Punkin’ Chunkin’: An Engineering Look at 30 Years of Propulsive Gourd Excitement

by James Herzing26 October 2016

Since 1986, Punkin Chunkin has been an ever-growing Fall tradition. Put on by the World Championship Punkin Chunkin Association (WCPCA), hundreds of teams from across the U.S. (and sometimes, other countries) converge upon the farmlands of Southern Delaware for one weekend to see who can launch pumpkins the farthest.  This year, alone, there are 15 individual categories in which teams can enter, including youth, “theatrical” and human-powered, but they all center around three dominant themes:  trebuchet/catapult/torsion, centrifugal and air.

So in the spirit of Punkin Chunkin fun, let’s take a look at some of the engineering factors that go into a successful chunk.

Energy Conversion

At the heart of each successful chunk is the machine’s ability to convert stored potential energy into kinetic energy, for it is this factor that directly determines the launch velocity of the pumpkin.  Air cannons, in particular can achieve up to 500mph without vaporizing the pumpkins (which, Chunkers refer to as “pie” as in “pumpkin pie in the sky”), although some message boards claim that given certain conditions, some can achieve upwards of 700mph, thereby breaking the sound barrier.  Energy conversion also depends upon the style of launcher:

Trebuchets

For trebuchets, there is a heavy counterweight at the front that causes the launching arm (or beam) to swing forward when released.  Additional momentum is created with the sling that acts as a second fulcrum.

Catapults

Catapults feature either a cantilever-type spring (this type of catapult is called a “Mangonel”) or torsional spring (this type of catapult is also known as an “Onager”) that is tightened (thereby storing potential energy) and then released.  They are not as efficient or accurate as trebuchets.

Centrifugals

Centrifugals, although inappropriately named, use car or truck engine motors (there are also human-powered versions, too) to accelerate the pumpkin to an optimal speed before releasing - think of a helicopter spinning on its side.

Air cannons

Lastly, air cannons use the potential energy stored in a compressed air chamber.  When a valve is released the compressed air escapes through the barrel of the cannon launching the pumpkin.  To date, the longest chunks have been recorded by air cannons.

Launch Angle

As in any study of ballistics, it’s necessary to take into account the launch angle (or pitch) of the pumpkin as it leaves its respective launcher.  From basic trigonometry and a bit of intuition, we know that a 45 degree angle will typically yield the optimum distance, which is true...provided you’re in a vacuum (elevation differential between launcher and target will also affect the angle, however this is usually negligible for Punkin Chunking).  Therefore, most Chunkers account for air resistance and reduce the launch angle to anywhere between 30 to 40 degrees.  The question then becomes, what is the ideal angle given that range?  This, of course, depends upon several factors:

Type of launcher

The rule of thumb is that low-speed, low-profile launchers (like catapults, launchers without springs or human-powered contraptions) will cause the launch angle to be closer to the 40 degree mark, whereas the higher speed air cannons will be closer to 30.

Arm & pin angle

For air cannons, angle adjustment is as simple as moving the cannon up or down, but for trebuchets and catapults, there is a bit more involvement.  Typically, launch angle is adjusted through angle of the release arm and the angle of the pin (in the case of machines with slings).  Arm angle varies depending upon viewpoints, but there are two basic schools of thought: the first which maximizes energy conversion by stopping the arm at the moment of pumpkin release, and the second (in the case of using slings), which stops after the sling has rotated through its arc, maximizing velocity.  For pin angle, the general idea is that if the pin is parallel to the arm, it will release earlier and if it’s perpendicular to the arm it will release later.

Length of sling

Bottom line: the shorter the sling, the earlier the release; the longer the sling, the later the release.

Wind

One of the variables that cannot be controlled are atmospheric effects like humidity and wind.  While not much can be done to account for the humidity, Chunkers will often lower launch angle in the case of headwinds.

Pumpkin Selection

According to WCPCA rules, pumpkins must be between 8 and 10 pounds.  That said, those experienced analytical Chunkers scour among the accepted varieties of pumpkins (Caspers, Luminas and La Estrellas are the most common) for the most aerodynamic ones for their arsenals.  For those teams with trebuchets and catapults, it is desirable to find small, spherical, dense pumpkins with small divots or dimples, much like that in golf.  These divots tend to be a deciding factor when one looks at the Magnus Effect.

The Magnus Effect is the phenomenon of an upward force that occurs due to the airflow around a rotating spherical object, causing the object (the pumpkin) to attain some additional “hang time” due to a pressure differential between the top and the bottom surfaces.  The divots in the pumpkins magnify this effect, but only to a point - if too big, they can have an adverse effect on drag.

Putting it all Together

The 2016 World Pumpkin Championship will be taking place in Bridgeville, Delaware from November 4-6, and will be broadcast on the Science Channel on November 26 at 8pm EST.  115 teams will be competing in a variety of categories and are registered under some rather entertaining names like “Bustomatic”, “Shooda Noed Beter” and “Chunk Norris”, but at the end of the day, there will be winners and there will be losers.  Knowing what you know now, think you can pick out the winner before the first launch?  If so, maybe you should be competing too!