French submarine Améthyste

The Rubis class was the second attempt at constructing a nuclear-powered attack submarine. The first, also dubbed Rubis, had been authorised in 1964 but cancelled in 1968, just as the first boat was about to commence construction. However, a new naval plan in 1972, called Plan Bleu, stated a requirement for 20 attack submarines of both nuclear-powered and conventionally-powered types.^[5] The new design, designated Type SNA 72 and called a sous-marin nucléaire de chasse (French: hunter nuclear submarine) was smaller than any contemporary nuclear-powered attack submarine design in other major world navies. The small hull design was capable due the development of a compact, integrated nuclear reactor-exchanger with turbo-electric drives.^[6] Even then, the project was only considered feasible if equipped with weapons and sensors already in service, with the fire-control, torpedo-launching and submarine-detection systems also found in the Agosta class.^[6][7]

The first four submarines of the class, Rubis, Saphir, Casabianca, and Émeraude were built according to the original specifications. They measured 72.1 metres (236 ft 7 in) long overall with a beam of 7.6 m (24 ft 11 in) and a draught of 6.4 m (21 ft 0 in). The Rubis class had a standard displacement of 2,265 tonnes (2,229 long tons), 2,385 t (2,347 long tons) surfaced and 2,670 t (2,630 long tons) submerged.^[7] The boats are of single-hull construction made of 80 HLES high elasticity steel and the forward diving planes are situated high on the conning tower. The Rubis class can dive to depths over 300 m (980 ft).^[6]

The submarines are powered by a CAS-48 pressurised water nuclear reactor creating 48 megawatts utilising 7% low-enriched uranium,^[8] driving two 3,950-kilowatt turbo-alternator sets. These power a single propeller creating 7,100 kW (9,500 shp).^[9] At low speeds, the submarines use natural circulation to reduce noise emissions. In case of a reactor failure, the Rubises have an emergency electric motor powered by batteries or a SEMT-Pielstick 16PA4 diesel generator set with an output of 480 kW (640 hp; 650 PS). On battery power, the vessel has a range of 50 nautical miles (93 km; 58 mi) and endurance for 15 hours.^[6][9] The submarines had a cruising^{[citation needed]} speed of 25 knots (46 km/h; 29 mph) and endurance of 45 days (60 days maximum). Alternating between two different crews, they have an active yearly service of 240 days. The crew initially^{[clarification needed]} numbered 66, including 9 officers.^[7][9]

Sharing sensors and weapons with the Agosta class, the Rubises have four 533-millimetre (21 in) torpedo tubes forward. They have stowage for a mix of 14 F17 Mod 2 torpedoes and SM39 Exocet anti-ship missiles. The submarines can fire and guide two torpedoes simultaneously. Instead of torpedoes, the submarine can embark^{[clarification needed]} FG 29 mines.^[10] The submarines was initially fitted with a DRUA 33 navigation/search radar, ARUR and ARUD electronic warfare systems, DSUV 22 multi-function passive sonar array, DUUA 2B active sonar, and DUUX 2 acoustic intercept sonar.^[7]