A ramjet is an engine used in aircraft. The engine design is simple when compared to most other jet engines. The source of the engine's power is the compressed air forced to enter the engine when it reaches high speeds. As such it is only functional once the aircraft or device has reached certain speeds, and not before.
In 1908, the ramjet was discovered and patented by Rene Lorin. Lorin attempted to construct a subsonic ramjet, but was unable. It was not possible to produce a working model, as the ramjet requires speeds approaching supersonic and above in order to function efficiently, which no aircraft was capable of reaching or maintaining during his lifetime. In 1933, shortly after Lorin's death, French engineer Rene Leduc patented a ramjet engine model. He created the Leduc 0.10 in 1949. The Leduc 0.10 used only ramjet propulsion to maintain flight once it had achieved to the necessary speeds.
The ramjet design is very simple and has no moving parts. Thrust is generated by compressing the air passing through the combustion chamber. The air slows and is ignited with some type of fuel. The exhaust created by this process then is ejected through a nozzle at speeds far greater than the speed at which the air originally entered.
Turbojet engines are commonly used in modern jet-propelled aircraft. They uses a rotary system to actively gather air in front of the engine and pass it into a combustion chamber. The air initially gathered is not suitably compressed to facilitate the combustion, so a device called a compressor is used. The many moving parts in these engines make them heavier and increase the potential for problems.
Due to the lack of moving parts, the ramjet design is significantly lighter than its turbojet counterpart. Unlike the turbojet, though, the ramjet is not capable of movement from a standing position. It must first reach a required speed in order for the necessary natural compression process to begin, making it inefficent for speeds below the subsonic level. This limits its application.
The ramjet reaches top efficiency at around Mach 3, or three times the speed of sound with is 2,283 miles per hour (3,675 km per hour) at sea level. The ramjet is capable of speeds ranging from Mach 0.5 up to Mach 6. The low pressure prior to Mach 1 produces very small amounts of thrust. Once the ramjet reaches speeds above Mach 5 it becomes unproductive.
In order to reach the required speeds, the ramjet powered craft must use another source of power. This might be another engine built into the craft, although increases cost, size and weight, or rocket propulsion during the initial flight. Once it has achieved sufficient speed the ramjet will begin to work and can power the craft. Ramjet engines will not function outside of the Earth's atmosphere as there is no air to compress.
Ramjets are used by militaries for use in missiles. Some aircraft, such as the high speed and altitude SR-71 Blackbird previously flown by the United States Air Force, use hybrid forms of ramjet-turbojet engines, which is considered an expensive practice. Research is being conducted to discover other uses of ramjet technology.