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Mark 8x series conventional bomb

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Mark 84 bombs; access hatches and inter-section seams visible

Mark 8x series of gravity bombs are a standard U.S. series, used by all U.S. military services[1], and many other nations. The basic munition is a low-drag gravity bomb (LDGB), with the minimal controls of substituting fins that maximize or minimize speed, and fuzes that mount either in the nose (e.g., for airburst or impact detonation) or tail (i.e., slight delay).

Rather than thinking of them as fixed configurations, it is most useful to consider them as modular systems, with the main Marx 8x section forming the warhead. Thus, they may be converted to guided bombs by adding alternate components. BLU- series warheads have alternative casings or fillers.

The low-drag means they have casings that are aerodynamically designed for minimum drag, so, when they are carried on the external wing mounting points of aircraft, they create the least possible wind resistance.

The "X" in Mark 8x is a code for the approximate total weight of the bomb. Roughly half of this amount is actual explosive content; the rest goes principally for the casing, fins, and fuzes. There are ballistically similar bombs, such as the BLU-109, which have much stronger cases, accepting much less explosive in order to be able to penetrate more deeply. Again, explosive fillers such as Tritonal are being replaced with PBXN-109 or other less sensitive explosives.

Modules of Marx 8x and BLU series

Adding intelligence

While the basic Mk. 8x is guided only by the accuracy of the releasing aircraft, using the modular design, most can be equipped with electro-optical guidance (e.g., GBU-15 with AN/AXQ-14 date links, home on a laser designator with PAVEWAY laser guidance, or use the Joint Direct Attack Munition (JDAM) with inertial guidance, often supplemented with Global Positioning System satellite navigation information kits, which replace or supplement the tail fins with steering surfaces, and perhaps glide wings.

Summary

USAF designation Total weight JDAM designation PAVEWAY laser guidance designation Comments
Mark 81 bomb 250 lb. GBU-29
Mark 82 bomb 500 lb. GBU-38 GBU-12 EBGU-12 with PAVEWAY, inertial and GPS; also called GBU-49
Mark 83 bomb 1000 lb. GBU-32 GBU-16
Mark 84 bomb 2000 lb. GBU-31 GBU-24 PAVEWAY III
Summary schematic, top view

"The bomb body is shipped with a plastic plug installed in the nose and tail fuze well to prevent damage to the internal threads and to keep out moisture. The aft end of the bomb body has a metal shipping cap installed. Plastic lug caps are installed in the suspension lug wells, and a plastic plug is installed in the fuze-charging receptacle well. Some bombs contain a hoisting lug packaged in the tail fuze well. Bombs are shipped on metal pallets. The number of bombs loaded on each pallet depends on the bomb size. For example, six Mk 82 bombs can be shipped on a pallet, three Mk 83 bombs can be shipped on a pallet, and two Mk 84 bombs can be shipped on a pallet."[2]

Nose group

Summary schematic, top view

"The bomb body is designed with a nose and tail fuze well. These wells are internally threaded to receive either mechanical or electric fuzes."[2]

There are two basic nose plugs used in general purpose bombs, the solid nose plug (MXU-735/B and MXU-735A/B) and the ogive nose plug. The ogive nose plug provides a pointed arch. A support cup is used in the nose well with the ogive nose plug to provide a solid structure to the bomb. The MXU-735 solid nose plug is designed to provide better penetration of hard targets, without the likelihood of nose plug shearing during oblique impact. The MXU-735 replaces the ogive nose plug and support cup. [3] Flatter MXU-735 cups improve stability for special applications where the bomb enters water.[4]

In both guided and unguided versions, the nose may contain part or all of a fuze intended for impact detonation, detonation after a brief delay, or, with a supplemental sensor that allows an airburst. With one of the most common fuzes, the FMU-139, only the ground proximity sensor is in the nose, and the actual electronics and detonating explosives of the fuze are in the tail.

FMU-139 fuxe in Mark 8x series conventional bomb case

Laser-guided bombs and bombs with electro-optical guidance need to have sensors, and usually control fins in or on the nose.

Warhead

While the original filling was TNT, there is a program of replacement with PBXN-109 insensitive high explosives to improve safety in crashes or emergency jettisons. Another program uses the PBX-9502 IHE as a booster in a demonstration Mark 82 with a main charge of the IHE AFX-645. [5]

Specialized fillers may be used, such as dense inert metal explosive that concentrates the blast, minimizing damage to anything near the actual target. There has been some use of concrete fillings in the Mark 8x case with precision guidance, for even more focused damage.

Hardback

At a minimum, the hardback includes mounting shackles for handling, and to attach the bomb to the carrying aircraft. "There are two suspension lug wells for the installation of suspension lugs. The suspension lugs are spaced 14 or 30 inches apart, depending upon the size of the bomb. Suspension lugs are used to attach the weapon to the aircraft bomb racks. An internally threaded well for the installation of a hoisting lug is located between the suspension lugs, at the center-of-gravity (CG) position on the bomb. The hoisting lug is used for handling purposes only."[6]

Actually dropping a bomb into the airstream below the aircraft can be nontrivial, so explosive bolts or other energetic devices may actually hurl the bomb rather than simply depending on gravity.

More advanced derivatives have the MIL-STD-1760 mechanical and electrical interface that confirms, to weapons control systems on the aircraft, that the bomb has fallen away. The -1760 interface also allows the bomb to be defined as an intelligent node on the MIL-STD-1553 data bus.

Tail group

Tail groups variously stabilize the free-falling bomb in high-altitude drops, or slow it for low-altitude delivery where the carrying aircraft needs to get clear of blast and fragments. Basic stabilizing fins are called conical.

Two main kinds of tail assemblies are used for low-altitude delivery. "Snakeye" fins stabilize, but also present considerable air resistance and braking. More recently, a soft device called a "ballute", for "balloon" and "parachute", also brakes the bomb.

References

  1. that have aircraft that drop bombs
  2. 2.0 2.1 U.S. Navy Nonresident Training, Chapter 1: Bombs, Fuzes and Associated Components, Aviation Ordnanceman Training Course, U.S. Navy, p. 1-17
  3. Aviation Ordnanceman course, p. 1-20
  4. Jillene Marie Bushnell (December 2009), Master's Thesis: Tail Separation and Density Effects on the Underwater Trajectory of the JDAM, U.S. Naval Postgraduate School
  5. John D. Corley, Gary H. Parsons, Alan C. Stewart, Fuzed Insensitive General Purpose Bomb Containing AFX-645
  6. Aviation Ordnanceman Training Course, pp. 1-17 to 1-18