Air Gun Fires Ping Pong Balls at Mach 1.2
The game of ping pong, or table tennis, originated in England in the 19th Century. Around the same time, the de Laval nozzle, an hourglass tube used to accelerate a hot, pressurized gas to supersonic speeds was also invented. But it took a 21st Century mechanical engineer to put the two together. The result is a cannon capable of launching a ping pong ball at speeds of over 900 miles per hour — about Mach 1.2. It’s so fast that the lightweight ball blows a hole right through a paddle.
Gizmag has a good explanation of how Mark French, a mechanical engineer at Purdue University in Indiana, and his graduate students Craig Zehrung and Jim Stratton built the de Laval tube:
In the simplest cases, these guns consist of an open-ended tube (usually PVC plumbing pipe) into which a ping-pong ball fits loosely. The tube is sealed at each end by a membrane strong enough to withstand atmospheric pressure, and the air in the tube is removed. To fire, the gun is mounted so that the ping-pong ball is near the rear membrane, which is then nicked (typically by a knife or sharp point). The ping-pong ball is accelerated by the inrush of air, which also blows out the front membrane.