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The W-71 nuclear warhead was a US thermonuclear warhead developed at Lawrence Livermore National Laboratory in California and deployed on the LIM-49A Spartan missile, a component of the Safeguard Program, an anti-ballistic missile(ABM) defense system briefly deployed by the US in the 1970s, superseding the W-50 that was designed for the LIM-49 Nike Zeus B - Spartan's predecessor.
The W-71 warhead was designed to intercept a cloud of incoming enemy warheads after their carrying missile's boost phase, at altitudes comparable to low earth orbit where there is practically no air, were the High-altitude nuclear explosion environment permits the passage of essentially all of the X-rays generated by the nuclear explosion. As part of the Safeguard ABM program the W71 was to serve as the first line of defense and as a complement to the other Safeguard missile-warhead pairing, the Sprint missiles enhanced neutron radiation warhead, the W66.
The W71 warhead had a yield of around 5 megatons of TNT (21 PJ) optimized for the directional production of thermal soft X-rays and minimal debris in an exoatmospheric detonation. The target would be damaged or destroyed by near-instantaneous x-ray vaporization/ablation of its surface resulting in an inward-propagating shock wave that would break the target up. 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 Spartan warhead(W71) produced great amounts of x rays, and needed to minimize fission output and debris to reduce the radar blackout effect that fission products and debris produce on Anti-Ballistic Missile radar systems.
To take advantage of the range of soft x-rays in space, the Lawrence Livermore National Laboratory is reported to have designed or "tailored" the warhead's secondary with a gold liner, instead of the usual depleted uranium or lead, which maximized x-ray production. When fast neutrons are slowed down, such as might occur in a gold containing Hohlraum or tamper/pusher of a thermonuclear secondary, the energy lost can show up as x-rays, and gamma rays. Moreover any neutron captures that occur during this slowing down process in stable Au-197, will form Au-198 which has a half life of 2.697 days and beta decay energy of 0.41 MeV, which is in the hard x-ray to gamma ray spectrum. Despite this, there is however some precedence for the use of gold in thermonuclear devices, having initially been used in thermonuclear weapons as "radiation mirrors" within the secondary assembly. Indeed the very first true "Hydrogen bomb"/Teller-Ulam device, the 1952 test shot Ivy Mike, is believed to have used a thin layer of gold on the secondary casing walls to enhance the blackbody effect, trapping more energy in the hohlraum case to enhance the implosion.
The design predecessors to the W71 were the explosive devices developed for Operation Plowshare-the US development of possible peaceful uses of nuclear explosives- which similarly required "clean bombs"/reduced outputs of residual radiation by reducing the "Fission fraction", or the quantity of fission products per kiloton and reducing the neutron induced radioactivity of the device casing and soil to make peaceful applications such as earth moving environmentally practical. The design approaches to reduce residual radiation in these early efforts proved critical to the development of warhead concepts that were deployed on the Spartan missile system in the early 1970s. One of these explosive device investigations, and when compared to the Ivy Mike test, somewhat closer in design to the W71 - due to it also requiring reduced fission product output per unit of explosive yield - resulted in the greatest amount of 198Au detected in any atmospheric US nuclear test, and is known as shot "Sedan", which was detonated at the Nevada Test Site on July 6, 1962.
Using the same principle of nuclear detonation generated soft x-rays to destroy incoming missiles and warheads, the 1980s Project Excalibur x-ray lasing concept of the Strategic Defense Initiative can be seen as the Spartan's W71 conceptual descendant.
Under good conditions, the W-71 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) miles against hardened warheads.
Thirty to 39, units were produced between 1974 and 1975. The weapons went into service, but were then taken right back out of service in 1975 and the warheads stored until 1992 when they were dismantled. The short service life of the W-71-Spartan and Safeguard Program in general, is believed to have been partly tied to it largely becoming obsolete with the development of Soviet offensive MIRV(Multiple independent re-entry vehicles) warheads, that unlike MRVs(multiple re-entry vehicles), can create a substantial spacing distance between each warhead once they arrive in space - and therefore would require at least about 1 Spartan missile launch to intercept each MIRV warhead. Fatally though, as the cost of the Spartan missile interceptor and an enemy ICBM were roughly the same, an adversary could afford to simply overwhelm the ABM system by adding ICBMs with MIRV warheads to its nuclear arsenal. (See more Spartan (missile)).
Probably because of unique design features associated with x-ray production and directional emission from the thermonuclear component of the warhead, it was decided to conduct a full-yield proof test of the W71, in preparation for this full-yield test, a calibration test known as Milrow of Operation Mandrel was conducted in 1969. Despite political and pressure group opposition to both tests, and in particular the full yield W71, coming from then US Senator Mike Gravel and the nascent Greenpeace, 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 of Operation Grommet in the world's largest underground nuclear test, on Amchitka Island in the Aleutian Islands off Alaska. The second highest yield underground test known, occurred in 1973, when the USSR tested a 4 Mt device 392
|Standard bomb in the "moderate" kiloton range||Enhanced/neutron bomb||Standard fission device warhead in space|
|Blast||50%||40% or as low as 30%||Approximately 0|
|Thermal energy||35%||25% or as low as 20%||70 to 80% as soft x-rays.|
|Instant ionizing radiation||5%||30–45%||Approx 10|
|Residual fallout radiation||10%||5%||Approx 10|
The W71, mounted in a Spartan missile body, was lowered 6,150 feet (1,870 m) down a 90 inches (2.3 m) diameter borehole into a man-made cavern 52 feet (16 m) in diameter. A 264 feet (80 m) long instrumentation system monitored the detonation. The full yield test was conducted at 11:00am local time November 6, 1971 and resulted in a vertical ground motion of 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 the Richter scale. A mile (1.6 km) wide and 40 feet (12 m) deep crater formed two days later.
Film of the test has been declassified and can be seen in the third of the Atomic Journeys documentaries Welcome To Ground Zero.
Nuclear weapon-generated X-rays are the chief threat to the survival of strategic missiles in-flight above the atmosphere and to satellites...The Neutron and gamma ray effects dominate at lower altitudes where the air absorbs most of the X-rays.