A pulse-jet is a type of air-breathing engine in which air is compressed by a series of spring-loaded, shutter-type valves located ahead of the combustion section. As in a ramjet, compression is achieved without the use of a compressor; however, whereas combustion in the ramjet is continuous, in the pulse-jet it is intermittent. Air is admitted through the valves, and combustion begins. This increases the pressure and closes the valves, preventing backflow through the inlet. As the gases expand through the rear nozzle to produce thrust, the pressure in the combustion section drops to the point where the valves open again to admit fresh air. The cycle is then repeated. The basic idea for the pulse-jet came from Paul Schmidt, of Munich, at the start of World War I but its most infamous manifestation was the German V-1 "buzz bomb."