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DAMBUSTERS

The development of modelling, simulation, and precision strike

A Lancaster bomber aircraft from the Battle of Britain Memorial Flight (BBMF) soars over the Derwent Valley Dam in Derbyshire. The famous ‘Dambusters’ of the RAF’s 617 Squadron trained in this valley during the Second World War for their mission.
It is 65 years since the Dambusters flew their daring raid over wartime Germany.

May 16th this year marked the 75th anniversary of the Dambusters raid. Thirteen Australian airmen took part in the mission with each and everyone of them recognised for their gallantry and extraordinary skill. The role of modelling and simulation was central to the success of the mission enabled by intelligence and a highly-coupled relationship between the military, science and technology experts, and the Defence industry.

By all measures, the Second World War was by far the most devastating conflict in human history.
In just six short years from 1939 to 1945, between 60 and 85 million fatalities – the majority of whom were non-combatants – were directly attributable to the actions of the opposing Axis and Allies coalitions.

After an initial territory-grabbing blitzkrieg, the war entered a period of comparative calm while both sides consolidated or regrouped. But operations in all three major domains soon rapidly escalated into total war as combat theatres were expanded and new theatres opened, and the Allies ramped up their industrial base and manpower to match and gradually exceed those of Germany and Japan.

The massive industrial ramp up also corresponded with equally huge leaps in both technology and tactics as each side sought to gain an edge over the other. Warships and tanks got heavier and faster thanks to advances in metallurgy and armour, and aircraft design surged thanks to new engine technologies and advances in aerodynamics.

By late 1942 both sides were more or less matching each other in warship, armoured vehicle, fighter, and bomber designs and numbers. But as the full political commitment and industrial might of the United States began to build, with a few exceptions, the quality and quantity of the Allies’ capabilities and platforms began to outpace those of the Axis nations who by then had reached a peak supply of raw materials and manpower.

A side elevation of Lancaster B.III (Special) G for George, the air raft 617Sqn CO WGCDR Guy Gibson flew on the Dambusters raid. Note the modified belly of the aircraft and the conformal mounting of the Upkeep bomb. Juanita Franzi

Defence industry was becoming vital ground with technology being delivered at the speed of war.
As new designs emerged the traditional test and evaluation schedules often measured in years were vastly compressed into months or even weeks, often being driven by an urgent capability requirement or tasking imperative.
One such imperative was a need to degrade Germany’s immense output of machinery and components from its industrial heartland in the Ruhr Valley.

The Ruhr in western Germany is an ideal region for light and heavy industry with many large and medium sized cities in the region such as Cologne, Düsseldorf, Dortmund, and Essen supporting a ready workforce; the confluence of several major navigable rivers and an intricate network of canals allowing the movement of raw materials; a ready power supply from nearby coal mines and power stations; and several large dams providing water for hydro-electricity and irrigation.

But the Ruhr is a large region, and the various factories that produced everything from ball bearings to aircraft were dispersed and often well disguised. The area was also heavily defended, so indiscriminate high-altitude bombing would have required an excessive amount of treasure, material and human resources for comparatively little effect, while battle damage assessment would likely have been inconclusive.

Even before the war the Ruhr region had been identified by the British as an important strategic target. But as the war widened, it was decided that the best way to disrupt the industrial effort of the Ruhr was to attack some of the dams of the upper Ruhr region. This would not only disrupt hydroelectric power generation capacity and river and canal traffic, but the surge of water would also destroy or damage those industrial facilities located on the river plains below the dams.

Planning for the raid, dubbed Operation Chastise commenced in earnest in late-1942. But it soon became clear that breaching the dams would be a difficult task, and one that would require an unconventional solution.

The six dams identified as targets were all large and robust gravity and masonry style dams built using steel reinforced concrete with rock and earth reinforcement on the upstream sides. Conventional bombing was deemed unlikely to be able to hit let alone breach the dams, while the Germans had rigged an intricate network of nets upstream of the dams to prevent torpedo attacks.

A range of modelling activities were undertaken by defence scientists to test the effectiveness of explosives against the dams. One of the first models was a 1/50th scale model of the Mohne Dam. It was based on the technical papers that were published during the construction of the dam around 1913 and was constructed with over 2 million scale concrete bricks! From here experiments to test the effectiveness of different types of explosives and the effect of different detonation points moved on to bigger representative targets to develop and test theories.

After several unsuccessful attempts against the disused Nant-y-Gro Dam in the remote Elan Valley in Wales, British wartime scientist Barnes Wallis was able to successfully demolish a centre section of the dam in July 1942. Despite Nant-y-Gro being much smaller than the Ruhr dams, the concept was proven. But a suitable delivery method for the necessary explosive force was yet to be devised.

Since 1941 Wallis had been working on a new type of bomb that was designed to attack battleships and other major surface combatants. Released at low altitude, the flat-sided ball-shaped ‘High Ball’ bomb was designed to skip across the water and impact the ship at or just above the waterline, exploding on contact. But initial tests were disappointing, with test bombs often breaking up when they hit the water, and the skipping action easily disrupted or deflected by ocean swells.

 

The Upkeep bomb or mine was only five feet long and just over four feet in diameter, yet it carried more than 6,000lbs of explosive. Imperial War Museum

On 17 March 1943 Squadron ‘X’ was formed at RAF Scampton in Lincolnshire under the command of Wing Commander Guy Gibson for the purpose of conducting a special mission. Gibson had recently completed an operational tour flying Avro Lancasters with 106 Sqn and had been posted to 5 Group headquarters. But instead, without knowing the nature of the mission Gibson was tasked with pulling together the new squadron using hand-picked and volunteer crews, initially to carry out just a single raid.

The squadron’s formation gave it little more than two months to equip and work up before water levels in the German dams were seasonally at their highest and thus most optimal for the bomb’s explosion to have maximum effect, usually in mid-May.

The squadron was soon given the designation of 617 Sqn, and flying operations using interim Lancasters IIIs drawn from other units commenced on March 31. The training emphasis was primarily on low-level over-water flying, and a rumour that the target was to be the German battleship Tirpitz which had been effectively locked up in a Norwegian fjord was spread amongst the squadron in order to disguise the true nature of the mission.

The Lancaster was chosen due to it having the required speed and range and a sufficient bomb load. “There are certain objectives in enemy territory which are vulnerable to air attack, and which are themselves very important military objectives,” Wallis was quoted as saying in Guy Gibson’s post-war book, Enemy Coast Ahead.

“However, these need a vast amount of explosive placed very accurately to shift them or blow them out… viaducts, submarine pens, big ships and so on,” Wallis said. “I have had my eye on such things for a long time, but always the problem has been too great, much too great. First of all there wasn’t an aeroplane with a high enough performance to carry the required load at the required speed. Then along came the Lancaster bomber, and this problem was solved.”

In the meantime, Wallis was refining the bomb’s design. While the unreliable ball-shaped High Ball design was retained for the anti-shipping role, the concept was developed into the barrel-shaped ‘Upkeep’ bomb or mine. The 9,200lb Upkeep – 6,200lb of which was comprised of torpex explosive – was 5ft long and had a diameter of just over 4ft, and was designed to be spun backwards at 500rpm using a motorised chain drive in the belly of the Lancaster before being dropped onto the reservoir surface.

“The next [problem] was the explosive itself,” Wallis continued in Gibson’s book. “It would have to take the form of either a very large bomb or a very large mine. But if it was to be dropped accurately enough to do its job it would have to be placed within a few yards of the right spot.

the bombsight developed for the Möhne Dam attack. The bombardier would look through the eyepiece at right, and would release the bomb when the posts at left lined up with the towers on the dam wall.

“There are three snags to this,” he said. “If bombing is to be as accurate as that then the attack will have to be at low level, which means below 300 feet. But with these great big bombs there’s always the danger that they may explode on impact from this height and you know what that means. And if they’re dropped above that height then accuracy diminishes, and the job can’t be done.”
The bomb was designed to skip on the surface of the reservoir to clear the torpedo nets, before impacting the wall and settling. As the bomb sank the backspin would keep it against the dam wall, before a pressure barometer detonated the bomb against the wall.

Simulation

Numerous drop tests were conducted of half-scale test bombs from Mosquito and Wellington test aircraft along the shallow waters of the Thames Estuary east of London and on the shallow estuary behind Chesil beach in Dorset from late 1942 and into early 1943, until optimum drop speeds and heights were determined.

The training continued day and night at 617 Sqn into April 1943. The problems of flying a big bomber over still water at night – initially at 300ft but later much lower – were not easily overcome, especially considering how inaccurate the radio altimeters of the era were. The crews trained over reservoirs in England and became proficient at manoeuvring in the tight valleys around the reservoirs.

Daytime training flights were often conducted with the pilot, co-pilot, navigator and bombardier wearing what RAAF wartime Spitfire pilot, POW and post-war author Paul Brickhill described as “synthetic night-flying gear”. This system, known as ‘two-stage amber’ or ‘two-stage blue’ saw the Lancaster’s cockpit windows covered with blue celluloid panels and the crew wear amber-tinted goggles, both of which combined to degrade daytime visibility to simulate what the crews would see if flying on a moonlit night.
Also in April, Gibson was told that the real targets would be dams, although not which ones, and the squadron began developing and flying attack profiles, most frequently against Derwent Reservoir in the Peak District near Sheffield.

The Derwent Dam with its masonry construction, size, large towers, and surrounding hills and forests most closely resembled the German dams visually and topographically, so it was an ideal practice target for 617 Sqn. Today, a commemorative plaque dedicated to 617 Sqn is affixed to the dam.

Precision

Barely two months after the squadron was formed, 617 took delivery of 20 modified Lancaster B.III (special) bombers from the Vickers factory. These aircraft had their bombs bays faired over and the spin drive installed, and the underside was modified to carry a single semi-recessed Upkeep bomb.
But it soon became necessary to hastily design and incorporate new tools and methods in order to meet the ever-increasingly demanding release parameters.

The testing of Upkeep had determined that the optimal drop height of the bomb would be just 60ft, little more than half the Lancaster’s 102ft wingspan, as it had been found that, if released any higher the bombs would frequently break up. Two searchlights were mounted on the Lancaster’s underside forward and aft of the bomb bay, and directed so they would intersect when the aircraft was at 60ft.

A post-war diagram shows how the Upkeep mine was used against the Ruhr dams.

The optimal release distance was determined to be between 400 and 450 yards from the upstream face of the dam, so a new bombsight was also required. With simplicity in mind, a wooden ‘Y’-shaped hand-held bomb sight about 18in long was designed. The aircraft’s navigator would look through a slot attached to the base of the ‘Y’ when approaching the dam wall, and small posts at the ends of the ‘Y’ would align with the towers on the dam to indicate the correct release point.

Modelling

In the meantime, reconnaissance flights over the Ruhr confirmed water levels were rising, and the model makers from RAF Medmenham’s V-Section unit constructed table-sized scale models of the target dams and their surrounding hills.

RAF Medmenham was an RAF unit located at a manor house in Buckinghamshire and specialised in photographic intelligence and interpretation. The unit has been described as being to imagery intelligence what the famous Bletchley Park – where the allies broke Germany’s ‘unbreakable’ Enigma code – was to signals intelligence.

It wasn’t until the day before the raid on May 15 that the 617 Sqn crews discovered what the actual targets would be. The primary targets, designated A, B and C were the Möhne Dam, the Eder Dam and the Sorpe Dam respectively, all located between 20 and 60 miles east of Essen. Three secondary targets in the same region were also selected, but were not attacked in the raid.

The crews were instructed to study V-Section’s models carefully and commit them to memory, as these gave them a perspective of the terrain and other obstructions and features surrounding the targets that aerial photographs couldn’t match.

The mission

Operation Chastise was conducted using 19 Lancasters in three waves. A 20th Squadron aircraft was unavailable for the mission because it had been damaged by the violent splash of an Upkeep weapon impacting the water after being released too low during a mission rehearsal several days prior.

The first wave comprised nine aircraft of three sections, each with three aircraft. Each section took off from RAF Scampton 10 minutes apart, and headed south-southeast over East Anglia into the southern North Sea, and crossed into enemy territory just north of the Dutch-Belgian border. The first wave’s primary target was to be the Möhne Dam, its secondary was the Eder Dam, and if it had any weapons left, the Sorpe Dam.

Aerial reconnaissance (vertical) photograph showing the breach in the Mohne Dam caused by No 617 Squadron, Royal Air Force’s raid on 16 May 1943.

The second wave of five aircraft actually departed first at 21.28 and took a longer easterly route across Lincolnshire into the North Sea, before crossing the enemy coast at Vlieland north of Amsterdam. It then turned southeast towards its primary target of the Sorpe Dam. Its secondary targets were to be targets D, E and F, the Ennepe, Lister and Diemel Dams respectively, but these targets were not attacked.

The third wave of five aircraft departed Scampton two hours later and followed the route of the first wave as a mobile reserve, and was to be recalled if all the primary targets had been breached.
All three waves flew at 300ft across the North Sea and enemy territory to avoid being seen on radar, and the routing deliberately avoided known flak batteries and fighter bases.

Legacy

It is now history that the Möhne and Eder Dams were fully breached, and the Sorpe Dam was damaged. But the cost was high – eight of the 19 aircraft crashed or were shot down and 53 crew members were killed, and some 1,600 civilians are reported to have died as a result of the raid, many of them Soviet POWs held in labour camps in the region.

The next day, Spitfire reconnaissance pilot Jerry Frey was flying a battle damage assessment of the raid, and years later described the scene in an interview with The Telegraph newspaper. “When I was about 150 miles from the Möhne Dam, I could see the industrial haze over the Ruhr area and what appeared to be a cloud to the east.

“On flying closer, I saw that what had seemed to be cloud was the sun shining on the floodwaters,” he said. “I looked down into the deep valley which had seemed so peaceful three days before, was now a wide torrent. The whole valley of the river was inundated with only patches of high ground and the tops of trees and church steeples showing above the flood. I was overcome by the immensity of it.”

Unfortunately, the damage to Germany’s industrial heartland was not as significant as Bomber Command had hoped. Many of the factories were either untouched by the floodwater, were quickly rebuilt once the waters subsided, or were relocated deeper into Germany or occupied countries in eastern Europe.

38 members of 617Sqn involved in the raid were decorated. WGCDR Gibson was awarded the Victoria Cross, while two Conspicuous Gallantry Medals, five Distinguished Service Orders, 10 Distinguished Flying Crosses and four Bar to Distinguished Flying Crosses, 11 Distinguished Flying Medals, and one Bar to Distinguished Flying Medal were also awarded.

The Legacy of Operations Chastise, more commonly known as the Dambusters Raid continues to this day. The combination of raising an elite unit with specially configured weapon systems, exquisite planning enabled by sophisticated modelling and simulation to practice and hone skills and target familiarity, and excellence in execution, in this case through terrain-hugging flight and coordinated attacks with precision weapons, are all concepts which continued to be developed throughout the Cold War and beyond.

In part 2 of this story in the next issue of ADBR, we will examine the many ‘firsts’ of Operation Chastise, and how many of these elements have been incorporated into contemporary training and mission planning.

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