In early 2021 France officially began development of its next generation of SSBNs (ballistic missile nuclear powered submarines). This is the SNLE-3G Program, with SNLE being SSBN in French. These new boats will be the third generation of French SSBNs. The announcement confirmed much of what had already been known about what the SNLE-3G would be. These new SSBNs will be about ten meters longer than the current 138-meter long SSBNs and probably displace (on the surface) at least 15,000 tons. This is about twice the displacement of the first generation SSBNs. The additional internal volume will be largely devoted to silencing, improved passive sensors, much upgraded electronics and better crew accommodations. This includes a towed sonar array in addition to built-in sound detectors. More powerful computers and software can more quickly identify known sounds and figure out what unknown sounds are likely to be and track them even if the sound signal fades in and out. There will be an upgraded nuclear power plant and new rudder and propeller designs using tech already installed on the new class of French SSNs (nuclear powered attack subs).
It was confirmed that the hull of the 3G SSBNs will be covered with sonar signal absorbing anechoic tiles. Inside the subs all machinery will be designed and mounted to minimize noise. French subs have crew with men and women and the improved accommodations make it easier to handle that.
For an SSBN the best protection is silence and stealth. All this stealth has a downside. This was demonstrated in early 2009 when France revealed that the Le Triomphant had collided with some unknown underwater object, causing some damage to its sonar dome. At the time, the crew did not know what they had hit, and assumed that it was a cargo container, floating underwater after having fallen off a ship. It's a common enough occurrence in bad weather. Nothing else seemed to make sense. Ten days later, after the British announced that one of their SSBNs had also collided with something underwater, they compared notes with the French and concluded that the two boats had run into each other. It appears that the bow (front) of the Le Triomphant scraped along the side of the HMS Vanguard. The French apparently believe that both boats were so quiet that neither boat detected the other, even after the collision. The French revealed that onboard the Le Triomphant the collision could be heard by the crew as bumping and scraping, then silence. There was a damage assessment drill, which came up clean, except for some damage to the sonar dome. That meant the sub was even less able to hear any noises coming out of the Vanguard. The Le Triomphant then proceeded on its way, only able to guess at what it had just hit. The Vanguard did the same, unaware that they had literally bumped into another stealthy SSBN.
The French later revealed that they did exchange general location data with NATO navies with regard to where their attack (SSN) submarines were operating. But they consider the SSBNs one of their strategic weapons and thus subject to a higher degree of secrecy. For decades, it's been understood that sharing information with all your NATO partners increased the chances of the secrets getting back to Russia. Since only one or two of these SSBNs are at sea at any time, it was believed that collisions would never be a problem. The odds of an underwater collisions were considered so high as to be virtually, but not absolutely, impossible. The U.S., Britain and France quietly got together to work out new procedures to avoid collisions between their increasingly stealthy subs. This is one reason why the French 3G SSBNs are spending a lot of money on improving passive sensors that will detect stealthy subs that get within collision range.
Le Terrible, the last of four current French Triomphant class SSBNs (ballistic missile nuclear powered submarines) entered service in 2010 and the first 3G SSBN is supposed to enter service in 2035. First generation French SSBNs began entering service in 1971 and they lasted 20 t0 23 years. Triomphant, the first second generation SSBNs is expected to serve for 30 or more years. By 2035 the youngest Triomphant class boat will have been in service for 25 years and one before it for 30 years. The first Triomphant is expected to retire in the late 2020s.
The Triomphant class boats displace 12,600 tons, have a crew of 101 and carry 16 M51 SLBMs (Sea Launched Ballistic Missile). At the time the other three Triomphants, already in service, carried the older M45 missile. These boats received the M51 after the "Le Terrible" entered service. The Triomphants replaced the six SSBNs of the Redoutable class. These were 9,000-ton boats that entered service in 1971 and began to retire by 1991. Each of these boats carried sixteen of the shorter range (5,000 kilometers) M4 missiles. The first Triomphant entered service in 1997 while the last of the six Redoutable SSBNs served until 2008.
In 2008, after more than a decade of development and delays, France finally completed testing the new M51 SLBM. In this case, the missile was fired from an underwater container similar to the silo on a submarine. There followed another successful launch from a French SSBNs. M51s weigh 52 tons each, carry six warheads and have a range of 10,000 kilometers. They replace the M45 (weighing 35 tons each, carrying six warheads and with a range of 6,000 kilometers). The M51 entered service on schedule in 2010 and has undergone several upgrades so that the 3G SSBNs will use it, at least initially. Currently there are no plans for a new SLBM.