Skipping Stones, Bombs, and Spacecraft

The physics of skipping stones across water has been studied by many. One engineer in particular wrote a book on the subject: The Secrets of Stone Skipping. For nearly three decades, Jerdone Coleman studied stone skipping in a huge variety of conditions, even working with MIT’s Strobe Project Lab to photograph the stones in motion.

Calculations published in nature magazine showing that 20° is the optimal angle.
Calculations published in Nature magazine showing that 20° is the optimal angle.

After the end of a relationship, Coleman found himself walking along a lake, skipping stones. The more stones he flung at the water’s surface, the better he became. One foggy day, with (unbeknownst to him) a group of people watching in mild interest, he tossed a stone that skipped beyond sight. The crowd of strangers applauded and Coleman later went on to set the world record of 38 skips in 1992. According to him, the perfect stone is both uneven (preferably 5-sided) and slightly heavy to prevent the wind from blowing it off course.

Skipping stones’ physics has actually led to other innovations and developments. During WWII, the Royal Air Force used “bouncing bombs” (designed by Sir Barnes Wallis) to target German dams. They were the attempted solution to avoid anti-aircraft weapons over the dams as well as the anti-torpedo netting underwater. Of the few bombs used, only one successfully destroyed a dam with others either detonating early or missed the target.

The floodwave from the bursting dam killed 1,579 people, mostly foreign laborers.
The floodwave from the bursting Möhne Reservoir dam killed 1,579 people, mostly foreign laborers.

Today, the properties of skipping are being used in space travel! “Skip reentry” is when a space craft skips off the atmosphere before total reentry. This both slows the craft down and allows for greater range. Doing this requires a great deal more calculation and precision than skipping stone, however. Too steep a dive and the craft can be destroyed, but too shallow a dive will send the craft careening into space, lost forever. That’s a risk no stone thrower has to consider.

An early, 1963 concept for the Apollo mission.

 

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