Almaz / X-Ray is the Soviet/Russian NATO-reporting-name family for the Project 1851 Nelma deep-diving special mission submarines. Open sources describe the 40-meter, titanium-hulled craft as carrier-borne vehicles with a diver lock-out, used for reconnaissance, cable tapping, rescue, and seabed recovery at about 1,000 meters.
RUSI places X-Ray among Russia's specialised GUGI submarines used for seabed warfare and critical undersea infrastructure threats, but the open-source record does not tie the class to a single named post-2015 conflict.
The X-Ray family is best understood as a carrier-borne atomic deep-water station rather than an independently deployed attack submarine. Open sources describe it as a small, unarmed, nuclear-powered vehicle intended to operate from a modified host submarine while carrying hydronauts and equipment for seabed work.
The vehicle was designed as part of a complex with a modified nuclear submarine carrier, with crew transfer through the carrier rather than normal pier-side deployment.
Published descriptions emphasize underwater reconnaissance, recovery from the seafloor, cable or infrastructure work, rescue tasks, and diver support rather than weapons carriage.
The class is associated with Russia's GUGI deep-sea organization, but individual missions and later service histories remain sparsely documented in public sources.
Open references split the family between the single Project 1851 / 18510 Nelma boat and the two improved Project 18511 Halibut / Paltus boats.
| Variant | Configuration | Designation notes |
|---|---|---|
| AS-23 | Project 1851 / 18510 Nelma boat | GlobalSecurity lists AS-23 as the first Project 1851 unit, laid down in 1981, launched in 1983, and accepted by the Northern Fleet in 1986. Sources: Project 678 / Project 1851 - Nelma class / X-Ray class |
 | Improved two-boat follow-on class | Covert Shores and GlobalSecurity describe AS-21 and AS-35 as modified Project 18511 follow-on boats that joined the original X-Ray-family design. Sources: Russian X-RAY Class Special Mission submarine, Project 678 / Project 1851 - Nelma class / X-Ray class |
Project 1851 sits inside a small family of Soviet and Russian deep-water station programs rather than a conventional submarine lineage. Published carrier details are treated cautiously because open sources describe modified mothership configurations rather than ordinary baseline hull classes.
| Related system | Connection | Why it matters |
|---|---|---|
| Improved follow-on | Project 18511 AS-21 and AS-35 extended the Project 1851/X-Ray concept into a two-boat Halibut/Paltus subfamily. |
 | Adjacent GUGI station | Project 1910 was a larger parallel deep-diving station family that appears alongside X-Ray and Paltus in open GUGI capability discussions. |
 | Later deep-water station | Project 10831 Losharik represents the later generation of carrier-supported Russian deep-water stations discussed with X-Ray, Paltus, and Kashalot. |
GlobalSecurity says the first Project 1851 submarine, AS-23, was laid down at Sudomech in Leningrad in September 1981.
Sources: Project 678 / Project 1851 - Nelma class / X-Ray class
GlobalSecurity and Modellmarine both place AS-23 in service in 1986, making it the first operational X-Ray-family deep-water station.
Sources: Project 678 / Project 1851 - Nelma class / X-Ray class, Russisches Spezial-U-Boot des Typs Almaz/Projekts 1851 (1/700, L'Arsenal) von Eberhard Sinnwell
GlobalSecurity records AS-21 entering service in December 1991 and AS-35 joining in October 1995 as the improved Project 18511 follow-ons.
Sources: Project 678 / Project 1851 - Nelma class / X-Ray class
RUSI's analysis of Russian activity against undersea infrastructure still treats X-Ray as part of the country's specialised GUGI capability set.
Sources: Stalking the Seabed: How Russia Targets Critical Undersea Infrastructure
