Bombs For Beginners

by Even Steven

At the outset of the Vietnam War, tactical aviation pilots were achieving a 750-foot circular error probable (CEP)--the radius from the aim point that half of the bombs dropped will fall within. This number is sufficient for the impact of a tactical nuclear weapon but is far from adequate for conventional weaponry. It took several years for the CEP to be lowered to a manageable 365 feet. [SOURCE] The advantage of guided bombs was revealed when compared with the F-105’ s work in Vietnam. The F-105s achieved a circular error probable (CEP) of 447 feet and 5.5 percent direct hits during the end of Rolling Thunder, compared with guided bombs’ CEP of 23 feet and 48 percent direct hits during the period of February 1972 through February 1973.

A bomb is an explosive filler enclosed in a casing. Bombs are generally classified according to the ratio of explosive material to total weight. The principal classes are general-purpose (GP), fragmentation, penetration and cluster bombs.

Approximately 50-percent of the General Purpose [GP] bomb's weight is explosive materials. These bombs usually weigh between 500 and 2,000 pounds and produce a combination of blast and fragmentation effects. The approximately one-half-inch-thick casing creates a fragmentation effect at the moment of detonation, and the 5O-percent explosive filer causes considerable damage from blast effect. The most common GP bombs are the MK-80 series weapons. General-purpose bombs were the type of ordnance most frequently employed in the Gulf War. According to Iraqi prisoners of war, formations of B-52s dropping general-purpose bombs were one of the most feared aircraft-weapon combinations of the war. GP bombs served as the basic building blocks for many of the other munitions used during the Gulf War.

Only ten to twenty percent of a fragmentation bomb's weight is explosive material; the remainder include specially scored cases that break into predictably sized pieces. The fragments, which travel at high velocities, are the primary cause of damage.

Penetration bombs have between twenty-five and thirty percent explosive filler. The casings are designed to penetrate hardened targets such as bunkers before the explosives detonate. Penetration is achieved by either kinetic energy of the entire projectile or the effects of a shaped-charge.

Cluster bombs are primarily fragmentation weapons. Cluster bombs, like GP bombs, can feature mix and match components (submunitions, fuzes, etc.) to produce the desired effect. Bomb Construction

Free-fall bombs have three sections. The bomb body is the casing containing the explosive material. The fuze section can be located in the nose and/or the rear of the bomb and determines the timing of the explosion. The tail section, or fins, determines how the bomb flies through the air. Desired weapons effects are achieved by selecting a particular combination of bomb body, fuzing, and tail section.

Bomb Bodies

Bomb bodies vary in size, weight, and thickness of casing. GP bombs have a thinner case and more explosive filler than penetrating bombs, whereas cluster bombs generally come in dispensers that open to release bomblets at predetermined altitudes. The bomb body casing (except for cluster munitions) houses the explosive filler. Upon detona- tion, the high-explosive filler creates an explosive train to achieve the desired weapons effect; detonation is triggered by fusing. Fuzes

A fuze initiates bomb detonation at a predetermined time and under the desired circumstances. Fuzes are located in the nose or tail of the munition, or both. They are armed by one, or a combination, of the following methods:

The arming vane, a small propeller, is rotated by airflow after weapon release. A specified number of rotations arms the fuse.

The arming pin is ejected or withdrawn by a spring action releas- ing the arming mechanism and allowing the fuze to arm.

The inertia fuze is armed by abrupt changes in the velocity of the bomb caused by the deployment of fins or ballutes.

The electric fuse is armed by a time-delay circuit powered by a thermal battery activated by extraction of the arming lanyard upon bomb release.

Different effects are obtained by mating different bombs to different fuzes. A fuze functions in one of the following ways. An impact fuze is designed to function on or after impact. Detonation upon impact is selected for targets such as supply dumps when the main destructive energy desired is blast. For a building, a delayed detonation might be selected so the bomb can penetrate several floors before exploding. A proximity fuze contains a miniature doppler radar set that senses height above the ground. When the explosion occurs above the ground, most of the destructive effect is caused by the bomb casing fragments.

Proximity-fuzed bombs are used against targets such as troops in trenches, radars, trucks, and other vehicles. In a timed fuze, the delay is normally initiated at bomb release rather than on impact. The timing element is a mechanical or electrical device. A hydrostatic fuze is employed in depth bombs used for underwater demolition work. The MK-36/40 Destructor is a special fuze with a sensor that can be mated to a bomb. It senses the presence of metallic objects such as trucks or ships, making it, in effect, a mine. These weapons can be used against either land or water targets. In Southwest Asia, the MK-36 (500-pound) detonators were used to mine the waters in the vicinity of Umn Qasr naval facility.

Stabilizing Devices

Bombs are stabilized in flight by either fin or parachute assemblies. These assemblies attach to the rear section of the bomb and keep the bomb nose-down during its descent. These assemblies can separate from the bomb after the bomb hits the ground. Two common types of fin assemblies used by foreign countries are the conical- and box-fin assemblies. The retarding-fin assembly is used by the US for most of its general-purpose bombs.

The conical fin was the tail section type most often installed on GP bombs dropped in Southwest Asia. The conical fin assembly helped stabilize the bomb in flight, allowing the bomb to exhibit the best effects of low drag and stabilization after release. A conical fin mated with a GP bomb results in a low-drag general-purpose bomb. Two types of high-drag retarders were used in Desert Storm. The first was the air- inflatable retarder tail assembly containing a ballute (combination balloon and parachute) device that deployed shortly after bomb release. There were two types of ballutes, the BSU-49 mated to a 500-pound MK-82 bomb, and the BSU-50 mated to a 2,000-pound MK-84 bomb. The second type of retarding fin was the Snakeye, which had four metal vanes that opened into the windstream to slow the bomb after release. Snakeye fins were used by Navy aircraft to deliver mines into the waters around Iraqi naval bases. These high-drag retarder tail assemblies were used to slow the bomb quickly after a high-speed, low-level release, thereby reducing the chance of an aircraft being damaged by its own bomb fragments.

Damage Mechanisms

There are five general categories of munitions damage mechanisms: blast, fragmentation, cratering, shaped charge penetration, and incendiary effects. A given target is usually most vulnerable to one particular damage mechanism, though it may be vulnerable (to a lesser extent) to several damage mechanisms. The factors governing determination of the primary damage mechanism for a given target are: target construction, target location (relative to the point of warhead detonation), warhead damage effects pattern, and the desired type and level of damage.

Blast is caused by tremendous dynamic overpressures generated by the detonation of a high explosive. Complete (high order) detonation of high-explosives can generate pressures up to 700 tons per square inch and temperatures in the range of 3,000 to 4,500º prior to bomb case fragmentation. It is essential that the bomb casing remain intact long enough after the detonation sequence begins to contain the hot gases and achieve a high order explosion. A consideration when striking hardened targets is that deformation of the weapon casing or fuze may cause the warhead to dud or experience a low order detonation.

Approximately half of the total energy generated will be used in swelling the bomb casing to 1.5 times its normal size prior to fragmenting and then imparting velocity to those fragments. The remainder of this energy is expended in compression of the air surrounding the bomb and is responsible for the blast effect. This effect is most desirable for attacking walls, collapsing roofs, and destroying or damaging machinery. The effect of blast on personnel is confined to a relatively short distance (110 feet for a 2000 pound bomb). For surface targets blast is maximized by using a general purpose (GP) bomb with an instantaneous fuzing system that will produce a surface burst with little or no confinement of the overpressures generated by excessive burial. For buildings or bunkers the use of a delayed fuzing system allows the blast to occur within the structure maximizing the damage caused by the explosion.

Fragmentation is caused by the break-up of the weapon casing upon detonation. Fragments of a bomb case can achieve velocities from 3,000 to 11,000 fps depending on the type of bomb (for example GP bomb fragments have velocities of 5,000 to 9,000 fps). Fragmentation is effective against troops, vehicles, aircraft and other soft targets. The fragmentation effects generated from the detonation of a high-explosive bomb have greater effective range than blast, usually up to approximately 3,000 feet regardless of bomb size. The fragmentation effect can be maximized by using a bomb specifically designed for this effect, or by using a GP bomb with an airburst functioning fuze.

The cratering effect is normally achieved by using a GP bomb with a delayed fuzing system. This system allows bomb penetration before the explosion. Since the explosion occurs within the surface media the energy of the blast is causes the formation of a crater. This effect is most desirable in interdiction of lines of communication (LOC) and area denial operations

Armor penetration is accomplished by shaped charges or kinetic energy penetrators. This is an effective damage mechanism for tanks, assault guns, armored personnel carriers, and other armored targets. A major problem associated with both shaped charges and kinetic energy penetrators is the lack of visible damage. This may result in repeated attacks to produce battlefield evidence that a target is no longer a threat.

Fire is effective in interrupting operations of enemy personnel and in damaging supplies stored in the open. Incendiaries produce intense, localized heat designed to ignite adjacent combustible target materials. The true incendiary produces no fireball and relatively little flame. The basic damage mechanism of firebomb weapons comes from the fireball and burning residual fuel globules, impact momentum of the fuel and container, and damage from fires started by the weapon. The sharp cutoff of casualty-producing mechanisms outside the incendiary pattern allows delivery close to friendly troops, usually parallel to the forward line of battle, with minimum risk. Munitions have been developed with full fragmentation and penetrating capabilities coupled with reactive incendiary devices. These improved incendiaries are highly effective against fuel and other flammable targets. A drawback, however, in planning for the employment of incendiary weapons is that incendiary/fire effects are not evaluated in current weaponeering methodologies.