The ramjet engine began to receive attention during the second half of the 1940s and reached its peak during the 1950s. The ramjet has been called a flying stovepipe, for the absence of rotating parts that characterize the turbine engine. The ramjet gets its name from the method of air compression-, it cannot operate from a standing start but must first be accelerated to a high speed by another means of propulsion. The air enters the spike-shaped inlet and diffuser (fig. 70) which serve the same purpose as the compressor. Fuel is injected and burns with the aid of flameholders that stabilize the flame. The burning fuel imparts thermal energy to the gas, and the expansion through the nozzle at speeds greater than the entering air produces the forward thrust. The ramjet, always needing an auxiliary propulsion system for starting, got squeezed between improved turbine engines and rockets during the 1950s and never recovered.