There’s not much out there on this system. Hansen for example has a grand total of two paragraphs on the system in Swords. In it, he describes it as the warhead for Minuteman and Poseidon before being cancelled in preference to the W68, that it had a yield of 150 kt and that it was only tested once in Crosstie Zaza.[1, pp. 459–460]

He is quite wrong on most of those points.

A Quick History

The W67/Mark 17 warhead began life as the Mark 12 warhead that eventually was deployed on Minuteman III with the W62 warhead.[2, pp. 51–52] During development of the reentry vehicle, significant time was spent on selecting the ideal weight and size for the RV which culminated in the “dual RV” concept where there would be a Mk12(L) for a light RV and Mk12(H) for a heavy RV, and that these two warheads would be deployed as a mixed force on Minuteman II.[2, p. 95]

During this time the US Navy became involved in the Mark 12 program.[2, pp. 23–24] There was considerable disagreement between the air force and navy over warhead hardening requirements. The navy wanted a higher level of hardening due to spacing requirements (I believe this means spacing between bursts) and the air force wanted a lower requirement due to weight concerns.[2, p. 50]

It was around this point that the Mk12(H) was spun off as the Mark 17.[2, pp. 51–52] Another official document states that the Mk12(H) was also spun off as the Mark 16,[3, p. 34] which might be because of uncertainty about RV size and weight as several heavy candidates were examined.[2, pp. 43, 46–47] Regardless, only the Mark 17 reached the stage of having a warhead number assigned to it, and was to be developed by Los Alamos.[4]

Once spun off, the navy remained involved in the Mark 17 program, hoping to put the weapon on Poseidon.[3, p. 37] Eventually, the navy decided to not go with the Mark 17 (or the Mark 12 for that matter) and chose to develop their own reentry vehicle (a reentry body in US Navy parlance).[5, p. 44] The reason was quite simple: the navy’s higher hardening requirements meant a heavier RV for the air force, while the much longer range of Minuteman meant a heavier RV for the navy.[2, pp. 23–24]

This was soon followed by budget concerns in the Minuteman program. Officials were faced with a question of what is preferable: assured retaliation or the ability to destroy hard targets for damage limitation? Being able to carry two or three warheads using the Mark 12, even though they were substantially lower in yield was preferable to carrying a single Mark 17. So the Mark 17 was cancelled in December 1967.[5, pp. 44–45]

Technical Details of the W67/Mark 17

Normally they are very cagy about reentry vehicle and RV warhead weights, so I was surprised to come across them.

The total RV and warhead weight was planned to be 410 kg (900 lb) and by November 1966 they had managed to get it down to 425 kg (938 lb). The bare warhead weight was 306 kg (675 lb).[5, p. 43] The yield is classified, but it was comparable to that of the W56/Mark 11, i.e. 1.2 Mt.[2, p. 22]

We can make some further assumptions from here. For example, the W56 warhead had a bare weight of about 255 kg (564 lb) and a yield of 1.27 Mt as tested in Dominic Bluestone, for a yield to weight ratio of 4.98 kt/kg.[6, pp. 172–173] If we assume that the W67 could obtain at least the same yield to weight ratio (a very reasonable assumption given the higher warhead weight class of the W67) then the lower yield estimate is 1.52 Mt.

An upper yield estimate might come from the DCI Briefing to Joint Chiefs of Staff which I have previously discussed. In it, they estimate that a 450 kg (1000 lb) warhead with a yield of 2 Mt could be produced for Polaris based on the results of the recent Operation Dominic and Nougat series of tests, and that in a future test series this could increase to 3 Mt.[7, p. 20] This comes out to 4.44 kt/kg and 6.66 kt/kg respectively.

Discarding the lower 4.44 kt/kg (which is deceptively low due to RV weight being included) at 6.66 kt/kg, the W67 would get 2 Mt using bare warhead weight or 2.8 Mt using total RV weight.

Another detail is that the W67 was designed to have the highest practical output temperature.[2, p. 62] What this means is the highest blackbody temperature so as to increase the average temperature of emitted x-rays which can cause damage to enemy warheads.[2, p. 54] However, this is only an effect at high altitudes as surrounding air would reduce the effective temperature, so presumably this was to add an x-ray pindown capability to the weapon. This appears to be backed by the requirement for the weapon to have a high altitude fuze (HAF).[3, p. 26]

Some other technical detail are available, for example its environmental safing device (ESD) was the MC2306 and through it passed two pre-arm signals, two arm signals and two fire signals.[8, p. 5] The “Sandia parts” (presumably the non-nuclear parts of the system) contained explosive components that were hazardous enough that during testing they were not to be handled once the ESD activated.[8, p. 4] As the gas bottle is part of the Sandia parts, I suspect that the two pre-arm signals activate the gas boosting system.[8, p. 3]

The warhead was only tested once, in a partial yield test.[2, p. 62] Crosstie Zaza is listed as having a yield of 160 kt. Presumably Hansen misunderstood that this was a partial yield test and not a full yield test of the warhead, hence him assigning the 150 kt yield figure to the device.

Initial plans called for 700 Mark 17 equipped and 300 Mark 12 equipped Minuteman missiles.[3, p. 37]

Summary

In summary, the W67/Mark 17 began life as the Mark 12 heavy RV, which was then spun off as the Mark 17. During that time the warhead was considered for both the Minuteman and Poseidon systems before the US Navy withdrew from the program. The warhead was then cancelled by the US Air Force for budgetary reasons in preference of the W62/Mark 12.

The W67/Mark 17 warhead weighed approximately 410 kg (900 lb) with a bare warhead weight of 306 kg (675 lb), and had a yield of somewhere between 1.5 and 3 Mt.

Bibliography

[1]          Chuck Hansen, Swords of Armageddon, vol. VI, 7 vols. 2007.

[2]          Daniel Ruchonnet, ‘MIRV: A Brief History of Minuteman and Multiple Reentry Vehicles’, Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States), COVD-1571, Feb. 1976. [Online]. Available: https://commons.wikimedia.org/wiki/File:1976-MIRV-A-Brief-History-Minuteman.pdf

[3]          Bernard C Nalty, ‘USAF Ballistic Missile Programs 1964-1966’, USAF Historical Division Liason Office, Mar. 1967. Accessed: Aug. 12, 2022. [Online]. Available: https://nsarchive2.gwu.edu/nukevault/ebb249/doc04.pdf

[4]          T. M. Josserand, ‘R&A for UUR_Weapon_History_Phases_20170206.’, Sandia National Lab. (SNL-NM), Albuquerque, NM (United States), SAND2017-2818C, Mar. 2017. Accessed: Jul. 25, 2021. [Online]. Available: https://www.osti.gov/biblio/1429158-uur_weapon_history_phases_20170206

[5]          Bernard C Nalty, ‘USAF Ballistic Missie Programs – 1967-1968’, Office of Air Force History, Sep. 1969. Accessed: Dec. 23, 2021. [Online]. Available: https://www.alternatewars.com/WW3/WW3_Documents/USAF/USAF_BM_1967-68.pdf

[6]          Chuck Hansen, Swords of Armageddon, vol. VII, 7 vols. 2007.

[7]          Director Central Intelligence, ‘DCI Briefing to Joint Chiefs of Staff’, 117940, Jul. 1963. [Online]. Available: https://commons.wikimedia.org/wiki/File:JCS_briefing_(July_30,_1963).pdf

[8]          F A Ross, ‘Preliminary Test Proposal on QEST for XW67’, Sandia Lab., Albuquerque, N. Mex., SC-DR-67-0412; ALSNL199800001620, May 1967. Accessed: Feb. 14, 2022. [Online]. Available: https://www.osti.gov/opennet/detail?osti-id=16340997