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Modern GP bombs use a thick-walled metal casing with explosive filler (typically TNT, Composition B, or Tritonal in NATO or United States service) comprising about 50% of the bomb's total weight. (The British term for a bomb of this type is medium case, abbreviated MC). The GP bomb is a common weapon of fighter bomber and attack aircraft because it is useful for a variety of tactical applications and relatively cheap.
General-purpose bombs are often identified by their weight (e.g., 500 lb, 250 kg). In many cases this is strictly a nominal weight, and the actual weight of each individual weapon may vary depending on its retardation, fusing, carriage, and guidance systems. For example, the actual weight of a U.S. M117 bomb, nominally 750 lb (340 kg), is typically around 820 lb (374 kg).
Most modern air-dropped GP bombs are designed to minimize drag for the carrier aircraft. Such weapons are called low-drag general-purpose (LDGP) bombs, nicknamed "slicks."
In low-altitude attacks, there is a danger of the attacking aircraft being caught in the blast of its own weapons. To address this problem, GP bombs are often fitted with retarders, parachutes or pop-out fins that slow the bomb's descent to allow the aircraft time to escape the detonation.
GP bombs can be fitted with a variety of fuses and fins for different uses. One notable example is the "daisy cutter" fuse used on Vietnam-era American weapons, an extended probe designed to insure that the bomb would detonate on contact (even with foliage) rather than burying itself in earth or mud, which would reduce its effectiveness.
GP bombs are commonly used as the warheads for more sophisticated precision-guided munitions. Affixing various types of seeker and electrically controlled fins turns a basic 'iron' bomb into a laser-guided bomb (like the U.S. Paveway series), an electro-optical guided bomb, or, more recently, GPS-guided weapon (like the U.S. JDAM). The combination is cheaper than a true guided missile (and can be more easily upgraded or replaced in service), but substantially more accurate than an unguided bomb.
Types of GP bomb
Modern American GP bombs: the Mark 80 series
During the Korean War and Vietnam War the U.S. used older designs like the M117 and M118, which had a higher explosive content (about 65%) than most current weapons. Although some of these weapons remain in the U.S. arsenal, they are little used, and the M117 is primarily carried only by the B-52 Stratofortress.
The primary U.S. GP bombs are the Mark 80 series. This class of weapons uses a shape known as Aero 1A, designed by the famous Ed Heinemann of Douglas Aircraft as the result of studies in 1946. It has a length-to-diameter ratio of about 8:1, and results in minimum drag for the carrier aircraft. The Mark 80 series was not used in combat until the Vietnam War, but has since that time replaced most earlier GP weapons. It includes four basic weapon types:
- Mk 81 (nominal weight 250 lb / 113 kg)
- Mk 82 (nominal weight 500 lb / 227 kg)
- Mk 83 (nominal weight 1,000 lb / 454 kg)
- Mk 84 (nominal weight 2,000 lb / 908 kg)
Vietnam service showed the Mk 81 "Firecracker" to be insufficiently effective, and it has been withdrawn from U.S. service.
Since the Vietnam War, United States Navy and USMC GP bombs are distinguished by a thick ablative fire-retardant coating, which is designed to delay any potential accidental explosion in the event of a shipboard fire. Land-based air forces typically do not use such coatings, largely because they add some 30 lb (14 kg) to the weight of the complete weapon.
Modern British GP bombs
As mentioned, the British describe general-purpose bombs as medium case (MC) bombs. The principal modern British bombs are 540 lb (245 kg) and 1,000 lb (454 kg), with a wide variety of fin, fuse, and retarder options.
The Russian term for general-purpose bomb is fugasnaya aviatsionnaya bomba, abbreviated FAB and followed by the bomb's nominal weight in kilograms. Most Russian iron bombs have circular ring airfoils rather than the fins used by Western types.
Beginning in the mid-1950s the Soviet Union developed a new series of FAB weapons with a streamlined shape, typically with a length/diameter ratio of about 15:1. The most common of these are the FAB-100, FAB-250, FAB-500, FAB-750, and FAB-1000, roughly corresponding to the U.S. Mark 80 series. These have seen widespread service in Russia, Warsaw Pact nations, and various export countries.
Larger bombs with less streamlined shapes also remained in the Soviet arsenal, usually for use by heavy bombers: in Afghanistan in 1988, Soviet Tupolev Tu-22M bombers used massive FAB-1500 (1,500 kg / 3,300 lb) and FAB-3000 (3,000 kg / 6,600 lb) weapons to devastating effect. The biggest Russian weapon of this type was the FAB-9000 (9,000 kg / 19,800 lb) weapon comparable to the wartime Grand Slam bomb. Its use in the postwar era was apparently never seriously contemplated, but it was used by Russian aircraft designers as a substitute for early nuclear weapons when determining the size and clearances of bomb bays.
French GP bombs
France's GP bombs, marketed by Matra and built by the Société des Ateliers Mécanique de Port-sur-Sambre (SAMP) are made in a variety of types with nominal weights from 50 kg (110 lb) to 1,000 kg (2,205 lb). The most common are the 250 kg (1,102 lb) EU2 and T25, 400 kg (882 lb) T200, and 1,000 kg (2,205 lb) BL4.
Other countries, including Brazil, Chile, Israel, Portugal, Romania, South Africa, Spain, and Sweden manufacture their own bombs, most of which are either licensed version of the U.S. Mark 80 series or close copies.
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