Re: How can I create a tennis ball launcher using propellants?

If you're looking for inexpensive propellant, compressed air is the best 
route. You can purchase low-cost tire pumps that plug in to a car's 
cigarette lighter, whioh you could use as a source of compressed air for 
whatever device you create. As to the workings of the device, you will need 
to create some means of storing the air and then releasing it when it 
reaches a predetermined pressure. A simple pressure valve connected to one 
end of a closed steel pipe would work, but you would need to explore the 
cost of that - contact someone in your city who handles compressed air 
supplies to find the going rate.