TCS Daily


Working to Reduce Collateral Damage

By Ralph Kinney Bennett - March 25, 2004 12:00 AM

One of the first "feedback" responses to my recent article on terrorist car bombs went on to the effect that the United States uses bombs, too, and that we have dropped more of them than all the car bombs combined thus far in Iraq.

This gentle reader missed the point entirely. American bombs have been directed by the military at military targets purely, and although there have been some accidental misses the efforts of U.S. forces to minimize civilian casualties are unparalleled in military history.

The U.S. defense establishment's ongoing concern over collateral damage has led to work on a new generation of bombs and munitions designed to largely confine damage to the target by controlling both the blast and the effects of flying bomb fragments in the "collateral damage footprint" around the target.

This remarkable work has been underway for several years at the Lawrence Livermore National Laboratory (Berkeley, Cal.), the Air Force Research Laboratory (Eglin AFB, Fla.) and the Naval Surface Warfare Center (Dahlgren, Va.).

Continuing improvements in precision guidance now allow for the delivery of a high percentage of bombs with pinpoint accuracy. Now the armed forces want bombs that will deliver sufficient destructive force against a bunker, building or fortified position while dramatically reducing damage beyond the structure itself. This is particularly important when the enemy purposely hides in structures close to or among civilian houses and buildings.

One of the big problems with employing standard steel-cased bombs against, say an army headquarters building located in a civilian suburb, is that, while a bomb may be literally delivered through one of the building's windows and its blast largely confined to the target building, the jagged fragments of the bomb casing would be propelled at speeds as high as 11,000 feet per second and travel out from the blast center as far as 3000 feet.

While this large fragmentation footprint may be desirable in bombs directed against troops and armor on the battlefield, it is a serious problem when they are directed at targets in a populated area, putting both civilians and friendly forces at risk.

But a team at the defense labs has come up with a promising solution employing "fine tuned" explosives and bomb casings fabricated from carbon fiber. A team at Livermore headed by engineer Michael Murphy has already tested scaled down versions of such munitions.

Two major challenges faced the team:

  • Part of the explosive power of an aerial bomb stems from the tremendous build-up of gases momentarily confined within the steel case, which can expand to 1.5 times its size before it flies apart. Would a bomb without the steel casing still have desired explosive force?

  • A bomb's steel casing is a big part of the weight and structural strength that helps it penetrate walls, floors, or the steel-reinforced concrete shell of a bunker (more than half the weight of a typical "GP" general purpose bomb is the steel nose, casing and tail mechanism). Would the much lighter-weight carbon fiber bomb casing be strong enough to achieve necessary penetration?

Dealing with the first challenge, Livermore experts have worked on an "enhanced-blast" munition based on triamino-trinitrobenzene (TATB), the bright yellow explosive long used as a conventional detonator in nuclear warheads. Although very powerful, TATB is a particularly "insensitive" explosive, extremely hard to detonate accidentally -- an absolute essential to the safety of nuclear weapons.

The TATB is about the consistency of toothpaste when it is packed into the carbon fiber casing. It hardens as it cures. The casing itself accounts for from 10 to 20 percent of weapon weight.

The Air Force is working with a more easily detonated and more powerful explosive -- cyclotetramethylene-tetranitramine, know as HMX. The ideal explosive for this new generation of munitions will have some trade-off between explosive power and enough insensitivity to obviate accidents during handling and transportation.

A Livermore team headed by engineer Scott Groves designed the smooth cylindrical carbon fiber case, which is tipped by a sharp steel nose. Could it penetrate a hardened target without tearing apart and thus dispersing the explosive payload before it could detonate correctly? Tests of its penetrating power, conducted at Eglin AFB, were more than encouraging.

A fully weighted test casing with its steel tip was fired into a simulated hard target at 1263 feet per second. It passed through a foot of high-strength concrete, then passed completely through the "soft-catch chamber" behind it consisting of almost 10 feet of sand sandwiched between multiple sheets of thick plywood and capped at the far end by a half-inch steel plate. The missile then buried itself six inches deep in the actual target, a solid block of high-strength concrete. It was still intact.

Enthused at the results, Groves speculated to the lab's Science & Technology Review magazine that the carbon fiber case may have "slipped" through the concrete with less friction than conventional steel.

Detonations of TATB in carbon fiber casings have proven hopeful thus far in confining blast damage to an immediate target radius and preventing fragmentation damage beyond. In one test at Eglin, for instance, bundles of insulating foam (to catch fragments) were placed 6, 10 and 16 feet out from a small TATB test charge.

The explosion obliterated the close-in bundle, but the other two bundles were left intact and found to have no fragments of carbon fiber imbedded in them. The casing had been blown to bits. The few pieces of carbon fiber recovered in the six-foot radius were less than a half-inch in size.

Researchers at Livermore are continuing work on perfecting the new munitions. While the characteristics of steel-cased munitions are well known (fuse timing, for example -- the precise instant that the bomb detonates once it has penetrated a target) many of the vagaries of carbon fiber munitions are still being learned through further tests and extensive computer simulations.

But the impulse for this costly and meticulous research is one for which credit is seldom given to the U.S. military -- the desire to confine war's death and destruction to combatants only, to protect friendly forces to the highest degree possible given the circumstances, and to minimize the physical damage that will have to be repaired or rebuilt once the fighting stops.


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