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Ping-pong gun fires balls at supersonic speeds

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February 3, 2013

When a paddle tries to return a supersonic ping-pong ball -- the paddle loses! (Photo: Mark French)

When a paddle tries to return a supersonic ping-pong ball -- the paddle loses! (Photo: Mark French)

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The fastest serve ever recorded by a ping-pong player moved at about 70 mph (113 km/h). Professor Mark French of Purdue University's Mechanical Engineering Technology department and his graduate students, Craig Zehrung and Jim Stratton, have built an air gun for classroom demonstrations that fires a ping-pong ball at over Mach 1.2 (900 mph or 1,448 km/h). As the picture above shows, that's fast enough for the hollow celluloid balls to blow a hole through a standard paddle.

There are plans all over the internet for ping-pong guns that use stored pressure one way or another to shoot ping-pong balls at velocities from 100-300 mph (161-483 km/h). 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. Even though some designs include a compressed air chamber, the laws of gas flow limit the velocity of the air in the tube to considerably less than the speed of sound.

How a de Laval (convergent-divergent) nozzle works (Image: Wikipedia)

Professor French and his students wanted to develop a demonstration of how a de Laval nozzle (also called a convergent-divergent nozzle) converts subsonic gas flow into supersonic flow. As you can see above, there is a pressure source at the inlet (on the left) of the nozzle. At that point, the pressure and temperature are large, but the velocity is low, as expansion of the gas is driven not by the total pressure, but by the local gradient of pressure. Another way of seeing this is that at the nozzle inlet, the gas is not expanding into a vacuum, but into a region whose pressure is nearly as large.

As the gas flows through the converging part of the nozzle, its speed increases. There is nowhere else for the gas to go, so to compress the flow through a smaller diameter requires that the velocity increases. At the throat (smallest diameter) of the nozzle, the gas velocity reaches the speed of sound. At this point, larger inlet pressures will not drive the gas any faster, a condition called choked flow. Now as the gas at the throat expands into the diverging (outlet) part of the nozzle, it expands, converting temperature and pressure into larger supersonic velocities.

Schematic diagram of the supersonic ping-pong gun (Image: Mark French)

To demonstrate the conversion of subsonic to supersonic flow, Prof. French and his team designed the gun shown above. The end of the pressure vessel is sealed with laminating tape. Both the nozzle and the barrel are evacuated so the the gas flow is unobstructed. Overall, the gun is a bit less than 12 feet (3.65 m) in length.

To fire the gun, the pressure is increased in the pressure vessel until the tape breaks. French found that two layers of tape ruptured at about 60 psi (414 kPa), and three layers at about 90 psi (620 kPa). The speed of the ball was measured using a high-speed camera viewing the ball moving against a calibrated scale. A typical velocity was a bit over 1,448 km/h (900 mph) – nominally a velocity of Mach 1.23, which is about the top speed of the Soviet-era MIG-19 fighter.

The lead photo should convince the reader that this ping-pong gun is not a toy. The energy and momentum of the ping-pong ball is roughly the same as that of a .32 caliber ACP pistol – not the best choice for defense, to be sure, but quite lethal under the right circumstances.

Prof. French gives a good explanation of the physics and design of the supersonic ping-pong gun in the video below. If you want to skip to the firing of the gun, it begins at about 5:40.

Source: arXiv.com (PDF) via MIT Technology Review

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About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer. All articles by Brian Dodson
8 Comments

so did the ball survive or was it vapor?

Mike MacDonald

re; Mike MacDonald

I am going to go with neither. The ball is destroyed but not vaporized.

The real question is how will this effect potatoe guns.

Slowburn

"Please do not try this at home." Like that is going to work on youtube. There are about 200 blokes yelling; Darling, your Dyson seems to have a blockage and stopped working, I need to take it out to the shed to see if I can blow it clear. Do we have any ping pong balls ??????

ELM

I was in the lab next door when this happened. Pretty awesome stuff!

Boiler Up!

Jordan Mansfield

If supersonic ping pongs are cool... supersonic golf balls are much cooler! www.youtube.com/watch?v=lHNV2nt821A&feature=youtu.be

Mobile Chernobyl

The term 'ping pong' is eighty six years out of date. 'Table tennis' was officially adopted in 1927.

Gerry Lavell

Hi, What material the nozzle is built from?

Dmitry Rybakov

How certain is everyone that the ping-pong ball is responsible for the hole in the wood? Perhaps a hole in the wood occurs whether or not the ping pong ball has been loaded into the gun or not. Has anyone gone through the firing sequence without the ping pong ball to see if the air blast alone is sufficient to put a hole in the wood?

BGriffin
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