Operation Chastise, the Dambusters, and the Bouncing Bomb Barnes Wallis Skipped Across a Reservoir to Break the Ruhr Dams

On the night of 16 May 1943, nineteen Avro Lancaster bombers of the Royal Air Force flew into occupied Germany at just 60 feet above the water to attack the great dams of the Ruhr valley. Using a spinning cylindrical “bouncing bomb” designed by engineer Barnes Wallis, they breached the Möhne and Eder dams and damaged a third, in a raid codenamed Operation Chastise. It remains one of the most technically demanding and audacious bombing missions in aviation history.

Why the Ruhr Dams Were a Target

The Ruhr valley was the industrial heart of Nazi Germany - steel, coal, and the factories that fed the war machine. All of it depended on water: for the steelworks, for the canals that moved coal, and for hydroelectric power.

Behind the valley sat several enormous dams holding back millions of tons of water. Break them, planners reasoned, and you could flood the mines, knock out power, and cripple production in a single night.

The problem was that no one knew how to break a dam from the air. The Möhne dam was a curved masonry wall more than 100 feet thick at its base. Conventional bombs barely scratched it, and torpedoes were useless - the Germans had strung deep torpedo nets across the reservoirs to snag anything dropped in the water.

How the Bouncing Bomb Worked

The solution came from Barnes Wallis, the aircraft designer behind the geodetic airframe of the Wellington bomber. He approached the dam as a physics problem, and the answer came from an unlikely place: skipping stones across a pond.

Wallis reasoned that a bomb spun and dropped correctly could skip across the reservoir’s surface, leaping over the torpedo nets entirely before striking the face of the dam. If it then sank flush against the wall before detonating, the surrounding water - which doesn’t compress - would drive the full force of the blast into the masonry rather than dissipating into open air.

The second stroke of genius was backspin. Wallis spun the bomb backward at roughly 500 revolutions per minute before release. The spin stabilized the skip across the water, and on impact it made the bomb crawl down the face of the dam, hugging the concrete so it sank to the ideal depth before exploding.

The finished weapon was codenamed Upkeep. It was not a sleek finned bomb but a 9,000-pound drum-shaped cylinder, packed mostly with the torpedo explosive Torpex - closer in appearance to a garden roller than anything you’d expect slung under an aircraft.

The Numbers the Crews Had to Fly

A clever bomb was only half the job. To make the skip work, the delivery had to be flown to figures that left no margin for error:

Altitude: exactly 60 feet above the water. Too high and the bomb shattered on impact instead of bouncing.
Speed: around 230 miles per hour. Too fast or too slow and the skip went wrong.
Release distance: roughly 1,400 feet from the dam. Too close and it wouldn’t reach the wall; too far and it bounced over the top.

All of this had to be done at night, in a heavy four-engine bomber, over enemy territory, under fire.

The Avro Lancaster was the finest heavy bomber Britain built, but it normally worked from 15,000 to 18,000 feet. Asking it to skim a lake at 60 feet was a radical departure. Selected aircraft were stripped and rebuilt: the mid-upper gun turret was removed, the bomb bay doors were taken off entirely because Upkeep was too large to fit inside, and the bomb was slung underneath in a cradle with two swing arms and a belt-driven hydraulic motor to spin it up before the drop.

To fly the mission, the RAF stood up a brand-new unit - No. 617 Squadron - which barely had a number when its crews arrived. They were told only to train for low-level night flying, lower than they had ever flown. Command of the squadron went to a 24-year-old wing commander, Guy Gibson, who had already flown some 170 missions.

Solving the Impossible With Light Bulbs and Plywood

Two problems nearly sank the whole plan, and the fixes are a masterclass in practical airmanship.

How do you fly at exactly 60 feet at night? A barometric altimeter simply isn’t precise enough to trust your life to at that height over dark water. The solution: two spotlights, one in the nose and one in the belly, angled so their beams converged into a single figure-eight on the surface only when the aircraft was at exactly 60 feet. The crew flew until the two pools of light kissed together, then held it there.

How do you release at exactly the right distance? Both main dams had two towers on top, a known distance apart. Someone built a simple handheld sight - a wooden Y with a peephole and two nails at the tips. When the bomb aimer lined the two nails up with the two towers, the geometry told him he was at the correct release distance. Two light bulbs and a piece of plywood solved problems the finest minds had called impossible.

The Night of the Raid

Nineteen bombers launched in waves, crossing the North Sea and hedge-hopping across occupied Europe to stay under German radar. The flying alone was lethal before a shot was fired. One Lancaster struck the sea and lost its bomb; others hit power lines and high-tension cables; flak towers along the route claimed more. Men died from the sheer violence of flying a heavy bomber at treetop height across an entire country in the dark.

Gibson’s wave reached the Möhne dam first. There was no diving in from altitude - the weapon demanded a long, flat, dead-steady run straight down the throat of the German flak guns positioned on the dam wall, holding 60 feet and 230 mph with no evasive maneuvering while tracer came up to meet them.

Gibson attacked first. His Upkeep bounced, struck the wall, sank, and threw a column of water hundreds of feet into the air - and the dam still stood. One bomb wasn’t enough. On the following runs, Gibson flew his own aircraft alongside each attacking crew, drawing the gunners’ fire onto himself to give the bomb aimer a clean shot. He did it again and again.

It took five bombs and five runs down that valley of fire before the Möhne finally breached and the reservoir poured out into the valley below.

The surviving aircraft with bombs still aboard pressed on to the Eder dam, set deep in steep hills that forced crews to dive over a hilltop castle, drop into the valley, get down to 60 feet, line up, and haul up hard over the far side before hitting the mountain. Some crews had to go around three or four times to get the geometry right - but they breached the Eder as well.

A third dam, the Sorpe, was damaged but not broken. It was an earthen embankment rather than a masonry wall, and the skipping attack simply didn’t suit it.

What It Cost

The price was severe. Of the 19 Lancasters that set out, 8 did not return. 53 of the 133 aircrew were killed - more than a third - and 3 more became prisoners of war.

On the ground, the floods killed well over 1,000 people. A hard truth worth stating plainly: many of them were forced laborers and prisoners with no say in the war, drowned in camps in the valley below the Möhne.

Did the Dambusters Raid Change the War?

No single raid wins a war, and the immediate strategic knockout Barnes Wallis hoped for didn’t quite land. German engineers patched the dams faster than anyone expected, rebuilding the Möhne within months.

But the effect was real. Germany had to pull tens of thousands of workers off other projects to repair the dams and defend against a repeat, and every reservoir in the country suddenly needed guns, nets, and men. Just as importantly, the story - nineteen bombers, 60 feet, a bouncing bomb, a plywood sight - traveled the world and delivered a jolt of morale to the Allies at a moment they badly needed one.

For pilots, it stands as one of the purest examples of airmanship under pressure on record: precision flying to the foot and the mile-per-hour, at night, at treetop height, into aimed gunfire, holding a straight-and-level run when every instinct demands you break away.

Guy Gibson was awarded the Victoria Cross for that night. He did not survive the war. Barnes Wallis lived a long life and designed many remarkable things, but by many accounts carried the weight of those 53 young men for the rest of his days - the engineer who dreams up the impossible weapon still has to read the casualty list in the morning.

Key Takeaways

Operation Chastise (the night of 16 May 1943) breached Germany’s Möhne and Eder dams and damaged the Sorpe, using a spinning “bouncing bomb” designed by Barnes Wallis.
The weapon, codenamed Upkeep, was a 9,000-pound Torpex-filled cylinder spun at 500 rpm to skip over torpedo nets and sink against the dam wall before detonating.
Delivery required flying an Avro Lancaster at exactly 60 feet, around 230 mph, releasing at about 1,400 feet - measured with two converging spotlights and a handheld plywood sight.
The raid was flown by the newly formed No. 617 Squadron under 24-year-old Guy Gibson, who was awarded the Victoria Cross.
The cost was heavy: 8 of 19 Lancasters lost, 53 of 133 aircrew killed, and over 1,000 people killed by the floods, many of them forced laborers.