Bomb fuzes designed by the U.S. forces can be classified in three ways; by their posi-tion in a bomb; by their method of arming; and by their firing action.

In current production the primary fuzes are of AN standardized types, the fuzes used against land targets being of Army design, and fuzes used against naval targets being of Navy design. In addition to the AN types, both the Army and the Navy have the own fu-zes designed prior to standardization. Most of these are obsolescent, but may still be found in the field. Between the two services the following types of fuzes can be found: nose, athwartship and tail fuzes using one of the following six methods of arming:

(1) arming vane

(2) arming vane with reduction gear

(3) arming pin

(4) arming pin with pyrotechnic delay

(5) arming pin with mechanical delay

(6) a combination of any two of the above

and functioning in one of the following ways: instantaneous, selective instantaneous or short delay, short delay, long delay, hydrostatically, or with serial burst.

Nomenclature:

When under deployment, Army fuzes carry temporary designation which are later dropped when the fuzes are standardized. Experimental fuzes controlled by the Army Ordnance Department are indicated by the letter "T" (e.g. T75); modifications incorpo-rated in the basic design carry the letter "E" (T51E1). Items in the developmental stage designed by the Chemical Warfare Service, such as fire bomb igniters, are indicated by the letter "E" rather than "T", and subsequent revisions in "E" designs carry the letter "R" (e.g. E9R17).

If the developmental fuze is standardized for Army use by the Ordnance Technical Comittee, the "T" or "E" designation is dropped and an "M" number is assigned. Standard-ized fuzes carry the designation "M" followed by a three digit number beginning with 1 (M100, M103 etc.). When a modification on a standard item is under development, the change will be given an "E" or "R" designation (e.g., M111E8); if the fuze thus modified is adopted as standard, it will take a designation in sequence in the "A" series, "A" indicat-ing an alternation in the basic design.

In undertaking the development of a new fuze, the Navy Bureau of Ordnance assigns a Mark number wich will designate the fuze in the experimental stage as well as in serv-ice use. No system of "T" and "E" designations is employed. The Mark is followed by a three digit Arabic number, beginning with 2 (e.g. Mk 234). Modifications are indicated by the abbreviation "Mod" followed by the proper Arabic number. Fuzes manufactured before the spring of 1943 were numbered by Roman numerals and used only two digit numbers.

When adopted as standard by the Joint Aircraft Committee, the prefix "AN" is placed before the M or Mk. designation. Once adopted, the service are obligated to use the standard item where it is applicable. The service which developed the item and whose type of designation is used has the engineering responsibility for the fuze, and no chan-ges may be made therein which affect installation or tactical interchangeability.

Explosive Train:

Typical firing trains employed in both Army and Navy fuzes use pointed strikers with sensitive primers for instantaneous action and blunt firing pins and percussion primers for delay action. The simple instantaneous explosive train in nose fuzes consists of a sensit-ive primer mixture of lead azide and lead styphnate, and upper detonator of lead azide, and a lower detonator of tetryl.

For a short delay, the blunt firing pin initiates a mercury fulminate percussion primer, which sends a flash through a chamber of compression to ignite a pressed black powder delay. This on completion of burning flashes a realy pellet of lead azide and lead styph-nate which detonates the lower detonator of tetryl.

The explosive train, including the booster, is usually incorporated in all Navy fuzes and Army nose fuzes, but in Army tail fuzes the explosive train does not include the booster, and may have the remainder of its explosives contained in an interchangeable primer de-tonator to allow optional selection of short delay. Three such primer detonator assemb-lies are now in use - the M14 used in the AN-M100A1 and A2 series fuzes; the M16 used in M112 and M115 series fuzes; and the M16A1 used in M112A1 and M115 series fuzes. Another primer detonator assembly, the M19, is used in the chemical long delay fuzes for safety in shipment since it is not assembled to the fuze until shortly before the fuze is assembled to the bomb.