View source for UGM-73 Poseidon

{{Infobox weapon
|is_missile=yes
|name = UGM-73A Poseidon C3
|image = File:Poseidon C-3 SLBM.jpg
|caption = A Poseidon missile lifts off after being launched from the submerged USS ''Ulysses S. Grant'' (SSBN-631) in May 1979
|type = Strategic [[SLBM]]
|origin=United States
|service = 31 March 1971 to September 1992 ([[Trident (missile)|Trident]] I phased in from October 1979)
|used_by = [[United States Navy]]
|vehicle_range = With 14 [[Re-entry vehicle|RV]]s: {{convert|2500|nmi|km}}, with 10 RVs: {{convert|3200|nmi|km}}
|filling = 10 or 14 [[W68]] warheads in Mark 3 RVs; {{convert|40|ktTNT}}<ref>{{Cite report| title = Minutes of National Security Council Meeting| location = Washington DC| series = Parity, Safeguard, and the SS–9 Controversy| date = 1969-05-05|url=https://history.state.gov/historicaldocuments/frus1969-76v34/d16|page=119}}</ref>
|engine = Two-stage [[solid-fuel rocket]], each single nozzle with [[thrust vectoring]]
|guidance = [[Inertial navigation system|Inertial]]
|accuracy = {{convert|0.3|nmi|m}} [[Circular error probable|CEP]]
|speed = {{convert|8000|mph|km/h|abbr=on}} (terminal phase)
|length = {{convert|34.1|ft|m}}
|diameter = {{convert|74|in|m}}
|weight = {{convert|64400|lb|kg}}
|payload_capacity = {{convert|2000|kg|lb}}<ref name = "missilethreat" />
|manufacturer = [[Lockheed Martin Space Systems|Lockheed Missiles Division]]
|unit_cost =
}}

The '''UGM-73 Poseidon''' missile was the second [[US Navy]] [[thermonuclear weapon|nuclear-armed]] [[submarine-launched ballistic missile]] (SLBM) system, powered by a [[multistage rocket|two-stage]] [[solid-fuel rocket]]. It succeeded the [[UGM-27 Polaris]] beginning in 1972, bringing major advances in warheads and accuracy. It was followed by [[UGM-96 Trident I|Trident I]] in 1979, and [[UGM-133 Trident II|Trident II]] in 1990.

==Development==
A development study for a longer range version of the Polaris missile—achieved by enlarging it to the maximum possible size allowed by existing launch tubes—started in 1963. Tests had already shown that Polaris missiles could be operated without problems in launch tubes that had their [[fiberglass]] liners and locating rings removed.

The project was given the title Polaris B3 in November, but the missile was eventually named Poseidon C3 to emphasize the technical advances over its predecessor. The C3 was the only version of the missile produced, and it was also given the designation UGM-73A.<ref>[http://www.spaceline.org/rocketsum/poseidon-c3.html Poseidon C3 at Spaceline.com] {{webarchive|url=https://web.archive.org/web/20060917002133/http://www.spaceline.org/rocketsum/poseidon-c3.html |date=2006-09-17 }}</ref>

Slightly longer and considerably wider and heavier than Polaris A3, Poseidon had the same {{convert|4600|km|nmi}} range, greater payload capacity, improved accuracy, and [[multiple independently targetable reentry vehicle]] (MIRV) capability. MIRV capacity has been given as up to either ten<ref>{{Cite web
  |title=W-68
  |url=https://nuclearweaponarchive.org/Usa/Weapons/Allbombs.html
}}</ref> or fourteen [[W68]] thermonuclear warheads<ref name="missilethreat">[http://www.missilethreat.com/missilesoftheworld/id.92/missile_detail.asp Poseidon C3 at MissileThreat.com] {{webarchive|url=https://web.archive.org/web/20070929090614/http://www.missilethreat.com/missilesoftheworld/id.92/missile_detail.asp |date=2007-09-29 }}</ref> contained in Mark 3 reentry vehicles to multiple targets.

As with Polaris, starting a rocket motor when the missile was still in the submarine was considered very dangerous. Therefore, the missile was ejected from its launch tube using high pressure steam produced by a solid-fueled boiler. The main rocket motor ignited automatically when the missile had risen approximately {{convert|10|m|ft}} above the submarine.

The first test launch took place on 16 August 1968, the first successful at-sea launch was from a surface ship, the {{USNS|Observation Island|T-AGM-23|6}} (from July 1 to December 16, 1969), earning the ship the [[Meritorious Unit Commendation]],  and the first test launch from a submarine took place on the {{USS|James Madison|SSBN-627|6}} on 3 August 1970. The weapon officially entered service on 31 March 1971. It eventually equipped 31 {{sclass|Lafayette|submarine|5}}-, {{sclass|James Madison|submarine|5}}-, and {{sclass|Benjamin Franklin|submarine|5}}-class submarines.

The Royal Navy also considered adopting Poseidon in the 1970s as an upgrade to its Polaris A3T boats, and like the US this would have kept the existing hulls. Although the Navy's favoured option, the British government instead adopted [[Chevaline]], a two warhead MRV system with decoys, on the existing Polaris airframes and later moved to the Trident D5 in new boats.

Beginning in 1979, 12 Poseidon-equipped [[SSBN]]s were refitted with [[UGM-96 Trident I|Trident I]]. By 1992, the [[collapse of the Soviet Union|Soviet Union]] had collapsed, 12 [[Ohio-class submarine]]s had been commissioned, and the [[START I]] treaty had gone into effect, so the 31 older Poseidon- and Trident I-armed SSBNs were disarmed, withdrawing Poseidon from service.

==Operators==
* {{USA}}
** [[United States Navy]]

==Gallery==
{{Gallery
|title=
|width=160 | height=170
|align=center
|footer=
|File:W68_Mk3_reentry_body.jpg
 |alt1=
 |The Mark 3 Reentry Body (RB) that would contain the W68 warhead carried by Poseidon.
|File:US_missile_and_rocket_hallway_(navy).jpg
 |alt2=
 |Mark 3 with objects for scale.
|File:Poseidon C-3 Missile1.jpg
 |alt3=
 |Poseidon missile on display at the USS Bowfin Submarine Museum & Park.
|File:Poseidon_C-3_Missile2.jpg
 |alt4=
 |Poseidon missile post-boost vehicle (PBV) showing 14 locations for warheads though only 8 are outfitted with warhead equipment.
|File:Polaris-Poseidon_family_of_SLBMs.jpg
 |alt5=
 |Polaris-Poseidon family of SLBMs.
}}

==See also==
* [[List of missiles]]

==References==
{{Reflist}}

{{Commons category|UGM-73 Poseidon C-3|position=left}}

{{US missiles}}
{{Authority control}}

[[Category:Cold War missiles of the United States]]
[[Category:Nuclear missiles of the United States]]
[[Category:Cold War submarine-launched ballistic missiles of the United States]]
[[Category:Lockheed Corporation]]
[[Category:Submarine-launched ballistic missiles of the United States|UGM-073]]
[[Category:Military equipment introduced in the 1970s]]