The Dambusters Case Study

Have you ever skipped rocks across the surface of a lake or a stream? Well in World War II the English skipped cylindrical bombs across the water at the top of dams to blow up the dams.

This was done to flood and destroy German farms, coal mines, and factories which in turn would slow their production to help the Allies to beat Germans. According to dambusters.org.uk they called this project Upkeep and it had two other applications that were spherical and were for anti-ship applications and were called Highball and Baseball and the whole group was codenamed Golfmine. The English were forced to rethink how to do it because they had already attacked once before causing them to put torpedo nets upstream from the dams. This is what caused

the English to come up with the idea of skipping the bombs. The bouncing bombs used two special physics principles to make them bounce farther.
The plan for the Upkeep project was to destroy farms, coal mines, and factories while also getting rid a lot of the water for drinking and use in industry. according to dambusters.org.uk the main reason they wanted to do this is because the Germans needed coal to make steel and the flooding would put out the steel furnaces which can take sometimes up to a week to get restarted and if the coal production stopped the steel production would slow and make it very hard for the Germans to make equipment and vehicles for the war.
Who was responsible for this idea? According to Dam Busters: Manual 1943 Onwards it was Sir Barnes Wallace who had a good track record for designing planes and bombs and his preliminary testing for the idea was done with marbles, a slingshot, and a tub of water. According to the “The Dambusters,”
Wallis was still faced with the problem of placing his hypothetical explosive charge against the wet face of the dam. It has been known since the dawn of artillery that cannon balls could be ricocheted off the surface of the sea and this technique had been used for years to extend the range of black powder guns. Although German physicists in the early 1900’s had identified and quantified the physics of bouncing solid objects off water it was not until Barnes Wallis turned his formidable intellect to the problem that it was proved back spin greatly extended the range of this ricocheting projectiles. Wallis by careful experimentation proved the “Magnus effect” whereby the spinning object is effectively given aerodynamic lift and the number of bounces thereby increased.
What is the Magnus effect? According to “The Magnus Force,” in terms of how it effects baseballs
The origin of the force which makes a spinning baseball curve can readily be appreciated once we recall that the drag force acting on a baseball increases with increasing speed. For a ball spinning about an axis perpendicular to its direction of travel, the speed of the ball, relative to the air, is different on opposite sides of the ball… The lower side of the ball has a larger speed relative to the air than the upper side. This results in a larger drag force acting on the lower surface of the ball than on the upper surface. If we think of drag forces as exerting a sort of pressure on the ball then we can readily appreciate that when the unequal drag forces acting on the ball's upper and lower surfaces are added together there is a component of the resultant force acting upwards. This force--which is known as the Magnus force, after the German physicist Heinrich Magnus, who first described it in 1853--is the dominant spin-dependent force acting on baseballs.
The way air pressure acts with change in speed of the air is the faster it moves the lower the pressure and the slower it moves the higher the pressure this is called Bernoulli’s principle and it also explains why airplane wings work. Bernoulli’s principle states that the faster a fluid moves the lower the internal pressure of that fluid and the slower it moves the higher the internal pressure of the fluid. Since the object is spinning the air is pulled over one side and pushed over the other side which causes one side to move faster while the other moves slower relative to the air because the air is moving around the object as it is spinning the air is being pushed and pulled around the object causing the air to go at different speeds causing a pressure difference around the object and that pressure difference pulls on the object while also pushing on it.
According to “The Dambusters” the bombs started off as spheres but they were hard to manufacture and got crushed against either the water or the dam face on the water side in testing and was proven not to drop straight.

Since there were many issues with the spheres they moved on to the next closest thing they could drop accurately a very wide but not very tall beer barrel shape which was composed of an outer barrel shaped case which was meant to make it bounce better and an inner metal cylinder which held the charge. This designs main flaw was its outside case was made of wood which would shatter on impact so they tried to salvage the idea because they did not think the inner cylinder case could withstand the bounces and hitting the dam. To salvage the idea they tried to better reinforce the barrel using more bands going along the outside of the case which was proven a futile attempt. After the barrel design was given up on they decided to try the cylinder casing bare. The inner casing did not break up on impact like they worried and so they decided to use the cylinder design which was much easier to manufacture and did not require any special loading equipment except for the bomb holder which hung under the plane and was used to spin the bomb while the plane was flying using a hydraulic motor and a belt system. Once the bare cylinder was the final design they could train the crews and design the bomb interior and put in safeguards to ensure the Germans could not use their

idea against them. The safeguards included three hydrostatic pistols(triggers the bomb at a set depth in this case the depth depended on the dam)which were meant to ensure the bomb went off and a self-destruct timer set for about 90 seconds that was triggered by a pin being pulled by the weight of the bomb as it was dropped.
The air is pulled across the smooth cylinder by friction between the cylinder and the air which is called fluid friction. If when they were designing this they could have increased the Magnus Effect by making the cylinder rough which would increase the pull on the air but may it could possibly make the bouncing off the water harder, but if they increased the diameter of the cylinder it would increase the amount of distance the air is pulled in a given period of time and increase the Magnus Effect on the bomb as it bounces. This was not needed because the bombs had enough power to work and how effective the bombs were as is for bouncing. The bombs sometimes bounced up and hit the tail of a plane or the splash would hit the plane and when it did it damaged the plane and normally ended in a crash. The issue with the splash and the bounce was mostly eliminated by pulling the plane up right after the bomb is dropped and it made the releasing of the bombs a bit safer for the people on the missions. These safety precautions lead to the Germans getting their hands on a bomb after a plane crashed that had not dropped the bomb that the time delay that was supposed to make sure they could not get a bomb but the Germans found one after one after a plane crashed. The Germans made detailed diagram of the bomb and attempted to start a program to destroy dams but could not build it before the war ended .
According to Dam Busters the Germans tried to make it harder to destroy dams by setting up nets under water and putting turrets on the dams they did this to avoid torpedo attacks however the bouncing bombs would bounce over the nets. After the dams were destroyed the Germans set up a number of defenses to protect their dams afterwards including mines in the water that could be set off to spray water up and damage the planes, floating barricades and nets above the water were set up to stop the bombs, search lights and smoke apparatuses were used to make planes visible and to make it so they could not see, and in addition to all that they added more guns around the dams so they could try to shoot the planes down and added cables to catch low flying planes. This was one of the reason the dams were not attacked at any time after the raids and also why the bombs haven’t been used in war since the raids. The other main concern was loss of civilian lives which were very high in the raids and this was the main reason they did not use the bombs against other high level targets like dams around Europe’s industrial areas that were taken over by the Germans which could have been easily attacked before the Germans could set up defenses.
According to Dam Busters the bombs made use of the Magnus Effect by the backspin they had while spinning at 500 rpm and the size of the bomb helped with the dimensions of 60 inches long and 51 inches in diameter and weighing in at 9,250 lbs.

the force can be calculated with this equation according to What is the Magnus Effect. Fm = S (w x v) where the Fm equals the force of the pull perpendicular to the spin with the S equaling the air resistance in the surface of the object and the w equals the angular velocity of the object and the v equals the velocity of the object. This means that you can calculate the Magnus Effect on the bombs before the first bounce which would slow it down and would be nearly impossible to

measure.
The Magnus Effect is a very peculiar thing since it makes objects act in ways that are predictable however it is hard to wrap your head around in the sense that it deals with changing the pressure in a fluid by changing its speed with the spin of the object. That in conjunction with the fact that the bouncing bombs were perfect examples of how physics principles and effects can be used in many applications which many people would never consider and make things appear to play tricks on your eyes. This is a perfect example of manipulating physics to confuse people when I first saw the original testing film from Ashley walk the testing range for many of the tests I thought my eyes were plying tricks on me or the film was playing at the wrong speed because the fake bombs they hadfor testing acted very odd compaired to the way something bouncing without spin should act because it bounced much higher than it should have and it stayed in the air much longer than it should have .
The two physics pricliples that the bombs used the Magnus Effect and Bernoulli’s principle allowed the English to accomplish what the Germans didn’t think was possible. Although the bouncing bombs don’t act like skipping rocks on a body of water they bounced and destroyed many high level targets in German controlled Europe towards the end of world war two.