The Super-fuze and the U.S. Nuclear Forces

While the United States and Russia/Soviet Union have signed several agreements that limit the number of nuclear weapons in each nation's stockpile, recent modernization of the U.S. nuclear forces have increased the killing power of the existing American ballistic missile forces.  A recent analysis by Hans Kristensen, Matthew McKinzie and Theodore Postol on the Bulletin of the Atomic Scientists website looks at how this new technology is undermining the concept of strategic nuclear stability between the United States and Russia.     

Since 2009, a device called the "super-fuze" (officially known as the MC7400 AF&F system) has been incorporated into the U.S. Navy's W76-1/Mk4A warhead used by America's submarine-borne nuclear forces.  The adoption of the super-fuze was part of a Life Extension Program (LEP) designed to extend the life of the W76-1/Mk4A warheads to roughly 60 years from 20 years.  Before the adaptation of this new technology, it was quite likely that the most accurate ballistic missile warheads might not detonate close enough to their targets to destroy hardened targets.  The new super-fuze is designed to detonate above and around the target, resulting in a much higher destroy-ratio.  What this means is that the U.S. submarine force has a much greater possibility of destroying hardened targets like Russian ICMB silos with far fewer missiles than it did in the past.  The authors of the study note that 10 years ago, only 20 percent of nuclear warheads carried by U.S. submarines had the super-fuze technology; they now estimate that all of the nuclear warheads carried by U.S. submarines in both the Atlantic and Pacific Oceans now have the capability to destroy hardened targets as shown on this graph:

How does the super-fuze work?  Let's start with a picture of the super-fuze:

The super-fuze is also known as the arming, fuzing and firing (AF&F) system which consists of a fuze, an arming subsystem which includes radar, a firing subsystem and a thermal battery to power the entire system (i.e. the detonator to laypersons).  The arming, fuzing and firing system is located in tip of the cone-shaped renters body above the nuclear explosive package.     

Back in 1994, the Defense and Energy Departments studied equipping the W76 warhead with a new fuze.  At the time, the W76/Mk4 warheads were equipped with a fixed height-of-burst fuze; this meant that the fuze could not adjust its detonation at a more optimal location if it were falling long or short of a target.  As a result, most submarine-based nuclear weapons were aimed at soft targets like military bases.  The Defense/Energy study found that a Mk4A equipped with a fuze that had an adjustable height-of-burst as it arrived at its target would result in far greater damage to hardened targets.  This means that a smaller number of Mk4 nuclear warheads could do far greater damage to Russia's military infrastructure than those equipped with the traditional fuze.  

Let's look at an example.  First, let's define the circular error probable (CEP), the technical name for a weapon's miss distance.  The CEP is defined as the radius of a circle around the aim point within which half of the warheads aimed at a target are expected to impact.  For the study, the authors assumed a 100-kiloton nuclear blast which generates 10,000 pounds per square inch or more of blast pressure and a circular error probable (CEP) of 100 metres, a miss distance that is comparable to the CEP of the Trident II sea-launched ballistic missile.  Here is a diagram showing the detonation spread for a conventional ballistic missile fuze with the horizontal axis showing the ground circular error probable and the dome-shaped volume showing the "lethal volume" within which a 100 kiloton nuclear explosion will general 10,000 psi or more of blast pressure on the ground:

As you can see, missiles with the traditional fixed height-of-burst fuze can easily overshoot or undershoot the "lethal volume", making them less effective.

The super-fuze is designed to measure its altitude before it arrives near the target at an altitude of between 60 to 80 kilometres above the surface of the earth.  If the warhead's altitude as measured by the super-fuze at a given time during its flight is exactly equal to the altitude expected for the intended trajectory, the missile is on target.  If the altitude is lower than expected, the warhead would likely hit short of its intended target and vice versa.  The super-fuze can shift the down-range aim point, resulting in a very high percentage of missiles that detonate in the "lethal volume", resulting in a much higher destroy-ratio.  

Here is a graph showing the probability that warheads will detonate close enough to a ground-target that is hardened to withstand 10,000 psi of blast overpressure for both the conventional fuze and the super-fuze:

The probability of destroying a Russian hardened missile silo with a super-fuze equipped ballistic missile is roughly 86 percent compared to 50 percent for a traditional fuze.  The 86 percent probability is similar to the probability that three traditionally fuzed ballistic missiles would destroy the same target.  This means that the super-fuze essentially triples the killing force of the W76/Mk4A warhead.  For a target that can only withstand 2000 pounds of blast overpressure, the probability of a kill rises to 99 percent compared to 83 percent for three warheads.

How would this play out during wartime?  Currently, there are approximately 890 nuclear warheads on U.S. ballistic missile submarines of which 506 are of the W-76-1/Mk4A type.   Russia has 136 silo-based ICBMs; this means that all Russian ICBMs could be destroyed by using 272 warheads (two warheads to each silo).  This would consume only 46 percent of the inventory of W76-1 warheads, leaving a total of 234 free to target other Russian installations.  

Let's close this posting with a quote from the article:

"The capability upgrade has happened outside the attention of most government officials, who have been preoccupied with reducing nuclear warhead numbers.  The result is a nuclear arsenal that is being transformed into a force that has the unambiguous characteristics of being optimized for surprise attacks against Russia and for fighting and wining nuclear wars...We estimate that the realist sof arms reductions with the increase in U.S. nuclear capacity means that the U.S. military can now destroy all of Russia's ICBM silos using only about 20 percent of the warhead deployed on U.S. land- and sea-based ballistic missiles.

We cannot foresee a situation in which a competent and properly informed U.S. president would order a surprise first strike against Russia or China.  But, our conclusion makes the increase se-base offensive and defensive capabilities we have described seem all the more bizarre as a strategy for reducing the changes of nuclear war with either Russia or China..." (my bold)

It's certainly starting to feel like the Cold War all over again.