If there’s one thing wrong with Ping Pong Guns, it’s that they can’t fire their ammunition at supersonic speeds. At least, that’s the feeling of Mark French, a mechanical engineer at Purdue University in Indiana, and his graduate students Craig Zehrung and Jim Stratton.
The dire performance of ping pong guns has forced these guys to take matters into their own hands. Today, they reveal the fruits of their work in the form of a bespoke gun capable of launching ping pong balls at speeds of over 400 metres per second. That’s about Mach 1.2.
Their design is simple. It consists of a pressure chamber sealed with two or three layers of Duck Tape. When the pressure exceeds about 620 kPa, the tape bursts allowing a pressure wave into a special nozzle and from there into the barrel where it propels a ping pong ball.
The design of the nozzle is important because it generates a supersonic pressure wave. For this, French and co use a so-called de Laval nozzle, named after the Swedish engineer who invented it in the 19th century.
A de Laval nozzle consists of a tube with a pinch in the middle. As the air enters the nozzle it accelerates as it is compressed. At the pinch, the air flow matches the speed of sound and then becomes supersonic as the tube begins to expand.
This supersonic flow then enters the gun barrel where it accelerates a ping pong ball to supersonic speeds.
French and co have used a high speed video camera to measure the ball velocity as it exits the barrel. They say they have video evidence of speeds in excess of 400 metres per second. That’s faster than an F-16 flying flat out at sea level.
That’s sterling work, no doubt. But let’s hope these guys never turn their attentions to toothpaste dispensers.
Ref: arxiv.org/abs/1301.5188: A Supersonic Ping Pong Gun