In 2015, I was fortunate to be the first media person to witness and report the existence of an Air Independent Propulsion (AIP) project by the Naval Materials Research Laboratory (NMRL) , a lab of the Defence Research and Development Organisation (DRDO), located in Ambernath near Mumbai. I have written about it in my book ‘Foxtrot to Arihant: The Story of Indian Navy’s Submarine Arm’.
The propulsion system was based on fuel cells. NMRL had already built a 20+ KVA metal/alkaline hydrate based prototype ready for concept demonstration. The project was housed in a 20-foot container. The membrane used was porous inorganic with silicon carbide. The catalyst was carbon paper coated with nano platinum particles. After the sanction of the project, the prototype was expected to be scaled to a 33KVA unit. NMRL proposed to add multiple units of the 33 KVA to achieve the required power generation of 140-220 KVA’s. One of the uniqueness of the Indian design was the exclusion of Carbon Dioxide (unique to French MESMA) or any other emission into the water. The waste products were to be stored in a separate container in a slurry form. Since there are no emissions into the water, there are fewer chances of the submarine being detected. The Indian Fuel Cell AIP effectively addressed two very important challenges, i.e., the storage of reactants and emissions. The metal/alkaline hydrate reactant and the waste product slurry required a very small storage space.
The Fuel cell AIP was planned to be introduced into newer submarines and even older ones by cutting open the submarine and then welded back with the Fuel Cell AIP module in between to maintain the buoyancy. The submarines were planned to have both conventional and fuel cell-based propulsion for operational reasons.
Submerged endurance for a submarine is the foremost asset as far as stealth is considered. The submarines in WW I & II were more of submersibles, which would dive to carry out torpedo attacks, make a getaway post the attack, at speeds of 8-10 knots and thereafter surface to charge their batteries. Most of their time was spent on the surface. In the great wars, they could do that because the Anti Submarine Warfare (ASW) was in its infancy. The submarines then were, of course, most vulnerable when on the surface, however, till the time the Air ASW (AASW) came to mature, these submersibles could still ensure surprise. Once the AAASW came to fore from the second half of the WW II, the submarine on surface stood at a disadvantage. Further, with the results of the development of Allied Submarine Detection Investigation Committee (ASDIC) work, the ASDICs (Sonar) installed onboard ships gave them some sort of detection capability and hence to an extent, acted as a deterrent to the submarine, which could no longer venture close to discharge her torpedo. These developments and the implementation of the convoy system defeated the submarine in the battle of the Atlantic.
The Germans developed the snorkel system on the Type 21 submarine, which enabled the boat to charge her batteries submerged at periscope depth. However, these boats appeared only at the fag end of the war and could not exert any influence in its outcome. The basic design of the Type 21 formed the basis of post war improvements in submarine design and stealth.
Walter’s Boat –In the mid-1930s Helmuth Walter, a German engineer, came up with closed cycle internal combustion engine using Hydrogen Peroxide. The first experimental submarine V 80 gave very encouraging results with an underwater speed of upto 23 knots. However, there were safety issues due to the storage of highly inflammable and unstable Hydrogen Peroxide in cylinders on board. Only a few boats of 650 Tons displacement were made, but a few mishaps ensured that these boats never saw active service and were scuttled at the end of the war in May 1945.
Post War Developments
The research on enhancing the submarine endurance submerged in the initial years after the WW II focused on operationalising the snorkel equipped boat of the Type 21 design and great success was achieved in this endeavour. The snorkel equipped submarine were able to enhance their radius of operation in a mix of dived deep and traversing at Periscope Depth (PD) using their diesel engines. However, they were vulnerable to detection at PD more so from air surveillance. With the advent of high resolution satellite based surveillance, this vulnerability only increased. This forced the submarine designers to look at alternative means of propulsion to increase submerged endurance.
Accordingly, research was oriented to harness nuclear energy for propulsion, which would give unlimited submerged endurance, mobility and speed. The efforts culminated in the commissioning of USS Nautilus and her famous signal “Underway on nuclear power’ ushered in the era of the nuclear-powered submarines. The hull design of Nautilus represented more of the Type 21 and was not optimal for efficient submerged hydrodynamic profile. Accordingly, the research focused on improving the hull design, which would be more conducive to underwater performance. The result was the re-birth of the ‘Tear drop’ shaped hull. It had been abandoned in the early part of the twentieth century. This form offered greater efficiency submerged as against performance on surface. Nevertheless, since the nuclear-powered submarine was primarily to operate underwater, the slight degradation on surface was not relevant. This hull shape also contributed in a lesser radiated noise profile. More silencing techniques were employed to further enhance the stealth of the submarine.
Development of AIP
As stated earlier, with the advancement of the Air ASW and more sophisticated sensors to detect a submarine, deployed from fixed wing and rotary wing aircraft, as also the fitment of hull mounted variable depth Sonars onboard surface ships, the gap between the stealthy conventional submarine and the ASW forces pitted against her, narrowed considerably. This led to a renewed interest in development of the AIPS. The word propulsion used in AIPS is a misnomer, as an AIPS is more of a power system and not a propulsion system. The system gives adequate power at 200-250 KW (Most AIPS are rated such) to cater for hotel load and power for the main electric motor to propel at 2.5-4 knots. For higher speeds, the power from the main batteries is drawn.
Types of AIP
There are basically four types of AIPS.
- Closed Cycle Diesel
- Sterling Engine
- Fuel Cell
Closed Cycle Diesel System – This system is an internal combustion engine using liquid Oxygen as the oxidant when submerged. To prevent burning of the metal in a pure oxygen environment in the engine, the oxygen is diluted with the recycled exhaust gas. At the start of the engine Argon is used to dilute the oxygen. Very few experimental boats were made using this system, by the Germans and the Russians but the limitation of storing liquid oxygen indefinitely, restricted the radius of operation and the system did not find acceptability. The last vessel using this technology was scrapped in 1970s.
MESMA-This is a French system, which generates steam to drive a turbo-alternator. It is a derivative of the nuclear propulsion system. The heat is generated by ethanol and oxygen, which are stored onboard in bottles at 60 Bars. At such a pressure the waste product the CO2, is discharged overboard without the need for a compressor. The system is installed on the three Agosta 90B submarines of the Pakistan Navy. The first boat Khalid was fitted with on commissioning in 1993 and the other two were retrofitted in 2011. This is a very cumbersome system and does not lend itself easily to ergonomics. Further, it is the least efficient of the AIP systems. It is for these reasons that it has not proliferated. It also requires a specialized shore-based infrastructure for support. It is interesting to note that the French do not use it in their navy and offer it only for export. The French Navy does not have any conventional submarine in its inventory.
Stirling Cycle Engine– Developed by Kockums of Sweden, it has been to-date the most successful system and is in use in the Swedish and the Japanese navies. It burns liquid oxygen with diesel fuel and therefore, does not require any specialized infrastructure ashore. Normal shop floors servicing the normal diesel ICEs are adequate to support such a system. It is reasonably more efficient in comparison to other AIP systems. However, the Japanese are now weaning away from AIPS after commissioning their first submarine with Lithium-ion battery, which offers higher capacity and lesser charging periods compared to lead-acid ones.
Fuel Cells – Several navies have been working on the fuel-based AIPS. The system requires a continuous source of Hydrogen and Oxygen to sustain a chemical reaction, which generates electrical power and water as a waste product. This implies carrying of Hydrogen and Oxygen on board in bottles. The gases at a ratio of more than 4% hydrogen constitute an explosive mixture, safety features required are very stringent. The Hydrogen bottles in the Type 214 design of the Germans, are stored below the keel along the length of the submarine. Nevertheless, the safety of the system is always an issue. The Indian DRDO is building the fuel cell system based on the Phosphoric acid fuel cell (PAFC) in collaboration with the French for fitment on the 5th and 6th submarines of the Scorpene design being built at Mazagon Dock Ltd (MDL). It is understood that the system is yet to be proven and therefore will be retrofitted on the boats during their commission.
Common Factors in AIP
- Except for the Stirling engine, all AIPS require specialized shore-based infrastructure for support and sustenance. This involves considerable investment and if there are multiple bases from which the submarines operate, the expenses are multiplied that many times. This is an important factor, which deserves serious consideration.
- The sustained speed that the AIPS of different hue afford varies from 2.5-4 knots with power ratings from 220-250 KW, despite claims to the contrary.
- The average endurance submerged under operational and not ideal conditions is not known to exceed 10-12 days, despite claims to the contrary.
Operational Factors affecting the Usefulness of AIP
- Role of the Submarine.
- Defensive Role. If a submarine is on a defensive mission operating close to own shores, then an AIPS may make sense and give an edge due to the extended submerged endurance that it provides. However, imagining a submarine designed purely for defensive missions is oxymoronic. Submarines by their very nature are meant to be offensive platforms in a war.
- Attack Submarine. In an offensive mission close to enemy shores or SLOCs, a patrol profile is, roughly in equal proportion in time, for search, attack and evasion if detected or a getaway after an attack. That would mean a third of the total time in each phase of activity. In attack and evasion or getaway phases, the submarine would need to manoeuvre at speed varying from 7-10 knots, which an AIPS cannot provide. In such a case the power from the main batteries will perforce be used. Once that happens the batteries would need to be charged at an appropriate tactical opportunity to bring the capacity to above 80%. In such a case, one would wonder whither an AIPS? Even in an evasion phase, one may argue that the AIPS would afford extended endurance. However, that would restrict the speed of the boat to 4 knots whereas the aim would be to move at a higher speed to put as much distance between the boat and the enemy ASW forces in the quickest possible time to enlarge the search area for the ASW forces. This area expands exponentially with each passing minute. At a speed of 8 knots, which a submarine can easily sustain for a couple of hours or more, the search area would expand by a simple formula of Pi-r2 that is in this case by 200 sq NM every hour. This imposes provision of a disproportionately large number of forces on the enemy to track down the submarine. Therefore, it is a considered view that instead of an AIPS, what is needed is a very powerful generator and a high capacity battery, which can take in a high amperage in a short time to reduce the charging time and thereby reduce the indiscretion rate. The Japanese seem to have realized this and have incorporated such a battery of lithium-ion.
- Weapon Outfit. If the weapon outfit is such that it affords delivery of the ordnance from a standoff range then one is tempted to question; “Of what use is the AIPS”? This is specifically the scenario in the Indian Navy. All submarines are equipped to deliver anti-ship and anti-land target cruise missiles from standoff ranges. However, the need is to have mobility and speed to optimally exploit such ordnance by having the ability to swiftly switch the scene of engagement.
- Penalties due to the addition of an extra section. The necessity to have an extra section in the pressure hull to accommodate the AIPS increase the length of the submarine and the surface displacement even though the section could be made neutrally buoyant in the submerged position. This increase in length and the surfaced displacement imposes penalties in the power to weight ratio, beam to length ratio as well as on the hydrodynamic profile of the boat. These have consequences on, the maximum speed that could be achieved, stability and the self and radiated noise profile of the submarine. It, therefore, becomes imperative to weigh these adverse effects against the envisaged advantages that the system may provide.
- AIPSs Mainly Used by Self Defence Navies. It is illustrative to note that the idea of AIPS first germinated from the navies which were inherently ordained as maritime self-defence forces; Sweden, Germany and Japan. As far as Navies, which have vast areas of interest at great distances from their bases and envisaged offensive missions are concerned, they did not opt for AIPS, instead focused on more efficient batteries and generators. In the case of the Indian Navy, this is the scenario.
The Hybrid Nuclear Option
The only true AIPS, which enables a submarine to have endurance, mobility and speed, is a nuclear-powered one. Whilst it is admitted that a larger nuclear-powered attack submarine may not be an ideal platform for operations in the littoral waters, there is a case for the hybrid nuclear-electric propulsion system. The Amethyst class submarine in service with the French Navy is a case in point. It is a boat with a submerged displacement of 2700 Tons and is capable of submerged speeds in excess of 20 knots for any length of time as the situation may dictate.
Eco- System to Operate Nuclear Powered Submarines
It is worth noting that the difference between a non-nuclear and a nuclear-powered submarine is not just the power source. Operating a nuclear powered submarine encompasses an entire eco system, which involves the creation of special basing and other infrastructure related to radiation safety and monitoring both on board and ashore, environmental safety, nuclear waste management, access control and containment measures in case of an emergency. Even for maintenance and repair the procedures that are followed are structured to adhere to safe practices to ensure radiation safety. The crew too has to get attuned to a mindset, which is alien to the conduct on a conventional submarine.
The Indian Navy already has experience of nearly three decades of operating nuclear-powered submarines and the entire gamut of the infrastructure required to operate such platforms as also ensuring radiation safety measures with best practices is already in existence. Inducting a hybrid nuclear-electric powered submarine would be a far easier option as against a non-nuclear AIPS with the attendant requirement to create fresh specialized infrastructure such as a fuel cell based one.
Project 75 (I)
This Project is the second part of the Phase I of the 30-year Plan for indigenous construction of six conventional submarines. In the original scheme of things AIPS was not part of the NSQRs and was added in 2006-07. The requirement now states that the AIPS should be preferably on fuel-cell technology. The primary role of these submarines is envisaged to be land attack capability with long-range cruise missiles to be delivered from stand off ranges. This capability is already available with the Indian Navy on the Kilo class boats in service. The submarines built under the Project 75 (I) must enhance this capability by being able to deliver the ordnance swiftly and with the ability to switch scene of actions with mobility and speed so as to optimally exploit this enhanced capability. The platform that affords this option is a hybrid nuclear-electric propulsion system with unlimited endurance, mobility and speed. Collaboration to design and build such platforms will be forthcoming from our long-standing partners in this sphere. (Also Read Strategic Partnership clause delays Indian Navy P-75 I submarine project )
Conclusion and Way Forward
We have examined the evolution of the Air Independent Propulsion Systems. Despite the interregnum in the 1960s, such systems were revived in the late 1970s/early 1980s. Three of them; Stirling Engines, MESMA and the fuel cell-based ones are in use presently. However, their utility must be scrutinized keeping in mind the backdrop of the roles of the concerned navies in general and the platforms in particular. When we do that the correct perspective emerges. What is good for the goose may not be, necessarily, so for the gander. Further, the operating philosophies as well as the experience of operating different propulsion systems must form an essential part of such analyses.
In the Indian context, where areas of interest are vast as well as dispersed in their location and expanse and where the primary role of the submarines is offensive, AIPS may not be the optimal solution. This gets more accentuated with the ability of the boats to deliver their ordnance from stand off ranges and the need to be able to switch scenes of action with speed and mobility to maximize their exploitation. At the other end of the spectrum, the hybrid nuclear-electric propulsion system seems a much more attractive alternative. The argument in its favour further gains strength, given our experience in operating nuclear-powered submarines, the in-place infrastructure to support them, and the eco-system nurtured and created for such platforms. In the longer run it offers a more cost-effective solution and obviates the need to create an altogether new infrastructure to support non-nuclear AIPS.
In view of the foregoing, the way forward suggests itself; which is that a non-nuclear AIPS does not really afford us the tactical advantage that we seek. A very powerful generator and a high capacity battery should suffice. However, if an AIPS is essential, then the option of a nuclear-electric AIPS is best suited to meet our requirement and therefore, a course correction in the implementation of the Project 75 (I) is needed and there is a need to seriously consider the nuclear-electric option, which incidentally is the best AIPS. It also affords endurance, speed and mobility to optimize the exploitation of the boats to be constructed under the Project. The indigenous AIPS under consideration is still to be proved at sea which is years away. Accordingly, the construction of the boats under P 75(I) should commence early without a non-nuclear AIPS and a mini reactor should be incorporated in successive platforms may be from the 4th onwards. In summary, the capabilities of endurance, mobility and speed to maximize the combat effectiveness of the submarines with the role envisaged under Project 75 (I), are best afforded by a nuclear-electric AIPS.