This weekend marks the seventy-first anniversary of one of the more famous operations of World War Two, the British attempts, over the course of two nights, to destroy the three main dams supplying hydroelectric power to Nazi Germany.  The mission was code-named Operation Chastise, and the objective was not only to deprive the German economy of electrical power, but to cause massive disruption of the German war machine as a result of flooding in the Ruhr and Eder valleys.

The mission succeeded in inflicting severe damage on two of the three dams (image at left of the Mohne dam), but in the final analysis it resulted only in delaying the Nazi war machine, as the Allies had not yet established air superiority over the German heartland, and so Hitler’s engineers were able to quickly repair the damage.

The mission has been highly publicized in the decades since the war ended, with at least one movie (“The Dam Busters”, from 1955, HERE), several books, and many articles and television shows.  Since it was only marginally successful, the interest in the mission was centered around the unique bomb that was used.  It was an unconventional design, based essentially on an explosive-packed 55-gallon steel drum laid on it’s side, that was dropped from a British Lancaster bomber only after a reverse “spin” had been imparted to the drum, and from a very, very low altitude.

The spinning (or bouncing, or skipping) bomb concept was necessary to defeat the defensive measures that the Germans had devised to protect the dams.  The had essentially hung a submarine net some distance away from the inside (impoundment) face of each dam, the idea being to snare any conventional bomb or torpedo before it could get close enough to inflict significant damage to the submerged dam wall.  To overcome this, the chief British designer, Barnes Wallis, came up with the idea of a bomb that would skip along the surface of the water until it hit the impoundment face of the dam, then would sink straight down until it reached a pre-determined depth, at which time it would detonate like an anti-submarine depth charge.  The force of the explosion would be contained and magnified by the pressure of the water, and by the close proximity to the dam wall.

Aside from the herculean task of designing, perfecting, and transporting these bombs through Germany to the heavily defended dam sites, there was also the problem of the altitude.  After much testing, the British realized that the only way to get the bombs to skip when they hit the surface of the water, rather than to immediately sink, was to drop them from a height so low that they would have much more horizontal momentum than vertical momentum.  The optimum height was eventually determined to be sixty feet.  SIXTY FEET!  At night, at about 240-mph, and with German anti-aircraft fire saturating the airspace!

The Lancaster crews initially intended to rely on their aircraft altimeters, but they soon realized that at sixty feet the altimeters were erratic and unreliable.  They finally came up with a simple and ingenious solution illustrated by my crude graphic, at right.  They mounted a spotlight at each end of the Lancaster bomber, and angled the projected beams until they would intersect at exactly sixty feet below the plane’s fuselage.  As the plane descended over the impoundment behind the dam, the bombardier would watch the two spots on the surface of the water through the open bomb bay doors.  When they converged into a single spot, he knew to tell the pilot that the altitude was perfect.

For more on this fascinating episode, the WikiPedia page is HERE, and a great article on the mission posted this week by the Royal Canadian Air Force is HERE.