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Vacuum Cannon drives ping pong ball at supersonic speed

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Here I will describe a cannon that can drive ping pong balls fast enough to go through plywood and aluminum cans. This "vacuum cannon" has been demonstrated in college physics classes, since it involves many basic physics concepts such as Newton's laws of motion, air pressure, and particle dynamics. There are some great videos online of these college class demonstrations, but details of construction are a bit sketchy. Some sources claim achieving a ping pong ball speed well over the speed of sound (340 m/s, or 760 mph), but others perform a mathematical analysis that indicates a limiting speed of about 280 m/s ( 625 mph). I made a ballistic pendulum and conducted many speed measurement tests, my average speed was 378 m/s (845 mph). So I am going to claim supersonic speed, and admit there is some controversy.

So here is the warning: At this speed, even a ping pong ball is very dangerous. We are talking about a projectile travelling as fast as rifle bullets. If you elect to build this device, you must treat it with respect and use appropriate safety measures. These include:

Set it up so nobody can be in front or behind the cannon.
Wear safety glasses and eye protection.
Do not put any part of your body in front of or behind the cannon.

Also - the cannon can and will fire at unexpected times, especially as you experiment at the beginning.











 
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omarmufasa1 month ago
You should use clear duck tape on each end.
iceng1 month ago

What happened to your excellent video ?

I wanted to show my wife you did this before Mythbusters.

A

Bill WW (author)  iceng1 month ago

I may have fixed it now, kind of forgot how to embed a video.

monty3241 year ago
this is so cool but i cant help but think how much better it would be if you replace the near side of it with a barrel sealing piston valve and a high pressure tank because theres the full force of upto 150 psi of pressure pushing a ball against no resistance. the ultimate spud gun.you could load potatoes into it and put them on the moon.
intergalactic potato launcher
Bill WW (author)  monty3241 year ago
Tanks for the comment, Monty.

Yep, I already have th materials for what you are thinking about. There will be pressure on one side of the near seal, vacuum on the other.
monty324 Bill WW12 months ago
glad you like my idea. if you decide to do it, could you post or update an ible and also is there some way you could make the far seal permanent like a piece of rubber on a hinge?
Great ible and idea by the way
Bill WW (author)  monty32412 months ago
Thanks Monty.

I will make a new Instructable, but I will first find another way to measure speed.

I also used just flat smooth discs cut from food containers, or soap containers. The disc can just be placed in front of the pipe, with the pimp running the vacuum will suck the disc tight .
monty324 Bill WW11 months ago
try measuring the speed by having 2 lasers pointing a 2 ldr's or other light sensors and have them 50 cm apart. then make some circuitry so when one beam is broken a timer starts and when the other one is broken it stops. then just S=D/T.
you could even wire the ldr up to a mic input of some device and record audio and use an app like Audacity to check the speed
Bill WW (author)  Oorspronklikheid10 months ago
Thanks guys, but electronics is not my strong suit. I understand the concepts, but you will have to tell me what a "ldr" is.
An ldr ( light dependent resistor ) is a simple device whose resistance changes depending on the amount of light it receives and can thus be used to detect a passing object if used in conjunction with a laser.
Bill WW (author)  monty32411 months ago
Thanks Monty -

I will work on this. Do you hav a suggestion for a timer?
monty324 Bill WW11 months ago
I would either use a standard stopwatch and solder new wires to the buttons or if you have any programming experience then use an arduino.
SG1Oniell1 year ago
Interesting project, but as you mentioned, it being supersonic is questionable. I'm used to firearms, and all supersonic rounds give off an audible and distinguishable crack as they break the sound barrier. This didn't. It might be close, but I don't think it made it.

Cool project though, straightforward 'ible. All things considered, nice work.
The audible "crack" is from a wall of sound waves building up in front of the projectile. As there is no air forward of the projectile until it exits the tube, and is only outside the tube for a few centimeters in the video, any supersonic crack would be very small.

But I'm also having a hard time understanding how the air behind the projectile can expand at a rate faster than the speed of sound when it's at (essentially) STP until the membrane is pierced.

I really wish you had a friend with a reliable chronograph.
SG1Oniell cchubb12 months ago
I don't think you have a great understanding of how a sonic boom works. In order for an object to be supersonic, it must be faster than those waves. For the pressure waves to be in front of an object, as you say, means that it is not supersonic. The object must lead the waves in motion, not the other way around. Also, those waves propagate behind the object and spread outward. Seems to me it could very well affect the atmosphere behind it and create a boom throughout its launch. Mind you, I'm not entirely certain if it requires atmosphere in front as well as behind, or only behind where the waves propagate. But I would point out that if the boom was as short as you say, you'd still be able to hear it. They are incredibly recognizable. The sound would be heard over others going on, and would reverberate off the walls at least allowing you to hear something of it. It'd be comparable to firing a suppressed gun with ammo that breaks the sound barrier at something a few inches away. The suppressor muffles only the noise of the expanding gasses, but does nothing to the sonic boom almost regardless of distance to target. One exception being firing into a solid with the barrel pressed against it.
the sound waves generated by an object radiate from it all around. As the velocity of that object increases, the sound waves in front of it are compressed as the waves catch up to one another until they form a front of extreme pressure at the leading edge of the object generating the sound. The "boom" is generated when it breaks through that pressure wave and surpasses the sound waves it is generating.
I'm with cchubb in thinking that the vacuum in front of the projectile would negate any sound waves from being generated. With no sound waves and no air, there would be no high pressure wave front to break through. The air entering the cannon can move as fast as 1640 fps. Sound travels at 1129 fps. In it's vacuum environment, I think the projectile will definitely be moving faster than sound, given it's low mass and speed of acceleration, but without any air pressure in front of it and thus no building sound wave front, there would be no "boom" as it surpasses the speed of sound. Once it hits the atmosphere, there isn't going to be enough inertial energy from the ping pong ball to displace that volume of air at a supersonic speed. I'm wondering if the can wasn't there, would the ball break apart on it's own when it hit the atmosphere?
You're incorrect in assuming that its always traveling through a vacuum. Rewatch the video, there is a space between the end of the barrel and the soda can that is not in vacuum, and in fact is open to the atmosphere. Based on that, your question regarding breaking apart is irrelevant. It hits the atmosphere before it hits the can, and the damage on the ball was done by the can.

Also, you're incorrect in stating that sound travels at 1129 fps. The speed of sound changes with temperature. You picked a value for a very specific temperature, which could be used to guess an altitude. Keep in mind that the speed of sound is not a constant value. for instance, people say Felix Baumgartner broke the speed of sound, when in reality he broke several speeds of sound.

Another thing, "inertial energy" doesn't exist. There is inertia, and there is energy, there is no "intertial energy". There is however, kinetic energy, which is what I think you're trying to refer to.

It is evident to me that you do not understand what you're trying to talk about with all of the errors you have made. Consider that the energy you're trying to refer to exists for all moving objects, and that it increases as velocity increases. Consider also that anything that moves through the air displaces the gas. Now, if something with as much energy as a ping pong ball moving at supersonic speeds (at whichever temperature you choose), cannot displace the air in front of it, then how does a ping pong ball tossed through the air displace the same volume? If what you said were true, then playing ping pong would not be possible, because the ball could not physically move through the air, and would act like it hit a solid the instant you released it.
Bill WW (author)  baratacus10 months ago
Thanks, Baratacus -

You said "The air entering the cannon can move as fast as 1640 fps."
How do you get that number? (Just curious).

No, the ball did not break up unless it hit the can or another object. However, I have wondered if it might heat up to its melting point if it was moving fast enough. All the kinetic energy would be converted to heat.

cchubb SG1Oniell12 months ago
Granted, I'm no expert at supersonic waves, but I have fired suppressed guns with projectiles both above and below the speed of sound. The "boom" from a supersonic round from a suppressed gun is loud, but I have never fired it at a target inches from the muzzle to determine if the sonic boom requires a significant stretch of non-turbulent fluid to pass through.

In the area within inches of the muzzle it's going to be very turbulent during and after the projectile leaves the barrel. I think that sound of a supersonic projectile is going to be very hard to detect over the sound of the membrane breaking, the can rupturing and other noises.

A chronograph is going to tell the whole story. Ideally one within the last couple of inches of the barrel, but one directly outside might have some bearing if the projectile is heavier than a ping pong ball, and more aerodynamic, like a golf ball, though attendant safety measures will be much more stringent with such a heavy projectile.
SG1Oniell cchubb12 months ago
I'm uncertain how much turbulence has to do with it. If a fighter flies through fairly turbulent storms at supersonic speeds, it'd still result in a boom. Though this is far different from that. I agree with you, a better measure is necessary to determine this cannons projectile speed.

Also, I must applaud you. It's rare that I find someone willing to have a pleasant discussion without resorting to personal attacks anywhere on the Internet. I feel you and I would get along quite well.
cchubb cchubb1 year ago
PS: Will you come organize my workbench?
Bill WW (author)  SG1Oniell1 year ago
Thanks for the comment. I'm still trying to understand this thing. I am working on an upgrade that could double whatever speed it gets now.
handy15710 months ago
To Prophes0r : I believe one part of your post may be flawed:

You state:
" Both will have to contend with the friction of the ball against the barrel." & "Both cannons also experience some loss as the ball smacks against the sides of the barrel since they are of differing diameters. "

If the projectile is round (as is the case with a ping pong ball) and is slightly smaller in diameter than the barrel, the ball will not touch the inner surface of the barrel because the air behind the ball is equally distributed on the surface of the ball. As the gun is fired, the pressure behind the ball will try to escape around the ball equally, causing a small 'cushion' between the ball and the barrel. This principle is the reason that the big guns on "Pumpkin Chunkin" can actually shoot the pumpkin projectile without obliterating the pumpkin. If there is a problem (probably due to the imperfect shape of the pumpkin or too much pressure), when fired, if part of the pumpkin touches the barrel, it "pies" as they call it, which just means the pumpkin is obliterated. With a cushion of air around the pumpkin, it can move through the barrel with no friction. You do have a loss of pressure which reduces the pumpkin's potential velocity as it travels through and out the barrel, but the pumpkin/barrel friction would be negated. I'd picture a vacuum cannon as having the same quality since the flow of air would still want to escape around the ball.

Bill WW (author)  handy15710 months ago
Tanks, Handy -

I believe you are correct about the air 'cushion' between the ball and the barrel. There would be minimal friction between ball and barrel anyway.
I have published a vacuum cannon video and have credited you for the inspiration in the description: http://www.youtube.com/watch?v=CVL99yIB3NQ
vincent752011 months ago
Fun !

But this is NOT a workshop… it's a LAB !!!!…
mainah vincent752011 months ago
You through now Vinnie.
vincent7520 mainah11 months ago
I'm sorry !
I was not aware I messed up that bad.
I really didn't mean to send more than one message … I simply thought it didn't get through as my screen froze.
I'm really sorry.
Bill WW (author)  vincent752010 months ago
Merci, Vincent.

I also send multiple e-mails, but you set a record :)

I can not decide if I want to be a woodworker or a science experimenter in my retirement.
vincent7520 mainah11 months ago
;)
bfk11 months ago
Very cool. Thank you. High speed cameras in the 1,000 fps category are in the "I can afford that" range now, but in order to use it you'll need to expand the view into the messy end of your shop :)
Bill WW (author)  bfk10 months ago
You are so wise, bfk. Yes, I moved all the junk out of range of the camera!
I am thinking about the high speed camera idea.
Appreciate your comments. Thanks.
ElmerJones11 months ago
Gosh do you have a nice clean workshop!
Bill WW (author)  ElmerJones11 months ago
You didn't see the other end of the workbench.
Prophes0r11 months ago
Interesting. Though some of your math, and physics, and other is wrong.

First. The ONLY rifle cartridge that travels this slowly is a .22. Many travel MUCH faster than this.  A NATO 5.56mm bullet usually travels at ~900m/s.

Second. The RMS of our atmosphere is ~500m/s at human comfortable temperatures. This means that regardless of the pressure on the high side, air will only fill the vacuum at a (average) speed of 500m/s. That is for unimpeded flow, which your cannon is not. It is important to repeat that this is not changed by the pressure differential. "air" at 10,000 atmospheres will still only flow at 500m/s into a vacuum at room temperature.

Lets compare 2 systems, and see how many variables we can identify. a "vacuum" cannon and a regular pneumatic cannon.

Both will have a 6 foot barrel of 1.5 inch PVC which has a nominal internal diameter of 40.894mm.  Both will use a regulation ping pong ball which is 2.7 grams and 40mm diameter.  Both will have a pressure differential of 1 atmosphere.  Both will have to contend with the friction of the ball against the barrel.  Both balls must travel through STP air after exiting the barrel.  Both cannons also experience some loss as the ball smacks against the sides of the barrel since they are of differing diameters.  This also causes some loss as air escapes into the low pressure side making the ball accelerate less.

There are a few small differences between the cannons which will change their performance.

The Vacuum cannon uses energy from the ball to break the far end seal, reducing it's velocity.  As the seal breaks, more air rushes into the barrel and presses back against the front of the ball (in what little room is left) further reducing velocity.

The regular pneumatic cannon must also move the ~730mm^3 of air in front of the ball which has a mass of ~0.94g.  This air will also have some water vapor in it, usually 2-3%, but water vapor is LESS dense than air so ~0.94g gives us a good worse case.  There will also be a small amount of friction between the air and the inner surface of the barrel.

So the question is.

Is less force required to accelerate that ~1g of air in front of the ball, or to break the leading seal?
Dream Dragon11 months ago
WOW! There's a lot of interesting information here, both in the instructable and in the comments especially from "Light_Lab". This is one of those "Instructables" that really captures the whole Ethos of "Free Exchange of information" and "Shared Experience".

Thanks guys!

It occurs to me that it SHOULD be possible to measure the elapsed time between the second membrane rupturing, and the projectile striking the Pendulum, for the purpose of comparison.

I also wonder, if the power comes from the pressure DIFFERENTIAL, could that be amplified by putting more pressure behind that first membrane? Ok, it would require a bit of trickery to release that, but...
Bill WW (author)  Dream Dragon11 months ago
Thanks for the comments, Dragon.

Measuring the elapsed time, or using high speed video, is next on my list. Seems everyone (including me!) is interested in verifying the speed.

I have the parts needed for phase II of the vacuum cannon; adding pressure. I will use a second, longer, length of PVC pipe, connected to the first using a flanged connection. The surfaces of the flanges will hold the membrane. The membrane will thus be between the vacuum in the cannon and the pressure in the second pipe. The membrane will be stronger, of multiple layers. Then, with vacuum in the cannon pipe, I will pressurize the second pipe until the membrane bursts. Will require a few trials. And for sure the speed will be supersonic, so will want credible speed measurement.
Light_Lab12 months ago
I like this Instructible a lot, so elegant in it's simplicity. I can suggest a few things from my experience with vacuum apparatus. With regard to the quality of the vacuum you are possibly worrying too much; the driving force for this is the pressure differential between atmospheric and the ~vacuum. There is not much difference between 14.9999 PSI and 14.9psi or for that matter 14.8psi. I wonder how well a modified electric tyre pump would work.
When I was doing ballistic research ~40 years ago we did extensive work with ballistic pendulums in comparison to a recently developed ballistic chronograph. We made ~ 100 pendulums of different designs and tested numerous firearms from air rifles up to high velocity hunting rifles. We found a very poor correlation between the pendulums and the highly accurate chronograph.
The reason was well known: the incomplete transfer of kinetic energy from the projectile to the pendulum. Using your device as an example: if the ping pong ball was not complete and round after the collision then part of the energy has gone to destroy the ball. We tried all sorts of bullet catchers: ballistic gels, cotton battens, sand, water sawdust etc with out much success. The interesting thing is that the pendulums always gave velocities lower than reality in our results!
Your device is an example of a pressure wave device; just like an organ pipe the air pressure is not distributed evenly along it's length at any time during firing. Yes, as some have pointed out, the air can't enter the tube faster than the speed of sound but we are not dealing with the speed of sound at atmospheric pressure.
The ball has inertia so the incoming air momentarily compresses behind the ball and then re-expands like a spring as the ball moves. All pressure wave devices from a two stroke engine exhaust, though pneumatic hammers to organ pipes are tuned for best performance. I believe that  the length of the tube and the position of the ball have a 'sweet spot' where best performance will be obtained.
It would be very interesting to use clear tubing, apply a smoke source near the inlet end and video it with a high speed camera.
Thanks for submitting. 
Bill WW (author)  Light_Lab12 months ago
Thank you for your comprehensive analysis, Light_Lab.

You said "The ball has inertia so the incoming air momentarily compresses behind the ball and then re-expands like a spring as the ball moves." Great observation, I had been wondering about that!
I plan to go to a local industrial supply store to get transparent PVC pipe; I know it is available. And will also look for high speed video camera.

I have several Aussie friends near Townsville, got my master's degree from James Cook University.
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