The Robust Nuclear Earth Penetrator is a proposed but (as far as is known) undesigned and unbuilt weapon, funded by the USAF Advanced Research Laboratory. Its primary purpose, as its name suggests, is to penetrate very deeply housed targets. The concept became one of great military interest when the fear of Iraq having Weapons of Mass Destruction in bunkers which might have been housed under a hundred feet of sand made all current weapons, including bunker-busting bombs obsolete. Even the MOAB and Daisy Cutter bombs are surface-blast weapons, and
not designed to penetrate so deeply as to destroy targets which are subterranean.
So research was undertaken to determine basically two things.
First, is it possible to build a rocket/bomb/other projectile which is capable of penetrating at least some of the way to the target? It is assumed that the blast alone cannot be enough to affect a target that is, say, 200 feet (61 m) under sand. The question becomes even more difficult to answer when the subject is not sand, but concrete. What sort of projectile would it take to penetrate 50 feet (15 m) of concrete? 100 feet (30 m)? 200 feet (60 m)? The engineering challenge is quite extreme.
Second, what sort of blast is required to entirely destroy said installation, provided our projectile is capable of giving us the mandatory 50 to 100 feet (15 to 30 m) of penetration? Shaped nuclear charges are possible, but what shape is necessary? What tonnage of explosive is actually necessary to destroy a target that has been penetrated?
Research challenges
Penetrating through 200 feet (60 m) of sand, 50 feet (15 m) of concrete, et cetera
Penetration by explosive force
Concrete design hasn't changed much in 60 years. In fact, the majority of protected concrete
structures in the US Military are derived from standards set forth in Fundamentals of Protective Design, published in 1946 (US Army Corps of Engineers). Various augmentations, such as glass, fibers, and rebar, have made concrete less vulnerable, but still nowhere near impenetrable. Moore [1] was able to create a "human sized hole" in 18 inch thick (45 cm) thick reinforced concrete with a mere 20 lb (9 kg) of explosive and a boltcutter. He performed this task in less than 48 seconds.
Additionally, when explosive force is applied to concrete, three major fracture regions are usually formed. The initial crater, a crushed aggregate surrounding the crater, and "scabbing" on the opposite side of the crater. Scabbing is the violent separation of a mass of material from the opposite face of a plate or slab subjected to an impact or impulsive loading. This is sometimes called Spalling.
The crater volume appears to vary approximately inversely with the square root of the concrete's compressive strength. That is to say, increasing the compressive strength of the concrete by 50% may yield a 25% smaller crater.
As the compressive wave propagates to the opposite side of the concrete and is reflected, fracture occurs, and we are left with scabbing on the interior wall.
What this means is that there is an asymptotic relationship between the strength of the concrete, and the damage that will be done between the crater, aggregate, and scabbing. One can never create a concrete which is impervious to penetration by explosive. The nature of concrete as a hard surface means that it it will suffer extreme damage when subjected to shockwave.
Penetration with a purpose-built hardened penetrator
This is a little trickier. If we were to drop a frictionless marble from an A-10 Warthog
flying at 40,000 feet (12,000 m), it would be travelling at 1,600 feet per second (500 m/s) at the end of its journey. Due to the fact that our marble is frictionless, it would then stop moving entirely once it hit the ground,
and make no noise nor dent in whatever it hit.
However, if we consider instead a GBU-12 laser-guided bomb, dropping
at 1,600 feet per second (500 m/s), the story is a little different.
Let us draw another fictitious situation. Let us say that our GBU-12 is able
to reach the same speed of 488 m/s. Let us also say that it weighs only 500 lb (227 kg), when that is in fact just the weight of its explosive payload. The energy released by our fictional GBU-12 simply striking the ground at that velocity is 27,029,344 joules, or 27 megajoules. This can be represented in heat as 25,641 BTU. That is to say, enough energy to heat a US gallon
of water (3.8 L) to 3,000 °F (1,647 °C).
In this very harsh environment, a projectile must not only not deform and melt, this may be possible with tungsten, which has a melting point of 3422 °C (6,192 °F), but it must continue to travel through the material protecting the target. In the case of concrete and sand, little research exists on the friction encountered by a projectile travelling through either. One can
imagine, however, that a projectile travelling through concrete would be subject to very high friction, and even tungsten would reach a point of deformation and even liquification.
Despite this seeming inevitable outcome of tungsten/concrete interaction, penetration research at Sandia National Labs and Eglin Air Force Base has found that there is a sort of "sweet spot" when penetrating, in particular, concrete.
If our projectile were to reach a speed of 10,000 feet per second (3,000 m/s) or approximately mach 9, it would simply vaporize when it hit the ground. If our projectile hits the ground at 1,600 ft/s (500 m/s), as described above, not much happens. You may get 15 feet (5 m) of penetration. Both ineffective.
However, if you get your projectile to 4,000 ft/s (1,200 m/s) or mach 3.5, as the projectile impacts with the concrete, the concrete begins to liquefy. Assuming a traditional ogive penetrator, very little damage is actually done to the projectile. The liquefied concrete actually damages the side of the penetrator some, and the rear, but it arrives essentially intact at the end of its journey. Penetrations of 100 and 150 feet (3 and 5 m) of concrete are possible with this sort of approach.
Of course, similar assumptions can be made about other substances, and the liquefaction of solids is well understood.
Much of this research is classified, and there are no real available sources outside of informal conversations with those involved in said research.
Outright destruction through light penetration and explosion
With light penetration, say, to a depth of 15 to 30 meters, the detonation of a nuclear weapon would transmit shockwaves very deeply into the Earth, which may possibly destroy deeply buried targets. This effect of course depends on the depth of the target, and of the yield of the explosion. Current discussions range from 0.3 to 340 kilotons.
Fallout and fallout containment
Nuclear weapons are perhaps most famous for their byproduct: nuclear fallout. Scientists in favor of an RNEP have stated that if a target is penetrated deeply enough, say 200 feet (60 m), and the warhead is sufficiently low-yield, and that it is designed to produce very little radiation, that the explosion occurring underground would be sufficient to contain the fallout.
The Union of Concerned Scientists points out that at the Nevada Test Site, the depth required to contain fallout from a nuclear test was between
100 and 500 meters. It is improbably that any type of bomb or missile could be
capable of penetrating so deeply.
Present weapons are capable of penetrating no greater than 30 meters. They
have been used effectively in war, but against targets which are closer to
the surface (such as missile silos), but not targets which are deeply buried.
Political ramifications
Assuming the RNEP is a technologically feasible concept, there are
political problems.
It is a violation of the nuclear test ban treaty to test nuclear weapons on
the Earth or in space. This has not stopped nations such as India and
Pakistan (who are non-signitaries) in recent times, however.
It is a violation of the nuclear non-proliferation treaty to develop new
nuclear weapons. Testing of nuclear stockpiles is permitted. Weapons are
allowed to get to "just before critical" to ensure that they are still safe to
use. There is some debate about this in that some construe this as research,
but that is beyond the scope of this article.
The United States has been attempting to circumvent this treaty by
modifying an existing weapon, the B61.
The "Can I use it?" factor
Presently, there are generally two types of nuclear weapons. There are the
multi-megaton "city busters", which are designed to do massive
damage to civilian populations. There are also weapons of smaller tonnage (in
the low kiloton range) which are designed to eliminate an enemy's military in
a first-strike scenario (see game theory, nuclear weapons design, and
mutually assured destruction for more on the science of conducting a
nuclear war). Both of these weapons are very unattractive
for small theatre warfare. If one were, for example, to conduct a conventional
war in the area of Iraq, the use of "city busters" would be globally
unpopular, and entirely unnecessary. Additionally, the existence of units like
the GBU-12 mean that war can be fought with conventional weapons very
effectively, rendering the "nuclear question" entirely moot.
However, if weapons are developed, which are low-radiation, and low-yield (say
in the sub kiloton range), the decision to deploy them becomes much less
difficult. Suddenly, the question is not whether to leave an area
irradiated for generations, to kill hundreds or thousands of
people, and have a giant mushroom cloud. Instead, we have a
subsidence crater and some seismic activity. If a target were deemed
important enough, somebody in the chain of command may feel that the use of a
nuclear weapon is justified.
The ramifications of this are ugly. Many people see the usage of a nuclear
weapon as a boolean decision. You either used a nuke, or you didn't. It
doesn't matter that it was a "really little" nuke, and left nothing but a
crater, and did not make the area uninhabitable. The use of nuclear weapons by
any nation, for any purpose may provoke another nation to retaliate.
This is a highly plausible scenario in the middle east, where the original
planning for the RNEP came about. The need to penetrate a deeply
protected target in a desert country.
Political conclusions
The use or creation of an RNEP (again with the caveat that one is feasible to
build with contemporary engineering) is not only a violation of several
treaties, it is also possibly a preclude to a theater nuclear war. For the
latter reason alone, the RNEP is probably best left on the shelf as an
interesting idea.
Sources used in this article
- Penetration Resistance of Concrete: A Review, James R. Clifton, for the Physical Security and Stockpile Directorate, Defense Nuclear Agency. January 1992.
Sources which were unobtainable at the Library of Congress but may be useful
- Moore, R. T.: Barrier penetration tests. National Bureau of Standards.
- ISBN 0415701996: Nuclear Warfare, World Politics - 21st century US Military Policy
- ISBN 1594542031: Nuclear Weapon initiatives: Low-yield R&D, Advanced Concepts, Earth Penetrators, Test Readiness
See also
