RAMJET ENGINE

A ramjet is a type of athodyd (where athodyd stands for Aero THermODYnamic Duct) airbreathing jet engine that was first proposed by Rene Lorin in 1913. It uses the forward motion of engine to compress the incoming air, without a rotary compressor. Thus a ramjet is much like a valve less pulsejet but it operates with continuous combustion rather than the series of explosions that give a pulsejet its characteristic noise.

The ramjet has been called a flying stovepipe, because it is open at both ends and has only fuel nozzles in the middle.

Construction

It consists of:

1. Supersonic diffuser

2. Subsonic diffuser

3. Combustion Chamber 

4. Discharge nozzle 

The function of supersonic and subsonic diffusers are to convert the kinetic energy of the entering air into pressure energy. This energy transformation is called ram effect and the pressure rise is called the ram pressure. The function of nozzle is to convert pressure energy of gas into kinetic energy.

Working

Air from the atmosphere enters the engine at a very high speed and its velocity gets reduced and its static pressure is increased by supersonic diffuser. Then the air passes through the subsonic diffuser and its velocity further reduces to subsonic value. Due to this, the pressure of air increases to ignition pressure. Then the high pressure air flows into the combustion chamber.

In the combustion chamber, the fuel is injected by suitable injectors and the air fuel mixture is burnt. The highly heated products of combustion gases are then allowed to expand in the exhaust nozzle section. In the nozzle pressure energy of the gas is converted into kinetic energy. So the gases coming out from the unit with very high velocity

Due to high velocity of gases coming out from the unit, a reaction or thrust is produced in the opposite direction. This thrust propels the air craft. Ramjet produces very high thrust with high efficiency at supersonic speeds. So, it is best suitable for high speed aircrafts. The air enters the engine with a supersonic speed must be reduced to subsonic speed. This is necessary to prevent the blow out of the flame in the combustion chamber. The velocity must be small enough to make it possible to add the required quantity of fuel for stable combustion.

Advantages

1. Ramjet engine is very simple and does not have any moving part.

2. Cost is low

3. Less maintenance

4. The specific fuel consumption is better than other gas turbine power plants at high speed.

5. There is no upper limit to the flight speed.

6. Light weight when compared with turbojet engine.

7. Higher temperatures can be employed.

Disadvantages

1. Since the take-off thrust is zero, it is not possible to start a ramjet engine without an external launching device.

2. The combustion chamber required flame holder to stabilize the combustion due to high speed of air.

3. It is very difficult to design a diffuser which will give good pressure recovery over a wide range of speeds.

4. It has low thermal efficiency.

5. Bad performance at lower speeds.

6. Needs booster to accelerate it to a speed where ramjet begins to produce thrust.

7. Maximum operating altitude is limited.

Applications

1. It is widely used in high speed aircrafts due to its high thrust and high operational speed.

2. Widely used in missiles - 

(a) Air-to-air missiles (MBDA Meteor, AIM-120 AMRAAM, Vympel RVV-AE-PD R77M (AA12 Adder), and Gorgon IV)

(b) Air-to-surface missiles (EADS/AMM, ASMP, ASMP-A, Radouga Kh41 Moskit and Zvezda Kh31P (AS17 Krypton) Radouga 3 M80/82 Moskit (SSN22 Sunburn), CPMEC C301 (coast to ship), and Hsiung Feng 3).

(c) Surface-to-air missiles (MBDA and LFK NG (USA), Bloodhound Mk2, Rapier and Sea Dart (UK), Mistral and Eurosam Aster (France).

(d) Anti-ship missiles (Machinostroenie 3 K55, Yakhont (SS N26), ANS (liquid fueled ramjet; reaches Mach 4 Range 25 km), and C101 & C103 (ranges 50 and 100 km).

3. Subsonic ramjets are used in target weapons.