France’s Rubis-Class Submarines: The Smallest Nuclear Attack Subs with Big Impact

French Rubis-class submarines are the smallest mass-produced nuclear-powered submarines in the world. Nevertheless, in terms of cost-effectiveness, experts unanimously consider the project highly successful. France's first attempt to create its own multi-purpose nuclear submarine failed: due to numerous technical problems, construction of the submarine laid down in 1957 had to be abandoned. It was later completed as an experimental missile diesel-electric submarine and, under the name Gimnote, was used for many years to test ballistic missiles.

Photo: commons.wikimedia.org by Richard Tanguy, https://creativecommons.org/licenses/by/2.0/

Sous-marin de la classe Rubis dans le goulet de Brest

Return to the Nuclear Submarine Program

The decision to build a multi-purpose nuclear submarine was revisited in January 1973. By that time, French shipbuilders had already gained the necessary experience by creating a series of nuclear ballistic missile submarines of the Redoutable class. They were now tasked with designing a compact and relatively inexpensive submarine with low noise levels and a highly reliable nuclear power plant. The diesel-electric submarine project Agosta was used as the prototype.

The new submarine's power system was based on the principle of electric propulsion. Development of the project, designated SNA 72, lasted more than two years, and the Cherbourg shipyard received the order for the lead submarine S601 in February 1976. Interestingly, the submarines were initially intended to be named after French provinces, and the first submarine was laid down as Provence. However, the decision was soon changed, and the submarines were ultimately named after precious stones. The lead boat entered service as Rubis ("Ruby”).

Originally, eight submarines were planned, but only six were built, as construction of the last two (S607 Turquoise and S608 Diamant) was canceled.

Design and Technical Features

The Rubis-class submarines feature a single-shaft, mixed construction design. Along most of the hull length, there is no outer light hull, with a double-hull structure present only in the bow torpedo compartment and near the electromechanical installation. The pressure hull is divided by watertight bulkheads into five compartments.

The bow compartment houses four torpedo tubes and storage racks for torpedoes, missiles, or mines. The second compartment contains the command center, navigation control post, living quarters, battery systems, compressors, and various equipment. The third compartment accommodates the nuclear reactor with its steam-generating unit. The fourth contains turbogenerators, while the fifth houses the main propulsion and auxiliary electric motors, as well as an emergency diesel generator and power control systems.

The main power plant is turboelectric, based on a pressurized water reactor CAS 48 with a capacity of 48 MW and two turbogenerators rated at 3,950 kW each. The submarine's small size required a reduced-power reactor, allowing natural coolant circulation in most operating modes, which improved reliability and reduced noise levels by eliminating pumps.

An auxiliary power unit includes a SEMT-Pielstick/Jeumont Schneider diesel generator with a capacity of 450 kW.

Modernization and Noise Reduction

The last two submarines of the series, S605 Améthyste and S606 Perle, feature a redesigned hull with a more hydrodynamic teardrop-shaped bow and an extended length by 1.5 meters. Their sail structures were also improved, and extensive use of soundproofing and damping materials significantly reduced noise levels.

Between 1989 and 1996, the first four submarines were upgraded following the same standards, bringing the entire class to a comparable level of combat capability.

Electronics and Crew Conditions

The submarines are equipped with advanced electronic systems, including the DLA-2B/DLA-3 combat control system, passive sonar systems DMUX 20 and DSUV 62C developed by Thomson Sintra/Thales, a navigation radar from Kelvin Hughes, and the ARUR-13/DR3000U electronic warfare suite.

Despite their compact size, the submarines provide relatively comfortable living conditions. Each crew member has an individual sleeping space, and the mess hall also serves as a wardroom. The onboard desalination system ensures an almost unlimited supply of fresh water.

Operations and Future Replacement

All six submarines are currently part of the French Navy and are based in Toulon. They have carried out numerous long-range missions and even circumnavigation voyages. Their endurance reaches up to 60 days.

Each submarine operates with two rotating crews ("red” and "blue”), minimizing turnaround time between deployments. Despite being outclassed by larger nuclear submarines of the United States, Russia, and the United Kingdom in some parameters, the Rubis-class remains highly effective due to its low-noise performance and advanced passive sonar systems.

The submarines are scheduled to remain in service until they are fully replaced by the new-generation Suffren-class nuclear attack submarines.

Key Specifications

• Surface speed: 15 knots
• Submerged speed: 25 knots
• Diving depth: 300 meters
• Endurance: 45-60 days
• Crew: 57 personnel (including 9 officers, two crews)
• Displacement (surface): 2,410 tons
• Displacement (submerged): 2,607 tons
• Length: 73.6 meters
• Beam: 7.6 meters
• Draft: 6.4 meters
• Reactor: CAS 48 (48 MW)
• Main armament: 4×550 mm torpedo tubes, up to 14 torpedoes
• Missiles: Exocet anti-ship missiles (optional)