|MadSci Network: Physics|
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.
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