A general-purpose bomb is an air-dropped bomb intended as a compromise between blast damage, penetration, and fragmentation in explosive effect. They are designed to be effective against enemy troops, vehicles, and buildings.
General-purpose (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 30% to 40% of the bomb's total weight. The British term for a bomb of this type is "medium case" or "medium capacity" (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, 227 kg). In many cases this is strictly a nominal weight (the counterpart to the caliber of a firearm), 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 (372 kg).
Most modern air-dropped GP bombs are designed to minimize drag for external carriage on aircraft lacking bomb bays.
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 fuzes and fins for different uses. One notable example is the "daisy cutter" fuze used in Vietnam War era American weapons, an extended probe designed to ensure that the bomb would detonate on contact (even with foliage) rather than burying itself in earth or mud, which would reduce its effectiveness. (This was not the first instance of such devices. As early as World War II, the Luftwaffe was using extended-nose fuzes on bombs dropped by Stuka dive-bombers and other aircraft for exactly the same reason. A blast several feet above the ground is many times more effective and has a far greater radius than one that is delayed until the bomb is below the surface.)
GP bombs are commonly used as the warheads for more sophisticated precision-guided munitions. Using 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 is substantially more accurate than an unguided bomb.
During WWII the British adopted a description of general-purpose bombs as medium capacity (MC) bombs. The 1000 lb MC was developed from 1942 to replace the existing 1000 lb GP (General Purpose) bomb. Initially using most of the components of the 1000 lb GP it was decided to give it a new tail and it was built with a half-inch-thick wall. Fillings could be Amatex, Amatol, Minol, RDX and others. Actual weight was around 1020 lb (464 kg). It was introduced to service in 1943 and about a quarter of a million were produced by the end of the war.
During the Korean War and Vietnam War the U.S. used older designs like the M65, M117 and M118, which had an explosive content about 65% higher than most contemporary 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 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 minimal drag for the carrier aircraft. The Mark 80 series was not used in combat until the Vietnam War, but has since replaced most earlier GP weapons. It includes four basic weapon types:
Vietnam service showed the Mk 81 "Firecracker" to be insufficiently effective, and it was withdrawn from U.S. service. However, recently, precision-guided variants of the Mk 81 bomb have begun a return to service, based on U.S. experience in Iraq after 2003, and the desire to reduce collateral damage compared to Mk 82 and larger bombs (e.g., when attacking a single small building in a populated area).
Since the Vietnam War and the 1967 USS Forrestal fire, United States Navy and United States Marine Corps 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. Fire is less a danger in a land-based facility, where the personnel can be evacuated with relative ease, and the building be the only loss. At sea, the crew and munitions share a facility (the ship), and thus are in much more danger of fire reaching munitions (which tend to be more closely packed, due to space limitations). Also, losing a munitions storage building on land is far cheaper than sacrificing an entire naval vessel, even if one could easily evacuate the crew.
All Mk80 bombs have both nose and tail fuze wells and can accept a variety of fuzes. Various nose and tail kits can be fitted to adapt the weapon for a variety of roles.
In the Mk 80 series bomb bodies is also used in the following weapons:
On August 14, 2020, Kaman Precision Products received roughly $57.3 million for a "cockpit-selectable" bomb fuze to be used on Mark 80 warheads (guided and unguided). The contract involved foreign military sales (FMS) to 25 unnamed countries.
Dumb Mk 80 bombs could be converted to smart bombs with attached kits:
The principal modern British bombs are 540 lb (245 kg) and 1,000 lb (454 kg), and are no longer in service. The smaller 540lb was retired with the demise of the Harrier GR9 aircraft with the larger 1000lb being retired in Apr 2019. Currently the UK only uses the 500lb class Paveway IV weapon system. The warhead is a modified enhanced Mk82 warhead.
The Russian term for general-purpose bomb is fugasnaya aviatsionnaya bomba (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.
In 1946 the Soviet Union developed a series of freefall bombs in four sizes 250 kg (550 lb), 500 kg (1,100 lb), 1,500 kg (3,300 lb), and 3,000 kg (6,600 lb) and sharing a single nose and a single tail fuze. The bomb could be dropped from up to 12,000 m (40,000 ft) and up to 1,000 km/h (625 mph). The original, 1946-series bombs had poor ballistic characteristics at supersonic speed, and their construction was fragile. As an interim measure, upgraded versions of the bombs were built with thicker walls and no nose fuze. The thick-walled version of the bombs were built until 1956.
The 1954 series of high-drag bombs was built in six sizes: 250 kg (550 lb), 500 kg (1,100 lb), 1,500 kg (3,300 lb), 3,000 kg (6,600 lb), 5,000 kg (11,000 lb), and 9,000 kg (20,000 lb). A feature of the 1954 series of bombs is the ballistic ring on the nose of the bomb which acts as a vortex generator to aid the bombs stabilizers. The smaller (less than 3,000 kg) bombs had a single nose and a single tail fuze, while the larger weapons shared a single nose fuze and two base fuzes. The FAB-9000 (9,000 kg/20,000 lb) weapon was roughly comparable to the wartime Grand Slam bomb. It was used by Russian aircraft designers as a substitute for early nuclear weapons when determining the size and clearances of bomb bays.
In 1962 a new series of streamlined, low-drag bombs was introduced, designed for external carriage by fighter-bomber aircraft rather than in internal bays. They come in only two sizes, 250 kg (550 lb) and 500 kg (1,100 lb). Both bombs have a single nose fuze.
Both the 54 and 62 series designs remain in use. The most common of these are the FAB-100, FAB-250, FAB-500, FAB-750, and FAB-1500, 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, primarily for use by heavy bombers. In the Iran–Iraq War, FAB-5000 (5,000 kg/11,000 lb) and FAB-9000 (9,000 kg/20,000 lb) bombs were dropped by Iraqi Air Force Tupolev Tu-22 bombers, generally against large, fixed targets in Iran. In Afghanistan in the 1980s, Soviet Tupolev Tu-16 and Tupolev Tu-22M bombers used massive FAB-1500, FAB-3000, FAB-5000NG, and FAB-9000 bombs to devastating effect during the Panjshir offensives.
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 (550 lb) EU2 and T25, 400 kg (882 lb) T200, and 1,000 kg (2,205 lb) BL4.
Other countries, including Australia, Azerbaijan, Brazil, Chile, Greece, India, Israel, Pakistan, Poland, Portugal, Romania, South Africa, Spain, Sweden, and Turkey, manufacture their own bombs, most of which are either licensed versions of the U.S. Mark 80 series or close copies.