On Wednesday, October 30, 2019, India’s Defence Research and Development Organisation (DRDO) went public in disclosing a visit by the Chief of Naval Staff to the Naval Materials Research Laboratory at Ambernath, Maharashtra to witness the functioning of a land-based Air Independent Propulsion (AIP) system prototype engineered to the form-and-fit of a submarine. This speaks of the maturity of the programme and its likelihood to meet the revised timelines set for its induction.
Air Independent Propulsion, as the name suggests, is a system designed to provide power to a submarine without the use of oxygen from the atmosphere. It is thus self-contained and has to rely on fuels that have to be carried on the host platform. Such a system is inherently complex and expensive.
It, however, has the ability to provide a fillip to the operational envelope of conventional submarines by reducing their requirement to periodically come to periscope depth to ingest vast amounts of air while snorkelling to run diesel and charge their large battery banks. This is becoming all the more important as above-water electronic and surveillance sensors improve, making even short exposures of submarine masts above the sea surface hazardous as they are becoming increasingly prone to detection.
To enhance the ability of submarines to remain submerged for prolonged durations, research has primarily focused on the adoption of nuclear power. This has matured considerably over the past several decades as evident by the highly refined nuclear-powered submarines that exist today. The downside of this approach is that they are difficult to design and exorbitant to build and maintain.
Designing plants to harness the heat emanated by a nuclear reactor to generate steam to drive turbines, all within the confines of a submarine, is a massive engineering challenge. Further, the rotating energy of these high-speed turbines has to be transferred to a much slower spinning propeller by a complex drive-train incorporating expensive reduction gearing. All this rotating machinery also tends to be noisy and therefore a vast amount of resources are devoted to silencing such platforms so as to increase their stealth. It, therefore, comes as no surprise that even though nuclear submarines have been in vogue for well over six decades, to date only six nations build and operate them. India is the most recent to join this select group.
While considerable experimentation concerning different approaches to air-independent submarines took place during World War II and in its aftermath, it was in the early eighties that research focused on finding a less expensive alternative to nuclear reactors began to show signs of fruition.
Several parallel approaches reached the production phase. These were either based on closed-cycle diesel engines, closed-cycle steam turbines, Stirling-cycle based heat engines or fuel-cells. Of these, the latter two have been far more successful with most of the current generation of AIP submarines incorporating one of them. The advantage of the fuel cell that essentially generates power by carrying out the reverse electrolysis of oxygen and hydrogen is that it is essentially solid-state and has very few moving parts. The downside, however, is that it requires hydrogen as a fuel which is highly combustible and difficult to store. Early generation of AIP boats stored hydrogen in the form of metal hydrides.
However, most modern systems generate their hydrogen on-board using special fuels put through a reformer. As all AIP systems tend to be relatively low powered, they cannot meet the high power demand of propulsion motors running at speed. For this, power stored in batteries comes into play. AIP submarines, therefore, retain their battery banks and diesel generators though the frequency of recharging batteries has diminished and is now much more flexible.
India’s quest for AIP began with the negotiations for the Scorpene Class of submarines from France under Project 75 in the 1990s. At the time, the system on offer by DCNS was the turbine-based ‘Module d’Energie Sous-Marine Autonome’(MESMA). It was still in its infancy and not in use with any Navy though orders had been placed by Pakistan for their Augusta 90B submarines. It was therefore decided to take a conservative approach by foregoing this requirement.
The door was, however, kept ajar for the incorporation of AIP systems on these boats by DRDO launching an indigenous programme based on fuel-cell technology. Cognizant of the immense technological challenges of designing, prototyping and productionizing such a system, it was initially planned to insert the AIP plug on the last two iterations of this six-submarine programme. However, in spite of the submarines being substantially delayed, these timelines could not be met. The intention now is for the plug to be inserted on all six boats as and when they come up for their first major refits.
The DRDO developed AIP variant, relies on Phosphoric Acid Fuel Cell (PAFC) technology that has several advantages over other types of fuel cells. Being more tolerant of fuel impurities, it is well suited to function in conjunction with reformers. Its life and efficiency are high thereby making it cost-effective. Such plants tend to be larger in size but the fact that the land-based prototype is already operating from a one-is-to-one scale model of what will be the final configuration implies that such challenges have been overcome.
Most importantly, this is a home-grown programme with substantial participation by industry. It can, therefore, be refined, scaled and adapted to meet all future requirements of conventional submarines. The progressive introduction of indigenous AIP system equipped conventional submarines in the Indian Navy will provide a substantial upgrade to its war-fighting capabilities in the undersea domain and has the potential to be a very significant techno-strategic accomplishment.
— Rear Admiral Monty Khanna (retd)
(The author is a former Commandant of Naval War College, Goa and a veteran submariner. He may be contacted on firstname.lastname@example.org)
(This article has been reproduced here in arrangement with the South Asia Monitor. It can be accessed at https://southasiamonitor.org)