Project 75(I): Redefining India’s Naval Power
How Air Independent Propulsion Is Transforming Underwater Warfare for the Indian Navy
India’s submarine fleet is entering a new era with Project 75(I), which introduces advanced Air Independent Propulsion (AIP) systems, making Kalvari-class submarines stay submerged longer, enhancing stealth and operational range. This article is a deep dive into how Project 75I AIP submarines become pivotal in Indian naval modernization.
Project 75 and Kalvari-Class Submarine Capabilities
The Indian Navy is strengthening the submarine capacities with the remarkable Project 75, comprising 6 Kalvari class submarines (Kalvari means “tiger shark” in Malayalam) – namely INS Kalvari, INS Khanderi, INS Karanj, INS Vela, INS Vagli and INS Vagsheer. These have been developed in the line of design of Scorpene class submarines, made by Naval Group, France.
Progressing Towards P75(I)
In the further bolstering P75, the Project P75(I) comes up with additional requirements in the submarines that make them stealthier and more resilient, with few additions after commissioning of the crafts under the previous project. Navantia (Spain) and Thyssenkrupp Marine Systems (Germany) are the stakeholders of the advancement project with Mazagon Dock Shipbuilders as the indigenous partners. Navantia has a functional design of the S-80 Plus class submarines already commissioned in the Spanish navy and the latter has proposed modified Type 214 class – and both of these have one thing in common – strong air independent system of power generation.
About Air Independent Propulsion Systems
The core objective of Project 75(I) is to integrate Air Independent Propulsion (AIP) systems that enable submarines to remain submerged for extended durations—by generating power without the need to surface for atmospheric oxygen.
In normal case, a snorkeling (coming on the surface to take up oxygen) period of a submarine comes after 2-3 days without this technology – which is about 2-3 weeks in case of AIP.
Types of AIP Systems
Majorly, an air independent system can be segregated into 4 types –
- Stirling Engine AIP – Stirling AIP uses external combustion with diesel fuel and liquid oxygen to drive a piston engine in a closed loop. It’s reliable, quiet, and simple, but bulky and less efficient than newer systems. Used by Sweden (Gotland-class) and older Japanese submarine
- Fuel Cell AIP – Fuel cell AIP generates electricity by reacting hydrogen with oxygen, producing only water and heat. It’s ultra-quiet, highly efficient, and ideal for stealth missions. Used in German submarines (Type 212/214) and under development in India with phosphoric acid fuel cells.
- MESMA AIP – MESMA (Module d’Énergie Sous-Marine Autonome) is a steam-based AIP system that burns ethanol with liquid oxygen to produce power through a turbine. It offers high output but is noisier and more complex. Developed by France, it is used in some Scorpene-class submarines like Pakistan’s.
- Closed Cycle Diesel AIP – This AIP uses modified diesel engines with stored oxygen in a closed system. It delivers high power but is noisy and outdated. Mainly used in older German and Soviet-era submarines.
PAFC: The Fuel Cell Technology Powering India’s AIP Submarines
PAFC or Phosphoric Acid Fuel Cell is the technology used for the AIP systems for P75(I) – starting from a 250 kW power requirement and planned for 500 kW needs, in case of higher size ships. The basic technological pros and cons of the PAFC are as follows:
Characteristics
- Operating Temperature: ~150–200°C
- Efficiency: 40–50% for electrical generation, up to 80% with cogeneration (combined heat and power).
- Fuel: Pure hydrogen or reformed hydrogen (e.g., from natural gas).
- Applications: Primarily stationary power generation, like backup power systems, hospitals, and small power plants.
Advantages
- Tolerant to CO₂ contamination (unlike alkaline fuel cells).
- Suitable for combined heat and power systems due to high-temperature operation.
Disadvantages
- Slow startup time.
- Expensive due to noble metal catalysts (usually platinum).
- Lower power density compared to newer technologies like PEMFCs or SOFCs
Explore an in-depth guide on PAFC fuel cell technology and how it supports India’s AIP systems here.
Role of PAFC in AIP Systems
AIP systems allow submarines to operate underwater for extended periods without surfacing or using snorkels to access atmospheric oxygen. This enhances stealth and endurance. The main features that make PAFC stand out are as follows:
- Silent operation: Unlike diesel engines, PAFCs produce minimal noise and vibration, critical for stealth.
- Long operational duration: Continuous operation for weeks is possible with hydrogen and oxygen storage.
- Moderate temperature: Operates at ~200°C, which is manageable in naval environments and allows for waste heat recovery.
The AIP Upgrade to Kalvari – Retrofitting
The Air Independent Propulsion (AIP) systems for P75(I) are being developed by DRDO, and the salient features of our system lies in the in-house production of hydrogen. As on date, it has passed all the major technical trials and has been slated to be retrofitted into INS Kalvari, the first of the class, by 2025-26.
The Process of Retrofitting
Retrofitting (in this case) involves integrating new AIP modules into existing submarine hulls, like INS Kalvari, without the need for full rebuilds—making it cost-effective and time-efficient.
Retrofitting a naval craft comprises of the following steps:
- Dry Docking: Submarine is taken out of service and docked for major overhaul.
- Cut and Insert: A new hull section containing the AIP module is inserted — a complex engineering operation.
- System Integration: AIP is integrated with the existing power, control, and propulsion systems.
- Sea Trials: Post-upgrade performance trials to validate underwater endurance, safety, and stealth.
- Operational Recommissioning.
Reason Behind Retrofit and Past Examples
A common question arises: why retrofit AIP systems into existing submarines despite the technical challenges involved? The answer lies in cost-efficiency and strategic practicality. Building a new submarine from scratch demands redesigning the hull, reengineering critical systems, and conducting extensive testing—resulting in significantly higher costs and longer timelines.
Retrofitting existing platforms, like the Kalvari-class under Project 75(I), offers a faster and more economical path to modernization. Additionally, delaying upgrades beyond a decade can lead to obsolescence and operational gaps in the Indian Navy’s submarine fleet.
To demonstrate the effectiveness of retrofitting in India’s naval history, here are notable examples:
INS Sindhurakshak / Sindhughosh-class (Kilo-class)
Multiple Kilo-class submarines were retrofitted with:
- Upgraded sonar and combat systems
- Kilo-to-Kilo missile upgrades (e.g., Klub-S)
- Improved batteries and habitability
- Carried out in Indian and Russian yards.
INS Vikramaditya
- Originally a Soviet-era Kiev-class carrier “Admiral Gorshkov”.
- Heavily retrofitted and transformed into a full-fledged STOBAR aircraft carrier
Frequently Asked Questions (FAQ)
Q1. What is Project 75(I)?
Project 75(I) is an Indian Navy program to build six AIP-powered submarines with advanced stealth and endurance capabilities.
Q2. How is Project 75(I) different from Project 75?
While Project 75 built Kalvari-class submarines, Project 75(I) adds AIP tech, greater automation, and local manufacturing.
Q3. What is an AIP system in submarines?
AIP (Air Independent Propulsion) lets submarines stay submerged for weeks without surfacing, enhancing stealth and endurance.
Q4. What AIP technology is India using?
India is developing PAFC-based AIP systems through DRDO, enabling quiet, long-duration underwater operation.
Q5. Why retrofit AIP into existing submarines?
Retrofitting is faster and more cost-effective than building new subs, extending life and capability without redesigning the hull.
Q6. When will AIP be added to Indian submarines?
The first retrofit is planned for INS Kalvari between 2025–26, after successful DRDO land trials.
Conclusion
With rising martial tensions and the geopolitical hazards, a country like India with professional armed forces in land, air and water, is vouching towards being an underwater dominator as well. The silent predator of enemy crafts in the depths of the seas, will inevitably rise India higher on the pedestal of defence superpower.
