Before any of you ask, I am not attempting to strap a rocket engine to my car. My real goal is to make a rocket powered boat(think aluminum fishing boat with single seat at front, jet ski steering, and a bigass rocket engine in the back). At the moment I am trying to make a small rocket engine for a little boat(non solid fuel). I have done a lot of research into rocket design and theory. I even sawed up a propane canister and put a simple nozzle on the end of it to spray the fuel and some holes to cause the rocket to "inhale" air. Unfortunately this design does not benefit from the burning of the fuel(propane)much at all. What I need help with at the moment is designing the rocket body. How do I calculate the dimensions of the rocket? I dont need super precision in the intial design, so simple percentile figures would be fine. Also, how do I begin the ignition proccess in the combustion chamber? I'm thinking a spark plug would be good. Any other links to resources and tips are appreciated!