To heed or not to heed?
Credit: From "Supersonic Ping Pong Ball Brain Waves" by purdueMET on YouTube
Nice serve. An engineering professor and his students have developed a ping-pong ball gun that shoots balls at supersonic speed. Their fastest shot? Mach 1.23, enough to punch a hole through a ping-pong paddle.
As Purdue University professor Mark French explains in a YouTube video, the supersonic ping-pong ball gun is actually a modification of less-speedy ping-pong ball air guns, directions for which apparently abound on the Internet (We never knew). French's modification includes a nozzle, called a de Laval nozzle, with a distinctive bell shape that also appears in supersonic rocket nozzles. French thinks the shape creates supersonic flow, although he cautions in the video that he hasn't analyzed the gun, so isn't sure if that's what's happening.
In addition, many homemade or lab-made ping-pong ball guns depend on atmospheric pressure to drive their flimsy plastic ammunition. French's gun includes a chamber of air pressurized over one atmosphere.
French explains the physics of the entire gun in his video, which is part of a long-running series explaining solutions to common engineering problems. He and his students show their gun at work at the end, including the money shot through the ping-pong paddle.