Air Force's New Skipping, Bouncing Bombs

By David Hambling
February 04, 2008 2:38:00 PM
Categories: Ammo and Munitions, Bizarro
Courtesy Of: Wired.com

If you want to break into a house, then the smart approach is not to shoulder-charge the thickest wall. It's much easier to kick down the door. The same goes for attacking bunkers: taking a sledgehammer approach to smashing through to them is not as smart as going through the main entrance.

My article in this month's Defense Technology International describes the Air Force's new tools for "kicking down the door" and attacking tunnel entrances. The big challenge is that the weapon needs to be going horizontally rather than vertically. The answer is a technique that was used in WWII – skip bombing.

The bomb is skimmed across the ground like a stone skipping on water. Although this was most notably done using Barnes Wallis' famous Upkeep "'bouncing bombs" to attack German dams, but a smaller version called Highball was developed to hit ground targets such as tunnels.

The same tactic was adopted by the US aviators in the Pacific using standard bombs:

Originally assigned to Australia as General Kenney's aide, Major Benn approached Kenney early in August with the idea of low-level skip bombing, a radical departure from the traditional Air Force doctrine of high-altitude attack. The R.A.F. had used skip bombing successfully and the U.S. Armament School at Elgin Air Field had tested the concept on a training level. Kenney became fascinated with the possibilities and gave Major Benn command of the B-17s of his 63rd Squadron to experiment with the method on a sand bar, and later on a wrecked ship in Port Moresby's harbor.

Using this approach required the hair-raising approach of flying into the teeth of enemy flak at low level, but it certainly proved effective:

...approaching the ship from 2,000 ft., then dropping down to an altitude of 200 to 250 ft. (maintaining the air speed of 200 to 250 m.p.h.) and releasing the bomb --equipped with a 4 to 5 second delay fuze-- 60 to 100 ft. away from the target was the way to do it. Thanks to the efforts of these men, the percentage of targets hit increased from less than 1% to 72%.

In fact the skipping technique goes back much further than WWII -- it was used by Napoleonic artillerymen to increase the lethality of cannonballs, bouncing them into enemy troop formations at head height.

Modern-day skip bombing is going to be carried out at higher speed and from higher altitude than the WWII variety, and this requires a special sort of bomb – the BLU-121/B:

Three particular features help to carry out the mission. First is the casing: a new ultra-hard type of steel called ES-1c developed at Eglin. Previously, the 'gold standard' for steels with high strength and toughness was Aermet 100. It alloys steel with nickel, chromium, molybdenum and cobalt, producing a metal that it hard but not brittle. Aermet 100 is very expensive (around $10/lb), which has limited its use. ES-1c used cheaper materials, incorporating vanadium and tungsten, and does not require a proprietary process to manufacture. Tests indicate that it should match or exceed Aermet's abilities.

The BLU-121/B also has a new thermobaric explosive filler, designated AFX-757. Crucially for this role, the explosive is highly insensitive: it will not be set off when the bomb skips on the ground or when it impacts a blast door, but only when detonated by the fuze. It produces a powerful blast of longer duration than standard explosives. This type of blast is highly effective at propagating through tunnel complexes.

Finally, the guidance software incorporates new algorithms for skip bombing. Details of exactly how accurate the weapon is are classified, but it seems to be well capable of hitting a standard portal.

And if one bouncing bomb is not enough to break the door down, my article in DTI goes on to describe another twist: new guidance software whith will allow the Air Force to hit the target with six skipping bombs at the same time: "The desired end result is to cause the multiple warheads to act as one large explosive charge by detonating the independent, closely spaced warheads simultaneously." That should do the trick...