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, where 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 production of X-rays and minimal debris in an exoatmospheric detonation. 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.
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.
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.
Prior to the W71 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
The W71 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.
- "W-71 warhead information at Globalsecurity.org "...the design of the warhead for Spartan, the interceptor used in the upper tier of the U. S. Safeguard Anti- Ballistic Missile (ABM) system. Spartan missiles were to engage clouds of reentry vehicles and decoys above the atmosphere and destroy incoming warheads with a burst of high- energy x rays...The Spartan warhead had high yield, produced copious amounts of x rays, and minimized fission output and debris to prevent blackout of ABM radar systems. Livermore also developed and first tested the warhead technology for the second- tier interceptor, the Sprint missile"".
- Allbombs.html entry on W71 at nuclearweaponarchive.org, Accessed June 6, 2007
- "Accomplishments in the 1970s: Lawrence Livermore National Laboratory". Archived from the original on 2005-02-17. Retrieved 2006-10-09.
- "W-71 warhead information at Globalsecurity.org "...The Spartan warhead had high yield, produced copious amounts of x rays, and minimized fission output and debris to prevent blackout of ABM radar systems"".
- "W-71 warhead information at Globalsecurity.org "...the possible peaceful uses of nuclear explosives through Project Plowshare. Reduced amounts of residual radiation-fewer fission products from the explosion and less induced radioactivity of the ground-were necessary to make feasible peaceful applications such as earth moving and power production. The design approaches to reduce residual radiation in these early efforts proved critical to the Laboratory's development of warhead concepts that were deployed on the Spartan and Sprint antiballistic missile systems in the early 1970s."".
- M. Todd Bennett (ed), "National Security Policy, 1969–1972", 2011, p. 41.
- Bennett 2011, p. 54.
- Gravel, Mike (1969-07-31). "Risks in Alaska Tests" (FEE REQUIRED). The New York Times. Letters to the Editor. Retrieved 2007-12-30.
- Richard D. Lyons (1971-08-23). "Underground A-Test Is Still Set For Aleutians but Is Not Final" (FEE REQUIRED). The New York Times. Retrieved 2007-12-30.
- "Witnesses Oppose Aleutian H-Blast" (FEE REQUIRED). The New York Times. 1971-05-30. Retrieved 2007-12-30.
- "The Amchitka Bomb Goes Off". Time. 1971-11-15. Retrieved 2006-10-09.
- "W-71 warhead information at Globalsecurity.org "...the Supreme Court ruled by a 4- 3 margin that the test could take place. On November 6, 1971, at 6: 30 a. m. in Amchitka, the go-ahead came from the White House on a telephone hotline"".
- US nuclear weapons list at nuclearweaponarchive.org
- Spartan warhead image from nuclearweaponarchive.org
- W-71 warhead information including at Globalsecurity.org
- Time magazine, Round 2 at Amchitka