The Future of the US Strategic Arsenal: Into the Fourth Generation?

Looking back over the last 60 years, it becomes apparent that the nuclear standoff between East and West was in large part responsible for the lack of great-power conflict.  The fact that this occurred in the face of an incredible increase in the lethality and mobility of conventional forces is historically astounding.  Since the end of the Cold War, the United States has kept its nuclear strategy more or less as-is, but it is possible that challenges posed by the changing global political situation and new technologies either recently developed or only now cresting the horizon of practicality will necessitate a major change to the makeup the US strategic deterrent.

The Bear and the Eagle

US nuclear strategy is based on the idea of using the American nuclear arsenal in a massive counterattack against an opponent who has initiated a nuclear (or possibly biological or chemical) attack upon the US or its allies.  It has traditionally been a primarily “counterforce” strategy which directly targets the military assets (nuclear and conventional) of the enemy, prioritizing those which have the potential to strike US and allied territory.  The strategic forces dictated by this doctrine are made up primarily of high-yield (> 100 kilotons) nuclear devices delivered by ballistic missiles and/or manned bombers.  The nuclear devices used by the US are thus generally two-stage thermonuclear devices which utilize the X-rays produced by an implosion-type first stage to initiate the implosive compression of a small amount of tritium and deuterium in an adjacent second stage.

In the coming century the US will face a more complex environment than the bipolar political order of the Cold War.  This may very well necessitate a more flexible nuclear deterrent.  It is at least as likely that we will find ourselves in a standoff with Iran or North Korea as Russia or China.  What technologies and doctrinal changes might American strategic planners take advantage of to ensure the continued security of the United States’ national interests?

Cutaway of modern US fusion warhead.

One revolutionary development that has already changed the face of conventional warfare is the increasing prominence of Precision Guided Munitions (PGM’s).  The guidance systems made possible by advances in computer and sensor technology increase the accuracy of modern weapons to a point where they can strike within meters of their targets.  Combining this with satellite reconnaissance so cheap it is becoming commercially available invites us to consider greatly increasing the accuracy of our strategic deterrent.  Increased accuracy of nuclear weapons delivery systems implies that devices with lower yields can destroy the same target.  A heavily reinforced missile-silo door which would have required hundreds of kilotons of yield to ensure destruction when weapons were accurate only to within hundreds of meters can be vaporized with only a few kilotons if an accuracy of several meters can be achieved.

Given the near real-time nature of much modern reconnaissance data, we can consider the possibility that targets like mobile ICBM Transporter-Erector-Launchers (TELs), such as those used by Russia, could be destroyed by ballistic missiles carrying nuclear weapons of very low yield, or perhaps even using only kinetic energy.  Multiple smaller warheads could be used to destroy dispersed targets like airfields and naval yards. Since destructive power scales with the 2/3-power of weapon yield (to make a bomb twice as destructive, it must be 3 times as powerful), using a greater number of smaller warheads can increase the destructive potential carried by a given missile or bomber. 

An advantage of smaller-yield (< 20 kt) warheads is that they can use simple, well understood pure fission implosion-based designs, obviating the need for the US to embark on a new nuclear test program before bringing them online.  Deployment of such weapons in the strategic arsenal would allow the DOE to save funds that are currently used to support complex infrastructure needed to maintain more complicated thermonuclear devices.  Another advantage is the possible decrease in collateral damage that could be realized through the use of more precise delivery systems, although this might prove a double-edged sword, as small fission weapons typically produce more fallout than thermonuclear devices.  Finally, having many lower-yield weapons available increases the flexibility possible in strategic planning by allowing a proportional response to smaller WMD threats for which much larger weapons would be “overkill.”  An example of such a situation would be the deployment of a small number of ballistic missile launchers against the US or allied nations.                                                                                                                                           

Transporter Erector Launcher (TEL) for Russian Topol M ICBM.

Another possible way to realize the advantages of using smaller-yield devices would be through the development of what are sometimes termed “fourth generation” nuclear weapons.  They derive their appellation from a schema which labels as the first generation of nuclear weapons the atomic bombs of the 1940’s, as the second generation the first hydrogen bombs in the 1950’s, and as the third generation those ideas that, like the neutron bomb, came of age in the 1970’s.  As for the hypothetical fourth generation:  

In a nutshell, the defining characteristic of fourth generation nuclear weapons is the triggering – by some advanced technology such as a superlaser, magnetic compression, nuclear isomers, antimatter, etc. – of a relatively small thermonuclear explosion in which a deuterium-tritium mixture is burnt in a device whose weight and size are not much larger than a few kilograms and liters.  – Andre Gsponer 

Using the experimental results of the National Ignition Facility (NIF) and similar apparatus, it is thought that it will be possible to build nuclear devices of very low yield (< 100 tons) that derive all of their explosive power from fusion (Gsponer 7).  This is not a near term prospect, but it often pays dividends for the strategic planner to consider the long term.

Should such weapons prove practical, the strategic picture could change to a significant degree.  First, due to their low yield and very small release of radioactive contaminants, such weapons would blur the line between conventional and strategic weaponry.  Indeed, it has been estimated that the radiobiological effects of the use of 400 tons of depleted-uranium anti-armor weapons in Iraq was the equivalent of using 600 kt worth of fourth generation nuclear weapons, the effects of which are primarily due to the release of unspent tritium and neutron interactions with the terrain.

Fourth-generation weapons would come with their own strategic limitations, of course.  In the case of antiproton-initiated fusion devices, one must either construct a dedicated industrial infrastructure to either manufacture the microscopic quantities of antimatter necessary or harvest it from Jupiter’s magnetosphere with magnetic-scoop equipped spacecraft.  Another difficulty is that it may not be easy to get their yield above a few hundred tons, relegating them to more of a tactical than a strategic role (though they may retain their usefulness for situations which do not require very great yield, and in which prevention of collateral damage is a priority).  Even if it is possible to construct such weapons, whether their production proves worthy of their cost will likely remain an open question for some time to come.

There’s antimatter in them thar’ orbits!

As far as the general idea of using smaller-yield warheads is concerned, it suffers from some problems of its own.  The small nuclear PGM’s assumed throughout the above discussion are highly reliant on good, up-to-date targeting data; micrometric accuracy is only good if the weapon is aimed at the right coordinates.  Attacking American satellites at the outset of a strategic standoff could have a detrimental effect on targeting abilities or post-attack damage-assessment, or on the weapons themselves if their guidance systems are dependent on GPS.  In this vein, it is worth noting that both Russia and China either have or are developing anti-satellite weapons.

Sadly, arms-restrictions treaties mean that we have no land-based ICBMs capable of carrying more than three warheads and our submarine-launched ballistic missiles (SLBMs) are limited to carrying four warheads.  They also set (dangerously) low caps on the total numbers of warheads that can be deployed by the US and Russia.  This implies that a move towards the use of a greater number of smaller warheads will require either the development of new delivery systems, a move to a much heavier reliance on bomber aircraft, or both; not to mention the negotiation of new strategic arms agreements or our withdrawal from the current ones.  Also, dealing with extremely hardened targets such as deep, subterranean facilities like those in the former Soviet Union and targets for which there is inadequate targeting data will still require high-yield weapons and it would be prudent to keep such weapons in our arsenal in significant numbers, even if only as a hedge against unexpected future developments.

Sources

Gsponer, Andre, and Jean-Pierre Hurni. “Antimatter Induced Fusion and Thermonuclear Explosions.” Independent Journal on Energy Systems and Radiation 49 (1987): 198-203. Web. 16 Dec. 2009. <http://arxiv.org/pdf/physics/0507125&gt;.

Gsponer, Andre, Jean-Pierre Hurni, and Bruno Vitale. “A Comparison of Delayed Radiobiological Effects of Depleted-Uranium Munitions Versus Fourth-Generation Nuclear Weapons.” Proc. of YUNSC-2002, Belgrade, Yugoslavia. 1 Dec. 2002. Web. 16 Dec. 2009. <http://arxiv.org/pdf/physics/0210071&gt;.

Gsponer, Andre. “Nanotechnology and Fourth Generation Nuclear Weapons.” Disarmament Diplomacy 67 (2002): 3-6. Web. 14 Dec. 2009. <http://arxiv.org/pdf/physics/0509205&gt;.

Younger, Stephen M. “Nuclear Weapons in the Twenty-First Century.” Proc. of LAUR-00-2850, Los Alamos, New Mexico. 27 June 2000. Web. 15 Dec. 2009. <http://fas.org/sgp/othergov/doe/lanl/la-ur-00-2850.pdf&gt;.

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