Measurements keep pumpkins sailing

A data-acquisition system monitors the tension on a torsion catapult.

Martin Rowe, Senior Technical Editor -- Test & Measurement World, 12/15/2011 4:50:12 PM

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On Thanksgiving Day, the chances are pretty good that you watched football on television. But there was another televised competition you could have watched instead: Punkin Chunkin on The Discovery Channel.
 
The 2011 Punkin Chunkin competition ran from November 2 to 4 and was broadcast a few weeks later. The point behind “punkin chunkin” is for competitors to build a machine that can hurl pumpkins. During the competition, the pumpkins, which weigh between 8 and 10 lb, fly through the air propelled by air cannons, torsion catapults, centrifugal-force machines, and other mechanisms.  The air cannons hurl pumpkins more than 4000 ft., while other machines tend to hurl at shorter distances. Torsion catapults, for example, typically send pumpkins sailing between 1000 and 3000 ft.

FIGURE 1. Torsion catapults use twisted rope to apply force to a swinging arm that hurls an object. Courtesy of John Camping, Team ETHOS.

John Camping is a member of Team ETHOS (Experimental Torsion Hybrid Onager System) from Beavercreek, OH, which came in third among six competitors in the torsion division. A torsion catapult is based on a classical Greek design that uses rope wrapped around two posts (Figure 1). The throwing arm goes inside the loop created by the rope. The lever pulls back, twisting the loop. When the lever is released, the ropes try to straighten, and the energy stored in the ropes forces the arm forward, which hurls the pumpkin.

A few years ago, the tension on the vertical posts caused some damage and the team had to rebuild the machine. At that time, Camping integrated a data-acquisition system from Measurement Computing (Figure 2) into the machine to monitor the tension that the ropes apply to the two posts. He wrote an application note that explains how the catapult works and how the data-acquisition module helps it (Ref. 1).

Team members, called “chunkers,” take the annual competition quite seriously. They spend the year between competitions designing new machines or modifying existing machines. Camping explained that he programmed the data-acquisition system and machined some of the parts himself. A local machine shop donated time to fabricate the more complex parts. He estimated that donated shop time is worth between $15,000 and $20,000. Much of the material for the machine was also donated, and Camping estimates that the total cost runs between $70,000 and $80,000. Air cannons may cost as much as $150,000 because they need compressors and are more complex than other chunking machines.

FIGURE 2. A USB data-acquisition system monitors the force on the vertical posts. Courtesy of Measurement Computing.

While chunkers continuously modify and redesign their machines, they’re learning that the pumpkin itself can affect the distance it flies. Camping explained that the size, weight, and distribution of mass in a pumpkin can affect distance, depending on the type of machine used to hurl it.

Although Team ETHOS didn’t win in its division, its machine didn’t “pie:” Each pumpkin sailed away as opposed to breaking up at the machine. T&MW

REFERENCE
Camping, John, “Building a USB DAQ System for a Championship Pumpkin Catapult,” Measurement Computing, www.mccdaq.com/appnotes/an116.aspx.

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A version of this article first appeared in Martin Rowe's "Rowe's and Columns" blog.