The Pershing II was a medium-ranged ballistic missile system with a range of 1,750 km, first deployed in 1983 to Western Europe and retired by 1991 due to the signing of the Intermediated-Ranged Nuclear Forces Treaty. It was a two-stage missile and the only US weapon to be deployed with a manoeuvrable reentry vehicle (MaRV). The weapon used a radar-based terminal guidance system.

As deployed, the missile carried the W85 nuclear warhead, which was an adaption of the physics package from the B61 mod 3 and 4 thermonuclear bombs, with a publicly estimated variable yield of 0.3 to 80 kt (probably four yield options). As a MaRV weapon with terminal guidance, it had a CEP of 30 m (100 ft). The reentry system can be seen below, with the radar and impact fuze at top, the warhead in the middle and the guidance system in the rear.

But the missile was initially planned to carry a second warhead design. This other warhead was the W86 Earth Penetrating Warhead, designed to destroy bunkers and command posts, and to crater aircraft runways. There were very few details on this system until recently.

So, without adieu the W86’s specification were:

Weight	184 kg (406 lb)
Diameter	170 mm (6.7 in)
Length	approximately 1700mm (67 in)
Penetration capability	7.2 metres (24 ft) into medium strength rock, 18.7 metres (61 ft) into low strength rock and 115 metres (377 ft) into silt or clay, assuming a peak allowable deceleration of 10,000 g (98,000 m/s2).
Type	boosted fission device, probably using linear implosion.
Yield	sub-10 kt, possibly 1 to 10 kt

Weight and diameter come from Effects of Nuclear Earth-Penetrator and Other Weapons,[1] wherein they discuss three weapons systems: the W86 (which is variously called the W86 and “low-yield EPW”), the strategic earth penetrator weapon (SEPW) which appears to be the W61 or B61-11 weapons (these are likely the same weapon with the B61-11 being based on the W61’s R&D), and a hypothetical “optimised EPW” which for their comparison is designed to provide maximum ground-penetrating capability. These figures are backed up by a March 1979 article in Los Alamos’ magazine Lab News, which describes a 400 lb W86 test unit undergoing penetrating testing.[2]

Length is less certain and is taken from an official diagram of the weapon.

In addition to the W85 warhead, a dotted outline is provided, which matches the shape of the W86 as seen in various images:

Taking some measurements of this outline and using the missile’s overall length of 10.6 m, I determined the length to be approximately 1700 mm (66 in). This figure appears reasonable given the few pictures of the system available.

The penetration figures come from Effects of Nuclear Earth-Penetrator and Other Weapons.

The type of weapon comes from Nuclear Safety Themes for Earth Penetrating Weapons, which describes in detail the safing themes for the W86, W61, SEPW and a modern gun-type device.[4] Though the document does not explicitly say it is an implosion type, it does discuss gun-type weapons separately and mentions that there have been no developments in that area in many years.

The weapon’s very narrow diameter (170 mm) puts it in the same range as nuclear artillery shells, like the W33 (8-inch, gun type), W48 (155 mm, linear implosion), W79 (8-inch, boosted linear implosion with a thermonuclear stage — an enhanced radiation or “neutron” weapon) and W82 (155 mm, same as W79). In theory it could be a two-stage device like the W79 or W82, but the reason those weapons were thermonuclear was because they were ERWs. A ground penetrating weapon does not need an ERW capability, so it would be more efficient from a nuclear materials point of view to use those neutrons to fission more Pu239 or U235. Therefore, it’s likely the weapon is a boosted linear implosion device, derived from nuclear artillery shell technologies.

The yield again comes from Effects of Nuclear Earth-Penetrator and Other Weapons. The study examines the use of both 10 and 300 kt earth penetrating weapons under various scenarios. The SEPW, being based on the B61-7 strategic bomb, is estimated to have a maximum yield of 340 kt. The 300 kt weapon examined appears to this yield rounded to obscure the classified weapon yield. We can therefore presume that the 10 kt figure is the actual or rounded figure for the W86 (i.e. between 5 and 14.9 kt).

Further, on page 80, they perform casualty modelling for yields of 1 to 10 kt, and on pages 82 and 83, they perform casualty analysis for yields of 10 to 300 kt. The minimum yield for the B61-7 is estimated to be 10 kt.

That said, in Potential NSNF Weapons Concepts for the 21st Century, they discuss the usefulness of 10-, 100- and 1000-ton nuclear weapons, which is immediately followed by a redacted page discussing the W86 warhead and then another redacted page discussing the adaption of the W33 as an EPW.[5] The upper end yield fits that of the lowest yield discussed above, but it may suggest the upper limit is lower than 10 kt, and that the lower limit is as low as 10 t.

It’s also not exactly clear if a maximum of 10 kt is a realistic figure for a 170 mm diameter weapon. The W79 (203mm diameter) was a variable yield weapon that is publicly estimated to have had yields between 0.1 and 1.1 kt in the ERW version. Meanwhile, the non-ERW version was supposedly 0.8 kt (reportedly converted by removing a component). It may be that the W79’s yield was limited for reasons of practicality rather than for technical reasons. The W82 does not help, as it is variously quoted as being 1 kt, or under 2 kt.

Similarly, it’s possible that free of length restrictions, the designers were able to get 10 kt in a 170 mm diameter package. The W33 has been described by some sources as having a yield of 40 kt in its maximum yield configuration and it could be argued that an implosion weapon should be capable of similar yields, but I am sceptical of the claim. For example, 1963 DCI Briefing to Joint Chiefs of Staff describes the 1958 developed 8-inch howitzer yield as being 1.9 kt, with 1961 and 62 developments pushing that yield to 10 kt.

To conclude, I believe that the weapon if unboosted gets about 1 kt, and boosted gets up to about 10 kt. The technology used would have been similar to nuclear artillery shells and therefore is likely a linear implosion device.

Update 9 Dec 2022: The OSTI staff rescanned the W86 images at higher resolution. I have replaced the originals with those images.

References

[1]          Effects of Nuclear Earth-Penetrator and Other Weapons. Washington, D.C.: National Academies Press, 2005, p. 11282. doi: 10.17226/11282.

[2]          ‘Pershing II Earth Penetrator Test Successful’, Lab News, vol. 32, no. 5, pp. 1, 4, Mar. 09, 1979.

[3]          Leland Johnson, A History of Exceptional Service in the National Interest. Sandia National Laboratories, 1997. Accessed: Feb. 04, 2022. [Online]. Available: http://large.stanford.edu/courses/2018/ph241/blair1/docs/sand-97-1029.pdf

[4]          N R Hansen, ‘Nuclear Safety Themes for Earth Penetrating Weapons, Issue C’, Sandia National Lab. (SNL-NM), Albuquerque, NM (United States), Apr. 1993. Accessed: Jul. 20, 2021. [Online]. Available: http://www.nukestrat.com/us/afn/97-14h_SNL040193.pdf

[5]          ‘Potential NSNF Weapons Concepts for the 21st Century – Perspectives, Warhead Technologies, and Delivery System Concepts’, Los Alamos National Lab., NM (USA), Dec. 1991. Accessed: May 22, 2021. [Online]. Available: https://www.osti.gov/opennet/servlets/purl/1045935.pdf