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Technology for storing energy in springs has come a long way. Each time I go to the park I marveI at the large springs which see second use as playground equipment, supporting a plane or horse or friendly whale which small kids can ride. Or grownups can ride.
I propose that a good human powered
pumpkin chunkin catapult could use coil springs to store human powered energy. Coil springs as used in big truck suspensions are cheap and robust. They are meant to be compressed. Also, the springs could be stored inside a strong pipe, which would keep them vertical and provide safety in the case of catastrophic failure. Springs could be in a group of 3 or 4, compressed with a manhole cover. These spring groups could stacked atop each other if necessary. Human compression would take place via a bicycle attached using gears. On release the springs would transfer energy to the catapult arm using a series of pulleys: total excursion of releasing springs would probably only be a few feet and so pulleys would be necessary to translate into high velocity for punpkin throwing arm.
Punkin Chunkin rules
There is a 1 human power class for machines. [bungston, Jun 21 2010]