Ramjet Guide, Meaning , Facts, Information and Description
This page is about the type of jet engine; for the method of interstellar travel, see Bussard ramjet.A Ramjet is a type of jet engine designed by William Avery.
Ramjets reduce engine complexity by eliminating most of the moving parts: they have no fans to do the compression in the front of the engine.
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 very effective, and can compress enough oxygen to efficiently burn the fuel for the engine all on its own.
Ramjets essentially do not work at all below half the speed of sound, and ramjets are inefficient due to low compression ratios until the airspeed exceeds 600 mph (1000 km/h). However, they outperform traditional jet engine designs at supersonic speeds, and although inefficient at the slower speeds, are still more fuel efficient than rockets.
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 hence is lighterweight than a turbojet and is particularly useful in applications requiring a small and simple engine for high speed use.
On the downsides they need to be flying at high speeds to start, making them less than useful for general tasks. 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, and they tend to work best at one designed speed and altitude.
They have found most 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.
Ramjets always slow the incoming air to subsonic speeds. Scramjets, or "supersonic combustion ramjet" are similar to Ramjets in that they rely on the ram effect for compression, but the air goes through the entire jet at supersonic speeds.
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This is an Article on Ramjet. Page Contains Information, Facts Details or Explanation Guide About Ramjet Aircraft Using Ramjets
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