Russia is saying goodbye to its last Soviet-era ballistic-missile submarines. Here's what's replacing them.

• Russia has announced plans to decommission the ballistic-missile submarine Ekaterinburg.
• That will be the beginning of the end for the subs that have long been the backbone of the Soviet and Russian fleet.
• They will be replaced by the Borei-class, Russia's most advanced ballistic-missile sub.

In April, the Russian Navy announced that the Ekaterinburg, its second-oldest Delta-IV-class nuclear-powered ballistic-missile submarine, will begin its decommissioning process in 2022.

The sub has spent almost two years laid up at port in Severodvinsk, and its decommissioning will be the end of a more than 36-year career, one with its fair share of mishaps and accidents as part of the Soviet and Russian navies.

Ekaterinburg's decommissioning is also the beginning of the end for the Delta-class series of nuclear-powered ballistic-missile submarine, or SSBN, that has been the backbone of the Soviet and Russian SSBN fleet for decades.

The Deltas will be replaced by the long-awaited and much-anticipated Borei-class.

The Delta series

Known in Russia as the Project 667BDRM Delfin-class, the Delta IV boats are the fourth and final iteration in a long series of 43 SSBNs, the first of which was introduced in the early 1970s.

At 544 feet long, Delta IVs have four torpedo tubes and 16 silos. They were originally armed with R-29RM Shtil submarine-launched ballistic missiles, or SLBMs, which were eventually upgraded to the R-29RMU Sineva in 2007. After 2014, some Delta IV boats were given the R-29RMU2 Layner SLBM as well.

The missiles are each capable of carrying four Multiple Independently Targeted Re-entry Vehicles, or MIRVs, each of which contain warheads that can be directed to different targets. The Shtil's MIRVs carried 100-kiloton nuclear warheads, while the Sineva's and Layner's carry 500-kiloton warheads.

Seven Delta IVs are in service with the Russian Navy. One of them, Podmoskovye, was converted into a Special Mission submarine in 2016 for intelligence missions.

Instead of carrying nuclear missiles, the Podmoskovye acts as a mothership, carrying underneath it smaller subs like the Losharik, a secretive nuclear submersible believed to be used for espionage and which suffered a deadly fire in July 2019. (Losharik could be out of service until 2025.)

Aside from the seven Delta IVs, one Delta-III-class submarine, Ryazan, is also in service. All Delta IVs are currently serving in Russia's Northern Fleet, while the lone Delta III serves with the Pacific Fleet.

Ekaterinburg

Ekaterinburg was the second Delta-IV-class boat to be built. Laid down in 1982 and commissioned in 1985, it has had an interesting history to say the least.

On August 6, 1989, during Operation Behemoth, Ekaterinburg attempted to launch all 16 of its R-29RM Shtil SLBMs while underwater - the first time any SSBN had tried such a feat. The first launch was successful, but a rocket-fuel leak in the second missile sparked a fire, causing the test to be terminated.

The missile itself was destroyed, but Ekaterinburg escaped without any serious damage. Exactly two years later, its sister-boat, Novomoskovsk, conducted the test successfully, launching all 16 missiles in three minutes and 44 seconds.

In 2011, a fire broke out on Ekaterinburg's bow while it was in a floating drydock in Murmansk. Attempts to extinguish the blaze were unsuccessful, and the fire burned for almost a full day before it was decided to submerge the submarine to put out the fire.

While the fire was out, Ekaterinburg was heavily damaged and had to undergo a three-year repair process.

It was later revealed that Ekaterinburg was actually carrying its full load of nuclear SLBMs when the fire broke out, a violation of normal procedure. The decision to submerge, then, prevented what could have been the worst nuclear disaster since Chernobyl.

After being repaired, Ekaterinburg served as any other SSBN in the Russian Navy. It was involved in a few missile tests and conducted a number of patrols with the Northern Fleet.

Borei-class

Ekaterinburg and the rest of the Delta-IV boats will eventually be replaced by the Borei-class.

Though design work started in the mid-1980s, construction of the first Borei-class boat, Yury Dolgorukiy, did not begin until 1996, and it did not enter service until 2013.

Despite being smaller than the famous Typhoon-class, the Borei-class is considered the most advanced SSBN Russia has built. Features like new sonar systems and a pump-jet propulsion system make it considerably quieter than its predecessors. It also has a new suite of electronics and control systems.

The Borei-class has six torpedo tubes and 16 missile silos that house new RSM-56 Bulava SLBMs. The Bulava can carry anywhere between six to 10 MIRVs, each with 100- or 150-kiloton yields.

Like the Yasen-class nuclear-powered guided-missile submarine, production of the Boreis hit repeated delays, which in turn allowed the Russians to refine the design.

As a result, the Borei-A subclass was created, with different dimensions and even more advanced tech.

There are currently four Boreis in service. The most recent of them, Knyaz Vladimir, was commissioned last year and is the first Borei-A in service. The next Borei-A, Knyaz Oleg, is currently undergoing sea trials.

Two of the Boreis are assigned to the Northern Fleet, while the other two are assigned to the Pacific Fleet. The Russian Navy plans to have 10 Boreis in service by the end of the decade.

The 10 Boreis were originally planned to be distributed evenly between the Northern and Pacific fleets, but after the Umka-2021 exercise, in which three Boreis surfaced through the Arctic at the same time, the Russian Defense Ministry reportedly decided to prioritize their delivery to the Northern Fleet.