Disney bomb

The Disney bomb, also known as the Disney Swish, officially the 4500 lb Concrete Piercing/Rocket Assisted bomb was a rocket-assisted bunker buster bomb developed during the Second World War by the British Royal Navy to penetrate hardened concrete targets, such as submarine pens, which could resist conventional free-fall bombs.

Devised by Royal Navy Captain Edward Terrell, the bomb was fitted with solid-fuel rockets to accelerate its descent, giving it an impact speed of —substantially beyond the free-fall impact velocity of the 5 tonne ''Tallboy'' "earthquake" bomb for comparable purposes.

The Disney could penetrate of solid concrete before detonating. The name is attributed to a propaganda film, ''Victory Through Air Power'', produced by the Walt Disney Studios, that provided the inspiration for the design.

The Disney bomb saw limited use by the United States Army Air Forces in Europe from February to April 1945. Although technically successful, it initially was insufficiently accurate for attacking bunker targets. It was deployed late in the war and had little effect on the Allied bombing campaign against Germany.

Background

During the Second World War, Barnes Wallis developed two large "earthquake" bombs for the Royal Air Force: the five-tonne Tallboy and the ten-tonne Grand Slam, for use against targets too heavily protected to be affected by conventional high explosive bombs.

These enormous weapons were designed to strike close by their target, to penetrate deeply into the earth, and to cause major structural damage by the shock waves transmitted through the ground. In practice, they proved capable of penetrating a significant thickness of concrete if they scored a direct hit, despite not being designed for that purpose by Wallis, who had to work within the accuracy limitations of current bombsights and the resulting low of the bombings.

The Disney bomb, by contrast, was designed from the start to penetrate the thick concrete roofs of fortified bunkers. Whereas the earthquake bomb's target was the bunker itself, the Disney bomb's target was the bunker's contents. To this end, the warhead was composed of an unusually thick steel shell containing a comparatively small amount of explosive. It was much slimmer than was usual for aircraft-dropped bombs, and a cluster of booster rockets accelerated the weapon as it fell. These features accord with Newton's approximation for impact depth and the empirical design equation known as Young's equation that states that the deepest target penetration is achieved by a projectile that is dense, long and thin (i.e. has a large sectional density), and strikes with a high velocity.

Description

The CP/RA Disney bombs were long and weighed . The diameter of the body of the bomb was , while the diameter at the tail was . They were composed of three sections.

The forward section was the warhead—an explosive charge of of shellite, contained within an armour-piercing casing of thick steel and fitted with two British No.58 MK I tail pistol fuzes at the base (i.e. furthest from the nose).

The second section was made up of nineteen rocket motors from the 3-inch Rocket Projectile—essentially metal tubes filled with cordite. In the third rear section, a tail cone contained the circuit that ignited the rockets. This was powered by a small generator with a vane spun by the airstream going past the falling bomb. Rocket ignition was controlled either by a time-delay switch, or a barometric switch.

There were six small fins at the rear of the bomb for stabilisation. The bomb was suspended from the aircraft by two weight-bearing lugs. Three arming wires also connected the bomb and the aircraft; as the bomb dropped, a brief tug from the wires would arm the warhead fuzes and the rocket-ignition circuit, and unlock the electrical generator, allowing it to spin freely.

For accuracy, the bombs had to be dropped precisely from a pre-determined height, usually . They would free-fall for around 30 seconds until, at , the rockets were ignited, causing the tail section to be expelled. The rocket burn lasted for three seconds and added to the bomb's speed, giving a final impact speed of , equivalent to or approximately Mach 1.29. Post-war tests demonstrated the bombs were able to penetrate a thick concrete roof, with the predicted (but untested) ability to penetrate of concrete.

Development and testing

According to an anecdote, the idea arose after a group of Royal Navy officers saw a similar, but fictional, bomb depicted in the 1943 Walt Disney animated propaganda film ''Victory Through Air Power'', and the name Disney was consequently given to the weapon. The Royal Navy developed the bomb even though the Fleet Air Arm operated no aircraft capable of carrying it. The navy was interested in a concrete-penetrating weapon due to the German navy's extensive use of fortified submarine pens to protect their U-boats and E-boats from air attack while docked.

The Disney Bomb was devised by a British naval officer, Captain Edward Terrell of the Royal Naval Volunteer Reserve, who served in the Directorate of Miscellaneous Weapons Development. Before the war, he had been a lawyer and the Recorder of Newbury. However, he was also an enthusiastic inventor and had filed several patents pre-war, including ones for a vegetable peeling knife and a bottle for fountain pen ink.

The bomb's development began in September 1943. Although there was support for the idea at the highest levels within the Admiralty, production of the weapon would have to come under the Ministry of Aircraft Production (MAP). The Road Research Laboratory provided theoretical formula for penetration from US data on the performance of shells against reinforced concrete, and the Chief Engineer of Armament Development at Fort Halstead prepared a preliminary design to present to the MAP.

In the face of opposition, the First Sea Lord prepared a memorandum for the Anti-U-Boat Committee, of which Churchill was the chairman. Terrell visited Churchill's scientific adviser Lord Cherwell to convince him that it was feasible technically. Due to the Prime Minister's absence through illness, it was not until January 1944 that Churchill expressed a desire that the bomb should be considered by the committee. Due to the number of departments involved there were meetings involving large numbers of technicians and scientists to confirm the technical feasibility.

Terrell received support through the Air Technical Section of the USAAC, and was able to show the Admiralty a mockup under the wing of a Boeing B-17 Flying Fortress. The Air Ministry was still opposed to its development on several technical grounds, and it took a meeting of the War Cabinet in May, which Terrell attended, to decide in its favour, giving it "P plus" priority. As a side effect the meeting focused attention on the issue of the U-boat shelters, and the RAF were directed to make attacks on them, dropping 26 Tallboys in August that year.

Despite being a British weapon, Disneys were used only by the United States Army Air Force, with the bombs becoming a joint project between the American Eighth Air Force and the British Royal Navy; they were never used by RAF Bomber Command. The 92nd Bombardment Group was initially tasked with their use. The bombs were also dropped by the 305th Bombardment Group and 306th Bombardment Group. The 94th Bombardment Group prepared to use the bombs, but flew no operations with them before the war in Europe ended.

The Boeing B-17 Flying Fortresses operated by these units carried the bombs in pairs; one was slung under each wing, as they were too long to be carried in the B-17's bomb bay. The Disneys were carried from the same external mounting used for the Aeronca GB-1 glide bomb. Cameras were also fitted to the aircraft so the bombs' trajectory and effect could be recorded.

Testing of the Disney bombs began in early 1945. Bombs were initially dropped on a bombing range near Southampton to photographically record their trajectory and calibrate bombsights. This was necessary as the flight-path of a rocket-accelerated bomb differed considerably from that of a free-falling bomb. Test drops were then conducted on the Watten bunker, German codename Kraftwerk Nord West (now known as the Blockhaus d'Éperlecques), a huge German concrete bunker near Watten in northern France. This was ideal for the purpose as the area had been captured by Allied forces in September 1944, so damage to the structure could be inspected after bomb tests. Four bombs, carried by two B-17s, were used and two hits scored on the target. The resultant damage was considered satisfactory by Royal Navy observers on the ground.

Combat

The first Disney attack was against the port of IJmuiden, Netherlands. This was the site of two separate fortified pens used by the German navy to house their Schnellboote (fast torpedo boats, known to the Allies as "E-boats") and Biber midget submarines. The older structure, codename Schnellbootbunker AY (SBB1), was protected by a thick concrete roof. The newer one, codename Schnellbootbunker BY (SBB2), had of concrete, with a further layer separated by an air gap.

The E-boats laid up in the shelters during the day, safe from air attack, and put to sea under cover of night to attack Allied shipping. The pens were priority targets as the torpedo boats they protected were a considerable threat to the supply lines serving Allied forces in western Europe. Since August 1944, the two bunkers had been attacked four times by No. 9 Squadron and No. 617 Squadron of the RAF, with a total of 53 of the five-ton Tallboy earthquake bombs. There had been numerous other attacks from bombers carrying smaller, conventional bombs.

Nine aircraft of the 92nd Bomb Group, carrying 18 Disneys, attacked Schnellbootbunker BY (SBB2) on 10 February 1945. Royal Navy intelligence learned the concrete had been penetrated, but the pens had been empty at the time of the attack. The 92nd therefore carried out an att…

Text taken from Wikipedia - Disney bomb under the CC-BY-SA-3.0 on April 13, 2023
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