Ramjet
A Ramjet is a type of jet engine designed by William Avery.The basic principle of a ramjet is the same as that in a jet engine: intake, compression, combustion, exhaust. (Note: it would be very helpful to understand the principles of operation of a turbojet jet engine in order to fully understand this discussion)
When air enters any jet engine, its speed decreases and its pressure increases, called the ram compression effect. At high speeds this process can be fairly effective, and can compress enough oxygen to efficiently burn the fuel for the engine all on its own. Typically, ramjets do not work effectively until the airspeed exceeds 600 mph (1000 km/h) - they do not outperform traditional jet engine designs until supersonic speeds.
Ramjets reduce engine complexity by eliminating most of the moving parts: the fans to do the compression in the front of the engine. A wide flight envelope (range of flight conditions), such as both low to high speeds and low to high altitudes, can force significant design compromises.
Ramjets are built to utilize the compression effect through a careful inlet design. Beyond that the engine is largely nothing more than a well-designed tube. A ramjet thus contains no (major) moving parts and is particularly useful in applications requiring a small and simple engine for high speed use. On the downside they need to be flying at high speed to start with, making them less than useful for general tasks. They have found use almost exclusively in missiles, where they are boosted to operating speeds by a rocket motor, or by being attached to another aircraft (typically a fighter). Today ramjets have been generally replaced by small turbofans or rockets.
A variant of the pure ramjet is a 'combined cycle' engines, intented to overcome the limitations of the pure ramjet. An example of this is the Air Turbo Ramjet (ATR). It operates as a conventional turbojet at subsonic speeds and a fan assisted ramjet at speeds below mach 6.
The ATREX engine developed in Japan is an experimental implementation of this concept. It uses liquid hydrogen fuel in a fairly exotic single-fan arrangement. The liquid hydrogen fuel is pumped through a heat exchanger in the air-intake, simultaneously heating the liquid hydrogen, and cooling the incoming air. This cooling of the incoming air is critical in achieving a reasonable efficiency.
The hydrogen then continues through a second heat exchanger positions after the combustion section, where the hot exhaust is used to further heat the hydrogen, turning it in a very high pressure gas. This gas is then passed through the tips of the fan providing driving power to the fan at sub-sonic speeds. After mixing with the air, it's then combusted in the combustion chamber.
An advanced version of the ramjet is the scramjet, or "supersonic combustion ramjet."
See also:
Aircraft Using Ramjets
Historical Development
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