine’s vulnerability as during snorting at least three masts (the periscope for visual lookout, the Electronic Warfare Support Measures mast for early detection and the snort mast) have to be exposed thus leaving a telltale ‘feather’ on the surface which is a dead giveaway in a calm sea.

Present Day AIP Systems

It was only in the late 1980s that the leading European submarine manufacturers began to develop different safe and reliable AIP systems. The Swedish Navy was the first to operationalise an AIP system based on the Stirling engine. Developed by the Kockums Shipyard in Sweden, it is in use on board the Gotland class submarines. The system uses liquid oxygen and diesel to run the generator which charges the submarine’s batteries. Each submarine has two 75 KW Stirling cycle propulsion units. The Singapore Navy’s two Archer class submarines (former Swedish Navy Vostergotland class) have also been retrofitted with the Stirling AIP System. The Japanese Maritime Self-Defence Force has also fitted the Stirling AIP built by Kawasaki Heavy Industries on their Soryu class submarines. Latest reports indicate that the Royal Australian Navy which has projected a future force level of 12 submarines may look towards collaborating with Japan for their future submarine programme. These submarines may also be powered by a Stirling engine AIP system. In 2012, a Gotland class submarine, deployed off the US East Coast transited the Atlantic Ocean fully dived without snorting.

HDW, the German submarine manufacturer, has developed a fuel cell AIP system which combines hydrogen and oxygen to produce water, electricity and heat. This system offers tremendous potential for the future because of the widespread application of this technology in other sectors including the automotive industry. It is based on the Polymer Electrolyte Membrane fuel cells. Safety is however an issue as the Hydrogen on board can be dangerous thought. This challenge is being addressed as the system gains in acceptance and popularity. The German Navy already has a fuel cell AIP on the Type 212 and 214 submarines and it will power their future SSK programmes, as well. The considerable export market for the HDW submarines will ensure that this system is adopted by other navies. South Korea already has the fuel cell AIP on its three Son Wonil class (Type 214) submarines.

The third proven AIP system is the MESMA (Moduled’ Energie Sous Marin Autonome) developed by DCNS of France and is deployed on the Scorpene and Agosta 90B submarines. It is a steam turbine system which burns ethanol and liquid oxygen at 60 atmospheres to generate heat which drives a turbo-electric generator. The expulsion of exhaust is also not limited by depth. Since France does not operate any SSKs, this system is meant for export. It is currently fitted on the three Pakistan Navy Agosta 90 B submarines.

In addition to these systems, other major submarine manufacturers are also developing their own versions of the AIP. Russia claims to have successfully developed a fuel cell AIP system called the Kristall 27-E which, according to Russian sources is being fitted on the Amur class and would be proven latest by 2016. The People’s Liberation Army (Navy) has also developed a system for the modified Song class submarines. It is believed that this system may not be as efficient as the existing western models but is an AIP nonetheless.

Spain is in the process of installing a modified and reportedly safer fuel cell system on its S 80 class currently under construction at Cartagena.

Whither India?

The Indian Navy does not have an AIP fitted submarine in its inventory as yet. However, surprisingly even the first four submarines of the under construction Project 75 (the French Scorpene class) will not be fitted with an AIP despite a proven system (MESMA) being available for this class of submarine. The option for AIP is planned only for the last two submarines which means, that the Indian Navy will not have an AIP submarine in its inventory until 2021-22. The first four submarines will probably be retrofitted with an AIP at a later date.

An option, if feasible, could be to retrofit a modular AIP system on the existing submarines if they undergo a Service life extension programme, though given the pace of decision making in the Ministry of Defence, this seems very unlikely.

The long awaited RFP for the Project 75(I) submarines is expected to include an AIP capability though the desired technology may not be specified to avoid a single vendor situation. This could further stymie the project which is already running well behind schedule and with the existing prevarication on its progress is expected to slip further.

The Indian Navy’s claim to be one of the leading submarine operating navies in the world is seriously under question with this debilitating deficiency in its undersea warfare capability and severely limits the ability of its submarines to shape the littoral battle space. The availability of an AIP on the three Agosta 90Bs on the other hand gives our western neighbour a major tactical advantage in its sea denial capability in a regional or bilateral conflict scenario.

It is understood that the Defence Research and Development Organisation is developing a fuel cell AIP system based on the use of phosphoric acid as an electrolyte. This is a proven technology in stationary applications since the 1960s but is not in use on board submarines with any other navy. The development of an indigenous AIP is indeed a noteworthy achievement but will perhaps take sometime to mature and become available for fitment on board. It is hoped that the delay in issuing the RFP for the Project 75(I) is not held up because of this. The Indian Navy should certainly encourage this development and incorporate its fitment into the future indigenous SSK design which constitutes the second phase of the CCS approved 30-year submarine construction plan.


The advent of AIP systems has greatly improved the dived endurance of conventional submarines thus overcoming a major limitation in their scope of operations. Coupled with the capability of modern submarines to deliver firepower at stand-off ranges, it has greatly widened the choices for the operational commander to shape the littoral maritime battle space to advantage. The Indian Navy would do well to incorporate this technology in its future submarine programmes including all six submarines of the Project 75 at the earliest to neutralise the advantage this system has provided to its principal adversary.

The author is a veteran submariner and a former Naval Adviser at the High Commission of India, London.