The fusing on the W76-1/Mk4a warhead for the Trident II SLBM played an important role in the argument of my Master’s thesis.
Hans Kristensen, Matthew McKinzie and Ted Postol have an important, and richly detailed, article on the W761-1/Mk4a “super fuse” or “smart fuse,” and its strategic implications at the Bulletin of the Atomic Scientists.
Those implications, to say the least, are not pretty ones.
In sum, the arming, firing and fuzing (AF&F) programme is a part of the life extension programme for the W76 warhead, and despite statements to the contrary, life extension significantly increases the strategic nuclear capability of the United States especially with respect to Russia.
The key concept here is the single shot probability of kill.
The lethality (K) of a nuclear missile is directly proportional to the two thirds power of yield (Y) in megatons of its payload and inversely proportional to the accuracy of the missile (in terms of circular error probable squared CEP) so that
K = Y^2/3 / CEP^2
The equation for single shot probability of kill is
SSPk = 1-e^-z
Where z = K r/2H^2/3
r is the reliability of the missile and H is the hardness in pounds per square inch overpressure of the target.
Increases in warhead accuracy are far more effective in increasing counterforce capability than increases in yield.
The super fuse on the W76-1/Mk4a warhead increases the reliability of the Trident II SLBM relative to a Trident II armed with the original W76 without increasing the accuracy of the Trident II itself.
That is the AF&F’s beauty.
Because accuracy is measured in terms of CEP there is always a certain probability that a missile will over or under shot its target. The idea with the super fuse is that prior to re-entry the W76-1/Mk4a warhead will determine whether it is on a trajectory that will undershot and overshot the target beyond the radius required to knock it out, especially super hardened Russian ICBM silos (10,000 PSI overpressure).
If that determination is reached the warhead will airburst, rather than groundburst, near the target in a volume of space such that the silo will be destroyed.
This procedure increase the SSPk of the Trident II W76 SLBM to 0.86 or 86%, making it a hard target killer and so thereby significantly increasing the hard target kill capability of the United States.
Kristensen, McKinzie and Postol don’t pull their punches. This is how they put super or smart fusing
This increase in capability is astonishing—boosting the overall killing power of existing US ballistic missile forces by a factor of roughly three—and it creates exactly what one would expect to see, if a nuclear-armed state were planning to have the capacity to fight and win a nuclear war by disarming enemies with a surprise first strike
The authors discuss the Russian ground based early warning radar system, something Postol has been researching for some time now and whose analysis should be carefully read, in the context of the super fuse arguing that it has significant implications for strategic stability given that the Russian system, as compared to the US system which includes space based infrared sensing (SBIRS), leads to lower warning times.
This is an article that I cannot highly recommend enough. One should read this very carefully because its importance cannot be overstated. I don’t have much to add to it, except allow me to offer three additional points.
Firstly, the analysis largely, as far as silo based ICBM vulnerability is concerned, focuses on the SS-18 and the Topol ICBMs. The SS-18 is going to be replaced with the RS-28 Sarmat ICBM, and this missile will be wholly or partly armed, according to reports, with a hypersonic glide warhead directly in response to Ballistic Missile Defense. This is often spoken of by Moscow as the main response to BMD. If this is the case, I think the implications that super fusing has for strategic stability are worse than that discussed in the article.
Secondly, SLBM flight times, and so hence warning time, can be lowered by launching SLBMs on depressed trajectories. The analysis presented by Kristensen, McKinzie and Postol is wholly couched within existing life extension programmes. However, we know that the US is going to go beyond LEP as part of nuclear modernisation. Combining super or smart fusing with projected re-entry vehicle modernisation, of which I shall not speak here, could well mean that we end up with super fused RVs capable of being launched by SLBMs on depressed trajectories. That might also make the implications that super fusing has for strategic stability worse than that discussed in the article.
Thirdly, all this occurred whilst liberal arms controllers told us Bambi was dreaming Prague dreams. Notice how programmes such as super fusing enable the US to enter a strategic arms control process whilst at the same time preserving, indeed augmenting, its first strike counterforce capability. This means that the peace movement and the broader citizenry are asleep at the wheel, ignorant of and immune to pending nuclear dangers.
There is a certain rationality that underpins programmes such as this, and I’m not just referring to US ones either. That is what makes them especially dangerous. At the individual level in a system of world order characterised by states deterrence has a rational basis, but at the collective, human level, it is irrational.
When irrationality, witness the global financial crisis or global warming, has a rational basis to it calamity becomes a very real prospect.