Attacking hardened and deeply buried target is one of the Air Forces biggest challenges. They are meeting this challenge with a devastating new approach: a focused underground shockwave that amounts to an artificial earthquake.

At present the kinetic approach  a.k.a. brute force  is favored; the most powerful weapon in the inventory is the BLU-113, a 4,600 lb weapon with a thick steel casing capable of piercing 22 feet of concrete -- or 100 feet of dirt -- before exploding. There are plans to go even bigger, with a monster 30,000 lb Massive Ordnance Penetrator which would take the maximum depth to 60 feet. Thats about as big as you can carry on a plane.



Ive described Deep Digger here previously. Unlike earlier weapons this is an active penetrator, a bomb that actually burrows into the ground by drilling a shaft with volleys from seven cannon. In a demonstration last year a Deep Digger prototype penetrated more than 30 feet of limestone. The makers were tight-lipped about how much further it could go.

This presentation from David Burns of the Medium Caliber Weapons Systems Branch of ARDEC reveals much more about the weapon than previously released. In particular, it is described as being able to dig down to 150 feet. Thats impressive on its own, but the Concept Of Operations in Slide 4 is staggering: an array of 20 Deep Diggers would be detonated together to produce a shockwave which will collapse all underground structures to a depth of 300 feet over a 200-yard square area.

Compare this to this description of the B61-11, the only bunker-busting nuclear bomb in the arsenal:

For a penetration depth of three meters and a yield of 0.3 kilotons, the B61-11 could destroy a target buried under roughly 15 meters [= 50 feet] of hard rock or concrete. For the same penetration depth and the maximum yield of 340 kilotons, the destruction depth would be roughly 70 meters [ =210 feet ] for a hardened target.

In other words, the Deep Digger array is more effective than a 340-kiloton nuclear weapon optimised to attack underground targets.

The secret is in effectively combining 20 separate explosions into a coherent pulse. This area has been researched for many years, in particular in the 90s under the name of ACE, for Array of Conventional Explosives. It takes a phenomenal amount of computing power to calculate the non-linear effects of multiple explosions combining in a three-dimensional volume (which may not be homogenous), and new software tools were developed for the job. In addition, real-world testing is needed to validate these models  hence exercises like the notorious Divine Strake which involves a underground explosion of 700 tons of explosives.

The last I heard the Array Of Conventional Explosives had been axed, in favor of simpler and more straightforward approaches, but Deep Digger has some key advantages over earlier weapons that make it more suitable:

- Deep penetration means that all of the blast goes into creating an underground shockwave, not just digging a crater. For blasting rock, its basic that the charge need to be drilled to a depth to be effective.

- Deep Digger parachutes down to a soft landing before digging in. Other bunker busters hit the ground very hard and experience a deceleration tens of thousands of g's. This affects their reliability, and the loss of a few warheads may make the whole array ineffective.

- Deep Digger may be able to maneuver underground, correcting the configuration of the array after it is in place.

And Deep Digger is only a first-generation active penetrator. Devices like General Dynamics Worm which Noah described last week may burrow much more effectively.

Or course, bunkers can always be dug deeper. One British Cold War plan involved relocating government centres to coal mines 5,000 feet underground. However, given that the Deep Digger array can collapse the entrance tunnels to a depth of 300 feet, any such deep bunker may become a tomb from which the occupants will never escape.

More importantly, such an array would make the vast majority of existing bunkers obsolete, or at any rate insecure. This would include nuclear facilities such as missile siloes.

((Of course the idea is not new. In WWII Barnes-Wallis used the shockwaves from Grand Slam bombs to bring down German railway viaducts when standard bombs proved ineffective. A Deep Digger array might also be used to cause the simultaneous collapse of blocks of skyscrapers, or dams or other large structures.))

Ive been advised that the Deep Digger program is undergoing a security review. If past experience is anything to go by, this means you wont be hearing any more news updates on the program.

Meanwhile, DARPA have started their Strategically Hardened Facility Defeat (SHFD) program which :

...seeks to leverage recent advances in non-nuclear earth-penetrating technologies for the defeat of strategically hardened targets. System and technology areas to be developed in this program include: new penetration technologies, robust self-contained aerial deployment options, sensing and navigation subsystems

This sounds like someone is going to start throwing a heap of money at something very much like Deep Digger. In the near future, digging yourself into a hole is not going to offer any protection -- and that could change a lot of things.

-- David Hambling