Cruise-missile submarine
A cruise missile submarine is a submarine that carries and launches cruise missiles as its primary armament. Missiles greatly enhance a warship's ability to attack surface combatants and strike land targets; although torpedoes are a more discreet option for submerged submarines, missiles give a much longer stand-off range, shorter time to impact the target, as well as the ability to engage multiple targets on different headings at the same time. Many cruise missile submarines retain the capability to deploy nuclear warheads on their missiles, but they are considered distinct from ballistic missile submarines due to the substantial differences between the two weapons systems' flight characteristics; cruise missiles fly aerodynamically using flight surfaces like wings or fins, while a ballistic missile uses its engine power alone as it may exit the atmosphere.
The United States Navy's hull classification symbols for cruise missile submarines are SSG and SSGN – the SS denotes submarine, the G denotes guided missile, and the N denotes that the submarine is nuclear-powered.
The earliest designs of cruise missile submarines had to surface to launch their missiles, while later designs could do so underwater via dedicated vertical launching system (VLS) tubes. Many modern attack submarines can launch cruise missiles from their torpedo tubes while some designs also incorporate a small number of VLS canisters, giving an overlap between cruise missile submarines and traditional attack submarines. Nonetheless, vessels classified as attack submarines are designed to use torpedoes as their main armament and have a more multi-role mission profile due to their greater speed and maneuverability. This is in contrast to cruise missile submarines which are typically larger, slower boats carrying a larger number of missiles and often possess a special compartment dedicated solely to the cruise missile tubes.
However, the line between SSGNs and SSNs is blurring, as new generations of SSNs increasingly carry cruise missiles. French Suffren-class nuclear submarines carry long-range MdCN cruise missiles launched not from vertical launchers but from standard 533mm torpedo tubes.
U.S. Navy
The U.S. Navy's first cruise missile submarines were developed in the early 1950s to carry the SSM-N-8 Regulus missile. The first of these was a converted World War II era,, which was fitted with a hangar capable of carrying a pair of Regulus missiles. Tunny was used as a test-bed for developing techniques of use for the missile system, before a second boat, was subsequently converted. Starting in 1957, these two boats undertook the first nuclear deterrent patrols.Subsequently, two larger diesel submarines of the were purpose built for the carriage of the Regulus missile, with each capable of accommodating up to four missiles, while a further boat, the nuclear-powered, could carry up to five missiles. Between September 1959 and July 1964, the five Regulus missile boats undertook deterrent patrols in the Pacific Ocean, in concert with the newly commissioned ballistic missile submarines in the Atlantic, until sufficient SSBNs were in service to replace them.
From 2002 to 2008, the U.S. Navy modified the four oldest submarines:,,, and into SSGNs. The conversion was achieved by installing VLS in a multiple all-up-round canister configuration in 22 of the 24 missile tubes, replacing one Trident missile with seven smaller Tomahawk cruise missiles. The two remaining tubes were converted to lockout chambers for use by special forces personnel. This gave each converted submarine the capability to carry up to 154 Tomahawks. The large diameter tubes can also be modified to carry and launch other payloads, such as UAVs or UUVs although these capabilities have not yet been fully implemented. In addition to generating a significant increase in stand-off strike capabilities, this conversion also counts as an arms reduction towards the START II treaty, because it reduces the number of nuclear weapons that are forward-deployed. USS Florida launched cruise missiles against Libyan targets as part of Operation Odyssey Dawn in March 2011.
Currently, Virginia-class submarines serve as universal ships, both attack submarines and cruise-missile submarines with 12 × Vertical launching system (VLS) for Tomahawk cruise missiles. The future Virginia-class submarines with 40 Tomahawk cruise missiles are slated to supplement and eventually replace the Ohio SSGNs when they are retired; USS Ohio itself is more than 40 years old.
Soviet Navy/Russian Navy
The Soviet Navy has operated a wide variety of dedicated cruise missile submarines :| Entered Service | NATO reporting name | Project Name and Number | Ship Class | Planform | Missiles carried |
| 1957 | Zulu | П611 | SSG | 1 x П-10 | |
| 1958 | Whiskey Single Cylinder | П613 | SSG | 1 x П-5 Пятёрка | |
| 1962 1964 | Whiskey | 613А 613АД | SSG | 1 x П-70 Аметист | |
| 1960 | Whiskey Twin Cylinder | 644 | SSG | 2 x П-5 Пятёрка 2 х П-5Д under Project 644Д and 2 х П-7 under Project 644-7 | |
| 1961 | Whiskey Long Bin | 665 | SSG | 4 x П-5 Пятёрка | |
| 1963 | Juliett | 651 651К | SSG | 4 х П-5/6 Пятёрка | |
| 1986 | Juliett | 651Э | SSGN | 4 х П-5/6 Пятёрка | |
| 1960 | Echo I | 659 | SSGN | 6 x П-5 Пятёрка | |
| 1963 | Echo II | 675 | SSGN | 6 x П-5/6 Пятёрка Later 8 x P-500 Базальт under the 675МК and 675МУ program and 8 x P-1000 Вулкан under Project 675МКВ | |
| 1969 | Papa | 661 "Anchar" | SSGN | 10 x П-70 Аметист | |
| 1967 | Charlie I | 670 "Skat" | SSGN | 8 x П-70 Аметист | |
| 1973 | Charlie II | 670M "Skat" | SSGN | 8 x П-120 Малахит | |
| 1992+ | Charlie III | 06704 "Chaika-B" | SSGN | 8 x 3 П-800 Оникс | |
| 1980 | Oscar I | 949 "Granit" | SSGN | 24 x П-700 Гранит | |
| 1986 | Oscar II | 949A "Antey" | SSGN | 24 x П-700 Гранит Plans for Project 949АМ upgrade to fit missiles compatible with VLS; П-800 Оникс, Клуб, 3M22 Циркон; triple-tube inserts | |
| 1987 | Yankee Notch | 667AT "Grusha" | SSGN/SSN | 32 x РК-55 Гранат | |
| 2013 | Yasen | 885 "Yasen" | SSGN | П-800 Оникс, Калибр family, 3М22 Циркон | |
| 2021 | Yasen-M/Yasen-II | 885М Yasen-M | SSGN | П-800 Оникс, Калибр family, 3М22 Циркон |
The Whiskey variants and Echo I cruise missile submarines deployed with a nuclear land attack version of the P-5 Pyatyorka from the late 1950s to 1964, concurrently with the US Regulus force, until the strategic land attack mission was transferred entirely to the SSBN force. Along with the Julietts and Echo IIs, these continued as SSGs or SSGNs with an antiship variant of the P-5 until circa 1990. The Echo Is were an exception; they could not accommodate the anti-ship targeting radar and served as SSNs after the land attack missiles were withdrawn.
Apart from true guided-missile submarines, late-Soviet attack submarines could launch various types of torpedo tube-launched missiles starting with the RK-55 and continuing with the Kalibr family of missiles. Cruise-missile capable Soviet submarines may have a different designation to incapable sister boats. Due to standardization of torpedo tube diameters, which are 533 mm, modern Russian attack submarine classes are capable of launching long-range strategic cruise missiles from their torpedo tubes, without needing specialized compartments for missile tubes.
File:DN-ST-84-01654-Papa class-Oct 1983.JPEG|K-222, the sole Project 661 submarine underway, 1983
File:DN-ST-86-11105-Juliett_class_submarine-11_Aug_1986.JPEG|A Project 651 boat underway, 1986
File:2023.09.29.143258 U-461 Peenemünde DE.jpg|K-24, Project 651 Juliett cruise missile submarine, with rear SS-N-3 Shaddock launch tubes in raised position
File:DN-SC-89-03179 INS Chakra submarine.jpg|Soviet K-43 underway, c. 1987
File:Oscar class submarine 2.JPG|Project 949 underway. The Oscar-classes are notably girthy; the very large P-700 missiles were placed outside the pressure hull, twelve launch tubes on each side
File:Oscar class submarine 1.JPG|Project 949A underway. Pr. 949A boats had a different tail fin which accommodated the towed-array sonar, an easy way to distinguish this class from its immediate predecessor