Me and two friends need to build a tennis ball launcher using nothing but stored mechanical energy to propel it. Our concept is similiar to a mortar or cannon. We tried some comercial springs from Home Depot and even a spring from inside a sprinkler head, but the tennis ball does not have enough velocity to even get out of the barrel. We are considering using a mattress spring, but need some ideas on how to increase the velocity. Any suggestions are welcome. Thanks in advance.