The Inflexible (S 615) is the sixth and final of the Redoutable class SNLE ("Sous-marin Nucléaire Lanceur d'Engins", "Nuclear Missile-Launching Submarine") of the FOST, the submarine nuclear deterrent component of the French Navy.

Construction began on March 27, 1980; launched on June 23, 1982 and commissioned on April 1, 1985.

Inflexible uses basically the same design as the other Redoutable-class vessels, but has yet benefited from technological advances over its predecessors.

She uses the M4 missile, which carries six independent 150 kilo of TNT equivalent nuclear warheads. Range is reported to be "over 4500 km".

Miscellaneous improvements were made in electrical systems, nuclear systems (improving safety and stealth), rudder and engines (improving reliability and stealth).

TIT (Traitement de l'Information Tactique, "Tactical Information Processor"), a cluster of French-designed computers and serial digital bus links for intersystem communication.
DMUX21 sonar.

Capability of launching the SM 39 Exocet anti-ship missile

The other Redoutable-class submarines have been modified to meet the standards of the Inflexible ("Refonte M4").