The Nilgiri-class frigates, formally classified as the Project-17 Alpha frigates (P-17A), are a series of multi-mission stealth guided-missile frigates currently being built by Mazagon Dock Shipbuilders (MDL) and Garden Reach Shipbuilders & Engineers (GRSE) for the Indian Navy (IN).

Designed by the Warship Design Bureau, the class is intended to serve as a complement to the currently-serving s (P-17) with improved design portfolios, such as low radar cross-section (RCS) and reduced infrared signature.

With a total of seven vessels, the construction of the frigates are currently divided between MDL and GRSE. As of 2024, all seven frigates have been launched and are intended to enter service with the IN between 2024 and 2027. The frigates will form a part of the Eastern Fleet as well as the future Carrier Battle Group (CBG) of . Further, they can be independently deployed as a surface combatant or with a surface task force.

Upon entering service, the class is to be complemented by an additional series of seven or eight frigates, under the codename the Project-17B series.

Design Development

The frigates were designed by the Warship Design Bureau (formerly Directorate of Naval Design).

The class' design was finalised by the DND in 2013; it was later unveiled to the public in April 2018, when MDL displayed a scale model of the frigate at the DefExpo 2018 military exhibition, held at Chennai, India. The P17A ships are geo-symmetrically larger by 4.54 % vis-à-vis P17, or the Shivalik-class frigates, and also features 75% indigenous content.

On 5 September 2018, Mazagon Dock Shipbuilders (MDL) and Garden Reach Shipbuilders & Engineers (GRSE) awarded Bharat Electronics (BEL) with a contract to supply seven Barak 8 LR-SAM air defence systems for Nilgiri-class frigates. In October 2018, Bharat Electronics signed a $777 million deal with Israel Aerospace Industries to help fulfil the Barak-8 order.

On 28 November 2023, Bharat Heavy Electricals Limited (BHEL) received a contract worth to manufacture 16 Upgraded Super Rapid Gun Mounts (SRGMs) or Strales Systems for in-service ships as well as the under construction Nilgiri-class frigates.

The state-owned firm Steel Authority of India Limited (SAIL) is supplying around 4,000 tonnes of steel for each of these ships. The steel is being manufactured in Bokaro, Bhilai, and Rourkela plants.

Features Stealth The frigate's design incorporates a significant degree of stealth and low radar-observability – through the utility of composite materials, radar-absorbent coatings and low-observable/radar-transparent technologies; the usage of the aforementioned materials assists the vessel in maintaining a low radar cross-section (RCS).

The vessel's physical profile also features a substantial level of stealth through the application of different physical forms – including an enclosed mooring deck, flush deck-mounted weapon systems and a reduced number of antennae.

The frigate's infrared signature, most specifically emissions from its propulsion exhaust and power generation machinery, are reduced through the application of Venturi effect and fluid injection, which helps to reduce the plume and hot metal temperatures of exhaust. To maintain acoustic silence, the frigate is equipped with propellers designed to have onset of cavitation at higher speeds to reduce hydrodynamic noise. In addition to the propellers, the vessel's hull also features special acoustic enclosures for some of the machinery to reduce the emission air-borne noise.

Modular profile The P-17A frigates are the first major class of Indian-designed warships to be built using the methodology of integrated modular construction – a manufacturing process in which multiple modules (or "blocks") of a vessel's hull are pre-assembled/pre-outfitted independently, before being aggregated for final assembly.

In December 2015, MDL contracted Fincantieri S.p.A. to provide technical assistance and essential expertise in the construction of the seven frigates using the "modular construction" methodology.

Naming The P-17A frigates were named after the former-s, which served in the IN between 1972 and 2013; the first six ships of the series were allotted the names utilised by the older class, namely – , , , , , and . The seventh and final vessel of the P-17A series, which did not have a namesake from the older class, was given the new name of Mahendragiri. The ships are given names of mountain ranges in India.

Instrumentation Armament Anti-surface warfare

As part of its anti-surface warfare (ASuW) capabilities, the class features eight VLS-launched BrahMos anti-ship cruise missiles, capable of speeds of up to Mach 3. The BrahMos is widely regarded as one of the most formidable anti-ship missiles currently in service, given the missile's extreme versatility and manoeuvrability.

The vessels are equipped with one upgraded OTO Melara 76 mm naval gun for anti-missile/anti-aircraft roles. The upgraded guns manufactured by Bharat Heavy Electricals Limited (BHEL) features the Strales system with DART ammunition firing capability. However, INS Udaygiri features the original, older Compact variant of the gun as noted during its commissioning.

Originally, there were plans to install the Mk 45 naval gun, manufactured by BAE; however, this plan was later scrapped in 2021 on account of financial constraints.

Anti-air warfare As part of its anti-air warfare (AAW) capabilities, the class features thirty-two Barak 8 surface-to-air missiles, with eight missiles present in four 2×4 VLS configurations – with two placed at the bow and two placed aft.

The LR-SAM is an extended-range variant of the original Barak 8 and is designed to neutralize various aerial threats, including fighter aircraft, helicopters, anti-ship missiles, cruise missiles, ballistic missiles and unmanned aerial vehicles (UAV). This variant has a range of about .

The ships are also equipped with two AK-630 each for close-in weapon system (CIWS) roles as well as multiple OFT-manufactured 12.7 mm M2 Stabilized Remote Controlled Gun.

Anti-submarine warfare

As part of its anti-submarine warfare (ASW) capabilities, the class features two 324 mm triple-tube lightweight torpedo launchers for firing TAL Shyena torpedoes.

The class also features two RBU-6000 (RPK-8) anti-submarine rocket launchers, capable of firing ASW projectiles to depths of up to . The launchers, designated Indigenous Rocket Launchers, have been manufactured indigenously by Larsen & Toubro.

Decoys During the class' unveiling in 2018, the frigate's design was noted to possess two anti-torpedo decoy systems and four decoy launchers – which presumably may be the NSTL Maareech torpedo-countermeasure system and the Kavach anti-missile decoy launchers, respectively.

Aviation facilities

As part of its aviation facilities, the class is equipped with a flight-deck and an enclosed aviation hangar, capable of accommodating a single naval helicopter – most likely the ALH Dhruv MK-III maritime-reconnaissance helicopters, or the Westland Sea King Mk. 42B ASW/ASuW helicopters – both of which are operated by the Indian Naval Air Arm.

Sensors Radar The class features the EL/M-2248 MF-STAR active electronically scanned array (AESA) radar as its primary radar suite. The EL/M-2248 is a multi-function, phased-array radar system featuring an azimuth of 360o, with the capability to track both aerial and surface targets, at a range of over .

The class also features the Indra LTR-25 'Lanza' surface-search radar, as its secondary radar suite. The LTR-25 is a solid-state, three-dimensional (3D), long-range radar capable of operating in a dense electronic environment and features a range of over , with the capability to track various aerial targets – including fighter aircraft and ballistic missiles.

The frigates are also equipped with an advanced combat management system, classified as "CMS-17A", which is reportedly equipped with sophisticated data links for high, efficient interoperability with other naval vessels.

Sonar The class is equipped with the BEL HUMSA-NG sonar, developed by the DRDO. The HUMSA-NG is a hull mounted "active cum passive" integrated sonar system capable of detecting, localizing, classifying and tracking sub-surface targets in both active and passive modes.

Electronic warfare The frigates feature the Shakti electronic warfare (EW) suite – developed by the Defence Electronics Research Laboratory (DLRL), for defense against anti-ship missiles and for the interception, detection, classification, identification and jamming of conventional radars. Shakti is equipped with wide-band electronic-support measures (ESM), electronic countermeasures (ECM), an integrated "Radar Finger Printing System (RFPS)" and a data-recording replay feature for "post-mission analysis".

Propulsion Each frigate is powered by two General Electric LM2500 gas turbines – manufactured by GE Aviation and two MAN 12V28/33D STC four-stroke engines – manufactured by MAN Diesel & Turbo, arranged in a combined diesel and gas (CODAG) propulsion configuration.

In December 2016, the IN contracted GE Aviation to supply fourteen LM2500 gas turbines for the seven frigates, which were license-assembled in India by the Industrial & Marine Gas Turbine Division (IMGT) of Hindustan Aeronautics Limited (HAL). Under the deal, HAL was additionally sanctioned to provide comprehensive services – including the supply of spare parts, maintenance inspections and equipment overhauls.

At the time of the order, HAL had already delivered eleven LM2500 turbines to the IN; the turbines are specifically used on multiple Indian warships, most notably on the Shivalik-class frigates.

In February 2019, MDL contracted GE to supply an assortment of auxiliary equipment to the IN to support the fourteen engines; under the agreement, GE also handled the design work of the frigate's auxiliary system and its fuel supply system.

In May 2016, the IN contracted MAN Diesel & Turbo to supply fourteen MAN 12V28/33D STC four-stroke engines for the seven frigates; under the deal, the requisite parts needed for the engines were sourced from India, while the engines' final assembly and testing were conducted at MAN's facility in Aurangabad, India.

In July 2026, Rolls-Royce announced that INS Dunagiri and INS Mahendragiri are equipped with four MTU 12V 396 TE54 generator sets.

History

Background

On 19 June 2009, the Defence Acquisition Council (DAC) — the main acquisition panel subordinate to India's Ministry of Defence (MoD) and chaired by the then Defence Minister of India A. K. Antony — cleared a proposal for the procurement of seven stealth frigates at a cost of under the designation Project 17A, as an advanced variant of s. The ships were to be constructed by Mazagon Dock Shipbuilders (MDL), Mumbai and Garden Reach Shipbuilders & Engineers (GRSE), Kolkata. The DAC also rejected the Navy's proposal of building two of the warships abroad. The construction of the warships will incorporate modular shipbuilding where 300-tonne prefabricated blocks would be put together similar to the construction style of . Earlier, on 4 December 2006, the Navy had sent out a pre-tender Request for Information (RFI) to French DCNS, Italian Fincantieri, American firms Lockheed Martin and Northrop Grumman besides shipyards in Russia and Korea for the construction of the first two ships by one of the foreign shipyards so as to ensure the timely absorption of the technology by the Indian shipyards. The project is expected to start in 2011 after both MDL and GRSE completes an upgrade to allow the modular construction. The deliveries were to begin 3–4 years after start of construction. All the firgates are to be inducted by 2022.