The Man-Made Earthquake 

A Short History of  Very Heavy Conventional Aerial Bombs

Copyright ©1998, 2003 and 2007  by John Godwin.  All rights reserved.  No part may be duplicated in any way without the expressed written consent of the Author and separate copyright holders.   Photograph Sources:  the author,  Kansas Aviation Museum, U.S. Army/Aberdeen Proving Ground, USAF, A.O. Smith Corporation, National Archives and Records Administration and the Imperial War Museum.  
1.    Genesis of the Earthquake Bomb

2.    The Tallboy

3.    The American Tallboy:  Bomb, GP, 12,000-lb, M109 (T10)

3a.    12,000-lb, Bell V-13  TARZON

4.     The Grand Slam

5.     The American Grand Slam:  Bomb, GP, 22,000-lb, M110 (T-14)

5a.    The Amazon: Bomb, SAP, 22,000-lb

6.     Aircraft Modifications

            A.     Avro Lancaster B.Mk I 

                    i.     Tallboy

                    ii.    Special (Grand Slam)

            B.     Boeing B-29  Superfortress

                    i.     B-29 w/ single Tallboy semi-external in bomb bay

                    ii.    B-29 w/ single Tallboy or Grand Slam semi-external in bomb bay

                    iii.   B-29 w/ Tallboy or Grand Slam mounted on external bomb racks under wings

            C.     Consolidated B-32 Dominator w/ Tallboy or Grand Slam in bomb bay

            D.     Northrop B-35 Flying Wing w/  Tallboy or Grand Slam on external bomb racks under wings 

7.     Postwar Developments

            A.     Project Ruby

            B.     Harken Project

8.     Bomb, SAP, 25,000-lb, T28E4

9.   Bomb, GP, 42,000-lb, T-12

10.   Epilogue

1.    Genesis of the Earthquake Bomb

After the abortive invasion campaign of Britain by Nazi forces during the summer of 1940, the order of battle changed from the high mobility of the Blitzkrieg to one of entrenchment and fixed fortification more characteristic of the First World War.  In order to protect Nazi assets along the French coast that were easily within reach of RAF bombers, an enormous program was undertaken to fortify and harden vital military installations.  By the spring of 1942, concrete submarine bases were being built that were thought to be bomb proof against any existing aerial bombs.  At the time,  the largest penetrating bomb available to the RAF weighed 1,000lb.  A small number of 2,000 lb anti-shipping armor piercing bombs existed but the small explosive charge and lightweight made them ineffective against heavily reinforced targets.1 

An answer to the problem had been available as early as the fall of 1940.   Vickers Armstrong, Ltd engineer Dr Barnes Wallis had anticipated the need to destroy targets of this type and proposed the use of an 'earthquake' bomb in his engineering study:  A Note on a Method of Attacking the Axis Powers.   In sixty pages of text, fifty pages of tables and appendix, Wallis made his case for building extra large earth penetrating bombs as a means of bombing massive, oversized, deeply buried or extra hardened targets by transferring the maximum kinetic energy of the explosive into the relatively incompressible earth.  The bomb Wallis had in mind weighed 10 tons and would be dropped from 40,000 ft.  Impacting the earth at supersonic speed and driven as much as 130 feet underground,  the detonation would form a camoflet and not even crater the surface.   The earthquake like pressure wave could damage even the most heavily constructed buildings by displacement of the foundation.  There was one major problem: there was no way to deliver such a heavy bomb to the target.                                                     

Over several months,  Wallis tried unsuccessfully to find a niche for his earthquake bomb in the Air Ministry battle plan.  Without an official government or military support and the fact that no aircraft in existence or the foreseeable future could bomb from such a height, the TALLBOY as the design was known, was shelved.

By the end of 1943, the RAF had begun delivering  High Capacity bombs in weights of 4,000 lb, 8,000 lb and 12,000 lb  with great effect against lightly constructed military and industrial surface targets.  These bombs were thin cased,  cylindrical shaped bombs with impact fuzes set in the ends.  Only the 12,000 lb models had tail fins fitted.  Intended as a destroyer of factories and warehouses, they were useless against structures buried deep underground or heavily reinforced.  Though delivered from fairly low altitude, the fact that weapons weighing 12,000 lb could be delivered by existing heavy bombers removed an obstacle that had blocked previous consideration of earthquake bombs. 

After the success of the Wallis’ bouncing Dambuster bombs on the night of May 16-17, 1943, there was reassessment of  specialized weapons like the ‘earthquake’ bomb.3   Though no RAF bomber could carry a 22,000 lb bomb to the desired 40,000 ft release height, there was considerable interest in using Wallis’ penetrating  bomb at a height that existing bombers could reach.  Wallis was asked to submit a report on the calculated penetration and effect of such a bomb dropped from around 20,000 ft.   In his report, Wallis suggested scaling down the 22,000 lb bomb to one weighing 12,000 lb and if such a bomb was dropped from at least 8,000 ft a measurable earthquake effect would still be achieved.  Wallis refined the design through late 1943, when a scaled down 12,000lb bomb, called Tallboy was ready for testing..4 

2.    The Tallboy

On July 18, 1943 the Ministry of Aircraft Production let contracts for  twelve scaled 4,000 lb ballistic trial bombs, 100 12,000 lb bombs and 100 22,000 lb bombs all under the codename ‘Tallboy,’ small, medium and large.  (Note: to reduce the confusion, the bombs will be referred to by their final codenames or weights.)  The limited capacity in the U.K. to manufacture steel casings for both the 12,000 lb and 22,000 lb bombs required a shut down of the 22,000 lb production on September 30, 1943 after producing only nine of the enormous casings.  Production of the 22,000lb eventually resumed in July 1944 with the new codename ‘Grand Slam.’   With the suspension of 22,000 lb bomb production, the order for the 12,000 lb bomb was increased to 325 with 125 casings built under license in the United States.  Less than a year had passed from the beginning of design work through manufacture, aircraft modification, testing to operational deployment.   

Operational drop of a Tallboy.  The bomb had just begun rotation  to the right.  The black dot in the the casing marked the bomb center of gravity.   IWM

The first use of the 12,000 lb bomb was on the night of June 8-9, 1944 against the Saumur Railway Tunnel, a vital link for German reinforcements trying to get to the Normandy invasion beaches.  19 bombs were dropped- at least one from as little as 6,000 ft- blocking the tunnel with tons of rock that required weeks to clear. 

Following this initial success, the production order for Tallboy was increased to 2,000 with half to be manufactured in the United States- some by the A.O.Smith Corporation of Milwaukee.  English Steel Corporation made up the bulk of U.K. production.5

In the months that followed, heavily reinforced V-Weapon assembly/launch sites, submarine and E-boat pens, tunnels, oil refining and storage facilities, viaducts, canals and bridges came under attack.  The Sourpe Dam, one of the original targets envisioned in Wallis’ 1940 earthquake bomb proposal and target of a “Dam Buster” raid in 1943, was attacked with 18 Tallboys in October 1944 scoring two direct hits on the dam face causing it to leak.  Even the heavily armored German battleship Turpitz was successfully bombed.  In all, 854 Tallboys were dropped from June 1944 through April 1945.    

The operational version of the U.K. manufactured Tallboy was 252” (21’) long (with tail assembly), 38 inches at maximum diameter, had a hardened steel case thickness of more than 4 inches in the nose, then tapering to 1.25 inches near the rear of the bomb casing and was loaded with 5,200 lb of Torpex explosive making up 41% of the 12,622 lb total weight.  A special hardened steel plug was fitted into the nose of the casing.   After filling, a one inch layer of TNT was added followed by four inches of woodmeal/wax then sealed with a 1/4 inch plywood plug with three holes cut to allow the booster assemblies to penetrate the explosive filler.  The primary boosters consisted of 2 ring and 2 solid C.E. (Tetryl) pellets that abutted 4 solid RDX/Beeswax pellets.   These were inserted into the secondary boosters which consisted of  2 ring and 2 solid large RDX/Beeswax pellets surrounded by a cardboard tube.   The bombs were shipped with the exploders in place but sealed with plugs.  Beginning in the first quarter of 1945, Tritonal (80% of TNT and 20% of Aluminum) was loaded into British bombs manufactured in the United States.6

Three No. 47 fuzes with delays up to 60 minutes or three No. 58 Mk I fuzes with delays from instantaneous to 11 seconds were screwed into pockets offset 120o forming a triangle in the base plate that was secured to the casing with 20 bolts.  

The explosive train for a No. 47 fuze progressed as follows:  Upon release, the retracted pullout wires broke a vial of Acetone that began dissolving a celluloid disk that kept a spring loaded striker from moving.  Depending on the thickness of the disk, a predetermined length of time elapsed before the weakened disk allowed the striker to punch through to the primer.  The primer held a small quantity of contact sensitive high explosive that would in turn detonate a booster explosive in a container about the size of a flashlight battery.  The booster contained sufficient explosive force to detonate the main charge of the bomb. 

Delay fuzing was used to allow the bomb to penetrate to maximum depth before detonation.  More importantly, delay detonation allowed individual bombers flying at night to drop their bombs within a time period without having the target obscured by smoke from prior bombs.  Reliability concerns with the No. 47 fuze is the likely reason that it was not copied for U.S. versions of the Tallboy.    One unexploded Tallboy fitted with three No. 47 fuzes set for 30 minute delay was discovered at the foot of the Sorpe Dam in December 1958 after being submerged for 14 years.

The No. 58 fuse was assembled from a No. 30 Pistol body that had the head cup filled with a brass plug.  A heavy striker was inserted into a hole machined in the plug.  A brass cross held by a screw was fitted into the top end of the striker to prevent travel.  Two arming wire holes were drilled through the assembled pistol body at 90o to each other to provide a choice of optimum angle for the pullout wire.  A transit safety pin was inserted through a third hole parallel to one of the arming wire holes.  

Fusing was accomplished as follows:  After the tail assembly had been attached to the casing, the armorer, working through access holes in the tail,  removed the fuse pocket plugs and then determined the depth of the cavity with a Gauge, Cavity Detonator, No. 2 Mark I.  The appropriate detonator was then inserted in the cavity and the pistol bodies screwed tight.  A single Wire, Safety, No. 10 Mk I was inserted into the pistol safety wire  hole that was optimal for pullout.  Two Clips, Safety No. 1 Mk I secured each pullout wire with approximately three inches protruding beyond the clips.  Arming supplies where stored prior to use in Box B. 346, which held 20 pistols, 20 safety wires and 40 clips.  Once loaded into the aircraft, the armorer attached the pullout wires to the aircraft, pulled the transit safety pins and secured the covers over the access holes.  (Note: the safety wire was always in place before the transit safety pin was pulled to prevent accidental travel of the striker and the cap over the head of the fuse body never removed.).  7

The explosive train for a No. 58 Mk I fuze progressed as follows:  Upon release, the single pullout wire for each fuze removed any restraint to the travel of the heavy striker pin except for a small brass cross that held the pin until the inertia of impact drove the pin into the primer.  The primer held a small tablet of contact sensitive chemical that would burn at a predetermined rate- up to several seconds-  before detonating Primary exploders in turn detonating the secondary exploders that detonated the main charge of the bomb8.

Demonstration of Tallboy assembly.  Note that three men could easily lift the aluminum tail cone while a fourth held the tapered midsection that streamlined the bomb between the tail and casing.  The attachment studs in the base plate of the casing that secured the tail cone are visible.  On top of the casing are a set of sockets for tightening the nuts inside the confined space of the tail cone. IWM

The long aluminum tail cone  No. 78 Mk 1was attached to twelve 1/2 inch studs located around the circumference of the rear of the bomb casing with simmonds nuts.   Circular plates covered holes in the tail permitted access to the fuze pockets and studs that held the tail cone to the casing.  Pullout wires passed from the fuzes through a small hole in the tail cone to the carrier aircraft for arming the bomb at the time of release.   Upon release from the the Lancaster flying at 18,000 ft and 200 mph, the tail fins canted 5o to the right caused the bomb to spin stabilize at 300 rpm  with an impact velocity of 1,097 ft/sec.  9

Overall the bomb casing was painted an olive drab green with the tail assembly left bare metal.  Identification markings included a red band painted just behind the nose plug and a green band with “Torpex. D1” stenciled just behind the red.  The official RAF name for the Tallboy was, Bomb, HE, Aircraft, MC 12,000 lb.  10  

3.    The American Tallboy:  Bomb, GP, 12,000-lb, M109 (T10)

The American version of the Tallboy was manufactured by casting or welding.  Officially called, the  Bomb, General Purpose (GP), 12,000-lb, M109 (T10), was 124 “ long, 38 “ in diameter and weighed 12,488 lb.  While the earliest versions were loaded with 5,200 lb of Torpex with a 1” layer of TNT at the rear, later production examples were loaded with 5,500 lb of Tritonal (43.8% of total weight).  An armor piercing plug was fitted to the nose of the cast casings.  Due to manufacturing demands during the war, a large number of casings were assembled from rolled plate steel welded together using techniques adapted from the manufacture of large pressure vessels.    

Welded  T-10s were made from five sections welded together.  The nose and tail ring were made of forged steel and the sidewalls were assembled from three pieces of rolled plate welded along the longitudinal axis.  11

Overall, the casing was painted olive drab green with a yellow band at the nose and tail.  Stenciled in black on the casing was the following information: Type of weapon (GP), weight, model, explosive filler, ammunition lot number, AIC symbol, ICC shipping designation, Inspector’s stamp, the letters “US”, displacement, and may have carried the shipping address and shipping ticket number.  Available photographs are likely inert training rounds which carried minimal markings or painted a high visibility black and white to aid tracking cameras.  Some inert training examples displayed for the public were painted silver or olive drab green overall.  Except for external markings, the American and U.K. versions appeared identical. 12


Three M169 (T708) tail fuzes (L) were fitted to the Tallboy & Grand Slam.  At the bottom of each fuze was an M40 primer-detonator (R) with the long end extending into an M118 booster.  The delays ranged from .025 to 30 seconds.  Selection of delay that would give the bomb time to penetrate to optimal depth before detonation would have been based on knowledge of the target construction and geology of the target area.  Both USAF

As with the U.K. models, three fuzes were fitted into pockets on the base plate offset 120o forming a triangle.  Access to the fuze pockets was facilitated by three holes in the tail assembly.  For reliability, three four inch long M169 (T708) impact fuzes with M40 primer-detonators and the 4 inch long M118 adapter booster were screwed into a fuze seat of 1.375-14TWS-1 thread to a depth of 2.45 inches.  

The M120 tail assembly was 132 1/8 inches long and 44 1/2 inches in diameter.  Twenty bolts held the M120 to the bomb casing bringing the overall length to 252” and weighing 12,622 lb fully assembled.  A circular metal band held in place by the joining of the bomb body and tail assembly, streamlined the bomb.13  

Army Air Force planners envisioned a whole family of huge earth penetration bombs that could be used as multi purpose bombs where huge near surface blast and fragmentation could be exploited along with camoflet.  Testing of the American prototypes was still in progress on V-J Day. 

The Ordnance Department of the Army broadened its engineering studies in early 1945 on Americanized versions of the Tallboy and Grand Slam.  These bombs differed from the British prototypes by having forged steel noses welded to rolled steel plate instead of casting.    Unsatisfactory features of the British Tallboy that were corrected by Ordnance engineers included an air-arming fuse and fin assemblies made of steel rather than aluminum.  Supersonic wind tunnel tests of subscale Tallboy models were conducted by Ordnance Dept engineers to evaluate how ballistic performance was effected by varying the length of the non-explosive tail.  The long tail of the earthquake bombs, which provided stability, made internal stowage in bomber aircraft impossible until the B-36 was in service.14  

3a.    12,000-lb, Bell V-13/ ASM-A-1  TARZON 


12,000 lb Bell B-13 TARZON bomb casing on display at the Museum of the United States Air Force, Dayton, Ohio.  Essentially a Tallboy with a new tail assembly that provided limited azimuth and range control via an AN/ARW-38 command transmitter and guidance receiver radio link with the carrier aircraft.  After release, a flare in the tail assembly aided the bombardier in guiding the weapon to the target.  Three modified B-29 bombers, similar to those modified to carry Tallboys semi submerged in the bomb bays, carried out a limited number of sorties against heavily defended bridges during the Korean War.  The enormous drag and loss of aircraft performance created after release of the bomb combined with the TARZONs marginal combat record limited its use during the war.  The large number of stockpiled American Tallboys left over after WWII were probably used to make the TARZONs.  During the Vietnam War, left over Tallboys and M121 bombs were considered for clearing helicopter landing zones in the bush prior to the deployment of the thin cased parachute retarded 15,000 lb BLU-82 Daisy Cutter Bomb.    Both J. Godwin Collection 


4.     The Grand Slam 

Production orders for 100 Tallboy (large) bombs were let on July 18, 1943.  Almost immediately it was determined that English Steel Corporation foundry capacity was seriously stretched producing both the Tallboy (medium) and the Tallboy (large).  Production of the Tallboy (large) was terminated on September 30 after only producing nine casings.  Production of the Tallboy (large) was resumed in July 1944 and renamed Grand Slam.  Manufacture of the large casings was slow and expensive.  Contracts were let with A.O. Smith in America for additional production.  The filling and fuzing of the Grand Slam was identical to the Tallboy described above except for scale.


Comparative sizes of the Tallboy (top) and the Grand Slam.  Note the large number of nuts used to hold the base plate to the casing  NARA

An incredible miscalculation by the British War Cabinet of an end to the war during the winter of 1944, resulted in a wholesale cancellation of new bomb production contracts including those for the Grand Slam.   Specialized modification of Lancaster bombers to carry Grand Slams was also halted.  Once it was realized that the war would continue well into 1945, production was resumed.   The delay prevented the deployment of the Grand Slam until the spring of 1945.

Though production had been held up, testing continued.  By October, development work on the original nine casings manufactured in 1943 was complete.  A single Avro Lancaster Mk 1 joined the test program that had been striped of unnecessary equipment & most defensive armament, a strengthened bomb rack and had the more powerful Merlin 24 engines installed. 

This photograph shows how the UK Grand Slams were hand loaded with a Torpex slurry that was poured into the casing.  One worker appears to be tamping, one is holding a scoop, one a lantern and as in many jobs of this sort, one  just watching.    The extreme size of the bomb can be appreciated when almost half is below the elevated platform that the workers are standing on.  In order to move the casing into this position, a plate was bolted to the rear of the casing that had a fitting the overhead hoist could hook onto. IWM


(L) Poor quality photograph of armored nose of Grand Slam in H Trolley.  Though this photograph is in monochrome, one can make out on the casing  the thin (red) ring followed by a wide (green) ring with stenciled "Torpex D1." (R) Moving a bomb the weight of the Grand Slam required heavy duty equipment.  All IWM

The first live test of a Grand Slam occurred on March 13, 1945.  The next day, a single Grand Slam was operationally dropped from 16,000 ft during a Tallboy raid carried out by 617 Squadron against the Bielefield viaduct.  Though bombed repeatedly with smaller conventional bombs, the massive spans had never been put out of action for long.   A very near miss by the Grand Slam and several Tallboys produced the predicted earthquake effect that reduced several spans to rubble and halted rail road traffic along the line for the remainder of the war.  In the weeks that followed, several more vital rail road bridges and canals were destroyed.  In all, 41 Grand Slams were dropped during the war. 

By October 1944,  plans were being drawn up to shift RAF bomber forces from duties over Europe to the Pacific Theatre for action against Japan in the fall of 1945.   Dubbed 'Tiger Force,'  up to forty Lancaster squadrons were to be based on Pacific islands captured by the Americans.  These plans included the use of 20 B.Mk 1 (Specials) each from No 9 and 617 Squadrons for dropping Tallboys and Grand Slams.  

At the time these initial plans were made, the nearest American air bases were over 1,500 miles from the Japanese mainland.  In order for the Lancaster to operate at this extreme range, some advanced solutions had to be considered:  in-flight refueling, auxiliary fuel tanks with a reduced bomb load or advancing deployment of the longer range Avro Lincoln.  

In-flight refueling was developed by Flight Refueling Ltd. which involved a tanker and a receiver Lancaster that would fly in formation and top off the receivers tanks after  passing a hose. 15   The second scheme explored was to add saddle tanks along the back of the Lancaster above the center of gravity.  Poor handling with only partial fuel loads lead to abandonment prior to full load testing.  In either case, the overload requirements for Tallboy and in particular for Grand Slam operations would have made successful missions questionable.

Later, plans were to deploy to Okinawa which was about 300 miles from the nearest Japanese mainland targets.  The end of the war ended the requirement for very heavy bombing operations and none of the Tallboy/Grand Slam Lancasters were deployed.


Manufacture of Tallboy and Grand Slam bomb casings at the English Steel Corp. foundry Sheffield.  Precision molded concrete cores were produced then lowered into a sand mould.  After the steel was poured and cooled, the concrete core was carefully chipped out.  The casing was precision machined inside and out to the prescribed dimensions then shipped to one of several Royal Ordnance Factories for filling.  NARA


The casing thickness around the hardened steel nose plug was 7 3/4 inches but tapered to only 1 3/4 inches at the base.  The Grand Slam bomb  with tail was 305 inches in length and 46 inches in diameter.   Overall the color of the bomb was dark green or gray while the tail assembly was left bare metal.  Tail Unit No. 82 Mk I for the Grand Slam was secured to the bomb end plate by twelve ½ inch studs with simmonds nuts.  The tail assembly and the streamline cowling were packed together in crate B.542.16    Markings on the casing were minimal.  At the nose of the casing was a thin red stripe followed by a wider green stripe with a stenciled "Torpex D1" indicating the explosive filling.   The official RAF name for the Grand Slam was Bomb, HE, Aircraft, MC, 22,000 lb.  The U.K. manufactured bombs were initially referred to as Grand Slam Mk I while the American manufactured bombs were called Grand Slam Mk II.  When dropped from 16,000 ft at a 200mph airspeed, the bomb would develop a terminal velocity of 1,097 ft/sec and be rotating at 60 rpm.  17





5.     The American Grand Slam:  Bomb, GP, 22,000-lb, M110 (T-14)

Like the American manufactured Tallboy, the American version of the Grand Slam was produced in cast and welded versions.  The Bomb, General Purpose (GP), 22,000-lb, M110 (T14)  as the American Grand Slam was called, was 150 “ in length, 46” in diameter, filled with 9,600 lb of Tritonal (41.6% of total) and weighed 22,850 lb.  Early production examples were filled with 9,200 lb (41% of total weight) of Torpex with a 1” layer of TNT across the rear of the casing weighing 22,113 lb fully assembled.  An armor piercing plug was fitted to the nose of the cast casings.  

Welded T-14s were assembled from five pieces: a forged nose piece approximately 26 inches long, three cylindrical sections of rolled plate with a longitudinal axis weld and a forged end ring held to the assembly with a stainless autinentic weld not heat treated for stress relief.  The casing was taper bored internally  then set in a die and hammered to form the correct exterior shape.  A quantity of wax was poured into the nose of the casing to act as a desensitizing medium prior to the pouring of the explosive filler. 18

Overall, the casing was painted olive drab green with a yellow band at the nose and tail.  Stenciled in black on the casing was the following information: Type of weapon (GP), weight, model, explosive filler, ammunition lot number, AIC symbol, ICC shipping designation, Inspector’s stamp, the letters “US”, displacement, and may have carried the shipping address and shipping ticket number.  Except for markings, the American and U.K. versions appeared identical. 

Available photographs of U.S. produced casings are likely inert training rounds which carried minimal markings or were painted a high visibility black and white to aid tracking cameras.  Some inert training examples displayed for the public are painted silver or olive drab green overall.

Three fuze pockets in a straight line (unlike the triangular arrangement found on the U.K. Grand Slams) were set in the base plate.  For reliability, three, four inch long M169 (T708)  fuzes with M40 Primer-detonators and the M118 adapter booster were screwed into a fuze seat of 1.375-14TWS-1 thread to a depth of 2.45 inches.   Access to the fuzes was accomplished through circular holes in the tail assembly.  The M121 tail assembly was 160 inches long and 53 1/4 inches at greatest diameter.  Twenty bolts held the  M121 to the casing which brought the total weight to 23,037 lb and overall length of the weapon to 305”.  The cone at the extreme end of the tail assembly could be removed so that the long weapon could be loaded semi-submerged into the joined bomb bays of a modified Boeing B-29.  Engineering studies were even funded to determine the feasibility of an external mating of the Tallboy or Grand Slam to the Northrop XB-35 Flying Wing.  The cavernous bomb bays of the Convair B-36 could easily carry the complete weapon.19 

5a.    The Amazon, Bomb, 22,000-lb, SAP  T28

Though weighing the same as the fabricated Grand Slam, the 22,000lb SAP Amazon T28 was designed to provide better penetration and still be carried by Project Ruby aircraft.  Amazons tested during Project Ruby were supplied in both cast and welded versions.  The welded Amazon was assembled in six sections: a forged steel nose, four 38 inch diameter cylindrical sections with 3 inch thick walls forming the body and a forged base ring.  All sections were welded together in six circumferential welds with a stainless austenitic weld holding the base ring to the body.  An endplate bolted to the casing held three welded fuse pockets forming a triangle.  T723 fuses with adapters and auxiliary boosters were screwed into the fuse pockets.  Though similar in length to the Grand Slam with the tail assembly attached, the casing of the Amazon was considerably longer.  This extra length placed the center of gravity farther aft than on the Grand Slam.  With the release and shackle assemblies fixed in the B-29s forward bomb bay and optimized for the Grand Slam/Tallboy, the Amazon tended to hang unbalance and slightly tail down.  This unbalanced loading caused two malfunctions in the release that resulted in lost casings over the North Sea.  20

15 Amazon bombs were tested during Project Ruby.   During the bombing trials from March 28 through July 19, 1946,  all four of the Amazon bombs dropped from 17,500 ft scored direct hits.  Two inert casings broke through the reinforced concrete roof intact and were ordered retrieved and to be reused (with new tail fin assemblies that had been stored in Germany) or returned to the US.   Though the bombing accuracy from low altitudes was a success, the variable weather prevented accurate bombing from the planned 25,000- 30,000 ft heights.  Overall the results of the Ruby tests proved disappointing.  None of the Amazon or Grand Slam bombs met their design criteria. 21 


7.     Aircraft Modifications

            A. Avro Lancaster B.Mk I   

                                        i.    Tallboy

The Lancasters B.Mk 1s operated by No. 9 and 617 Squadrons had bulged bomb bay doors so that the Tallboy could be carried internally.  These enlarged doors were originally used to carry the 8,000lb and 12,000lb High Capacity Blockbusters.  When loaded with a Tallboy, the Lancaster carried 1,650 imperial gallons of fuel and had a 250 gallon reserve with a 700 mile radius of action. 22 

Loading the Lancaster with such a heavy object as the Tallboy and later Grand Slam required specialized support equipment.  For transport and hoisting into the awaiting aircraft, the H Trolley was modified.  It carried the load on four tires (80 lb nominal pressure) that mandated paved roadways to operate on and had manual jacks for stability while loading.  A 10 ton crane with a power lowering capability carried the bomb on a special Vickers designed cradle and hoisting sling to the trailer lift cradle.  Two chains strung over the top in a cris-cross fashion secured the casing to the trailer while it was transported from the bomb dump to the aircraft.  Loading was accomplished by backing the trailer under the nose of the Lancaster.  (note: An H Trolley was evaluated by the USAAF during the Spring of 1945 for suitability with the B-29 but was found to be too tall to fit under the aircraft and curiously was not supplied with the rolling and traverse gear.)  23 


The H (Heavy) Trolley loaded with a 22,000lb Grand Slam bomb in the spring of 1945.  NARA

Four manually operated 4,000 lb U type winches raised the bomb to contact the sway braces.  Long sway braces were fitted for the Tallboy and a short set was used for the Grand Slam.   The setting template 7/22888 was used to align the tail cone of the bomb along the long axis of the aircraft.  The nose template 1/22888 was also provided but was seldom used.  Small lateral adjustments were accomplished with a displacement gear.  The weapon could be rotated by placing pry bars into slots on the lift cradle rollers.  Typical loading took between 30 minutes to an hour.  A spring loaded pin fit into the center of gravity depression on the top of the bomb casing to prevent rolling while suspended under the aircraft.  As the bomb was winched up, the pin contacted the casing and traveled inside the aircraft on a guide that indicated when the correct height had been reached.  This could only be observed from inside the aircraft.  

The uncomplicated bomb rack on a Lancaster B.Mk 1 Special (Grand Slam).  Adjustable sway bracing could easily accommodate the smaller diameter Tallboy by replacing the turn buckles with longer ones.   The telescoping alignment pin projected  through the hole in bomb bay ceiling.  The links of the support chains hinged from the ceiling of the bomb bay.  The cables used to retract the chain ends into the bomb bay hang down in front of the alignment pin. NARA

The bomb was held fast to the aircraft by two lengths of chain connected by a Vickers release mechanism that was located underneath the casing at the center of gravity.  The release could be operated in either electrical or manual mode.  The five point electrical connection was located fore (Tallboy) or aft (Grand Slam) of the spring loaded center of gravity pin depending on which weapon was carried.  Two cables attached to the chain ends were employed to prevent the chains from striking the aircraft after bomb release.  These cables were then used to draw the chains into the aircraft through a hole in the bomb bay ceiling. 24 

                                   ii.    Avro Lancaster B. Mk I Special (Grand Slam)          

Modification to the Avro Lancaster B.Mk 1 Special (Grand Slam) included removal and fairing over the nose and dorsal gun turrets, removal of the H2S blister.  The bomb bay doors were removed and the more durable tires and wheels of the Avro Lincoln bomber installed.  The front and rear ends of the bomb bay were contoured to improve airflow around the bomb.    

General arrangement view of Avro Lancaster B. Mk1( Special) and Grand Slam bomb setting on an H trolley.  

Avro Lancaster B.MK 1 (Special) PB592/G was manufactured on the Avro production line in Manchester in 1944.  It was assigned to the AAEE at Boscombe Downs along with PB995 in early 1945 for loading and flight testing.  PB592/G did not incorporate all the standard modifications of the B.Mk1 Special as it retained its nose and dorsal gun turrets at least up to the time this photograph was taken.  This photograph shows a Grand Slam loaded with the tail fins aligned in a cruciform and with what appears to be a tarp enclosing the tail and the rear bomb bay.   With this departure from the usual "X" configuration, The upper tail fin might even have contacted the bomb bay ceiling.  NARA

            B. Boeing  B-29 Superfortress  

                                i.     B-29 w/ single Tallboy in semi-external bomb bay installation

B-29 No. 42-63577, was modified at Bell Aircraft Corporation, Marietta, Georgia for use in suitability tests of the Tallboy hoisting, carrying, and releasing.  Arriving at Eglin Field on March 23, 1945, Phase One testing by Wright Field personnel consisted of determining aircraft performance with the bomb.  Phase Two consisted of functional tests with the mechanics of hoisting, carrying and releasing the weapon.  Both phases of the program were expected to take about two weeks each.25  

The AAF Proving Ground Command simultaneously tested a B-29 modified at Boeing Wichita that was capable of carrying either a Tallboy or a Grand Slam bomb and was to supplant the Tallboy only configuration once engineering and testing proved the configuration successful.  A third test program was underway during the summer of 1945 at Boeing Wichita that involved the addition of two under wing racks capable of holding two Tallboys or Grand Slams per aircraft. 26 


Single Tallboy conversion for B-29 looking fore and aft.  Both USAF



Loading the Tallboy into the bomb bay of  a B-29 (L).  The cutout bomb bay doors (C) provided good streamlining around the bomb casing that extended half outside the aircraft.  Once the Tallboy was dropped, the large hole in the bomb bay doors created an enormous amount of drag.  Note the difficulty in clearing the wing carry through that separated the two bomb bays and the standard bomb racks that were retained in the rear bay to allow 500 lb bombs to be carried on a Tallboy mission (R).  All USAF

Modifications to the B-29 consisted of shortening and strengthening the forward and aft bomb bay doors providing a cut out for clearance for the weapon that was carried semi externally due to its large size.  The fixed bulkhead under the wing spar that separated the two bomb bays was modified into two longitudinal bulkheads with a cut out providing clearance for the top half of the bomb, which extended into the second bomb bay.  Oxygen bottles, tubing and other equipment that was formerly installed in the area were relocated.   A removable H frame, attached by 4 bolts to fitting near the front wing spar, featured sway bracing, a fore and aft load centering pin and an attachment point for an A-4 bomb release with D-7 shackle.  Four removable pendant-type manually operated hoists attached to the corners of the H-frame were used to hoist the weapon.  Special metal hoisting slings with chocks were used to support and position the weapon on the ground prior to loading.  Proper positioning under the rack was accomplished when a shallow 2-1/8 inch hole in the bomb casing,  marking the weapon center of gravity, was aligned under a plumb-bob suspended from the  carrying pin on the H frame.   A pair of triple width chains carried the load.  A small windlass in the navigator’s station attached to a cable threaded through the chain links provided retraction.  A turnbuckle attached at the end of each chain to the H frame ensured that the loaded bomb was held fast.  The loading procedure required nine men:  two to operate each  of  the four chain hoists and one additional man to oversee the hoisting process and  keep the bomb centered in the rack. 27  

Testing uncovered several problems requiring a fix prior to deployment:  

During practical tests of the configuration, fourteen Tallboy casings were sand loaded to the proper weight and dropped at various altitudes with good accuracy.   Even though several casings exhibited yawing during fall, the 10,000ft average circular error was only 50.65 ft; 20,000ft avg. cir. Err was 185.6 ft; 30,000 ft avg. cir. Err. Was 159.4 ft. 

The final testing report dated June 11, 1945 recommended that a B-29 group equipped to carry the 12,000 lb Tallboy be formed as quickly as practical and only be used against specialized targets selected by the Joint Target Group.28  


                                ii.    B-29 w/ single Tallboy or Grand Slam in semi-external bomb bay installation

Boeing Wichita modified B-29A loaded with a single Grand Slam bomb. This same photograph was also used in Boeing publicity releases to illustrate the single 42,000 lb T-12 installation.  A check at the Boeing Archives has confirmed that this photograph depicts a B-29 loaded with a Grand Slam  Thien Collection of the Kansas Aviation Museum via Deborah Livengood

Early summer 1945, three Boeing B-29s were modified to carry a  22,000 lb Grand Slam on external bomb racks under each wing between the inboard engine and fuselage.  After experiments at Eglin AAF in Florida, an order for fifty modified aircraft was placed.   If the war hadn’t ended with the dropping of the atom bombs, dual Grand Slam B-29s might have been available to see action over Japan by the first week of  September.  Postwar publicity photographs never show more than three of the dual bomb modified B-29s flying in a formation.  That may have been all that were converted.  Another B-29 was modified so that the lower part of the double bomb bay section would permit the semi external carriage of one large bomb (Tallboy, Grand Slam or T-12?). 29  

                                iii.    B-29A w/ under wing bomb racks for Tallboy & Grand Slam Bombs

An order of 50 B-29s to be fitted with under wing racks was placed in the early summer of 1945.  Conversion work was undertaken at the Boeing Wichita Plant.  These racks could carry one Tallboy or Grand Slam under each wing.  Alternate loads of four 1,000lb, four 2,000lb or four 4,000lb bombs could be carried.  This modification did not materially alter the existing bomb bay and allowed additional fuel tankage in both bomb bays when the wing racks were in use.  Preliminary flight testing at Wichita uncovered no unexpected buffeting or vibration even when provoked with trick power settings and deliberate stalls.  These aircraft would also have had the latest improvements: APQ-13 Radar w/Modil Computer for bombing through overcast, Curtiss electric propellers and fuel injected engines.

The fifteen aircraft assigned to the 886th Squadron of the Twentieth Air Force were to be based at North Field, Tinian and to be tactically available in mid September 1945.  Only if it became required by practical necessity would the 886th be transferred to the 8th Air Force.  

Since the range of the B-29 with two Grand Slams under the wings was only about 300 miles, it was incompatible to fly with Tallboy missions where the range was upwards of 2,100 miles.   It was also determined that the number of targets suitable for the Grand Slam was considerably less than those for Tallboys, with an estimated monthly requirement of only 125 Grand Slams versus 600 Tallboys.  Because of these differing requirements, it was decided that the 886th would carry only Grand Slams and a second unit of 15 B-29s, attached to the 450th Group would carry the Tallboys.30 


Detail of a B-29 carrying two Tallboys on pylons located near the wing root.  At least three B-29s were converted to carry Tallboys or Grand Slams in this configuration.  USAF

                        C.  Consolidated B-32 Dominator

The Consolidated B-32 was the runner up design placed in production as a hedge in case the Boeing B-29 failed to meet expectations.  Production problems delayed the program to the extent that only 115 were built by the end of WWII.  A single squadron of 15 actually saw combat in the PTO.  

In late spring 1945,  B-32-20-CF 42-108535 was modified under Project Albert, to carry the Tallboy and Grand Slam bombs internally.   This aircraft was attached to the Army Air Forces Proving Ground Command, Very Heavy Bombing Unit (Squadron E, 611th AAFBU), Eglin Field, Florida until the B-32 program was officially terminated in October 1945.  


Modifications were to include  replacement of the sliding bomb bay covers (that caused a 9 mph drop in speed during the bomb run) with snap opening hinged doors similar to the type found on the B-29, the relocation of numerous antennas that were mounted along the keel like partition that divided the large bomb bay into side by side parts.   This keel structure, which provided a catwalk connecting the front and rear of the aircraft, may have been heavily modified or removed altogether.   Once modified, the B-32 would have been able to carry two Tallboys, or one Tallboy and one Grand Slam side by side within the bomb bay.   The Tallboy would release vertically while the Grand Slam would have exited the aircraft at a 17.5o angle.  The engineering challenges of loading and releasing a Grand Slam at such an angle had not been overcome prior to cancellation of the project.

The length of the 305 inch Grand Slam (with tail) also proved too long to fit the 297 inch long bomb bay so the tail was shortened 18 inches.  This short tail design caused considerable confusion and delay with the engineering of the semi-external bomb bay installation for the B-29 as previous design and ballistic data had been developed around the full length round.  

By late July, the B-32 test program was going so slow it was clear to planners that it would not be ready for very heavy bomb sorties for the foreseeable future and the short tail was cancelled.  Since the 305 inch Grand Slam was going to be the only version to go to war, work on the B-29 bomb bay conversion got back on track without having to worry about ordnance optimized for the B-32s limitations. 31  


8.     Postwar Developments

            A. Project Ruby

War time analysis of Tallboy and Grand Slam attacks revealed that their effectiveness against reinforced concrete structures was questionable. Problems associated with casing break up and explosive sensitivity required testing under conditions similar to those on the battlefield order to better understand why these specially designed bombs failed to penetrate hardened targets. 

Post war testing by The RAF began in June 1945 when the V-weapon structure at Watten was bombed.  Additional testing was undertaken with American cooperation in August 1945 at the submarine assembly factory at Farge, Germany.  AAF participation was limited to providing a B-17 attached to the 40th Combat Wing to drop Disney rocket assisted concrete penetration bombs for the RAF.  Prior to the arrival of the test ordnance from the US, the AAF detachment maintained proficiency by dropping 100 lb sand filled bombs and inert filled CP/RA bombs at the Ordfordness bombing range.  Additional practice bombing with sand filled bombs but not CP/RA bombs may have been conducted at the Rushford Bombing Range.  

A joint Anglo-American test program, called Project Ruby was initiated in the spring of 1946 to determine the cause of earthquake bomb failures during the war and test new penetration bomb designs against captured Nazi fortifications under ideal conditions.   No controlled tests of large penetration bombs had ever been made prior to Project Ruby and after an expenditure of $380,000 for the US Army Ordinance Corps. to develop munitions capable of penetrating 32 ft of reinforced concrete, it was essential that practical tests take place.   At one point, the AAF considered deep penetration aerial bombs as second in importance only to the atom bomb insofar as the US offensive war potential was concerned.

The original objectives of these tests were to study the strength of casings, determine the detonation and fuzing system sensitivity,  reliability of fuzes/primers/boosters and the sensitivity of the explosive filling of several penetration bomb designs.  The target for these tests was the U-boat assembly pens at Farge that had a steel reinforced concrete roof structure 16 ft thick.  The RAF had dropped 13 Grand Slams and 4 Tallboys on the Farge complex on March 27, 1945.  Two Grand Slams successfully penetrated the roof during that raid.   The mission objective to determine if the explosive filling was too sensitive and the cause of pre detonation problems was later dropped when a survey of the Farge site revealed that many important roads and industrial sites were so close that live trials posed too great a risk.  Since the tests were to determine depth of penetration, not blast effect, the bomb casings were sand filled.  Live trials were moved to the abandonded Heligoland U-boat shelter in the British Sector while inert loaded/penetration trials took place at Farge.   32 

Demobilization after the war threatened to derail further American participation due to personnel loss and UK base closures.  The infrastructure required to support a single B-17 had become very difficult.  As a result, a decision was made to create a permanent AAF detachment of three B-17 aircraft and support personnel at RAF Station, Mildenhall.  The unit was activated on January 10, 1946.  In March of 1946, three B-29s and four B-17s were added to the Mildenhall detachment and the combined group moved to RAF Station, Marham for the test program that was now called Project Ruby (UK codename: Operation Front Line).  All bombs supplied by the U. S. Director of Armament Research and Development carried the special markings: "PROC Q.3304" and a serial number.  Headquarters, bomber Command for the ETO provided one 10 ton crane and the required trailers for the tests. 33 

Since the British designed H Trolley was too tall to fit under the B-29, a novel method of loading bombs larger than the Tallboy was devised.  A trench was excavated about one foot deep in order to give a Boeing trailer loaded with a Grand Slam enough room to clear the tail skid of the bomber.  The aircraft was then backed over the trench until the bomb bay was directly over the weapon.  The bomb was then raised to the aircraft using the internal hoists.

The American bombs tested included the fabricated (welded plate) Tallboy (T-10) and fabricated Grand Slam (T-14), the 2,000 lb SAP (M103) and a new 22,000 lb SAP design called Amazon (T28).  The British bombs included the cast Tallboy and Grand Slam, the Disney and the 1,650 lb scaled version of a proposed 12,000 lb rocket assisted concrete penetrating bomb. 34 


The “Valentin” submarine assembly factory at Farge, Germany was approximately 14 miles downriver from Bremen on the Waser River.  It was designed for the building of the 1,600 ton displacement Type XXI submarine.  Work began on the structure in March of 1943 and was 90% complete by the end of the war. 

The structure was 1400 ft long and measured between 318 ft and 220 ft in width.  The massive concrete footings extended to a depth of 55 ft in some places.  The outer walls were poured in layers 67 ft long and 14 ft 9 in thick.  An effort to make the walls a uniform 23 ft thickness was underway but less than 1% had been completed by the surrender.  

The average height of the building was 74 ft with a roof thickness of 14 ft 9 in.   At the west end of the structure, extending 267 ft along the east/west centerline, the roof was 17 ft thick and sloped to 14 ft 9 in at the north and south walls for drainage.   At the east end of the structure, approximately 40% of the roof thickness had been increased to 24 ½ ft along the east /west centerline and sloping to the north and south walls where the roof was 23 ft thick.  The height of the structure over this area was about 82 ft above grade.   Four different types of roof construction were employed but the primary one consisted of pre stressed reinforced concrete arch trusses. 

The concrete used during construction either employed a slag mix or Portland cement.  The design strength of 5,675 psi was typically not met due to quality control problems in the mixing and the forced use of low grade aggregate. 35 


Since live bombing trials were too risky at Farge,  all explosive sensitivity tests were undertaken at the U-boat shelter on the uninhabited North Sea island of Heligoland.  The structure was 506 ft in length and 310 ft wide.  The roof thickness was a approximately 10 ft. 36 

Chart showing comparative sizes of Project Ruby bombs.  USAF  

Tests revealed the following: 

None of the bombs tested could penetrate the Farge roof at the 23 foot thickness.  All explosive fillers tested were insensitive to impact detonation.  The types tested were:  British (cast) Tallboy:  though able to penetrate reinforced concrete to 5’7” at 1150 ft/sec impact velocity, all casings suffered total breakup at impact at velocities over 620 ft/sec. None could be expected to survive a drop from 15,000 ft!    The hardened steel nose plugs were fractured and in many cases the bomb would ricochet at impact.  The British Tail Pistol No. 47A Mk II installed on the cast Tallboys and Grand Slams functioned reliably during the tests.  

American (welded) Tallboy:  would survive better than 50% of concrete impacts at velocities of 610 ft/sec and would likely ricochet and dent.  As the tests progressed, attempts were made to strengthen the welds that secured the end ring to the bomb body.  One casing had several weld beads added to the inside of the ring while two others had additional welding on the outside of the end ring.  The welding on the exterior of the casings was done solely because the desensitizing wax had already been applied to the interior of the casings prior to the decision to weld the end rings.  One of each reinforced bombs hit the target and both of these suffered end ring separations.  Failure of these rear welds, even when reinforced proved to be where casing breakup would begin. 

American (welded) Grand Slam: would survive better than 50% of concrete impacts at velocities of 610 ft/sec with likely ricochet and denting, but breakup at impacts of 850 ft/sec or greater.  The casings broke up after secondary impact with the crater wall which caused the failure of the stainless austenitic welds that joined the base ring to the casing. 

The tests showed that the Tallboy and Grand Slam shapes were not properly dimensioned for concrete penetration.  The welded American casings held up better than cast British but none could withstand breakup after impact from high altitudes.  The explosive fillers proved to be insensitive enough to survive the impact from high altitude drops.  The D-9 shackle used on the B-29 was better suited for the Grand Slam but not the Amazon due to differences in diameter and center of gravity.  Two Amazon 'hangs' were attributed to the fixed shackle location being too far forward of the center of gravity for a clean release.

The American (welded) Amazon was able to survive initial impact on concrete at 1100 ft/sec.  Side wall failure would occur upon secondary impact after passing through 14’9” concrete roof.  Breakup of the bomb casing was due to failure of the rear stainless austenitic weld.  All other welds held.  The T723 fuzes installed in the end plate did withstand all impacts but would not survive a breakup of the casing.  Tests conducted by Picatinny Arsenal on the M39 Special Primer proved that it might not reliably ignite the delay powder charge.  The auxillary boosters filled with Composition A were insensitive to impact detonation in all tests.  While the Amazon was properly dimensioned for concrete penetration, the sidewall strength was insufficient to prevent side impact breakup and the stainless austenitic welds in the rear were too weak to hold.  Vertical penetration was calculated to be 11 ft  2 in.  If dropped from 35,000 ft,  the Amazon was calculated to penetrate as much as 22 ft- still short of the 23 ft at the thickest part of the Farge roof.  This also assumed that the casing could withstand the much higher impact velocities and would impact at a near vertical angle.

Recommendations resulting from the tests asked for a  new design featuring similar weight/charge as the Amazon but with a more pointed nose, smaller diameter and a casing resistant to bending by use of multiple layer walls, internal ribs/ corrugations or use of special alloys.   It was also desirable to develop a means of delivering the bomb so that the line of travel was perpendicular to the surface at the moment of impact to maximize penetration and reduce sidewall stress.  Develop a universal bomb shackle/release that would handle bombs of all weights, lengths, diameters and centers of gravity.  37 


            B. Harken Project

As a follow on to Project Ruby,  Harken Project was initiated to begin testing refined weapons from Ruby as well as more recent designs beginning April 1, 1947.  The Military Air Attaché, London was instructed to make inquiries for the use of an airbase on British soil that could handle three modified B-29s operating at a gross weight of 135,000 lb. and approximately 200 personnel.  A port facility to receive bombs weighing 25,000 lb, with a length of 218 inches (without tail fin assembly), diameter of 32 inches and with suitable transportation facilities to move the bombs overland to the airbase of operations.   In exchange for participation and logistical support, the RAF was allowed to observe the test of an improved 22,000 lb T2S Amazon bomb.  Once again, the submarine assembly pens at Farge, in the British Sector were be the target.38  

The testing of two new penetration designs was to be ready beginning April 1, 1947 and would be conducted simultaneously.  35 large inert bombs of two types, each weighing about 25,000 lb, code named Amazon II (T28E1) and Samson (T28E2) were tested during a program that was to take approximately 4 to 6 months to complete.39 

Results were expected to be good as stateside range bombing with the Amazon II and Samsom bombs which showed that actual range errors were less than 1 mil from provisional bombing tables 22,000-B-0 used with bomb T28E1 and 25,000-A-0 used with bomb T28E2.  Because of previous experience with casing breakup,  Matt Dilot of bomb manufacturer A.O. Smith Corp. was authorized to observe the tests.  40  

The Amazon II (T28E1) weighed close to 25,000 lb while the Samson (T28E2) weighed 25,200 lb at the time of the Harken Project tests.  The comparative specifications of the two bombs was as follows:  

    Basic profiles of the Amazon II and the Samson bombs taken from APG, Ballistic Research Laboratories Report No. 712 

T-28E1  Amazon II:  

    38"             diameter

    154"           casing length (w/o tail assy.)

    4.5"            sidewall thickness

    20"             maximum thickness of nose section

    4,200 lb      weight of explosive (17% of total wt.)

    25,000 lb    overall weight


T-28E2 Samson:

    32"             diameter

    196"           casing lenght (w/o tail assy.)

    4.125"        sidewall thickness

    20"             maximum thickness of nose section

    3,900 lb     weight of explosive (15% of total wt.)

    25,200 lb   overall weight

Examination of the recovered bomb casings revealed that the maximum stress on the bomb bodies occurred at the join of the ogive nose section and the sidewall.  Calculations estimated the Amazon II and Samson casings underwent impact accelerations of -30,700 ft/sec2 and -37,000 ft/sec2 resulting in compression forces on the order of 29,000 lb/in2 and 42,700 lb/in2-  all values well below the maximum compression specification of  100,000 lb/in2.  Testing identified a weakness in the rear weld of the base ring to the bomb body of the Amazon II which lead to numerous breakups upon perforation of the roof or upon secondary impact.  Several of the bombs were recovered intact and undeformed.  Overall the tests showed that both bombs could be expected to survive impact velocities of up to 1100 ft/sec on reinforced concrete stuctures. 41

9.    Bomb, SAP, 25,000-lb, T28E4  

Development work on the 25,000 lb SAP bomb probably began in 1946 with practical testing during Harken Project of 1947.  Project Ruby had pointed out short comings in the Grand Slam design which failed on several occasions to penetrate the massive, though uneven quality concrete roof of the Farge site.  From the photograph below, the overall length of the bomb was about the same as the Grand Slam.  The bomb casing was 199 1/2 inches in length and had a diameter of 32 inches.  The tail assembly was mounted to the casing by sixteen studs in the base plate.  Another source described the 25,000 lb bomb as being 218 inches long with a 32 inch diameter.  The tail section measured 87 inches in length in stowage position and weighed about 800 lbs.  Unlike the Grand Slam, the nose of the bomb was a solid piece without an armor piercing plug.  Development work on this bomb was still underway in 1950.42   

This is a montage photograph showing what appears to be an Amazon II T28E1  (L) and a much smaller diameter bomb (R) that likely is the 25,000 lb T28E2  SAP Samson.   A.O. Smith Corp. via Charles S. Wright


10.  Bomb, GP, 42,000-lb, T-12

Faced with the possibility of fighting a war in Europe without the benefit of bases in England, the USAAF initiated an intercontinental bomber program in April 1941 for the design of aircraft capable of carrying a 10,000lb bomb load 10,000 miles.  Two aircraft designs were funded: the Northrop XB-35 Flying Wing and the XB-36 Peacemaker. Neither aircraft design would realize a flying prototype before the end of the war and only the B-36 would be built in quantity.  By late 1942 the threat of a Nazi invasion of Britain had passed, but German U-boats were still taking a staggering toll on Allied ship convoys in the North Atlantic.  

The safe haven for these U-boats were the immense submarine pens under construction on the West coast of France.  With roofs thought to be made of steel reinforced concrete up to 50 feet thick, these pens would have been impenetrable to the heaviest aerial bombs in existence.  Even the 12,000 lb Tall Boy bomb developed for the RAF to destroy hardened or deeply buried targets could not penetrate.43  

The Nazi benchmark for a bomb proof structure was steel reinforced concrete 23 ft thick.  It was calculated that it would take a 27,000 lb bomb striking at a precise angle and at Mach One to punch through such a structure.  This benchmark fails to take into account that it is not necessary to completely penetrate a structure to cause damage inside.  The shock waves produced by a large explosion can displace the reinforcement bars causing a 'scab' to detach from the face opposite the one impacted.  

The Tall Boy bomb could penetrate concrete to almost 10 ft and scab to a depth over 18 ft.  If the target roof was only 16 ft thick, then a large scab could be detached from the inside face of the structure crashing down on anything underneath it. The heaviest weapon used during the war was the 22,000 lb Grand Slam bomb which could penetrate concrete to 12 ft and scab to a depth of almost 23 ft. Because the danger of falling scabs would have been greater than that of outright penetration, a mattress of steel beams was added on the underside of many structures to provide additional strength and catch concrete detached from the ceiling.44  

Modeled somewhat on the Tallboy and Grand Slam bombs, design work began in 1942 on what would become the enormous T-12 general purpose bomb. Though designed to weigh approximately 42,000 lb, the practicalities of manufacturing drove the weight to 43,600 lb,  41% (17,600 lb) of which was high explosives.  The resulting bomb was nearly twice the size and weight of the Grand Slam.  The bomb was 200 inches long, 54 inches in diameter and assembled from six sections of steel welded together.  At a total length (with tail assembly) of 322 inches, the T-12 was not merely a scaled up Tallboy or Grand Slam.   Several improvements were made by the Ordnance Corps. engineers at the U.S. Army's Aberdeen Proving Ground to correct features that had been copied by war time expediency directly from the Tallboy.  By the end of the war, the T-12 had emerged as a purely American bomb design.  45  


This T-12 casing stands on its nose at the courtyard of the Aberdeen Proving Ground Museum building (L).   Note the short tail cone when compared to the line drawing below  from official scale drawings.  Several different tail fin configurations were tested and the one used in this display might have been designed for delivery by Boeing B-29 where there was limited space in the bomb bay.  (C) Close up of the rough finish and weld lines in the nose area.  (R) Shot of tail cone showing large tail fins offset 5 degrees to good advantage.  J. Godwin collection

The long length of this weapon made it impossible to stow completely inside carrier aircraft during the war and created enormous drag and ballistic problems when carried externally.  But a long tail provided the stability essential for precision delivery to the target. The original British design called for tail fin assemblies made of aluminum. These were replaced with assemblies made of steel. The assemblies were attached to the base plate of the bomb by 24 bolts.  One suggestion by the AAF was to design a collapsible fin assembly. This scheme was rejected on the grounds that it was overly complicated, prone to delay or failure while opening, which in turn would cause range and deflection errors.  Precise placement of these earth quake bombs was essential to achieve maximum effect.  British fuzes and detonators had been incorporated into the design initially to save time.  These fuzes had no in flight arming feature and this problem was one of the first tackled by Ordnance Corps engineers.46   

Work to convert the Renton built Boeing B-29A-70-BA, s/n 44-62263 to carry this weapon was undertaken at Boeing, Wichita during the summer of 1945.  Flight testing continued through 1946.  Modifications required were the removal of the fuselage center section between the bomb bays, making a cutout in the forward and rear bomb bay doors to fit the contours of the bomb,  instruments to record dynamic forces on the airframe during release and recording cameras were installed.  Testing  followed at Muroc AAFB.  Six inert casing drops at an altitude of 25,000 ft were made from March 5, 1948. 

Once available, a Convair  B-36 bomber joined the test program.  The B-36, first flown in 1946, had enormous bomb bays that could easily carry two T-12s or an alternate load of one T-12 and two Tall Boys or Two Grand Slams. On January 29, 1949, B-36B-5CF, s/n 44-92043 flew a 2,900 mile round trip from Ft. Worth, TX to the Muroc AAFB bombing range dropping two T-12s from 35,000 ft and 40,000 ft.  At least one fully armed T-12 was tested during the program.47   


Illustrations of the loading and lift sequence for the 50,000 lb hydraulic bomb lift.  USAF via Dave Cross

From early on it was determined that no practical hoist system could be installed inside an aircraft so Boeing developed the USAF Model No. X50J29603 hydraulic bomb lift.  Designed as a low-slung trailer-type vehicle, it could be used to lift bombs from ground clearance blocks then transported over short distances for loading into aircraft.  Bombs weighing up to 50,000lb and of diameters from 32 to 54 inches could be handled.  A maximum towing speed of 20 mph was possible on paved surfaces when loaded, 30 mph unloaded.   The trailer was equipped with electric brakes on the rear wheels powered by the tow vehicle.  In the event of a breakaway, the trailer brakes lock automatically.   A lever located near the rear wheels allowed manual brake operation.  Loading was accomplished by backing the trailer down the length of the bomb.

The hydraulic system was electrically powered.  A 28-volt receptacle for external power was located at a control pedestal on the left side at the front of the trailer.  Six control levers manipulated pitch, leveling jacks, lift cylinders and roll motors while yaw movement was performed manually.  Hydraulic leveling jacks at the front stabilized the trailer.   Slings run under the bomb casing were connected to four initial lift cylinders that raised the bomb from ground level to a sufficient height to permit the 5,000lb main lift cylinder carriage to be moved underneath.  Mounted on four grooved wheels in steel tracks, the main lift cylinder carriage was locked at the front of the trailer while the initial lift cylinders raised the bomb.  After the main lift carriage was rolled under the bomb center of gravity, the initial lift cylinders were lowered until the bomb was at rest on the main lift cradle. The main lift cylinder was made of four telescoping sections with a total height range up to 150 inches with a 22,000lb Grand Slam bomb and to structural limits of 85 inches with the T-12.  The main lift cylinder could lift a 50,000 lb bomb 85 inches in about 90 seconds.  With a multi-axis movable bomb cradle at the end of the lift piston, the bomb could be manipulated four inches side to side, ten inches forwards and backwards, tilt up and down 6 degrees or be rolled 360 degrees.  Because of low ground clearance with the B-29, the lift was rolled into a pit and the aircraft backed over it.48      

Multi-axis bomb cradle could precisely load a T-12 bomb in the rack.  With some modification, the bomb lift for the T-12 was used to transport and load the 64 inch diameter  TX-14 Thermonuclear Bomb that weighed upwards of 50,000 lb.  Other TN weapons, like the TX-17/24 followed, which weighed approximately 42,000 lb.

Additional development and testing was carried out until the program was terminated on August 11, 1954.  Earth penetrating nuclear weapons that weighed a fraction of the T-12 had made large conventional bombs obsolete.  The multi-megaton first generation Thermonuclear weapons tested during the Summer of 1954 had been given the green light for production taking the 50,000 lb bomb lifts that had been attached to the T-12 program.  The slings, H-frame and shackles already in B-36 bombers  that had been developed for the T-12 were readily converted for the new TN weapons.49  

The Army Ordnance Department had placed a war time order of 100 but at the time of contract cancellation only 57 casings had been produced.  By that time of cancellation, 50 forged steel and seven cast steel bombs had been assembled by the A. O. Smith Corporation. All seven of the cast bombs and a few of the forged bombs were inert loaded to test ballistics and mating with aircraft.   Tritonal was loaded into 20 of the forged casings. Several experimental tail assemblies designated T104, T104E1 and T104E2 were tested with the T-12 bomb. At least one bomb with a T104 assembly was recovered from the hardpan at Edwards AFB after a test drop from 25,000 ft at 300 mph TAS for analysis of the angle of entry, depth of penetration and path through the ground.  The results of those tests prompted the manufacture of 36 T104E3 fin assemblies from which seven were sent to Edwards AFB for proposed higher altitudes ballistic tests.  It is unclear if these tests were ever carried out since at the time of termination a B-36 aircraft had not been available.50 


  The Earthquake Bomb family:  (from L to R)  12,000 lb Tallboy, 22,000 lb Grand Slam, 42,000 lb T-12, and the 25,000 lb SAP Samson 


11. Epilogue

This history would not have been possible without the help of many interested people.  In particular:  David Cross, Deborah Livengood, Charles S. Wright of A.O. Smith Corporation, Nan Myers, Government Documents Librarian, WSU, and to a friend from Scotland who wishes to remain unnamed, many thanks for guidance and information over the years.  


1 The Aeroplane, May 31, 1946, Bombs Versus Concrete,   p. 634

2 Barnes Wallis, A Biography by J.E. Morpurgo,  St. Martin’s Press, New York, 1972,   p. 279

3 Bombs Gone: The development and use of British air-dropped weapons from 1912 to the present day, by Wing Commander John A. MacBean and Major Arthur S. Hogben, Patrick Stephens Limited, Wellingtonborough, Northhamptonshire, England, 1990, p. 138-139 

4 Barnes Wallis, A Biography  p. 279 

5 Bombs Gone: The development and use of British air-dropped weapons from 1912 to the present dayP. 140-141, 142;  During WWII, A.O.Smith Corporation of Milwaukee, Wisconsin used innovative methods to produce nearly 5 million bomb casings from 3,400 miles of welded pipe.  Most were of the 500, 1,000 and 2,000 lb types but they also produced the giant 22,000 lb and 44,000 lb casings that were assembled with the same hand welding methods used in the production of pressure vessels.  History of A.O.Smith Corporation, Chapter IX- The War Years, no author, A.O.Smith Corporation, Milwaukee, WI,  Xerox copy of mimeograph edition c.1960? provided by Charles S. Wright of A.O.Smith   p. 10-11 

6 Ibid. p. 155;  The United States Army in World War II,  The Technical Services, The Ordnance Department:  Procurement and Supply, by Harry C. Thomson and Lida Mayo, U. S. Government Printing Office, 1960.  p. 147;    British Explosive Ordnance; Naval Ordnance Systems Command; Navord OP 1665, 10 June 1946 p. 49-50

7 Report of the Army Air Forces Board, Orlando, Florida, Information Compiled by Large Bomb Commission, European Theatre of Operation, Study of the Requirements, Employment and Effectiveness of Large Bombs, 26 April 1945, Project No. 4614A471.6  P. 62-63, 66

8  Bombs Gone: The development and use of British air-dropped weapons from 1912 to the present day,  p. 155, 292-293

9  According to  British Explosive Ordnance; Naval Ordnance Systems Command; Navord OP 1665, 10 June 1946 p. 49-50,  7/16 inch studs held  the tail cone to the after body. 

10 Ibid,  p. 146  Some sources refer to the bomb as a D.P. (Deep Penetration) type since the design was for special operations requiring deep penetration, not armor or concrete piercing.

11  Comparative Test of the Effectiveness of Large Bombs against Reinforced Concrete Structures (Anglo-American Bomb Tests- Project RUBY), Air Proving Ground Center Eglin AFB, FL, October 31, 1946, DTIC copy.   p. 29  

12 Bombs for Aircraft, TM 9-1980- AFM-136-7, United States government Printing Office, Washington, 1950, P. 12, 16, 198;  The largest bombs were made of rolled plate because (early in the war) there was no seamless tube made of sufficiently large diameter.  The United States Army in World War II,  The Technical Services, The Ordnance Department:  Procurement and Supply, by Harry C. Thomson and Lida Mayo, U. S. Government Printing Office, 1960.   p. 120-121

13  Bombs for Aircraft, TM 9-1980- AFM-136-7, United States government Printing Office, Washington, 1950,  p.309, 198

14  The United States Army in World War II, The Technical Services, The Ordnance Department:  PLANNING MUNITIONS FOR WAR,  by Constance McLaughlin Green, Harry C. Thomson, and Peter C. Roots.  U.S. Government Printing Office, 1955.  p. 471-472

15  This method was adopted by the USAF for extending the range of post war strategic force B-29s.   Tankers were designated KB-29M and receivers were B-29MR.

16  Report of the Army Air Forces Board, Orlando, Florida, Information Compiled by Large Bomb Commission, European Theatre of Operation, Study of the Requirements, Employment and Effectiveness of Large Bombs, 26 April 1945, Project No. 4614A471.6 P. 66-67  

17  Bombs Gone: The development and use of British air-dropped weapons from 1912 to the present day, p. 199, 155;  British Explosive Ordnance; Naval Ordnance Systems Command; NAVORD OP 1665, 10 June 1946 p. 51

18 Comparative Test of the Effectiveness of Large Bombs against Reinforced Concrete Structures (Anglo-American Bomb Tests- Project RUBY), Air Proving Ground Center Eglin AFB, FL, October 31, 1946, DTIC copy.  p. 29-30 ;  British Explosive Ordnance; Naval Ordnance Systems Command; NAVORD OP 1665, 10 June 1946 p. 51

19  Bombs for Aircraft, TM 9-1980- AFM-136-7, United States government Printing Office, Washington, 1950  P. 199,  309.

20  Comparative Test of the Effectiveness of Large Bombs against Reinforced Concrete Structures (Anglo-American Bomb Tests- Project RUBY), Air Proving Ground Center Eglin AFB, FL, October 31, 1946,   p.37

 21  Ltr from Army Air Forces Proving Ground Command HQ-Project Ruby, RAF Station Marnham, England to Commanding General, AAF, Washington, D.C. via AAF Center Liaison Office;  memo from SAC Andrews Fld. to USAFE (CO Harken Detachment Giebelstadt, Germany),  14 September 1947.

22  Report of the Army Air Forces Board, Orlando, Florida, Information Compiled by Large Bomb Commission, European Theatre of Operation, Study of the Requirements, Employment and Effectiveness of Large Bombs, 26 April 1945, Project No. 4614A471.6  p.16

23  Ibid p.17, 67

24  Ibid

25  Preliminary Report on Test of Tallboy Bomb Installation in B-29 Airplane, Tests conducted by AAF Proving Command Eglin field, Florida, Project No. 4474C471.6, Report of the Army Airforces Board, Orlando Florida, 11 June 1945. p. 6, 19

26  Ibid, p. 3

27  Ibid, p. 10, 11, 12, 45

28  Ibid, p. 12-16, 53, 3 

29   Boeing Magazine, February 1946, p. 13

30  It seems absurd to base underwing GS operations on Tinain as it was well beyond range of front line targets.  Okinawa would have been a more logical choice since it was about 300 miles from targets on the extreme southern end of Kyushu and  Formosa.  Okinawa was also the basing of choice for RAF Tiger Force Lancasters of the 9th & 617 Squadrons. 

31  Dominator, The Story of the Consolidated B-32 Bomber by Stephen Harding & James I. Long, Pictorial Histories Publishing Company, Missoula, Montana, 1984.  p. 9, 18, 28-29, 57;  Consolidated B-32 Dominator, The Ultimate Look from Drawing Board to Scrap Yard by Dr. William Wolf, Schiffer Publishing Ltd., Atglen, Pa, 2006, p. 154, 158; Weekly News Letter, Head Quarters United States Army, Strategic Air Force (Rear Echelon), Washington 25 D.C., 20 July 1945,p. 4  Weekly News Letter, Head Quarters United States Army, Strategic Air Force (Rear Echelon), Washington 25 D.C., 27 July 1945, p. 2

32  Memo:  Use of Sub Assembly Pens at Farge, Germany, HQ AAF-AFACC,  22 October 1946. ;  The Aeroplane, May 31, 1946, Bombs Versus Concrete   p.635-636;  Ruby Project report, typed report by (?) Lt. Baum (sic), no date, Project Ruby files, Maxwell AFB, AL USA. 

33  Comparative Test of the Effectiveness of Large Bombs against Reinforced Concrete Structures (Anglo-American Bomb Tests- Project RUBY), Air Proving Ground Center Eglin AFB, FL, October 31, 1946, DTIC copy.   p. 62  Testing began on March 25th  and was completed on October 30th.

34  The Disney bomb was a ~4,500 lb rocket assisted concrete/armor piercing bomb.  Designed to be dropped from 20,000 ft, a tail mounted set of rocket motors would ignite at 5,000 ft accelerating the weapon to a high impact velocity.   The forged and hardened steel cylinder body with a sharply pointed nose contained about 500 lbs of Shellite (similar to US explosive compostition D).  In the base of the bomb, two British No. 58 Pistol fuzes were mounted.  The tail unit attached to an adapter plate mounted to the base of the bomb.  Two suspension lugs were bolted to the bomb casing.  The tail unit contained 19 3” rockets bundled together weighing about 900 lbs.  Each motor contained 12.5 lbs. of cruciform shaped flashless cordite rocket propellant SU/K/X.  An M111 mechanical time fuze activated a switch allowing electrical ignition of the rocket motors that was powered by a wind driven generator mounted on a conical fairing at the tail.   The bomb was carried on external racks.  Individual pullout wires were used to arm the M111 time fuze, the pistol fuzes and to allow free spinning of the wind generator.  USSTAF Armament Memorandum No. 3-133, 28 January 1945, p. 1-2.  

35  Comparative Test of the Effectiveness of Large Bombs against Reinforced Concrete Structures (Anglo-American Bomb Tests- Project RUBY), Air Proving Ground Center Eglin AFB, FL, October 31, 1946  p. 85-87 

36  Ibid

37  Ibid

38  John F. McBlain, Brig Gen, USA to Military Air Attaché, American Embassy, London, England, 27 January 47; Memo AGWAE to USFET in re: USAFE  3 August 1946.

39   AGWAR SGD Spaatz, for action:  USFET, W-87834 17 December 1946;   Bombs, Aircraft-Attack of Reinforced Concrete, Remarks of Armaments Board No. Q5,073, 0 May 47, p.2; The code name for the Samson bomb is often spelled "Sampson" in official correspondences.  Since the code name was never applied to a quantity stockpiled weapon, the author uses the name Samson only for the sake of clarity.  Samson was a ancient Hebrew hero of great strength.

40  Memo from WAR to  USAFE, 16 July 1947;  WAR to USML, London (for Col Bomar),  in re: WLRX-84522, 10 August 1947.

41   Bentz, William H., Penetration and Deceleration of 25,000-lb Bombs in Massive Concrete Targets, Report No. 712, Army Ballistic Research Lab, Aberdeen Proving Ground, MD, December 1949.  p. 3, 5,13,18  

42  Bombs for Aircraft, TM 9-1980- AFM-136-7, United States government Printing Office, Washington, 1950, p. 206;  Memo CG, Continental Base Section, John F. McBlain, Brig. Gen USA, to CG, USFET, Jan 28, 1947.  This program was likely canceled by the summer of 1954 when multi-megaton thermonuclear weapons of similar weights made conventional earth penetration bombs of this size obsolete.

43   Though Nazi fortifications varied in their thickness of bomb proofing, most had a maximum thickness of about 12 to 16 ft.  One large mass of reinforced concrete formed the dome of the giant V-2 assembly complex at Wirzenes, France.  The attacks on the Wirzenes complex is a textbook example of earthquake bomb use.  The large domed structure was under construction on a chalk bluff overlooking an old quarry.  Some Tallboys hit the structure itself doing little damage.  Tallboys that landed  at the base of the bluff caused a landslide that greatly undermined the foundation and left portions of the structure leaning towards the pit below.  A number of buttresses for the dome had broken under their own weight after the loss of earthen support.   Rather than try to rebuild such a heavy fortification, the Nazis abandoned the site.  Once the site was overrun by Allied troops, a close examination revealed that far and away  the greatest damage was caused by the near miss rather than the direct hit.

44  Vengeance, Hitler's Nuclear Weapon: Fact or Fiction?, by Phillip Henshall, Alan Sutton Publishing Ltd., U.K. 1995, p. 99-100. 

45   United states Army in World War II,  The Technical Services, The Ordnance Department: Planning Munitions for War, by Constance McLauglin Green, Harry C. Thomson and Peter C. Roots, office of the Chief of Military History, Department of the Army, Washington, D.C. 1955 p.471-472.

 46   Ibid p.471-472; Bombs for Aircraft, TM 9-1980- AFM-136-7, United States government Printing Office, Washington, 1950, p.199.

47   A Cold War Legacy, A Tribute to Strategic Air Command 1946-1992 by Alwyn T. Lloyd, Pictorial Histories Publishing Company, Inc., Missoula, MT, 2000, p. 81-83. 

48   Munitions Specialist, Vol. 3, Prepared by Air Training Command, USAF Extension Course Institute, Air University, Gunter Air Force Base, Alabama, November 1955, p. 20-21;   A Cold War Legacy, A Tribute to Strategic Air Command 1946-1992 by Alwyn T. Lloyd, Pictorial Histories Publishing Company, Inc., Missoula, MT, 2000, p. 81-83. 

49   The MK-8 was an improved Little Boy gun type atomic bomb that configured for earth penetration was 132 inches long, 14.5 inches in diameter and weighed only 3,200 lbs.  Up to 40 Mk-8 weapons were produced between November 1951 until May 1953.  The Mk-8 could penetrate up to 40 ft of sand, 60 ft of loam and 100 ft of clay.   US Nuclear Weapons: The Secret History by Chuck Hansen, Orion, 1988, p. 139-141;  The Mk 17/24 Thermonuclear bombs were first stockpiled in April 1954 as Emergency Capability (EC-17) until fuzing and suitable parachute delivery was developed.  Each weapon was 25 ft long, 62 inches in diameter and weighed approximately 42,000 lb.  US Nuclear Weapons, p. 147

50   Ordnance Committee Item 35543, 7 Oct 54, Department of the Army, Office of the Chief of Ordnance, Sub Committee on Bombs, Subject: Bomb, General Purpose, 44,000lb, T-12-Termination of Development, p. 1-2.;  A Cold War Legacy, A Tribute to Strategic Air Command 1946-1992 by Alwyn T. Lloyd, Pictorial Histories Publishing Company, Inc., Missoula, MT, 2000, p. 81-83. 



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