A ramjet, sometimes referred to as a stovepipe jet, is a type of jet engine. The idea was patented as early as 1908 by René Lorin, but it only became reality with the works of René Leduc (who was stopped by France's occupation in World War II and resumed after the war) and William Avery in the United States.
Ramjets reduce engine complexity by eliminating most of the moving parts; the speed of an aircraft must be enough to compress air at the ramjet's inlet, eliminating the need for fans.
The basic principle of a ramjet is the same as that in a jet engine: intake, compression, combustion, exhaust. 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 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 with a combustor in the middle and a supersonic nozzle. A ramjet thus contains no (major) moving parts, and hence is more lightweight than a turbojet and is particularly useful in applications requiring a small and simple engine for high speed use.
Normally the combustor must be capable of operating over a wide range of throttle setting, for a range of flight speeds/altitudes. Usually a sheltered pilot region enables combustion to continue when the missile intake undergoes high yaw/pitch, during turns. Overfuelling the combustor can cause the normal shock, within the intake system, to be pushed forward beyond the intake lip, resulting in a substantial drop in engine airflow and net thrust.
Ramjets generally give little or no thrust below about half the speed of sound, and they are inefficient (less than 600 seconds due to low compression ratios) until the airspeed exceeds 600 mph (1000 km/h). Even above the minimum speed 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 optimised for one designed speed and altitude (point designs). However, ramjets tend to outperform traditional jet engine designs at supersonic speeds (mach 2-4), and although inefficient at the slower speeds, are still more fuel-efficient than rockets within the atmosphere.
They are found almost exclusively in missiles, where they are boosted to operating speeds by a rocket engine, or by being attached to another aircraft (typically a fighter).
Ramjet propulsion is used in the British Bloodhound (no longer in service) and Sea Dart surface-to-air missiles.
A number of missile projects currently under development use ramjet engines to achieve better fuel efficiency (and thus longer range) at supersonic speeds than a rocket-driven approach. These include the British MBDA Meteor air-to-air missile and the Russian-Indian BrahMos supersonic cruise missile.
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.
A variant of the pure ramjet is the 'combined cycle' engine, intended to overcome the limitations of the pure ramjet. One example of this is the SABRE engine. Another example of this is the Air Turbo Ramjet (ATR) which 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.
During the cold war, the United States designed and ground-tested a nuclear-powered ramjet called Project Pluto. This system used no combustion - instead, a nuclear reactor heated the air. The project was ultimately canceled because ICBMs seemed to serve the purpose better, and because a low-flying missile would have been highly radioactive.
The SR-71's Pratt & Whitney J58 engines act as ramjets at high-speeds (Mach 3.2).
Aircraft using ramjets
- D-21 Tagboard
- Lockheed X-7
- Ram accelerator
- Aircraft engines
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