I'm thinking of making a liquid/solid fuel cannon sometime in the future, and I'm sharing my current sketch with the community to get feedback. The image was made to get the idea across, so some details are missing/inaccurate.
To shoot the gun,
1. pump air through the valve till the tank is at a sound operating pressure
2. close the valve and disconnect the pump
3. load the fuel through the barrel
4. ignite the fuel or fill the barrel with an inert gas(to prevent premature ignition)
5. load the ammunition through the barrel
6. heat the filament to the autoignition point of the fuel(in the case of gasoline ~500 °F)
7. and open the valve
PFAQ(predicted frequently-asked-questions)
Q: Why use a filament instead of a spark plug?
A: Due to the laws of thermodynamics, a filament will stay hot longer than a spark, giving the fuel a better chance to ignite
Q: Why liquid/solid fuel over gas?
A: It's denser and cheaper
The design was inspired by the NightHawkInLight project https://www.youtube.com/watch?v=q508d3k9518.
Feel free to experiment with the design. I don't have that much time or money on my hands, so it might be a few months(or even years) until you see progress from me. If you want the DXF, you can download it at https://dl.dropboxusercontent.com/u/102110774/LiSCa.dxf