# Supersonic ping-pong-ball gun leaves cartoonish ball-shaped hole in hapless paddles

The finest moments in physics instruction always involves something going bang, blam, or boom, and this is no exception: Purdue's prof Mark French and grad students Craig Zehrung and Jim Stratton built a supersonic ping-pong-ball gun that attains supersonic muzzle velocity:

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.

Ping-pong gun fires balls at supersonic speeds [Gizmag/Brian Dodson]

(via DVICE)

### 47

1. Tell us more about these “right circumstances”.

We have to be in the vacuum chamber for the ping-pong ball to kill us, right?

Recommendation:  do not weaponize.

1. fuzzyfuzzyfungus says:

The vacuum chamber is only inside the barrel, and the target is outside, so a little atmosphere won’t protect you(though drag probably brings a ping pong ball down from that kind of muzzle velocity pretty quickly).

What may be more likely to save you is size: .32 ACP, as its name suggests, is .32 inches in diameter, 7.65mm. A ping pong ball is either 38mm(2000 and previous) or 40mm(newer). (Though, size has its tradeoffs: above a certain size, impact is spread out enough to prevent penetration and deeper injuries. This is good. Below that size, though, the bigger the projectile the bigger and messier the hole, extra credit if it fragments or tumbles. If the projectile is going deep, or all the way through, you want a very small, very neat, little hole, not a huge gaping one. Not sure how far away you’d have to be before size goes from curse to blessing). Projectile crumpling may also help some, a celluloid ball full of air is going to crumple somewhat more easily than an FMJ full of lead.

I certainly wouldn’t be first in line to test it though.

1. awjt says:

start from way across the room (or parking lot) and then move closer until you can’t take it anymore

1. That’s the way, uh-huh, uh-huh — !

2. Realist.  I want to be killed by the ping-pong ball inside the vacuum chamber.

Like in 2001: A Space Odyssey, where David Bowman is making a suitless jump through hard vacuum, and HAL-9000 (deranged from singing “Bicycle Built for Two” too many times) fires a ping-pong ball into the open airlock.

But seriously:  thank you for explaining the physics.

2. Recommendation:  do weaponize.

FTFY ;)

2. Paul Renault says:

So, it’s like the Canadian Air Farce’s Chicken Cannon, but scaled down, eh?

3. E MM says:

I did the math when I first heard this story.  If I didn’t drop a digit somewhere, the ping pong ball carries roughly the same kinetic energy as a frozen turkey travelling at 60mph.  Probably not suitable for classroom demos…but maybe out on the soccer field?

1. Tribune says:

You do your calculations in frozen turkey equivalents?

1. awjt says:

So THAT’S what FTE stands for.  I have been wondering that for years when our budgeters keep talking about FTEs this and FTEs that.

2. Ian Anthony says:

Ah, but didn’t it instantly generate a mental image that made you understand exactly what he was talking about?

2. bzishi says:

I think there is an error in your calculation.

KE = 0.5 m*v^2
KE_1 = KE_2
m_1/m_2 = (v_2/v_1)^2

This gives a ratio of 225 for the mass. Apparently a ping pong ball has a mass of 2.7 grams, so the other mass would be 607.5 grams. This is way too small for a turkey, but it is close to that of a basketball at 60 mph.

1. SamSam says:

The energy of a 2.7g ping pong ball travelling at 900 mph is instead equivalent to that of a 3 kg (6.6 lb) turkey travelling at 27 mph.

Regardless, I don’t want to be on the receiving end of this turkey gun.

4. Jake0748 says:

Funnest thing I’ve seen all day.  (No, I’m not gonna try it at home).  I wish they had showed some of the high-speed camera footage that he talked about.

5. fuzzyfuzzyfungus says:

Any word on the state of the ball after its no-doubt-traumatic deceleration?

1. Jake0748 says:

What ball?

6. awjt says:

I bet Zhang Jike could return that, no sweat.

7. As described, we’re dealing with one version which uses one atmosphere and no nozzle, and another which uses two or three atmosphere and a nozzle. It feels incomplete without at least discussing a version with two or three atmospheres and no nozzle, and why this can’t break the sound barrier.

1. nixiebunny says:

The nozzle is described as the thing that produces the supersonic air velocity. It is a rather nonlinear effect. Some would call it magic.

1. So you still need something to show the pressure itself doesn’t.

8. It should be interesting on the air cannon line come the November punkin chunk.  The only problem might be getting the barrel’s pumped down within the time limit.  (for those that haven’t seen one in the flesh, a “competitive” barrel is 10″ in diameter, and over 50′ long.  (with some of the winners sporting 120′ long tubes).

[added: “pump down in the time limit” – you have to put the gourd in the machine with the marshal watching. You then have 15 minutes to bring your pressure vessel up to launch pressure, and fire. For mechanical catapults, the time limit is only 5 minutes.]

1. ldobe says:

Interesting proposition.

I wonder if the vacuum might have a positive effect on keeping the pumpkin from going pie?  There might be less opposing force, since there’s no air drag on the leading aspect of the gourd…..Although, the gourd would be slamming into a wall of air at the end of the tube.  If it’s going mach 1.23, the heating might make it start to get all sloppy, or cause air pockets inside to expand some.  But then again, the pumpkin was already in a vacuum, which would make the air pockets expand as well.

Fascinating.  I’m gonna have to go do some math now.

1.  It occurred to me to wonder what was going to happen to the pumpkin when it got exposed to vacuum.  There is a whole lot of water in the thing, that will be wanting to change state.  I suppose “best” case, you get some sublimation, and cooling.  You would get a lighter, and tougher gourd.  Worst case, you get a cracked mess, and/or DQ’d for modifying the gourd.

(for those that haven’t tried it, if you pull a vacuum on a container of water, the stuff first boils, then ice crystals form.)

1. fuzzyfuzzyfungus says:

I know that a lot of produce carries the ‘Coated with food-grade vegetable-, petroleum-, beeswax-, and/or shellac based wax or resin to maintain freshness.’ tag and that that coating process is largely about reducing moisture permeability. Would such a mod disqualify a pumpkin?(It wouldn’t save you from what happens to a gourd with 1 atmosphere of pressure inside and ~0 outside; but it could keep the amount of fluid bleed to a minimum over the relatively short pressurization interval allowed by the contest…

1.  The pumpkin gets sprayed with paint at weigh in.  (different color each day).  That would provided the equivalent of the wax.  I would do the experiment, but my vacuum pump is deeply buried.

9. There will be a CSI where this is the murder weapon out by the end of the year, bet on it.

1. morcheeba says:

Yeah, but they’ll fundamentally mess up the science and/or politics somehow. Remember the 3d printed gun episode? The protagonist wanted police to print guns as needed because… well, I guess he thought police had trouble acquiring guns. They totally missed the social implications.

1. JonS says:

They’ll probably have the ping pong ball gun having an effective range of, like, 800m. Instead of 8m.

Because a massive contraption that can barely hurt you from across the room just isn’t that scary, whereas a silent killer that leaves no traces and can getcha from three city blocks away … oooooh, scary.

10. Winslow Morgan says:

Work on the vacuum spelling and give Venturi some credit..Mkay?    (Giovanni Battista Venturi,  {1797 Venturi effect}).

1. Another deserving a little credit would be de Laval, who discovered the convergent/divergent nozzle shape.

11. I demand that Myth Busters shoot supersonic pingpong balls at pig cascaras or other human analogs now! So I don’t have to. Because I don’t need to spend tomorrow trying to find the best price for a pig locally and seeing if I end up on a FBI watch list for trying to source the parts from Home Depot…

1.  I’d prefer they use ballistics gel with a high speed camera; that would be a lot more interesting to watch.

12. Preston Sturges says:

If they use nitrocellulose ping pong balls, I would think the impact would make it detonate.  If not, they should work on it.

1. fuzzyfuzzyfungus says:

Having set a number on fire for, um, purely educational purposes, my experience has been that not-a-whole-lot of nitrocellulose, not enclosed in any sort of pressure vessel, flares up quite pleasingly but doesn’t really ‘detonate’ in the sense that leaves you using base 8 when you count on your fingers…

13. Pat R x 2 says:

Hehehe! I saw this on the arXiv blog. They have a paper on it here: http://arxiv.org/abs/1301.5188

14. Yeah this was great. I like the warning about “don’t try this at home we have college degrees and special equipment”. If you look at their Youtube channel you can see them blasting soda cans http://youtu.be/VyCCo9i8Hwc and their special equipment for hearing protection consists of putting their fingers in their ears. No other safety equipment is visible, and one guys is standing about 1m away filming it on his phone.

1. I thought that warning was pretty ridiculous the first time I saw it and even more when repeated. They could have just said, “please don’t try this at home; it is very dangerous” and left it at that.

I’m guessing that years of experience is more relevant than 8 college degrees in this instance. And, while it’s great to have high-speed cameras and such, those do more to explain after the fact than to keep safe in the first place. Also, how’d they get those years of experience if they were following similar warnings?

1. s2redux says:

I expect that experience is commensurate with 8 college degrees via the “yet another friggin’ lab course” factor.

15. emo hex says:

like a supersonic pingpong ball,
leaves a hole
in my heart!”

“thank you very much”

16. Jack Daniel says:

So when he says do not, DO NOT, try this at home, he actually means do…right?

17. Jon Smith says:

Great video, great presentation, but why would any professor from a science department at an accredited program even mention imperial measurements?

18. Gregg Grose says:

And the science gets done and you make a neat gun…

19. Steve Tedder says:

The danger involved in building one of these things isn’t what you might do with the ping-pong ball. The danger is the possibility that the pressure vessel might explode and injure someone. If you aren’t completely confident of your ability to build such a vessel, you should leave this project alone. And remember, pressure vessels should be designed with a safety factor of at least five times the operating pressure! If you don’t know how to do that, just say no!

1. Scratcheee says:

I noticed a complete lack of personal safety equipment in the soda can video.  No glasses, nothing.

20. jimkirk says:

I’m not suggesting anyone try this ay home, but FYI, schedule 40 PVC 1 1/2  or 2 inch pipe operating pressure is greater than 11 atmospheres (198 psi for 1 1/2 inch, 166 psi for 2 inch).  Bursting pressure is about 5 times that.  But seals, joints and stuff tend to be weak points, so be careful…nudge, nudge, wink, wink.

http://www.engineeringtoolbox.com/pvc-cpvc-pipes-pressures-d_796.html

21. I’ve been thinking of two things, regarding this:
1)  Could you improve the range a bit by machining the nozzle to provide “rifling” to cause the air flow to spin?
2)  Supersonic Golf Ball Cannon…