Although the Direct Strike Hard Target Weapon concept was unfunded as of 1997, in early 2002 it was reported that Northrop-Grumman and Lockheed Martin were working on a 30,000-lb. earth penetrating guided conventional weapon, said to be known as "Big BLU" or "Big Blue" [which is also the nickname of the 15,000-lb surface burst BLU-82]. Big BLU will be GPS guided and feature cobalt-alloy penetrator bomb body that enables it to penetrate to depths of up to 100 feet below the surface before detonating. The bombs are so large that a bomber such as the B-2 could carry one of them. As of March 2002 reportedly three Big BLUs had been ordered by the Air Force on an urgent basis ["Inside The Ring," By Bill Gertz and Rowan Scarborough, Washington Times March 15, 2002 Pg. 10].

The Air Force is also investigating whether a similar size weapon could be used in a blast-only configuration, to replace the BLU-82 Daisy Cutter blast weapon dropped from the MC-130.

The U.S. Air Force is developing a new, 2nd generation, ten ton large, low air burst bomb. It will replace the older "Daisy Cutter" 7.5 ton bomb developed during the 1960s. This was a 7.5 ton bomb using a semi-liquid explosive for clearing landing zones in the Vietnam jungle. The terms "Daisy Cutter" actually comes from the four foot probe at the bottom of the bomb which triggered the explosion without creating a crater (helicopters don't like to land in craters.) The probe was later replaced with a radar altimeter fuze, but the nickname "Daisy Cutter" stuck. The official designation was BLU-82 (or "Big Blue"). Until the BLU-82 came along, the biggest non-nuclear explosion obtainable was with a FAE (Fuel Air Explosives). FAE  works by dropping a bomb that is actually a large aerosol dispenser. When the FAE "explodes" it first dispenses a large cloud of flammable material (anything like gasoline or propane will work). The cloud is then ignited and huge explosion results. There's one drawback, the size and density of the aerosol cloud depends a lot on the wind, air temperature and humidity. So the power of the explosion will vary a lot. But it's difficult to get a FAE to work in a bomb larger than 2000 pounds. So the replacement for the BLU-82 bomb, called MOAB (Massive Ordnance Air Burst) simply uses more of the  slurry of ammonium nitrate and powdered aluminum.  In dry, dusty conditions, the Daisy Cutter produces a mushroom cloud similar to that created by a nuclear explosion (and for the same reason, the sheer size of the explosion creates an upward pull that sends up a "mushroom" of smoke and dust on a column of smoke).

A Massive Ordnance Air Blast weapon is prepared for testing at the Air Armament Center here March 11. The MOAB is a precision-guided munition weighing 21,500 pounds and is the largest non-nuclear weapon in existence. (DOD photo)
Air Force workers prepare the Massive Ordnance Air Blast weapon for testing at the Air Armament Center on March 11. The MOAB is a precision-guided munition weighing 21,500 pounds and is the largest non-nuclear weapon in existence. (DOD photo)

In addition to a more  powerful explosion, MOAB doesn't need a parachute, like the Daisy Cutter, but uses a GPS (like JDAM) and an aerodynamic body to detonate the bomb at a precise area. Thus the MOAB can be dropped from a higher altitude (like outside the range of machine-guns and rifles). Like the Daisy Cutter, MOAB is shoved out the back of a cargo aircraft (usually a C-130, but since the MOAB uses GPS and higher altitude drops, the C-17 can probably be used as well.) MOAB is a highly destructive and terrifying weapon. If used in Iraq, it would demoralize any Iraqi troops in the vicinity who survived the explosion. The force of a MOAB explosion is sufficient to knock over tanks and kill any people within several hundred meters of the detonation. After the 1991 Gulf War, the United States started to get rid of it's various FAE weapons. But some were left in the inventory when the Afghanistan came along and the success of Daisy Cutters there, plus the new Russian research in FAE weapons, led to the new American research effort. There may be larger, or simply more powerful, FAE weapons in the works. But for the moment, MOAB, using pretty old fashioned technology, is the biggest non-nuclear bomb around.

The Direct Strike Hard Target Weapon is a 20,000 lb. class precision guided, adverse weather, direct attack bomb employed on the B-52 and B-2 aircraft. It will make use of the GCU developed by the JDAM program which uses GPS aided INS for adverse weather guidance. Precision accuracy will be attained by using differential GPS (DGPS) technology demonstrated on programs such as Enhanced Differential GPS for Guidance Enhancement (EDGE) and Miniature Munition Technology Demonstration (MMTD). The weapon will make use of the JDAM interface under development for the B-52 and B-2 aircraft and would be carried internally using new suspension hardware within the bay. The warhead will be a 20,000 lb. penetrator with dense metal ballast. This concept uses the Hard Target Smart Fuze (HTSF), an accelerometer based electronic fuze which allows control of the detonation point by layer counting, distance or time. The accelerometer senses G loads on the bomb due to deceleration as it penetrates through to the target. The fuze can distinguish between earth, concrete, rock and air.

During the Second World War the British designer Barnes Wallis developed the largest conventional bombs used in combat. Wallis first designed the "Upkeep" bouncing bombs that were used during the Dambusters Raid on 16th May 1943. The rotating bouncing bomb exploded at the base of the retaining wall of the dam, producing heavy floods and damaging German production in the Ruhr.

Wallis next produced the 12,000-lb Tallboy, also known as the earthquake bomb. Tallboy was 21" long, with an overall diameter of 3'8", while the bomb body itself was 10'4" long and 3'2" in diameter. It weighed a total of 11,855 pounds, of which 5,200 pounds was Torpex D1 explosive. The weight of the case was thus a high proportion of the weight of the bomb. Dropped from 20,000 feet, a Tallboy made a 80ft deep crater, 100ft across. The bomb had a high terminal velocity, variously estimated at 3,600 and 3,700 feet a second [much faster than sound], and at these speeds it could go through 16ft of concrete. It was used for attacks on tunnels, V-1 Flying Bomb launch sites, and other high-priority targets. Its most important use was in the sinking of the German battleship Tirpitz at anchor in Norway, on 12 November 1944. Over 700 Tallboys were dropped during the War.

In 1945 Barnes Wallis developed the 22,000lb Grand Slam, which remains the largest conventional bomb ever used in action. The 10 ton (22,000 pound) "Grand Slam" was 26-feet, 6-inches long. Its hardened casing was cast in a single piece in a sand mold, using a concrete core. The "Grand Slam" could reportedly penetrate though 20+ ft of concrete. The first one was dropped on Germany on 14th March 1945. It hit the ground about 80 feet from the target, but it created a crater over 100 feet deep. This bomb was used with great effect against viaducts or railways leading to the Ruhr and also against several U-boat shelters. In one raid on 27 March 1945 against the U-Bootbunkerwerft "Valentin" submarine pens near Bremen, two Grand Slams penetrated 7 meters (23 feet) of reinforced concrete, bringing down the roof. In total, 41 Grand Slams were dropped during the war.