Mark 8x series conventional bomb

Mark 8x series of gravity bombs are a standard U.S. series, used by all U.S. military services, 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).

Modules of Marx 8x and BLU series

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.

While the basic Mk. 8x is guided only by the accuracy of the releasing aircraft, all can be equipped with the Joint Direct Attack Munition (JDAM) kit, which replaces the fins with movable fins that can steer the bomb. Steering commands come from a guidance assembly attached to the nose, which can use inertial guidance, inertial+GPS, or, recently, home on a laser designator. Other, more intelligent, guidance and fuze designs are in development.

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

Nose group

In both guided and unguided versions, the nose will 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.

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 to attach the bomb to the carrying aircraft. 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

One of the tail delay variants can deploy a braking parachute, so the aircraft can drop at very low altitude, but be far enough from fragments when the bomb hits.