Muzzle Velocity of a Potato Cannon

I played with a "Hypothetical" potato cannon once. I completely shattered a perfectly sound 1x6 Cedar fence board from about 30 ft. Fun "toy" er... "weapon"? Definitely do not want to be on the wrong end when the BBQ igniter is triggered.

_{Hint: Hair-spray will muck up the igniter, and make the end-cap hard to screw/unscrew, try starter fluid, or WD-40.}

I just hypothetically conceived of exactly the same setup. About the same size. I found that wd40 did not work AT all (virtually speaking). . Brought out the hair spray, put a bit too much in (since things had not gone well over the past half hour), kaboom! this virtual spud was still heading up over 150 foot trees 60 yards away.

Thank you.

I was about to add a follow up, but I did a little Googling and came up with my own answer on this one.

Using the Google calculator I arrived at 0.5 lb = 3500 grains.

Then at http://www.pyramydair.com/site/articles/formulas/ I plugged in 3500 grains and 154 ft/s and got 184.36 ft-lbs or 249.96 joules for the muzzle energy.

Jumping to http://www.recguns.com/Sources/VIIE8.html I found the muzzle energy to be above most 22s, about equal to a 38 and less than a 9mm.

Does it seem like everything I did was correct? Small diameter lead is very different from large diameter potato, but the comparison is interesting anyway.

Thanks,

Bruce

Yep, that all looks correct. To think of it another way, your (hypothetical) potato projectile, fired into a 184lb man, would lift him 1 foot off the ground! not an insignificant impact.

If you have an interest in the effects of small-diameter projectiles vs. large-diameter ones with the same energy, investigate what shooters refer to as meplat - it is a method of comparing how different-shaped projectiles transfer energy to the target.

Hypothetically-speaking, of course!

No surprise to those of us who worship at the altar of Hatcher (caliber and bullet weight). For real thrills, try oxy-acetylene in STEEL pipe launching tennis balls into low orbit. Hypothetically of course.

What does "g" represent?? Gravity?

good ole mr. newton, he would love this question.

using the equations above, i arrived upon the same muzzle velocity.

although the kinetic energy at the muzzle is the same for both the potato and the .308, the bullet obvesouly will hold its kinetic energy for much longer due to its aerodynamics and centrifugal stabilization properties from the rifiling, therefore as distance increases away from the muzzle, the bullet gets exponentially more deadly than the potato.

essentially the potato is only as deadly as the bullet for the first few couple hundred feet. still not bad tho.

just a suggestion... but i found that propane and nitrous oxide works much better as a fuel...

O₂ and gasoline work well, too. I had 1/4 inch wall, Al tube tennis ball cannon.

A Coors light can would just slip down the bore. It launched a tennis ball outta sight.

Pleeease tell me you properly disposed of the contents of the Coors can prior to using it as a projectile!

I don't drink Coors Light, but we had someone with us who did. Yes. The can worked best if it was about 1/2 full of water. The top, empty portion of the can would collapse.

Sounded like a shotgun and got us kicked out of a USFS campground near Crown King, Az.

In response to #6 & #8:

I'm not sure I grasp the use of the Coor's Light can. Was it just providing a better pressure seal than the tennis ball? Was the tennis ball too small for the tube? What was the water for?

Did you use gasoline in liquid form or did you try to vaporize the gasoline by shaking the tube like people did years (decades) ago with lighter fluid?

Sounds good, but I didn't know that nitrous oxide (in gaseous form) was available to the public. Was the N2O a medical product or somehow used for racing?

P.S. This should be under #5. Somehow I still don't have the hang of how to get a reply in the correct place.

AquaNet seems to pack the best pop... hypothetically, of course.

My favourite hypothetical fuel was calcium carbide. Use an ounce of water, drop the calcium carbide in, load the spud. Wait an experimental length of time then ignite with match by a small flash hole.

If you wait too long you have a nice little candle flame. Just right it packed a punch.

Hypothetically of course. Dad never figured out why his carbide cave lamp was always out of carbide!

No need to be hypothetical unless banned by state or local ordinance, I used to keep a copy of the BATF ruling that made spud guns "not weapons".

To the best of my knowledge it holds.

And Aquanet was gooey, but always ignited best for us.

Hypothetically rook scarers, the agricultural fireworks on a rope for bird scaring, give not only the propellant, and hypothetically an old brass shell case 2" diameter with a hole at the base where you can light the fuse might work like a charm. Just ram the spud on the top for a perfect seal. of course if you bore a rook scarer diameter hole into the spud, and insert a second rook scarer in the potato, fuse pointing back into the combustion chamber, you have a really evil baked potato delivery system.

I was a lot younger then, and had consumed excessive quantities of C2H5OH so questions like muzzle velocity, launch angle and personal survival were a long way down the list of concerns.

We christened this discipline "practical potato".

Simon

Thank you.

Any chance you might wish to share the equations for the graphs?

The equation provided is a good first order approximation and will usually get you close enough.

It doesn't include drag, which can be significant for a projectile of this size and weight. I ran a simple ballistic simulation which included air drag and came up with ~175 fps for the muzzle velocity. About 15% higher than the ideal equation. An even higher value, over 200 fps, would not surprise me.

I have seen many variants of the spud gun in action and believe the muzzle velocity can easily approach 300 fps under the right conditions. I also believe the shots that came out as just "mashed potato spray" would probably have exceeded 300fps.

These entertaining devices should be treated with care as the PVC shrapnel from a catastrophic failure can cause severe injury.

I made one of these around 1990. It was an Aquanet cannon. We bought super sized spuds, pushed them thru a steel die to form a perfect fit od and cut the ends square at 3 1/2 inches long. It was impressive. I put a small but powerful magnet into 10 spud projectiles and shot it through a chronograph. 217 to 245 fps. Our combustion chamber was 1.8 x the cubic capacity of the barrel and the barrel was 6 feet long. We used this ratio for all of the cannons we built.

I forgot to mention in my previous post that a mentor of mine who grew up in the 1920s used to take those 10 gallon metal milk cans and drill a hole in the base for a wick. Add a few cups of water and a handful of miners carbon (for acetylene outgassing), clamp on the lid and let it work for a bit. Maintaining elevation by holding a handle while straddling the can, ignition was applied and that 3 or 4 lb can lid would be launched 200 yards or so. Claims that a lot of windows in his school fell prey to his cannoneering skills.

I feel fairly free in revealing this in that I'm sure that the statute of limitations has run out on his juvenile delinquency.

I may have been involved in a period of spud-gun engineering and the fun that results. In a hypothetical competition we discovered barrel length had no effect on the distance the russet traveled. The BFG 8000 (8' barrel) didn't have the range of my launcher with a 4 1/2' barrel. Both where constructed similarly and both used ether for a propellant. The only difference was my 4 1/2' gun had the chamfer around the end of the barrel to shave the potato while forcing it into the barrel as well as a slight chamfer on the inside of the end of the barrel to slightly compress the potato as it was muzzle loaded. This creates a higher pressure launch. This was a highly guarded secret that I never shared with my competitors but I openly reveal to all my fellow CR4ers.

Warning; Always shoot from the hip - it's safer and it looks cooler.

As for the real-world verification, every paintball field near you will have a chronograph - most radar based - that are specifically designed for each of correctly measuring muzzle velocity.

I like the shoot-from-the-hip recommendation for "looking cool," but our adventure in applied hypotheticals ended up with an adjustable sighting-tube on the side and fired on the shoulder.

We postulated that two styrofoam cups full of paintballs, attached and masking-taped top-to-top, would form a nicely fitting football shape, similar to a potato.
Or a potatoe.
A third cup taped to the bottom made the assembly look cool and aerodynamic, though once launched, all appearance of shaped styrofoam dissipated into a nice cloud of snow, allowing the paintballs to rain down over a large area. Emphasis on the raining-down part, as the only remotely safe use of this multiple-warhead delivery system was from 100-yards-plus away, aimed well over the heads of intended recipients.

hypothetically...

Also hypothetically, since we only launched them (hypothetically) from a 3-man sling-shot (surgical rubber tubing, doubled, 15 feet long, attached to a pouch made from an old pair of levis, ... hypothetical design, that is), a tennis ball, filled with home-made napalm, makes a good fit to the proper sized PVC tube, and launches with extreme velocity. Makes a really cool spash of fire at the other end of the flight.

Also, in the hypthetical spirit of the day, using an Estes rocket, with a D-6 motor (that's a big sucker with a lot of boost), in a rocket body with the proper diameter, put a 12-gauge shotgun round in the upper section of the rocket body, primer aft, with a shortened and filed down nail-point touching the primer, and a small weight taped to the nail's flat head to increase forward moving energy in the end phase of the flight, insert the whole assembled rocket in a slightly larger piece of PVC (say a 1 or 1 1/2 inch ID tube, depending on your rocket model). Shoulder fire that, or better yet, fire it remotely from 20 feet away, but in any case toward a solid object (trees, hypothetically, work wonderfully) and at the moment of impact, you have a shot-cloud projecting forward, right into the tree. Of course, a paper or plastic shotgun round hull tends to burst in ALL directions, but then it just acts like an impact-fused grenade.

Pretty spectacular (hypothetically!!!) to do either of these at night. And yep, we had lots of ideas about how to increase the military usefulness of either of these. Hypothetically, of course!

Then hide before the police come round to see what's up.

Only hypothetically, of course.

Use 213 ft/s from #17.

Plug it into the two links from #3, use 3500 grains again, and get 352 ft-lb & 478 Joules.

Thus, an almost perfect match for the energy of a 9mm, hypothetically.