The BLU-109 (B
nit ver. 109) is a penetrating gravity bomb
, i.e. it is designed to be dropped from aircraft and is unpowered.
The weapon is based on the widely-used Mk.80 LDGP family of weapons, specifically the 2,000lb. Mk. 84. In the 1970s, the U.S. Air Force and U.S. Navy became concerned with reports which had come back from the Vietnam theater regarding the effectiveness of aerial bombardment vs. buried fortifications. As the Viet Cong and NVA were adept at the creation, concealment and utilization of underground facilities (including, in some places, entire small towns hidden beneath the surface) the U.S. forces found themselves bombing known or suspected shelter locations quite frequently. The problem was that the existing family of aerial weapons was designed primarily for fragmentation and blast effects. The U.S. Navy's prime strike platform, the A-6 Intruder, specialized in low-level delivery of retarded weapons such as the Rockeye cluster bomb; these weapons were dropped at speed low down, and struck their target areas moving mostly horizontally. This made for poor penetration characteristics. High-altitude bombs such as the Mk. 82 used by the B-52 Stratofortress in the ARCLIGHT missions were not accurate enough to warrant pinpoint use against point targets, and in any case the high-alititude bombing platforms were optimized for the dispense of large numbers of smaller (500lb.) weapons.
The only real ray of hope was the recent introduction and subsequent decent performance of the first generation of PGMs, such as the TV-guided Walleye and the laser-guided Snakeeye GBUs (Guided Bomb Units). These had the potential to strike the surface directly atop buried facilities. However, the munitions used by these weapons was still the standard blast/frag type, and offered poor performance.
The BLU-109 was the response. It is a heavily modified version of the 2,000lb Mk. 84 LDGP case. The casing of the weapon is narrower and constructed of reinforced steel; both of these aid in penetration. There is no provision for a nose fuze (used to detonate bombs at or above ground level) - instead, only the tail had a fuzing receptacle. This allowed the nose of the weapon to be further reinforced. The tail fuze meant that the weapon could be detonated after penetrating the surface, as well as ensuring that the fuze was in a more protected location and thus would likely function after impact even if used with substantial (hours) delay.
The design criteria for the weapon was that it needed to be able to penetrate six feet of steel-reinforced concrete, and detonate its primary charge only after passing through this barrier. The plan was that if dropped on tunnels or bunkers, the bomb would pierce the ceiling of the structure and detonate inside it.
This capability remained unused on a wide scale until the Gulf War. Coalition military planners were keen on reducing Iraqi air force assets on the ground if possible; however, the Yugoslavian government had (by contract, several years prior) constructed a series of hardened aircraft shelters. These, decisionmakers feared, would protect Iraqi aircraft from harm, as well as make proper intelligence counts of remaining enemy aircraft much harder to assess. The shelters (which Coalition forces were quite familiar with, having seen and used their ilk all over Europe) turned out, in the end, to be a bad investment. Because the Iraqi leadership was so intent on preserving its best platforms, the shelters became natural preferred targets, especially when not in populated areas. After the war, the GWAPS indicated that in fact the BLU-109 had been quite successful at piercing the hardened tops and sides of these shelters and making moist scrambled eggs out of whatever was hiding beneath it.
Most of the now well-known video clips of weapons hitting shelter targets (you know the ones, they feature long still shots of buildings which finish with a weapon zipping into frame to detonate against the top or entrance) show BLU-109 strikes. After the war, investigations onsite showed images of shelters which appear mostly intact from the outside, but inside were rubble.
The BLU-109 is certified for release from most of the medium attack aircraft used by the NATO nations, as it is based on the common Mk. 80 family. It serves as the core munition for the 'penetration' variants of several modern weapon systems such as the JDAM and WCM.