A ‘bigger bang’ – the 22,000 lb MC ‘Grand Slam’ bomb, Brooklands Museum


Careful consideration of the facts can sometimes lead one to incorrect conclusions. However, if you were asked to name the top British aeronautical designers who had contributed most to the war effort during the period 1939 – 1945, you would have to include Sidney Camm (Hurricane, Tempest), Reginald Mitchell (Spitfire), and Roy Chadwick (Lancaster), but you would also have to consider Barnes Wallis, too. This Derbyshire-born genius – and that is not too strong a term – was responsible for the Vickers Wellington (the backbone of RAF Bomber Command for two years) the ‘Upkeep’ mine (which breached the Ruhr Dams) and the ground-breaking and ground-shaking ‘Tallboy’ (12,000 lb) and ‘Grand Slam’ (22,000 lb) bombs, the largest conventional munitions dropped during WW2. It was found to be possible to construct a simple 12,000 lb blast bomb, by bolting three of the existing 4,000 lb ‘Cookie’ HC (High Capacity) units together, but only the Avro Lancaster, of the RAF’s WW2 bombers, could carry this weapon. High Capacity bombs had a very thin skin, which enabled them to carry around 75% of their weight as explosive – usually Amatol – but they were not aerodynamic, and could not really be aimed. They relied for their effectiveness on their superior blast effect, which was felt over a wide area, hence the nickname ‘blockbuster’, plus the fact that they were notoriously sensitive, and could NOT be jettisoned ‘safe’- in other words they would explode, anyway.

The famous armaments company of Vickers, Sons and Maxim Ltd first began to build airships in 1908, and eventually constructed aircraft, guns, ships and tanks. Barnes Wallis had been employed on airship design by Vickers before moving on to design aircraft such as the Wellesley, Wellington and Warwick, but he is perhaps best known for his weapon designs. Unlike the ‘area bombing’ form of attack, favoured by certain sections of the RAF, the Air Ministry and Winston Churchill’s scientific adviser, Frederick Lindemann, Barnes Wallis believed in precision attack by aircraft carrying the largest weapons possible, against ‘high value’ enemy targets. Wallis’s ‘Upkeep’ weapon, technically a mine, (but almost always referred to as the ‘bouncing bomb’), was used by 617 Squadron, RAF – forever after known as ‘The Dambusters’ – to attack the Ruhr dams on 16/17th May, 1943, with mostly successful results. After this, Air Chief Marshall Harris of RAF Bomber Command and Vickers were happy to let Barnes Wallis continue with the development of his pet project, the earthquake bomb. Initially, Wallis wanted a ’10 ton’ bomb (c. 22,000lb), which would, ideally, have been released from an aircraft flying at 40,000 ft! Since an aircraft capable of doing this did not exist at the time it was decided to scale the weapon down, and a 12,000lb D.P. bomb (for ‘deep penetration’) was authorized; this was codenamed ‘Tallboy’ and was 21 feet long, with a diameter of 38″, and carried a charge of 5,200lb of Torpex D1. Torpex was named for its intended underwater use – as in torpedo warheads – because the 18% of aluminium powder that the explosive mixture contained extended the ‘explosive pulse’ and therefore ensured the maximum damage due to over-pressure. Other constituents of the explosive mixture included TNT (40%) and RDX (42%). RDX, a codename meaning Research Department Explosive, and sometimes called T4 or cyclonite (cyclotrimethylenetrinitramine), was around 50% more powerful than TNT, so the Tallboy packed a powerful punch. This destructive power was used to create a void deep under the earth after the body of the bomb, its nose made from tough forged steel, had punched its way underground. As this void collapsed, it left a large crater into which structures such as bridges and rail tunnels collapsed. Not only that, but foundations were shaken and weakened by the ‘ground waves’ caused by the explosion which could be up to 150 yards away and still be effective. Even if total destruction did not occur, the amount of damage inflicted would often be enough such as to make the structure uneconomic to repair. This contrasted sharply with the fact that a 12,000 blast bomb could detonate as close as ten yards away from a major stone viaduct, and no serious damage would be inflicted on the structure (as had happened during raids on the Anthor Viaduct in France and the Dortmund-Ems Canal in Germany).

The only RAF aircraft capable of carrying the Tallboy was the Avro Lancaster, rapidly becoming RAF Bomber Command’s standard heavy bomber. The Lancaster’s bomb bay doors had to be modified to accommodate the weapon, and it was decided that only a few squadrons (including 9 and 617 Squadrons) would be given the training and special SABS MkllA bomb sight needed to operate with the weapon. After a period of work-up these squadrons began attacking strategic targets such as the Samur Railway Tunnel in France, blocking it completely and preventing vital German reinforcements from reaching the D-Day beaches from the South of the country. An even more impressive feat was the sinking of the ‘Tirpitz’, a German battleship which was moored in Norwegian waters following damage in an earlier bombing raid, by Lancasters operating at their maximum radius of action from the Scottish airbase at Lossiemouth. On 12th November, 1944, Tallboys dropped by both Nos 9 and 617 Squadrons smothered the ‘Tirpitz’ and three direct hits were obtained; the battleship turned turtle.

Following the initial operational successes, in July, 1944 Barnes Wallis was given the green light by the Ministry of Aircraft Production for his monster ’10 tonner’, the ‘Grand Slam’. The Vickers Don Valley facility in Sheffield was one of the very few forges which could handle the production of such a massive weapon. The steel casing took two days to cool down after being cast, before the difficult task of machining the body could begin. When ready, the bomb casing was filled with molten Torpex explosive. Amazingly, this took one whole month to set and cool; the final charge/weight ratio was calculated at 43%. Consequently, because production of the bomb was so slow and expensive, aircrews were instructed to bring their weapon back to base if they were unable to drop it on target.

On top of the 9,200lb of cooled Torpex, a one inch layer of TNT was poured, (the explosion of which would initiate the Torpex), followed by 4 inches of a wood-meal and wax mixture which was then sealed with a half-inch plywood washer with three holes drilled through it to take the triple No. 58 fuse pistols. Finally, a heavy steel plate was bolted over the end of the bomb casing, then the special No 78 Mk1 tail with its four fins (offset at 5° to induce a right-handed spin of over 300rpm and increase accuracy) was bolted on using a dozen 7/16″ studs. The bomb was painted mid-green, with a single one inch red stripe near the nose to indicate that it was filled with explosive. To carry this enormous weapon, even more changes had to be made to the Avro Lancaster bomber. The bomb bay doors were completely removed, as were both the mid-upper and the front turret (a lightweight aerodynamic fairing was fitted in its place). Some of the standard armour plate on the aircraft was taken out, and higher power Rolls-Royce Merlin 25 engines fitted. The resulting aircraft were designated the Avro Lancaster B.Mk I (Special).

It was decided to drop just one weapon – now known as the ‘Bomb, Medium Capacity, 22,000lb’ – as a test (no more could be afforded). The remote Ashley Range in the New Forest in Hampshire was chosen, and on the 13th March, 1945, the first live ‘Grand Slam’ was dropped; the bomb was released using special Vickers-designed gear fitted to the Lancaster, and accelerated to near sonic speed. It impacted close to the target and the resulting explosion caused a crater 70 feet deep and 130 feet across; the weapon was cleared for use. The following day, a Grand Slam was used against the Bielefeld Viaduct in Germany. This vital rail link was attacked by 617 Sqn, 28 aircraft of which were carrying Tallboys and one a Grand Slam. The 22,000 pounder landed within 80 yards of the target and 11 seconds later a huge crater was created, which fatally weakened the structure; nearby Tallboy misses finished the work, and two spans of this vital transportation link collapsed. Raids on railway bridges, viaducts and submarine pens across Germany continued throughout March, 1945, with devastating results. The German Reich collapsed, along with these and other high-value targets, and the European campaign drew to a close on VE-Day, 8th May, 1945. No less than 854 Tallboys and 41 Grand Slams had been delivered, many with deadly accuracy.

Post-war, in early 1946, the RAF and the USAAF decided to conduct live bombing trials with deep penetration weapons; these trials were called ‘Operation Ruby’. The test target chosen was the partially destroyed Valentin submarine pens at Bremen, Germany and Grand Slams were some of the weapons used. The RAF used Lancasters, of course, and the USAAF the B-29 Superfortress, which was the only other WW2 aircraft capable of carrying the Grand Slam; actually, the B-29 could carry TWO of the giant bombs externally, one under each inner wing!

Following these trials, the USAF built up a stock of US-manufactured versions of the Grand Slam. These were designated T-14, and were built in five sections, a forged nose cone and tail, and three rolled and welded steel sections in between. One of these is preserved at the United States Air Force Armaments Museum near Eglin Air Force Base, Florida. The American authorities even developed a monstrous 44,000lb deep penetration bomb (the T-12), which the B-29 could only just carry. However, the aircraft fuel load had to be reduced by so much that it was, to all intents and purposes, useless as a weapon of war. An example of the T-12 is on display at the U.S Army’s Aberdeen Proving Ground in Maryland.

Five Grand Slams are preserved in the UK. One is held by the RAF Museum, one at the Battle of Britain Memorial Flight’s Visitor Centre at RAF Coningsby, one at the Dumfries & Galloway Aviation Museum in Scotland, and another at RAF Lossiemouth, where No 617 Squadron were stationed for many years. A main body of a Grand Slam (minus tail) is on display at Kelham Island Museum, Sheffield, not far from the site of the Don Valley Works of Vickers Ltd, where the bombs were made. The one you can see here is a casing, complete with tail, on display at the Brooklands Museum. This is quite appropriate as the former Vickers plant on this site was where Barnes Wallis worked for many years.

Strangely, there are also two Grand Slams on display in Pakistan – one at Karachi and the other at Sargodha! They were on their way to the Pacific, to be used by 617 Squadron against Japanese targets, when two other bombs – of the atomic kind – brought the Second World War to a close. Following the partition of India, which gave rise to the two new countries of Pakistan and India, the Grand Slams found themselves on the Pakistani side of the line of partition.

There was one other Grand Slam which had been on display for many years outside the Main Gate at RAF Scampton, the WW2 home of 617 Squadron. In 1958, the road outside the Main Gate needed to be widened, and an attempt was made to lift the bomb, which was thought to be an empty casing. Shockingly, it was found to be full of ‘live’ explosive, and it was gingerly towed away to the remote military ranges at Shoeburyness on the Thames Estuary. There it was safely detonated. Here I must quote from a document I found – “Some safety distance calculations were done, however, about the effect of a Grand Slam detonating at ground level in the open. Apart from the entire RAF Station, most of the northern part of the City of Lincoln, including Lincoln Cathedral, which dates back to 1250, would have been flattened.”

The Grand Slam and Tallboy – bombs that showed there was a viable alternative to the crude weapon of area bombing used on Germany in WW2, and proof of the genius of Sir Barnes Wallis.

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One comment on “A ‘bigger bang’ – the 22,000 lb MC ‘Grand Slam’ bomb, Brooklands Museum”

  1. Incredible story about the live weapon standing as a gate guard those many years! The U.S. version if the Grand Slam bomb is indeed at the USAF Armaments Museum and you’d be happy to know it is on prominent display at the entrance. Interesting about the number of missions as I had no idea that the mission count was so high into the hundreds — thanks 🙂

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