Supersonic Ping Pong Bazooka
Ping pong bazookas have been used in physics demonstrations for quite some time, but something’s been holding back their true potential. Typical designs restrict ping pong projectiles from reaching the sound barrier, limiting their destructive awesomeness.
A team of engineers at Purdue University decided to make a few design changes, and they’ve broken through Mach 1. Ping pong balls shot from their modified bazooka have enough force to burst through lines of soda cans and blast holes through ping pong paddles, as you can see in the video below.
Supersonic by Design
Engineer Mark French and his students posted their design on the arXiv preprint server. Their paper serves as not only a glimpse into this great demonstration but also a primer on basic fluid dynamics and compressible flows.
To break the sound barrier, the team added a pressure chamber and a divergent-convergent nozzle – also known as a de Laval nozzle – for added acceleration. The nozzle looks like a misshapen hourglass, featuring a narrowing section that leads to a choke point: the narrowest part of the nozzle. After this point, the nozzle widens again to form an asymmetric tube.
In such a nozzle, the flowing gas reaches a choke point at the throat of the nozzle. This is the point where the gas flow reaches Mach 1, the speed of sound. After reaching this choke point, the flow of gas will continually lose pressure in the converging section of the nozzle and gain speed.
A diagram of the team's bazooka.
Image Credit: Mark French, Craig Zehrung, and Jim Stratton/Purdue University
If the nozzle is designed correctly, the gas flow will keep accelerating after the choke point until reaching the end of the nozzle. Acceleration stops when the flow reaches a shock wave. For the ping pong bazooka, the team accelerated the balls to 406.4 meters per second, or Mach 1.23.
With ping pong balls flying at these speeds, the team cautioned that they only did these tests in a safe laboratory environment. Don’t try this at home, readers.
The team also notes in their paper that changing the shape of the nozzle could lead to dramatic changes in the velocity of the ping pong balls. That sounds like a perfect excuse for some more ping pong bazooka research.