AN Series Bombs

The creation of the Army-Navy Standardization Board resulted in the standardization of the bombs of these two services into the AN series. This series was very similar to the M series except for these modifications:

1. A third suspension lug was added at the center of gravity and diametrically opposite the dual suspension lugs, to fit British release devices.

2. The base plate was changed to a male plug to increase the strength on low-angle penetration of targets.

3. The bombs were painted an overall olive drab with one inch yellow bands around the nose and base and a 1/4 inch yellow band around the center of gravity.

The bombs in this series included the following weights: 100, 250, 500, 1,000, and 2,000 pounds. These bombs are all general purpose high explosive bombs. Though other types of bombs — armor piercing, semi armor piercing, fragmentation, incendiary, depth, etc. — have been subsequently standardized and given AN designations, it is with the AN general purpose bombs and their modifications that this introduction shall be concerned.

AN GP Series: The AN series was subsequently replaced by the AN GP series to make it possible for these bombs to be used in antisubmarine work as well as for general bombardment. This modification was incorporated into the 500, 1,000, and 2,000 pound bombs only, their new designations becoming AN-M64, AN-M65, and AN-M66, and consisted of a change from the Adapter Booster M102 to the Adapter Booster M115. With the Adapter Booster M115 it became possible to use either standard Army tail fuzes or the Tail Hydrostatic Fuze AN-Mk 230. The standard filling of these bombs was 50/50 Amatol until the supply of TNT became ample in 1943, at which time TNT became the standard filler. At the present time, 25% of the production of these bombs is filled with Composition "B'’.

AN GP A1 Series: The A1 modification consists of two steel pins in the base plate which lock the base plate to the main filling, thus preventing removal of the base plate, once the bomb is filled. In this series the adapter boosters have also been modified to the M102A1 and the M115A1. This modification consists of a hole through the adapter booster and a groove in the threads of the base plate to receive a locking pin which is shipped with all anti-withdrawal tail fuzes. Insertion of the pin which is held in by the fuze body prevents withdrawal of the adapter booster. This series is the current production GP bomb series and is used jointly by the Army, Navy, and British forces.

The following table will assist in understanding the development of the designations that have been successively applied to general-purpose bombs used by the Army and Navy.

Weight (lbs)

M Series

Navy

AN Series

AN GP Series

AN GP A1 Series

100

M30

Mk 4 Mod 4

AN-M30

AN-M30

AN-M30A1

250

M57


AN-M57

AN-M57

AN-M57A1

300

M31





500

M43

Mk 12 Mod 2

AN-M43

AN-M64

AN-M64A1

600

M32





1000

M44

Mk 13 Mod 2

AN-M44

AN-M65

AN-M65A1

1100

M33





2000

M34


AN-M34

AN-M66

AN-M66A1

Body construction: The body construction of American GP bombs may be in one, two, or three pieces. Methods of manufacture include (1) one piece cast or spun, (2) two piece cast and welded or (3) three piece cast and welded. The ogival nose tapers to join in the thin parallel side walls, which terminate in a boat-tailed shape at the after end. The threaded nose opening is closed by the fuze-seat liner, and the threaded base opening is closed by a male plug, the tail fuze pocket being made by the adapter booster.

Suspension: Dual suspension lugs for horizontal suspension are welded directly to the bomb case, being spaced 14 inches apart on bombs up to 2,000 pounds, and 30 inches apart on bombs 2,000 pounds and over. A single horizontal suspension lug is also welded to the bomb case at the center of gravity and diametrically opposite the dual lugs. The lugs are eyebolts, shaped from bar steel and formed in the shape of a U. "M" series bombs could lie carried on single suspension racks by using an additional single suspension lug welded on a band fitting around the bomb body at the center of gravity.

For suspension in dive bomb displacement gear, trunnions are provided on a separate band which may be one of two types. The first type provides the trunnion only: and the second, a more common type, provides a single hoisting lug in addition to the trunnions. On some of the newer designs of AN bombs of Navy manufacture, the suspension and hoisting fittings are not attached to the bomb case by welding, but are held to the case by threaded bolts fitting into holes tapped and threaded into the body. For suspension in torpedo slings, the torpedo sling guide key found on Navy bombs can be made on AN bombs by using the base of the single suspension lug with the lug removed by gentle hacksawing.

To hoist bombs into Navy planes, the AN bombs not equipped with hoisting lugs must have a hoisting lug furnished by either an expendable band with single or dual lugs, or by a removable hoisting band. The removable band is preferred, since it does not affect the terminal velocity of the bomb. The newest and best of these bands is the Universal Hoisting Band Mk 8. For Army planes, bombs are usually hoisted by canvas, slings, which also have the advantage of not affecting air trajectory.

Tail fin construction: The tail construction is known as the box-type tail and consists of the following parts: a cast-steel sleeve secured to the body of the bomb by a fin locking nut, and four sheet-steel fins supported by four sheet- steel struts in the shape of a box. One fin and one strut are pressed from a single piece of metal, and the four pieces are then welded to the sleeve. “A” indicates heavier construction for high-altitude bombing.

Color and markings: The standard color scheme employed on these bombs since 11 March 1942 has been an olive drab body with yellow bands to indicate the HE filler. The banding system for Amatol and TNT fillers is a one-inch yellow band nt the nose, a one-inch yellow band at the tail of the bomb body, and a l/pinch dotted band at the center of gravity. Because Composition “B” is more sensitive and requires more careful handling, it is given an additional marking consisting of a second one-inch yellow band at the nose nnd at the tail. On these bands “Comp. B” is stencilled in black paint. Recent production eliminates the 1/4 inch band, since the center of gravity is located accurately enough by the single suspension lug.

The following standard markings are painted on the bomb body in black paint: type, weight, and name of bomb, type of filling, lot number, place and date of filling, and inspector’s initials.

Additional indestructible marking is stamped into the metal of the bomb case on the rear conical surface of the bomb body: type, size, name, maker's initials, lot number, and date (i.e., GP 500 lb. AN-M64 CSCO Lot 57 4/42).

Explosive filling: The filling of these bombs is accomplished in the following manner. With the nose fuze seat liner in place, the bomb is placed on end nose down and an Auxiliary Booster M104 is positioned behind it. The M104 is a bakelite tube containing tetryl pellets. The initial pour of the explosive is sufficient to secure the auxiliary booster when it cools. Then the remainder of the filling, 50/50 Amatol, TNT, or Composition “B”, is added until a depth of approximately six inches remains to be filled. A second Auxiliary Booster M104 is then inserted in all bombs except the 100 pound, and the tail surround is added to complete the filling. A wooden former is inserted in the tail fuze cavity as the filling cools. After cooling, the former is removed and the appropriate adapter booster is inserted. The adapter booster consists of the fuze seat liner with an additional steel cup containing a tetryl pellet. The Adapter Booster M102 (and M102A1) has an internal thread diameter of 1.50 inches. The adapter Booster Ml 15 has an internal thread diameter of 2.0 inches, and an adapter ring for further reduction of the diameter to 1.50 inches when smaller fuzes arc used.

Tests conducted by the Army indicated that the presence or absence of Auxiliary Boosters M104 does not alter the effectiveness of the detonation of bombs. On the basis of these tests, the Army started to eliminate the Auxiliary Boosters M104 from Composition B loaded 500 pound, 1,000 pound, and 2,000 pound GP bombs during the latter part of 1944. Elimination of Auxiliary Boosters M104 in TNT and Tritonal loaded GP and SAP bombs was started in March (TNT) and June (Tritonal) of 1945. At the present time, all productions of Composition "B", TNT and Tritonal loaded GP and SAP bombs, and Composition B loaded Frag bombs, exclude Auxiliary Boosters M104.

The bombs from which the Auxiliary Boosters M104 are now eliminated are as follows:

GP (Comp B, TNT, Tritonal filling)

SAP (TNT filling)

Frag(Comp B filling)

100 lb AN-M30A1

500 lb AN-M58A2

220 lb AN-M88

250 lb AN-M57A1

1000 lb AN-M59A1

260 lb M81

500 lb AN-M64A1



1000 lb AN-M65A1



2000 lb AN-M66A1



Tests conducted by the Army have shown that inert nose and tail surrounds improve safety in handling of GP and SAP  bombs. On the basis of these tests, the majority of GP and SAP bombs loaded at the present time have inert nose and tail surrounds. This covers Composition B, TNT, Tritonal, and Picratol filled bombs. Formerly Composition B loaded bombs had TNT surrounds.

The ingredients of the inert surrounds used at present are as follows: ester gum, 51.0%; castor oil, 10.5%; kaolin, 31.5%; and wax, hydrocarbon, 4.0%.

The thickness of these surrounds is as follows :

GP Bombs

SAP Bombs

Nose - 1 1/4" + 1/4"

Nose - Fills nose cavity and extends not more than 1/3" beyond the bottom of the fuze seat liner.

Tail - 3/4" + 1/4"

3/4" + 1/4"

The thickness of the surrounds is such that most, if not all, of the explosive portion of the adapter booster and that of the nose fuze booster (surrounded by fuze seat liner) extends into the main filler, with exception of the SAP nose set-up.

Anti-ricochet attachments: To give a more nearly vertical impact to 100 pound, 250 pound, and 500 pound GP  bombs and to break their rapid descent when dropped from low flying planes, the Anti-Ricochet Devices M16 and M17 have been developed. The device consists of a parachute unit, a modified AN-M112A1 series tail fuze (M151) using an anemometer-type arming vane, and a fuze adapter.

See Also

Adapter Booster, M102, M102A1

Booster, Auxiliary, M104

Bomb, 100 lb GP, AN-M30, AN-M30A1

Bomb, 250 lb GP, AN-M57, AN-M57A1

Bomb, 500 lb GP, AN-M64, AN-M64A1

Bomb, 1000 lb GP, AN-M65, AN-M65A1

Bomb, 2000 lb GP, AN-M66, AN-M66A1, AN-M66A2

Source(s)

OP 1664, Volume 2 - US Explosive Ordnance (1947)