a simple propulsion engine utilizing unheated atmospheric air as the propellant , and driven by a single cycle engine with internal combustion cylinder and free piston is disclosed . a free piston with an annularly arranged thrust piston to divide a dual - diameter cylinder into two combustion chambers and two thrust chambers is provided . scavenge feeder lines connected the thrust chambers to the combustion chambers via check valves provide exhaust scavenging , additional thrust output through exhaust nozzles , and feeding of fresh air into the combustion chambers . also , pressure - actuated fuel injectors utilize pressure changes in respective combustion chambers to inject fuel at the appropriate time . the fuel injector includes an intensifier piston and pintle to raise the fuel pressure .