February 20, 2012:
Russia is equipping its elderly Delta class SSBNs (nuclear powered ballistic missile submarine) with improved SLBMs (Sea Launched Ballistic Missiles). The current R-29RMU missiles will be replaced with the new R-29RMU2 ("Liner") missiles. The R-29RMU was introduced five years ago, replacing the much older (1980s) R-29RM design. The R-29RMU was supposed to be the last update of the R-29 line of SLBMs but apparently there were second thoughts about this. Now the R-29RMU2 will be the end of the line for Russian liquid fueled SLBMs.
Apparently the R-29RM missiles are feeling the effects of age as well the subs that carry them. The five Delta III boats entered service in the late 1970s. The seven Delta IVs did so a decade or so later. A significant number of replacement SLBMs (RSM-56 Bulava) are more than a decade away. It's also possible that R-29RMU2 was developed (and successfully tested last year) as a backup, in case Bulava did not get fixed.
The R-29RMU2 is an improved R-29RMU. The R-29RMU2 is the last liquid fuel Russian SLBM design. Liquid fuel missiles are more complex than solid fuel missiles because of all the plumbing needed to put liquid fuel through an engine. The fuel can be stored for long periods inside the missile but is still inherently more troublesome than solid fuel rockets. Unable, for a long time, to develop the technology for solid fuel rockets, Russia made the most of this and developed some very effective "storable liquid fuel" rockets. It was only near the end of the Cold War that Russia finally mastered the solid fuel rocket construction techniques. The Borei class SSBNs will have a solid fuel SLBM (the Bulava) based on a successful ICBM (Topol M).
Although Russia has only a dozen SSBNs in service, they are all aging Cold War era Delta class boats. These were supposed to be retired by now because of safety and reliability issues and the high expense of running them. But the delays in getting the Bulava working properly meant that the new Borei class SSBNs were slow to be built. There are only two in service and they will not have all their Bulava SLBMs for over a year. The Bulava was only approved for mass production this year.
The 10,000 ton Delta IIIs and 11,000 ton Delta IVs SSBNs have been kept in service via some refurbishment and a lot less use. These boats rarely go to sea. When they do it's usually for short range training missions, which often last a few days or just a few hours. But the true measure of a fleet is the "combat patrol" or "deployment." In the U.S. Navy, most of these last from 2-6 months. In the last decade Russian SSBNs have carried out, on average, only one or two combat patrols a year. But the SSBNs, while tied up in port, could still fire their missiles if they had to. But a growing number of those R-29RM missiles are apparently too old to safely fire. Given the delay in getting ten new Borei SSBNs into service (that will take 10-20 years), the Deltas will have to limp along. Keeping them in port can help but the old SLBMs have to be replaced. The new R-29RMU2 SLBM is apparently the same size (40 tons, 14.8 meters long and 1.8 meters in diameter) as the R-29RMU. The R-29RMU2 can carry 12 warheads and countermeasures for anti-missiles systems. Each Delta boat carries 16 SLBMs.