W71

For other uses, seeW71 (disambiguation).

The W71 nuclear warhead

Warhead being lowered into the borehole

TheW71nuclear warhead was a USthermonuclear warheaddeveloped atLawrence Livermore National Laboratoryin California and deployed on theLIM-49A Spartanmissile, a component of theSafeguard Program,ananti-ballistic missile(ABM) defense system briefly deployed by the US in the 1970s.

The W71 warhead was designed to intercept incoming enemywarheadsat long range, as far as 450 miles (720 km) from the launch point. The interception took place at such high altitudes, comparable tolow Earth orbit,where there is practically no air. At these altitudes,x-raysresulting from the nuclear explosion can destroy incomingreentry vehiclesat distances on the order of 10 miles (16 km), which made the problem of guiding the missile to the required accuracies much simpler than earlier designs that had lethal ranges of less than 1,000 feet (300 m).

The W71 warhead had a yield of around 5 megatons of TNT (21 PJ). The warhead package was roughly a cylinder, 42 inches (1.1 m) in diameter and 101 inches (2.6 m) long. The complete warhead weighed around 2,850 pounds (1,290 kg).

The W71 produced great amounts of x-rays, and needed to minimize fission output and debris to reduce theradar blackouteffect thatfission productsand debris produce on anti-ballistic missile radar systems.

Design

The W71 design emerged in the mid-1960s as the result of studies of earlier high-altitude nuclear tests carried out before thePartial Nuclear Test Ban Treatyof 1963. A number of tests, especially those ofOperation Fishbowlin 1962, demonstrated a number of previously poorly understood or underestimated effects. Among these was the behaviour of x-rays created during the explosion. These tended to react with the atmosphere within a few tens of meters at low altitudes (seerope trick effect). At high altitudes, lacking an atmosphere to interact with, themean free pathof the x-rays could be on the order of tens of kilometers.

This presented a new method of attacking enemy nuclear reentry vehicles (RVs) while still at long range from their targets. X-rays hitting the warhead's outermost layer will react by heating a thin layer of the material so rapidly thatshock wavesdevelop that can cause theheat shieldmaterial on the outside of the RV to separate or flake off. The RV would then break up during reentry.The major advantage of this attack is that it takes place over long distances, as great as 30 kilometres (19 mi), which covers the majority of thethreat tubecontaining the warhead and the various radar decoys and clutter material that accompanies it. Previously the ABM had to approach within less than 800 feet (240 m) of the warhead to damage it through neutron heating, which presented a serious problem attempting to locate the warhead within a threat tube that was typically at least a kilometer across and about ten long.

Bell received a contract to begin conversion of the earlierLIM-49 Nike Zeusmissile for the extended range role in March 1965. The result was the Zeus EX, or DM-15X2, which used the original Zeus' first stage as the second stage along with a new first stage to offer much greater range. The design was renamed Spartan in January 1967, keeping the original LIM-49 designation. Tests of the new missile started in April 1970 from Meck Island, part of theKwajalein Test Rangethat had been set up to test the earlier Nike Zeus system. Because of a perceived need to rapidly deploy the system, the team took a "do it once, do it right" approach in which the original test items were designed to be the production models.

The warhead for Spartan was designed byLawrence Livermore National Laboratory(LLNL), drawing on previous experience fromOperation Plowshare.A nuclear explosion at high altitude has the disadvantage of creating a significant amount of electronic noise and an effect known asnuclear blackoutthat blinds radars over a large area. Some of these effects are due to the fission fragments being released by the explosion, so care was taken to design the bomb to be "clean" to reduce these effects. Project Plowshares had previously explored the design of such clean bombs as part of an effort to use nuclear explosives for civilian uses where the production of long-livedradionuclideshad to be minimized.

To maximize the production of x-rays, the W71 is reported to have used a gold tamper,rather than the usualdepleted uraniumorlead.The lining normally serves the primary purpose of capturing x-ray energy within the bomb casing while the primary is exploding and triggering the secondary. For this purpose, almost anyhigh-Zmetal will work, and depleted uranium is often used because the neutrons released by the secondary will cause fission in this material and add a significant amount of energy to the total explosive release. In this case the increase in blast energy would have no effect as there is little or no atmosphere to carry that energy, so this reaction is of little value. The use of gold may have been to tailor its transparency to x-rays.In Congressional testimony on potential dismantling of the W71, a DOE official described the warhead as "a gold mine".

In 2008, theUnited States Department of Energydeclassified the fact that the radiation case of the W71 containedthoriummetal.

Lethality

Under good conditions, the W71 warhead had a lethal exo-atmospheric radius as much as 30 miles (48 km),although it was later stated to be 12 miles (19 km) against "soft" targets, and as little as 4 miles (6.4 km) against hardened warheads.

Production & service history

There were 30 to 39units produced between 1974 and 1975. The weapons went into service in 1975, but were retired that same year, and the warheads stored until 1992 when they were dismantled. The shortservice lifeof the W71, Spartan andSafeguard Programin general, is believed to have been partly tied to it largely becomingobsoletewith the development of Soviet offensiveMIRV(Multiple independentre-entryvehicles) warheads, that unlikeMRVs(multiple re-entry vehicles), can create a substantial spacing distance between each warhead once they arrive in space, hence would require at least approximately oneSpartan missilelaunch to intercept eachMIRVwarhead. As the cost of the Spartan and an enemyICBMwere roughly the same, an adversary could afford to overwhelm theABMsystem by adding ICBMs with MIRV warheads to itsnuclear arsenal.

Shot Grommet Cannikin

Main article:Cannikin

Prior to the W71 test, a calibration test known asMilrow of Operation Mandrelwas conducted in 1969. Despite political andpressure groupopposition to both tests, and in particular the full yield W71, coming from then US SenatorMike Graveland the nascentGreenpeace,a Supreme Court decision led to the test shot getting the go-ahead,and a W71 prototype was successfully tested on 6 November 1971 in Project Cannikin ofOperation Grommetin the world's largest underground nuclear test, onAmchitka Islandin theAleutian IslandsoffAlaska.The second highest-yield underground test known occurred in1973, when the USSR tested a 4 Mt device392

The W71 was lowered 6,150 feet (1,870 m) down a 90-inch-diameter (2.3 m) borehole into a man-made cavern 52 feet (16 m) in diameter. A 264-foot-long (80 m) instrumentation system monitored the detonation. The full yield test was conducted at 11:00 am local time November 6, 1971 and resulted in a verticalground motionof more than 15 feet (4.6 m) at a distance of 2,000 feet (610 m) from the borehole, equivalent to an earthquake of magnitude 7.0 on theRichter scale.A 1-mile-wide (1.6 km) and 40-foot-deep (12 m) crater formed two days later.

See also

• AmchitkaMilrowandCannikintests

External links

• US nuclear weapons list at nuclearweaponarchive.org
• Spartan warhead image from nuclearweaponarchive.org
• W71 warhead information including at Globalsecurity.org
• Timemagazine, Round 2 at Amchitka