General Purpose Bombs
A blast warhead is one that is designed to achieve target damage primarily from blast effect. When a high explosive detonates, it is converted almost instantly into a gas at very high pressure and temperature. Under the pressure of the gases thus generated, the weapon case expands and breaks into fragments. The air surrounding the casing is compressed and a shock (blast) wave is transmitted into it. Typical initial values for a high-explosive weapon are 200 kilobars of pressure (1 bar = 1 atmosphere) and 5,000 degrees celsius.
The shock wave generated by the explosion is a compression wave, in which the pressure rises from atmospheric pressure to peak overpressure in a fraction of a microsecond. It is followed by a much slower (hundredths of a second) decline to atmospheric pressure. This portion is known as the positive phase of the shock wave. The pressure continues to decline to subatmospheric pressure and then returns to normal. This portion is called the negative or suction phase.
For a fixed-weight explosive, the peak pressure and positive impulse decrease with distance from the explosion. This is due to the attentuation of the blast wave. The rate of attenuation is proportional to the rate of expansion of the volume of gases behind the blast wave. In other words the blast pressure is inversely proportional to the cube of the distance from the blast center (1/R3). When a bomb is detonated at some distance above the ground, the reflected wave catches up to and combines with the original shock wave, called the incident wave, to form a third wave that has a nearly vertical front at ground level.
This third wave is called a "Mach Wave" or "Mach Stem," and the point at which the three waves intersect is called the "Triple Point." The Mach Stem grows in height as it spreads laterally, and as the Mach Stem grows, the triple point rises, describing a curve through the air. In the Mach Stem the incident wave is reinforced by the reflected wave, and both the peak pressure and impulse are at a maximum that is considerably higher than the peak pressure and impulse of the original shock wave at the same distance from the point of explosion. Using the phenomenon of Mach reflections, it is possible to increase considerably the radius of effectiveness of a bomb. By detonating a warhead at the proper height above the ground, the maximum radius at which a given pressure or impulse is exerted can be increased, in some cases by almost 50%, over that for the same bomb detonated at ground level.
Approximately 30% of the energy released by the explosive detonation of a General Purpose bomb fragments the case and impart kinetic energy to the fragments. The balance of available energy is used to create a shock front and blast effects. The fragments are propelled at high velocity, and after a short distance they overtake and pass through the shock wave. The rate at which the velocity of the shock front accompanying the blast decreases is generally much greater than the decrease in velocity of fragments, which occurs due to air friction. Therefore, the advance of the shock front lags behind that of the fragments. The radius of effective fragment damage, although target dependent, thus exceeds considerably the radius of effective blast damage in an air burst.
Whereas the effects of an idealized blast payload are attenuated by a factor roughly equal to 1/R3 (R is measured from the origin), the attenuation of idealized fragmentation effects will vary as 1/R2 and 1/R, depending upon the specific design of the payload. Herein lies the principle advantage of a fragmentation payload: it can afford a greater miss distance and still remain effective because its attenuation is less.
Fin assemblies, used with the Mk 80/BLU 100 (series) GP bombs, provide stability to the bomb. They cause the bomb to fall in a smooth, definite curve to the target, instead of tumbling through the air. The typical BSU-33/conical fin assembly is steel, conical in shape, and has four fins to provide stability. Access covers, attached by quick-release screws, are located on the sides of the fin body, providing access for dearming and inspections. There is a drilled or punched hole at the top and bottom of the forward end of the fin body. This hole is used to install an arming wire when the bomb is being configured for electric tail fuzing. The fin is attached to the aft end of the bomb, and is secured in place by tightening the fin setscrews into the V-groove of the bomb.
In the unretarded mode of delivery, the weapon is released from the aircraft and the fins remain in the closed position. The weapon free-falls to the target. In the unretarded mode of delivery (without pilot option), the cotter/safety pin installed in the fin release band is not removed or replaced with an arming wire. However, the safety tag that reads REMOVE BEFORE FLIGHT is removed.
In the retarded mode of delivery, the fins open to retard or slow the weapon. Since the aircraft and the weapon are traveling at the same speed when the weapon is released, the weapon and the aircraft arrive at the target at the same time. During low-level bombing, the aircraft could be damaged by the blast; therefore, the retarded mode of delivery is used during low-level bombing to ensure the aircraft is clear.
The BSU-85/B bomb fin attaches to the Mk 83/BLU 110 GP bomb. It is an air-inflatable retarder designed for very low altitudes. It can be dropped in either high-drag (retarded) or low-drag (unretarded) mode. The BSU-85/B fin attaches to the bomb body by eight setscrews. The four fixed fins provide low-drag aerodynamic stability. The wedges installed on the trailing edges provide stabilizing spin during both low-drag and high-drag release. When stored in its original shipping/storage container, the bomb fin shelf life is 20 years.
The BSU-86/B bomb fin is used with GP bombs, Mk 82 Mods/BLU 111 (series), or the practice bomb BDU-45/B. The fin provides a retarded (high-drag) or unretarded (low-drag) bomb delivery capability for the aircraft. The BSU-86/B fin is attached to the Mk 82/BLU 111 or BDU-45/B bomb by eight setscrews. A 25-degree wedge is located at the tips of each fin to impart spin. The air stream drives the fin open rapidly, when the MAU-199/B spring arming wire (SAW) is activated. The spring load under each fin blade initiates fin opening.
Alliant Precision Fuze Co., Janesville, Wis., was awarded on January 13, 2004 a $7,999,697 firm fixed price contract modification to provide for DSU-33B/B proximity sensor, a nose-mounted Radio Frequency Proximity sensor use on M117 and MK-80 series general-purpose bombs and the Joint Direct Attack Munition (JDAM). The DSU-33B/B provides a sensor fire pulse to a fuze, which in turn initiates bomb detonation at a height-of-burst (HOB) nominally 20 feet. The HOB detonation capability enhances the performance of general-purpose bombs and JDAMs for above ground targets. The DSU-33B/B can provide a fire pulse to the following fuzes: FMU-139/B, FMU-139 A/B, FMU139B/B, and the FMU-152/B. Power for the DSU-33B/B is provided by an internal battery, which is initiated by either an FZU-48/B or the FZU-55/B and FZU-55A/B in the Air Force configuration, or the fuze function control set in the Navy configuration. The sensor is qualified for use on the following aircraft and each variant in their series: Marines AV-8, Navy/Marine F/A-18, and Air Force A-10, F-15, F-16, F-22, B-52, B-1 and B-2 including foreign military sales versions. This action is for procurement of 7,078 additional DSU-33B/B Proximity Sensors and Cables under Option 3 of the contract. Total funds have been obligated. This work will be complete by March 2006. The Air Armament Center, Eglin Air Force Base, Fla., is the contracting activity (F08635-02-C-0049, P00010).
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