I'm trying to figure out a way to get better results in a "pumpkin launching" contest. The goal is to launch an 8-12 lb pumpkin (or bowling ball) as far as possible. One method is going to use elastic, bungee or large rubber/latex bands as the method of propulsion. The problem is there are limiting factors which impede this design the larger it gets (cost being one of them). I was thinking of a way to help infuse more power into the system with other means of accelerating the projectile but am unsure if it will aid in the overall performance. I have considered using compressed coil springs, each with a 2000lb compression capacity and 8-10 inches of expansion capacity. The springs (4) would be used to move the entire launch device in the same angle and direction of launch (obviously causing stress on the launch device upon reaching terminus). At some point during the expansion of the springs, the primary firing mechanism would release the elastic bands. Obviously 8-10" of movement allows very little time to initiate launch so I have considered using a block and tackle with multiple pullies to increase distance & time of travel, reduce damage to launch mechanism. As the springs decompress, the entire mechanism accelerates later initiating the final launch. My question is if this theory in using two methods to power the launch is scientifically sound. If the launch device is moving at say 3ft/.1 seconds = 30 fps = 20mph, how will this affect the item being launched?