Tallboy (bomb)

The Tallboy or Bomb, Medium Capacity, 12,000 lb, was an earthquake bomb developed by the British aeronautical engineer Barnes Wallis and deployed by the RAF in 1944. It weighed five tons and, carried by the Avro Lancaster, was effective against hardened structures against which earlier, smaller bombs had proven ineffective.

History
Wallis presented his ideas for a 10-ton bomb in his 1941 paper A Note on a Method of Attacking the Axis Powers, which showed that a very large bomb exploding deep underground next to a target would transmit the shock into the foundations of the target, particularly since shock waves are transmitted through the ground more strongly than through air.

Wallis designed the "Victory Bomber" of 50 tons, which would fly at 320 mph at 45000 ft to carry the heavy bomb over 4000 mi, but the Air Ministry were against a single-bomb aircraft, and the idea was not pursued beyond 1942.

Following Wallis' 1942 paper Spherical Bomb — Surface Torpedo and the design of the "bouncing bomb" for the Dam Busters of Operation Chastise, the design and production of Tallboy was done without a contract on the initiative of the Ministry. As such, the RAF were using bombs they had not bought and that were still the property of the manufacturer, Vickers. This situation was normalised once their capabilities were recognised.

Accomplishments of the Tallboy included the 24 June 1944 Operation Crossbow attack on La Coupole — along with Grand Slams — which undermined the foundations of the V-2 assembly bunker, and a Tallboy attack on the Saumur tunnel on 8–9 June 1944, when bombs passed straight through the hill and exploded inside the tunnel 60 ft below the surface.

The last of the Kriegsmarine's Bismarck-class battleships, the Tirpitz, was sunk by an air attack using Tallboys.

Design
Most large Allied World War II aircraft bombs had very thin skins to maximize the weight of explosive a bomber could carry—this was an improvement on the early part of the war, when the explosive content of British bomb designs was low.

To be able to penetrate the earth (or hardened targets) without breaking apart, the casing of the Tallboy had to be strong. Each was cast in one piece of high-tensile steel that would enable it to survive the impact before detonation. At the same time, to achieve the penetration required, Wallis designed the Tallboy to be very aerodynamic, so that when dropped from a great height it would reach a terminal velocity much higher than traditional bomb designs.

In the final design, the tail of the bomb was about half the overall length of the finished weapon—the bomb casing was some 10 ft of the overall 21 ft length. Initially the bomb had a tendency to tumble, so the tail was modified—the fins were given a slight twist so that the bomb spun as it fell. The gyroscopic effect thus generated stopped the pitching and yawing, improved the aerodynamics and improved accuracy.

Tallboy was designed to be dropped from an optimal altitude of 18000 ft at a forward speed of 170 mph. Impacting at 750 mph, it made a crater 80 ft deep and 100 ft across and could go through 16 ft of concrete.

The weight of the Tallboy (approximately 12000 lb) and the high altitude required of the bombing aircraft meant that the Avro Lancasters used had to be specially adapted. Armour plating and even defensive armament were removed to reduce weight, and the bomb-bay doors had to be adapted. Even then the Lancaster was not capable of reaching the bomb's intended dropping height of 40000 ft.

At the same time No. 617 "Dambusters" Squadron trained in the use of a special bombsight, the Stabilizing Automatic Bomb Sight (SABS). For accuracy, multiple corrections had to be made for temperature, wind speed, and other factors. It was only effective if the target could be identified, and several missions were cancelled or unsuccessful because of difficulty in accurately identifying and marking the targets.

For use on underground targets, the bomb was fitted with three separate inertia pistols (firing mechanisms). These triggered detonation after a pre-set delay, which gave the bomb sufficient time to penetrate the target before exploding. Depending on mission requirements, the time delay could be set to 30 seconds or 30 minutes after impact.

To guarantee detonation, a total of three separate Type 47 long delay fuzes were fitted inside the rear of the bomb. This dramatically improved reliability of the weapon: even if two of the fuses failed to function, the third would trigger detonation. Despite this elaborate system, at least one Tallboy failed to explode during the second attack on the Sorpe dam and was found during repairs in late 1958 when the reservoir was emptied.

Although the bomb was aimed at the target during an operation, and proved capable of penetrating deep into hardened reinforced concrete when it hit, this was not the primary intention of Barnes Wallis's design. The bomb was designed to impact close to the target, slide into the soil or rock beneath or around the target, and then detonate, transferring all of its energy into the structure, or creating a camouflet (cavern or crater) into which the target would fall.

This 'earthquake' effect caused more damage than even a direct hit that penetrated the armour of a target, since even a burst inside a bunker would only damage the immediate surroundings, with the blast dissipating rapidly through the air. An earthquake impact shook the whole target, and caused major structural damage to all parts of it, making repair uneconomic. The attack reports below should be considered with this in mind.

The construction of each Tallboy was labour intensive because each was largely hand-made, requiring much manual labour during each separate manufacturing stage. The materials used were costly, with very precise engineering requirements with regard to casting and machining. For example, to increase penetrative power, a large and specially hardened steel plug had to be precisely machined and mated to a recess in the nose of the bomb. The ogive had to be machined into a perfectly symmetrical shape to ensure optimum aerodynamic performance. This was no easy task when manipulating a bomb casing with the size and weight of a Tallboy.

The Torpex filling was poured into the base of the upturned casing by hand, after melting it in "kettles". The final stage of explosive filling required that a one-inch layer of pure TNT be poured over the Torpex filling, followed by sealing the base with a 4 in layer of woodmeal-wax composite with three cylindrical recesses fitted with the explosive boosters and into which (when the bomb was finally armed), a total of three chemical time-fuses were inserted.

Tallboys were not considered expendable, and if not used on a raid were to be brought back to base rather than safely jettisoned into the sea. The value of the weapon offset the additional risk to the aircrew.

Given their high unit cost, Tallboys were used exclusively against high-value strategic targets that could not be destroyed by other means. When it was found that the Lancaster could be modified to carry a bomb larger than the Tallboy, Wallis produced the even larger Grand Slam bomb.

June - August 1944

 * Saumur rail tunnel — The sole operational north-south route on the Loire. Nineteen Tallboy-equipped and six conventionally equipped Lancasters of 617 Squadron attacked on the night of 8–9 June 1944. 617 Squadron were guided on to the target by 83 Squadron Pathfinder Force. This was the first use of the Tallboy bomb and the line was destroyed — one Tallboy bored through the hillside and exploded in the tunnel about 60 ft below, completely blocking it. No aircraft were lost during the raid.

Operation Crossbow sorties
Operation Crossbow was a set of offensive and defensive measures that were carried out to deal with the threat of the German V-1 flying bomb ("buzz bomb" or "doodlebug") and V-2 rocket weaponry. As part of the operation, a number of sorties using Tallboys were deployed by the British to destroy several missile sites.


 * 19 June 1944 - Watten
 * The nearest Tallboy dropped by 617 Squadron landed 50 yd from the target, a heavily fortified V-2 launch site under construction The bunker was rendered useless.


 * 24 June 1944 - Wizernes
 * The target was a V-2 assembly and launch site linked with the Watten site. Several Tallboy hits undermined the foundations but did not penetrate the dome. The bunker was abandoned.


 * 25 June 1944 - Siracourt V-1 bunker
 * Lancasters of 617 Squadron scored three direct hits with Tallboys without loss.


 * 4 July 1944 - Saint-Leu-d'Esserent
 * 617 Squadron used seventeen Lancasters with Tallboys, supported by one Mosquito and one Mustang, in an attempt to collapse the limestone roof of the caves used as storage depots. Aircraft from No 5 Group followed up with 1000 lb bombs.


 * 6 July 1944 - Mimoyecques
 * Tallboys hit one of the V-3 cannon's shafts and blocked galleries with earth and debris


 * 17 July 1944 - Wizernes
 * Sixteen Lancasters, led by a Mosquito and a Mustang, bombed Wizernes -- three Lancasters managed to drop Tallboys (one caused the dome to shift out of alignment, two others blocked the entrance).


 * 27 July 1944 - Watten
 * One Tallboy hit the target but did not penetrate the structure.


 * 31 July 1944 - Rilly La Montagne
 * Both ends of the railway tunnel were collapsed by Tallboys dropped by 617 Squadron. William Reid's Lancaster at 12000 ft was hit by a Tallboy dropped from 18000 ft

Sorties against German dockyards
Shipping in the English Channel and the Atlantic Ocean were threatened by u-boats and e-boats stationed in France. U-boat docks were protected against conventional aerial bombardment by thick concrete roofs.


 * 14 June 1944 - Le Havre
 * Part of the first massive RAF daylight raid since the end of May 1943, two waves attacked e-boat facilities at Le Havre: No 1 Group first, No 3 Group second. Just before the first wave, 22 Lancasters of 617 Squadron and 3 Mosquito marker aircraft attacked, several hits were scored on the pens, one bomb penetrated the roof.


 * 15 June 1944 - Boulogne harbour
 * 297 aircraft - 155 Lancasters, 130 Halifaxes, 12 Mosquitos - of Nos 1, 4, 5, 6 and 8 Groups attacked Boulogne harbour. One Halifax was lost. A French report described the great destruction as the worst raid on Boulogne. During the raid 22 Lancasters of No. 617 squadron bombed the e-boat pens with Tallboys. Due to cloud cover ten planes returned to base with their bombs. However the raid was considered a success as the e-boats retired to IJmuiden on the Dutch coast where they were better protected but less able to interfere with naval traffic supporting the Normandy invasion.


 * 5 August 1944 - Brest
 * 15 Lancasters of 617 Squadron attacked the U-boat pens at Brest and scored six direct hits with Tallboys penetrating the concrete roofs. One Lancaster was shot down by flak. Subsequent attempts to reinforce other sites with even thicker concrete diverted resources from other projects.


 * 6 August 1944 - Keroman
 * Flight Lieutenant Thomas Clifford Iveson dropped one Tallboy.


 * 7 August 1944 - Lorient
 * The planned Tallboy mission against the U-boat pens was cancelled


 * 8 August 1944 - La Pallice
 * Flight Lieutenant Thomas Clifford Iveson dropped one Tallboy.


 * 28 August 1944 - IJmuiden
 * Flight Lieutenant Thomas Clifford Iveson dropped one Tallboy.

September - November 1944

 * 23/24 September 1944 - Dortmund-Ems Canal near Ladbergen, north of Münster
 * During the night attack 617 Squadron scored six direct hits with Tallboys.


 * 7 October 1944 Kembs Dam north of Basle
 * The dam waters could have been kept in reserve to flood the area of a US advance. The Dambusters destroyed the lock gates with Tallboys dropped at low level, releasing the stored water.


 * 15 October 1944 Sorpe dam
 * This target of the original Dambusters raid survived a second attack by 9 Squadron (617 Squadron did not participate in this raid). The Tallboy bombs were seen to hit the dam but did not breach it.

Bombing sorties against Tirpitz
The German battleship Tirpitz was a threat against convoys sailing to and from the Soviet Union.


 * 15 September 1944 (Operation Paravane)
 * One Tallboy hit near the bow of the Tirpitz and caused considerable damage. The shock-wave from the hit damaged the battleship's engines, causing the Germans to convert the ship into a semi-static heavy artillery battery.


 * 29 October 1944 (Operation Obviate)
 * No direct hits were scored but one near miss bent a propeller shaft.


 * 12 November 1944 (Operation Catechism)
 * In the final operation the Tirpitz was sunk when two Tallboys hit her, and four others fell close by. The battleship rolled over and capsized in shallow water.

December 1944 - April 1945
Bombing of U-boat pens, December 1944-April 1945


 * 15 December 1944 IJmuiden on the Dutch coast,
 * 617 Squadrons attacked U-boat pens with Tallboys. A smoke-screen hindered the bombing, and the results went unseen.


 * 12 January 1945 Bergen
 * "32 Lancasters and one Mosquito of Nos 9 and 617 Squadrons attacked U-boat pens and shipping in Bergen harbour. Three Lancasters of 617 Squadron and one from 9 Squadron were lost; the Germans told the local people that 11 bombers had been shot down. A local report said that three Tallboys penetrated the 3½-metre-thick roof of the pens and caused severe damage to workshops, offices and stores inside".


 * 3 February 1945 IJmuiden & Poortershaven
 * 36 Lancasters of No 5 Group attacked U-boat pens at IJmuiden (9 Squadron) and Poortershaven (617 Squadron) with Tallboys. Hits were claimed on both targets without loss.


 * 9 April 1945 Hamburg
 * 617 Squadron attacked with Tallboys and Grand Slams. Some of the bombs hit their target and no aircraft were lost.


 * 18 April 1945 Heligoland
 * 969 aircraft - 617 Lancasters, 332 Halifaxes and 20 Mosquitos of all groups bombed the naval base, airfield and town "almost into crater-pitted moonscapes". Three Halifaxes were lost; the islands were evacuated the following night.


 * 19 April 1945 Heligoland
 * 36 Lancasters used Tallboy bombs against coastal positions.


 * 8 December and 11 December 1944
 * The Urft Dam, (30 mi south west of Cologne) was attacked to prevent it being used to flood the area as American troops advanced. The lip of the dam was damaged, but the Germans prevented further damage by lowering the water level.

617 Squadron.
 * 21 December 1944 Politz

The viaducts were attacked by 617 and 9 squadrons with Tallboys and the first Grand Slams. The Arnsberg viaduct withstood the attack but 100 m of the Bielefeld viaduct collapsed through the 'earthquake effect' of the Grand Slams and Tallboys.
 * 14 March 1945 Bielefeld and Arnsberg viaducts


 * 15 March 1945 Arnsberg viaduct
 * The Arnsberg viaduct was attacked again by 9 Squadron, it did not collapse.

The Lützow was attacked by 617 Squadron. Despite intense flak, 15 aircraft managed to bomb the target with Tallboys or with 1000 lb bombs. One near miss with a Tallboy tore a large hole in the bottom of the Lützow and she settled to the bottom in shallow water. One Lancaster was shot down, the Squadron's last loss of the war.
 * 16 April 1945 pocket battleship Lützow

Hitler's vacation home, the Berghof, near Berchtesgaden was attacked on with a mixed force that included six Lancasters of 617 Squadron dropping their last Tallboys. The bombing appeared to be accurate and effective.
 * 25 April 1945 Berghof

Use by the United States
The T-10 was an American-made version of the 12000 lb Tallboy modified to use standard American components. Development was started in late 1944 and plans were made to drop them on the fortified island strongholds of the Pacific to aid in softening their defences before amphibious assaults. None were ever used in combat, since the capitulation of Japan following the atomic bombings of Hiroshima and Nagasaki negated their need. In the late 1950s the T-10 was re-designated the M-121. During the Korean War a number of T-10s were converted to the radio guided Tarzon bomb and were used to destroy railroad bridges and reservoir dams, being dropped by the Boeing B-29 Superfortress.

After the Korean War ended and the B-29 and B-36 bombers were retired, the United States Air Force no longer had an aircraft that could drop the completely assembled M-121 and they were put in storage. Production of the T-10 ended in 1955. The B-36 was the last operational aircraft that could drop a fully assembled Tallboy type bomb in the conventional way. During the Vietnam War, some M-121s, minus their rear streamline shroud and tail fin assembly were shipped to Vietnam for Commando Vault missions where the warhead was incorporated in a BLU-82 weapon dropped by C-130 using radar control. The warhead was mounted on a platform and pulled by a parachute from the rear loading ramp of the C-130. After clearing the aircraft the large extraction chute and pallet were cut away and small triangular chute stabilized the large warhead till impact. A three-foot nose probe detonated the bomb at the right stand-off. One of the last of the World War II Tallboy designs was dropped during a Commando Vault mission to clear a landing zone for helicopters on a ridge during the famous Battle of Hamburger Hill. Dropping from 10000 ft, the bomb hit exactly where it was needed. The Commando Vault missions were more accurate in bomb delivery on target than the more modern B-52s.

Work still progressed on the 43000 lb T-12 Cloudmaker, which the Convair B-36 had been redesigned to deploy.