R-27 Zyb (SS-N-6)

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The R-27 Zyb (NATO designation: SS-N-6 “Serb”), was a submarine-launched, liquid-fueled ballistic missile developed and deployed by the Soviet Union. Elements of the R-27 are believed to be the basis for some of North Korea’s ballistic missile programs.

Zyb (SS-N-6 “Serb”) at a Glance

Originated from
Soviet Union
Possessed by
Soviet Union
Submarine-launched Ballistic Missile (SLBM)
9.65 m
1.5 m
Launch weight
14,200 kg
Single-stage, liquid propellant
1 mT (Mod 1, 2), 200 kT (Mod 3)
2,400 km (Mod 1), 3,200 (Mod 2, 3)
In service

R-27 Development

Development of the R-27 began in 1962 and flight tests began in mid-1965. The missile was first revealed to the public during a Moscow military parade in 1967.1 The Mod 1 version of the missile officially entered service in 1968 and was deployed beginning in 1969.  It was shortly after followed by the Mod 2 and Mod 3.2

The R-27 was the world’s first sea-based liquid-fueled missile. Unlike other liquid-fueled missiles of its time, such as the Titan or Titan II, the R-27 could store its fuel, obviating the need for a time-consuming fueling process prior to launch.3 This capability allowed to missile to be fired within ten minutes of receiving launch orders.4

The missile was designed for use on the Project 667A Navaga or Yankee-class submarines. Each sub could carry 16 of the missiles, and could launch one every eight seconds.5 Of the nearly 2000 R-27s produced, only 192 remained in service as of 1991, due mainly to arms control treaties, modernization efforts, and the general decline of the Soviet Union. In 1994, this number dropped to 32.  The last set of R-27s were decommissioned alongside the final Yankee-class sub in 1996.6


Over its 28-year service span, three different variants of the Serb were built (with a fourth type designed and tested, but never fielded).

R-27 Mod 1

First deployed in 1969, the Serb Mod 1 was a 2,400 km range missile with a single 1 mT warhead. The Mod 1 was estimated to have a 1.9 km CEP, which would have allowed the missile to serve as a counter-value weapon, targeting cities, industrial centers, and unhardened targets.7

R-27 Mod 2

The Mod 2 (R-27U), released in 1972, was designed to be an extended range version of the Mod 1. To increase the missile’s range without having to change dimensions of the missile, the fuel tanks and engines were redesigned to improve weight efficiency and fuel capacity. One way that this was done was by compacting the fuel and oxidizer tanks, allowing them to use some of the same parts. Additionally, the guidance system and rocket engine were placed inside the oxidizer and fuel tanks.8 The redesigned interior was able to boost the Mod 2’s range to 3,000 km. Guidance system upgrades also increased the accuracy to 1.3-1.8 km CEP.9

R-27 Mod 3

The Mod 3 was a Mod 2 missile with a redesigned warhead package introduced in 1975.10 Where the Mod 1 and Mod 2 missiles had used a single 1 mT warhead, the Mod 3 added a multiple reentry vehicle (MRV) capability in the form of three, 200 kT warheads.  Unlike modern missiles, the Mod 3’s warheads could not be individually targeted, but could result in greater saturation of the target area.11

R-27 Mod 4

The Mod 4 (R-27K) was a cancelled project (1970-1974) which aimed to create a variant of the Mod 1 that could target coastal stations or ships. The Mod 4 originated with the initial 1961 proposal for the R-27, which called for an antiship ballistic missile that would also be capable of striking strategic coastal targets such as radar stations.1213 To be able to hit smaller targets such as these, the Mod 4 was redesigned to have a terminal guidance radar and a maneuvering reentry vehicle (MARV). But the additional weight of these necessitated lowering the warhead’s magnitude. Since the Mod 4s counted as strategic missiles under the Strategic Arms Limitations Talks (SALT 1) agreement in 1972, in 1974 the Soviets chose to abandon the project in favor of building more Mod 2 and Mod 3 missiles.14

Possible Proliferation to North Korea

In April 2009, North Korea attempted to orbit a satellite using its Unha-2 space launch vehicle. Following the launch, many experts believed that the Unha-2’s second stage rocket was a R-27.15 Among the reasons cited for these beliefs was that the Unha-2 second stage functioned perfectly despite never having been tested before and that the stage physically resembled the R-27.

However, a 2010 report by the International Institute for Strategic Studies that considered the possibility of R-27 proliferation to Iran suggests that due to the large number of R-27s produced, it is possible that some missiles or components were shipped or stolen from Russia to North Korea or Iran.  However, the report concluded that it is “highly improbable that complete or disassembled R-27 missiles were exported by Russia.”16

After analyzing debris from the 2012 Unha-3 flights, some of the experts who previously believed that the Unha-2 used a R-27 changed their assessment.17 They suggested that the second stage was, in fact, a heavily modified Scud missile. To support this, they point to the long, low-thrust burn made by the second stage which would be highly inefficient in a ballistic missile design.18 Additionally, they assert that the Vernier thrusters from the Unha-3, previously thought to have been proliferated components, used an entirely different than the R-27.19

The BM-25 Musudan is another North Korean missile that may have originated from R-27 technology.  It is estimated that the Musudan program began in 1992 after engineers from Russia’s Makeyev and Isaev rocket design bureaus attempted to enter North Korea and work on a R-27-based space launch vehicle.20 Many of the engineers were arrested, but some may have made it into North Korea.21

Due to the R-27 and the Musudan having a common fuselage diameter, some experts believe that the missiles share a similar engine assembly and fuel system in addition to their superficial appearance.22


    1. Federation of American Scientists, “R-27 / SS-N-6 SERB,” July 13, 2000, https://fas.org/nuke/guide/russia/slbm/r-27.htm.
    2. Duncan Lennox. “R-27 (SS-N-6 ‘Serb’ and RSM-25 Zyb/4K10),” in Jane’s Strategic Weapon Systems (Obsolete Weapons), (London: IHS Global 2011).
    3. Global Security, “R-27 / SS-N-6 SERB & SS-NX-13,” March 17, 2017, http://www.globalsecurity.org/wmd/world/russia/r-27.htm.
    4. Lennox.
    5. Ibid.
    6. Ibid.
    7.  Pavel Podvig. “R-27 (D-5 Missile System, SS-N-6) and R-27U (D-5U Missile System, SS-N-6),” in Russian Strategic Nuclear Forces, (Cambridge, MA: MIT Press 2001), 321.
    8. Federation of American Scientists.
    9. Podvig, 321.
    10. Astronautix, “R-27,” March 21, 2017, http://www.astronautix.com/r/r-27.html.
    11. Lennox.
    12. Podvig, 319
    13. Federation of American Scientists.
    14. Ibid.
    15. Union of Concerned Scientists, “SS-N-6 Missiles: How Many Could Have Been Transferred?,” September 8, 2010,  http://allthingsnuclear.org/dwright/ss-n-6-missiles-how-many-could-have-been-transferred.
    16. International Institute for Strategic Studies, “Iran’s Ballistic Missile Capabilities: A net assessment,” May 10, 2010, http://www.iiss.org/en/publications/strategic%20dossiers/issues/iran–39-s-ballistic-missile-capabilities–a-net-assessment-885a.
    17. Union of Concerned Scientists, “Markus Schiller’s Analysis of North Korea’s Unha-3 Launcher,” February 22, 2013, http://allthingsnuclear.org/dwright/markus-schillers-analysis-of-north-koreas-unha-3-launcher.
    18. Ibid.
    19. Ibid.
    20. Nuclear Threat Initiative, “North Korea Missile Capabilities,” May 1, 2010, http://www.nti.org/analysis/articles/north-korea-missile-capabilities/.
    21. David Wright, “North Korea’s Missile Program,” July 17, 2009, http://nautilus.org/wp-content/uploads/2011/12/Wright.pdf.
    22. Nuclear Threat Initiative.
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Missile Defense Project, "R-27 Zyb (SS-N-6)," Missile Threat, Center for Strategic and International Studies, May 16, 2017, last modified April 23, 2024, https://missilethreat.csis.org/missile/ss-n-6-r-27-serb/.