The dropping of the Massive Ordnance Air Blast Bomb (MOAB) in Afghanistan a few weeks ago reportedly killed 94 terrorists. It also generated a flurry of articles breathlessly dissecting the capabilities of the mighty MOAB and hailing the absolute destruction it visited upon ISIS fighters hunkered down in caves and tunnels. But as preliminary satellite imagery, photos, and actual video start to come in documenting the damage wreaked by the MOAB, it is clear that much of the initial reporting on the damage it inflicted is contradictory and uninformed. Some reporters even seem to be under the impression that we dropped something akin to the Massive Ordnance Penetrator (MOP). While there is little doubt that the MOAB, the GBU-43/B, is absolutely lethal to anything on the surface, such certainty when it comes to caves and tunnels is not warranted.
Before looking at what the single use of the MOAB accomplished, let’s quickly address some of the wild claims made in the media regarding the MOAB. First, the MOAB under no circumstances will generate a lethal blast radius anywhere near a mile. The MOAB will not puncture ear drums at five miles. The MOAB will not produce a crater 300 yards in diameter. These are a few things the MOAB will not do. But what was the MOAB designed to do?
Unlike our general-purpose bombs, which are designed to inflict damage via both the initial blast and the fragments produced from their heavy steel bomb casings, the MOAB’s raison d’être is to produce intense pressure. Indeed, assuming level terrain, an observer or structure 300 feet from a MOAB detonation would experience approximately 20 pounds per square inch (psi) of pressure (reflected overpressure). That may not seem like much given that car tires are typically inflated to pressures of 30 to 35 psi — and a woman in stiletto high heels can strike the ground with 2,000 psi, about 100 times greater than the MOAB blast’s overpressure.
But expert, in-depth analysis reveals some nuanced differences between high heels and the MOAB, which gives the 21,700-pound bomb a decisive edge. Whereas high heels generate their 2,000 psi over a tiny area of about one-sixteenth of a square inch, at 300 feet from ground zero, the MOAB’s hemispherical blast wave generates its 20 psi of reflective overpressure over a massive surface area greater than 100 million square inches, while simultaneously producing winds that will briefly reach over 500 miles per hour. This is enough to destroy or damage even heavily built concrete structures. So, while the pain of having your foot stepped on by a pair of stilettos is not to be underestimated, when everything is taken into consideration we find that, for most military scenarios, the MOAB is actually more dangerous than even the most lethal of high heels.
Providing some further perspective, the MOAB is about 30 times more powerful than the improvised bomb that nearly sunk the 9,000-ton Arleigh Burke-class destroyer, the USS Cole, in the port of Aden in 2000. It is about the equivalent of the total destructive power (kinetic plus explosive energy) of ten 16-inch broadsides of high-capacity rounds fired from an Iowa-class battleship. Indeed, the explosive yield is in the ballpark of the least powerful nuclear weapon ever deployed: the Davy Crockett recoilless rifle that fired a Mark 54 nuclear device with a yield of ten or 20 tons of TNT. The MOAB does not deliver the lethal radiation of the Mark 54 nuclear device, but, beyond a doubt, it is an incredibly powerful conventional bomb.
But as powerful as it is, the lethal blast radius for the MOAB is nowhere near the mile reported by some news media. Instead, at about one-quarter of a mile away, assuming flat terrain, the MOAB will collapse most residential structures and injure many, some fatally. At a half mile, the explosion would still be deafening and some windows might shatter, but injuries would be light and infrequent.
The reason the lethal blast radius is not larger is that the explosive force experienced by an object is in the inverse ratio of the cube of the distance from the point of explosion (ground zero). Translated into English, the destructive power of an explosion diminishes very, very, very rapidly with distance in open space. Conversely, significantly expanding the destructive radius of an explosion requires copious amounts of explosives. To illustrate this: To create an explosion capable of doing the kind of damage at a full mile equal to what a MOAB can do at one-quarter of a mile, you would need about 1.2 million pounds of TNT.
Of course, a conventional weapon weighing 1.2 million pounds is neither practical nor desirable. And in designing the MOAB, the Air Force was eminently practical. The designers’ goal was to produce the most intense non-nuclear airborne blast possible for a given weight and size — while still being deliverable by plane — at a reasonable cost and with precision-guided accuracy.
In designing the MOAB, the Air Force was eminently practical.
To keep weight down, as well as maximize the force of the blast, the MOAB was designed with a very thin aluminum bomb casing. The thin, light, aluminum casing allows for a bomb that is 85 percent explosive by weight. In contrast, general-purpose bombs typically contain less than 50 percent of explosives by weight. The thin, relatively weak casing also means that the vast majority of the explosive energy (about 85 to 90 percent) goes into producing a blast wave. In contrast, our general-purpose bombs use about 40 percent of their explosive energy to fragment their casing, with 60 percent left over to produce the blast wave.
Its long, cylindrical shape strongly suggests that the blast wave produced by the MOAB is primarily designed to scour the surface of targets rather than drive a lot of energy into the surface to create the ground shock necessary to attack buried targets. And its thin, aluminum casing, along with its name, tell us the MOAB is not designed to be a penetrator. So, any talk of the MOAB plunging 100 feet into rocky soil before detonating is inaccurate. However, even though most of the MOAB’s blast energy will be delivered outward, not downward, it is so powerful that a relatively shallow tunnel could be collapsed by the MOAB if it is detonated on the surface or very near to the surface. The superiority of surface blasts to air blasts in producing ground shock waves (seismic) is well documented, including a comprehensive study by the U.S Nuclear Regulatory Commission:
Air bursts, with the exception of nuclear explosions, generate moderate to low amounts of ground shock because of limited coupling with the ground. Surface bursts have a high severity of ground shock due to the potential for a significant amount of coupling between the explosive charge and ground. Buried charges can generate the most severe amounts of ground shock.
Clearly then, the MOAB, which was not designed to be a penetrating-type bomb nor optimized to produce ground shock, is not the ideal weapon for crushing tunnels. But if detonated very near a vulnerable tunnel or cave with a properly situated entrance, it will send an intense blast wave down the tunnel, destroying equipment and killing personnel. And there is some preliminary evidence that suggests this may have occurred in the case of at least one tunnel entrance. In this role, it will be superior to multiple smaller bombs or even larger bombs that expend much of their energy generating fragments. So, the MOAB is devastating to surface targets. And it can also be used to attack caves and tunnels, but with far less probability of success. In other words, there are a lot of caveats surrounding a MOAB’s effectiveness in attacking caves and tunnel systems.
Developing a large thermobaric bomb could also have been a possibility. But because a thermobaric bomb relies on atmospheric oxygen, it cannot be used underwater and it also loses a significant percentage of its power at high altitudes. And it can also be affected by adverse weather. Further, while the MOAB might not produce as much total explosive energy as a very large thermobaric bomb, at ground zero it will produce far more intense pressure. It turns out the MOAB is more reliable and can be used in wider variety of situations.
On the other end of the spectrum from air-blast weapons is the Massive Ordnance Penetrator (MOP) — the weapon many in the media seemed to believe we deployed in Afghanistan. While the 30,000-pound MOP is 38 percent heavier than the MOAB, it only carries 28 percent as much explosive. Instead, the MOP’s weight comes from a tremendously strong steel bomb casing designed to allow the MOP to survive smashing through earth and concrete prior to detonation. According to the Air Force, the MOP can penetrate 200 feet into the earth before detonating. Consequently, the MOP is in its element when attacking hardened targets deep beneath the surface of the earth that cannot be effectively attacked by normal bombs or missiles — or even a MOAB.
The MOP is definitely impressive, but given that its effective blast wave, after bursting its casing, is only going to be around one-sixth of that of the MOAB, it will not be nearly as effective in scouring the surface of structures as the MOAB. Assuming flat, unobstructed terrain, the MOAB will destroy most residential structures within 1,000 feet of the point of detonation. At 700 feet from ground zero, most buildings will be collapsed. And at 300 feet from ground zero, even heavily built concrete structures will be severely damaged or destroyed. As a rough estimate, you can pretty much halve these numbers for the MOP. But using it as an air-burst weapon would be a serious misuse of a very expensive asset.
For more details on blast effects at distance, see the following table.
(Click Table to Enlarge.)
Assuming a MOAB was detonated right on the surface in the kind of rocky terrain making up the surface of the ISIS complex that was attacked, what kind of crater would we expect? On such terrain, the diameter of its blast crater would be in the neighborhood of 80 feet, maybe a bit less. For comparison, the Oklahoma City Bombing, estimated to be equivalent to about 4,000 pounds of TNT, produced a crater of about 30 feet. Even the 1 million pounds of TNT detonated by the U.S. Navy in the 1965 Sailor Hat exercises produced a crater of under 300 feet in diameter. Therefore, any talk of the MOAB producing craters of 300 yards (900 feet) across are way off.
In terms of shaping the battlefield, it has been posited that the MOAB’s massive explosions are so much larger than other weapons that it takes “shock and awe” to a new level. Couple that with its ability to attack enemies in locations formerly thought secure, and you have a weapon that creates FUD — Fear, Uncertainty, Doubt — in the mind of the enemy. If you can create FUD in the mind of the enemy, you are shaping the battlefield to your advantage.
Going beyond theory, preliminary analysis of satellite imagery and video footage released of the actual blast site reveals that some 38 surface structures that were part of the ISIS compound have been reduced to rubble. Trees were also shredded and torn out by the roots. Undoubtedly, anyone in those structures or who was caught in the open perished. But detonating a MOAB on uneven, rocky, mountainous terrain such as that found at the ISIS complex is not the same as detonating it over flat terrain. This reality is demonstrated by the photos taken after the explosion showing intact trees with leaves just a few hundred feet from ground zero. Apparently, the trees were protected by the terrain and did not experience winds of hundreds of miles per hour.
Also of interest is that the pictures and video that we have so far do not show any significant cratering. This strongly suggest that the MOAB was detonated far enough above the ground to be considered a pure air burst. That it did not produce significant cratering, or any crater at all, means that it is unlikely that even relatively shallow tunnels were collapsed — air bursts are mediocre at producing ground shock. This, however, does not preclude the MOAB from having damaged tunnels via vulnerable entrances. But because fighting still rages all around the area of the blast, it may take some time to obtain an independent assessment. Sadly, just three weeks ago two U.S. soldiers were killed in fighting close to where the bomb was dropped.
History has taught us that taking out tunnels and caves is extremely difficult and usually requires feet the on ground. A weapon like the MOAB does not change that fact. Short of the liberal use of tactical nukes, including deep penetrating nukes, there is no silver bullet for taking out caves and tunnel complexes. With that said, the MOAB does give us an incremental situational ability to attack enemy caves and tunnel systems. That its first use may not have destroyed every cave or tunnel near ground zero does not mean that the MOAB is not an effective weapon. In fact, in an era where one anti-air missile can cost $3 million, and the Massive Ordnance Penetrator costs over $3 million per bomb in addition to well over $200 million in development costs, the $170,000 per bomb for the MOAB verges on being a bargain. That it was designed, developed, and built in-house by the Air Force — which repurposed existing hardware and technology to keep costs and development time relatively short — is admirable. Also admirable is that the in-house development was far less costly than it would have been if the program had been outsourced to defense contractors.
As is the case for all professions, using the right tool for the job is a key to success. And the military profession is no exception. The wide variety of capabilities that our general-purpose bombs bring to the table, coupled with the specialty capabilities of the MOAB and the MOP, give our military leaders a wide range of tools. With that said, using the MOAB does not guarantee the destruction of large complexes of tunnels. And using the MOAB in contested airspace would be difficult as its delivery vehicle, a MC-130 (a C-130 variant), is very vulnerable to enemy fighters and surface-to-air missiles. Finally, we have relatively few of them, about 20, so the MOAB should only be used when there is a chance of taking out truly high value targets.
Still, assuming the unit cost is somewhere near $170,000, the MOAB, used judiciously, delivers more bang for the buck than what we have typically been getting from our defense contractors. And that is a good thing.
— Mike Fredenburg is a regular contributor to National Review Online and the founding president of the Adam Smith Institute of San Diego.
Editor’s Note: This article has been emended since its original publication.