Ramjet Engine

RAMJET ENGINE:

A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets therefore require forward motion through the air to produce thrust.

The ramjet was invented in 1913 by French inventor René Lorin, who was granted a patent for his device. Ramjets require considerable forward speed to operate well, and as a class work most efficiently at speeds around Mach 3. This type of jet can operate up to speeds of at least Mach 5.

DESIGN:

A ramjet is designed around its inlet. An object moving at high speed through air generates a high pressure region in front and a low pressure region to the rear. A ramjet uses this high pressure in front of the engine to force air through the tube, where it is heated by combusting some of it with fuel. It is then passed through a nozzle to accelerate it to supersonic speeds. This acceleration gives the ramjet forward thrust.

Ramjet componets can be divided in various groups as classified as below:

1. Inlets-Ramjets try to exploit the very high dynamic pressure within the air approaching the intake lip. An efficient intake will recover much of the freestream stagnation pressure, which is used to support the combustion and expansion process in the nozzle.The Inlet is divergent, to provide a constant inlet speed of Mach 0.5

2.Combustor-As with other jet engines the combustors job is to create hot air. It does this by burning a fuel with the air at essentially constant pressure. The airflow through the jet engine is usually quite high, so sheltered combustion zones are produced by using flame holders that stop the flames blowing out.

3. Nozzle-The propelling nozzle is a critical part of a ramjet design, since it accelerates exhaust flow to produce thrust.
For a ramjet operating at a subsonic flight Mach number, exhaust flow is accelerated through a converging nozzle. For a supersonic flight Mach number, acceleration is typically achieved via a convergent-divergent nozzle.

RAMJET TYPES:

Ramjets can be classified according to the type of fuel, liquid or solid; and the booster

A ramjet generates no static thrust and needs a booster to achieve a forward velocity high enough for efficient operation of the intake system. The first ramjet powered missiles used external boosters, usually solid-propellant rockets, either in tandem, where the booster is mounted immediately aft of the ramjet.

Integrated boosters provide a more efficient packaging option since the booster propellant is cast inside the otherwise empty combustor. This approach has been used on solid, for example SA-6 Gainful, liquid, for example ASMP, and ducted rocket, for example Meteor, designs. Integrated designs are complicated by the different nozzle requirements of the boost and ramjet phases of flight. Due to the higher thrust levels of the booster a different shaped nozzle is required for optimum thrust compared to that required for the lower thrust ramjet sustainer. This is usually achieved via a separate nozzle which is ejected after booster burnout.

FLIGHT SPEED:

Ramjets generally give little or no thrust below about half the speed of sound, and they are inefficient (less than 600 seconds) until the airspeed exceeds 1000 km/h (600 mph) due to low compression ratios. Even above the minimum speed a wide flight envelope (range of flight conditions), such as 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 generally outperform gas turbine based jet engine designs and work best at supersonic speeds (Mach 2-4). Although inefficient at slower speeds they are more fuel-efficient than rockets over their entire useful working range up to at least Mach 5.5

THE below video gives a slight detail about the pulse jet ramjet engine:

http://www.youtube.com/watch?v=b0KJwa5iWTY